MALARIA EPIDEMICS DETECTION AND CONTROL FORECASTING AND PREVENTION '

WORLD HEALTH ORGANIZATION Distr.: LIMITED

?L'$

WHOh4ALl98.1084

'

ORIGINAL: ENGLISH

MALARIA EPIDEMICS DETECTION AND CONTROL FORECASTING AND PREVENTION

J.A. Najera, R.L. Koumetsov and C. Delacollette

World Health Organization Division of Control of Tropical Diseases

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TABLE OF CONTENTS

LLST OF GRAPHS

. . .

v

LIST OF TABLES

. . .

vii

I

.

GENERAL CHARACTERISTICS OF MALARIA EPIDEMICS 1 1.Introduction

. . .

1

1

.

1 . Historical background

. . .

2

l

.

2 Major determinants of malaria epidemics

. . .

4

2

.

Epidemic Waves and Periodicity

. . .

5

2.1. Epidemic waves of different Plasmodium species

. . .

5

2.2. Epidemic periodicity

. . .

8

3

.

Classification of Major Epidemic Types

. . .

1 1 3.1. True epidemics

. . .

1 1 3.2. Resurgences or failures of control

. . .

20

4

.

Classification of Epidemic Risk

. . .

22

. . . .

11 EARLY DETECTION

AND

CONTROL OF EPIDEMICS 25 5

.

Recognition and Epidemiological Investigation

. . .

25

5.1. Confirmation and initial assessment

. . .

25

5.2. Epidemiological investigation

. . .

27

6

.

Control of Epidemic Situations

. . .

28

6.1. General principles of control

. . .

29

6.2. Formulation and implementation of a control plan

. . .

30

6.2.1. Relief operations

. . .

30

6.2.2. Mobilization of resources and logistics

. . .

30

6.2.3. Planning transmission control

. . .

31

.

7 Disease Management

. . .

32

7.1. Diagnostic and treatment facilities

. . .

32

7.2. Improving the quality of care

. . .

33

. . .

7.3. Public information and communication 35 8

.

Transmission Control

. . .

36

8.1. Emergency control

. . .

36

8.2. Prevention and control of transmission

. . .

38

8.2.1

.

Indoor residual spraying

. . .

39

8.2.2. Impregnation of bednets

. . .

43

iii

I11

.

EPIDEMIOLOGICAL SURVEILLANCE. FORECASTING

AND . . .

PREVENTION OF EPIDEMICS 44

. . .

9

.

Epidemiological Information Systems 44 9.1. Identification of indicators of epidemic risk

. . .

46

. . .

9.2. Field investigations 47

. . .

9.3. Geographical information systems 51

. . .

10

.

Risk Detection and Forecasting 52

. . .

10.1. Monitoring of morbidity and mortality 53 10.2. The spleen rate as an indicator of herd immunity

. . . .

55

. . .

10.3. Monitoring entomological variables 55

. . .

10.4. Monitoring meteorological variables 56

. . .

10.5. Monitoring socioeconomic variables 58

. . .

10.6. Comprehensive monitoring of epidemic risk 61

. . .

11

.

Emergency Preparedness and Epidemic Prevention 63

. . .

11.1 Preventive measures 64

REFERENCES

. . .

67 ANNEX

. . .

75

LIST OF GRAPHS

Graph 1 Separate and combined curves of epidemic

. . .

6

Graph 2 Monthly malaria morbidity. Posada. Sardinia. 1930

. . .

6

U Graph 3 Malaria cases diagnosed in Palestine

. . .

7

Graph 4 Paraquinquennial periodicity in the northern para-equatorial

. . .

zone S

.

Graph 5a Epidemic malaria death rate due to A Albimanus

. . .

9

.

Graph 5b Epidemic malaria death rate due to A Darlingi

. . .

9

Graph 6 Malaria cases reported in Nicaragua and Honduras (1962 . 1990)

. . .

10

Graph 7 Malaria cases reported in Iran (1 962 1990) .

. . .

10

Graph 8a Seasonal malaria incidence in relation to temperature and humidity. Qatif Oasis. l946

. . .

14

Graph 8b Seasonal malaria incidence in relation to temperature and humidity. Qatif Oasis. 1947

. . .

14

Graph 9 Registered malaria morbidity & mortality in Italy

. . .

15

Graph 10 Registered malaria cases and deaths in Spain

. . .

16

Graph 11 Malaria prevalence rates in Gerzira. children 2-9 years old

. . .

21

Graph 12 Malaria cases reported in Sri Lanka

. . .

22

Graph 13 Nile river levels at Khartoum

. . .

48

Graph 14 Monthly rainful at Khartoum

. . .

49

Graph 15 Blue Nile levels at Wad Medani: normal channel (1 970- 1990)

. . .

50

Graph 16 Brazil . malaria cases reported

. . .

58

Graph 17 Malaria positive cases in the Cereno district (1970-1974)

. . .

59

Graph 18 Malaria incidence in Costa Rica compared with banana production (1980-1992)

. . .

60

Graph 19 Malaria incidence in El Salvador compared with cotton production (1980 1990) .

. . .

60

b

LIST OF TABLES

. . .

Table 1

.

Insecticides used for residual indoor spraying ⁴¹

. . .

Table A True Epidemics 77

. . .

Table B Resurgences or Failures of Control 81

vii

I. GENERAL CHARACTERISTICS OF MALARIA EPIDEMICS

1. Introduction

Malaria has been one of the major causes of devastating epidemics in the past.

They occur periodically following excessive rains and floods in arid areas or periods of drought in fertile river valleys, compounding the misery often brought about by such meteorological calamities. Malaria epidemics have obstructed efforts to colonize jungle areas or to irrigate dry lands and have plagued displaced populations and refugees. They erupt following the devastation of war and spread with the demobilization of armies, thereby hampering the quick reconstruction of rural life.

The etymology of the word epidemic, from the Greek f ~ n t 8 q p i ~ 1 , arrival or stay in a country (similar to the English visitation), from the verb krr6q~6o-Q, to come or to reside as a foreigner, shows the essential element of this strange phenomenon. The concept of 'epidemic' is itself a relative one. In principle, any sudden increase in disease incidence beyond what is considered normal will constitute an epidemic. As Brks (1986) comments, it would be an error to consider as an epidemic a hitherto unrecognized endemic situation or a mere seasonal increase in the incidence of a disease. It would also be an error to neglect the significance of a single case of a new disease in a country, which might well be the prelude to a further dramatic spread of disease.

An essential prerequisite for the occurrence of an epidemic is the existence of a large enough number of susceptible persons who are likely to become clinically ill when suddenly exposed to infection. There is therefore an inverse relationship between endemicity and epidemicity: malaria epidemics cannot affect the populations of highly endemic areas who develop sufficient immunity early in life.

The actual impact of epidemics depends not only on the increase in specific morbidity, but also on the general health of the affected population. Many malaria epidemics coincide with periods of famine, economic crisis, war or civil disturbances, affecting impoverished or displaced populations that are not only physically weak, but often affected by other diseases and unable to obtain appropriate medication. Malaria epidemics do not always cause dramatic emergencies; instead, they more frequently affect economic development.

They may flare up suddenly and subside at the end of one season and not return for several years, or build up over several transmission seasons. The early stages of an epidemic may often pass unnoticed, as malaria is still easily treated and antimalarial drugs are widely available almost everywhere.

Page 2

The presentation and spread of malaria epidemics varies considerably. At one extreme, they may arise in an apparently explosive way, affecting areas covering hundreds of thousands of square kilometres at practically the same time and then subside after a year or two. At the other extreme, they may progress slowly from locality to locality, or valley to valley, taking several years to spread and showing little tendency to subside.

1.1. Historical background

From historical accounts, it may be difficult to ascribe to malaria some of the major fever epidemics of the past with certainty, but during this century, there have been specifically diagnosed malaria epidemics, which clearly show the magnitude of the impact that they can produce:

the explosive and disastrous epidemic which affected large parts of India in 1908 was officially estimated to have attacked 100 million people and caused about one million deaths;

m the aftermath of the First World War and in the midst of civil war, the 1922-1923 epidemic in the Soviet Union caused more than 10 million cases and at least 60 000 deaths;

the Ceylon (Sri Lanka) epidemic of 1934-1935 caused nearly 3 million cases and 82 000 deaths;

the epidemic in north-eastern Brazil, following the invasion by A. gambiae, caused over 100 000 cases and at least 14 000 deaths in

1938;

in 1942, a similar invasion by A. gambiae of Lower Egypt caused some 160 000 cases and more than 12 000 deaths;

in 1958, an epidemic in Ethiopia caused more than 3 million cases and 150 000 deaths;

in 1963, in Haiti, hurricane Flora disrupted the ongoing malaria eradication campaign and caused 75 000 cases;

the epidemic in Sri Lanka in 1968, which occurred while most of the country was in the consolidation phase of the eradication programme, caused 1.5 million cases during 1968-1970, thus revealing the fragility of this particular phase of the campaign;

the epidemic of 1976-1977 in the Indian subcontinent caused more than 7 million cases;

that of south-eastem Turkey in 1977-1978 caused some 270 000 cases (Bruce-Chwatt, 1985).

Page 3 Malaria epidemics are not a phenomenon of the past. Severe epidemics have recently occurred:

in Afghanistan since the beginning of the civil war in 1979;

in northern Iraq and southern Turkey, 1993-1995;

in Tajikistan, 1993-1994;

in Azerbaijan, 1993-1994;

in north-western India, particularly Rajasthan, 1995- 1996;

in southern and east Africa (Zimbabwe, Botswana, Mozambique, Swaziland and South Africa), 1996.

Apart from cyclical meteorological phenomena, new factors increasing the risk of severe evidemics include the economic crises and the widestlread civil unrest affecting many countries of the so-called third world.

The disastrous epidemic of 1908 in the Punjab gave rise to a sustained programme for the study of malaria epidemiology, with particular emphasis on meteorological and physiographical conditions (Gill, 1928). The postwar epidemics of 1920- 192 1, which affected many countries of Europe, particularly the USSR, and pushed the limit of malaria transmission beyond the Arctic Circle, spurred governments to support malaria control and to pay special attention to the prevention and control of epidemics. In the Punjab, Gill (1923) was asked to elaborate a system of epidemic forecasting and control, which was progressively developed and adapted to other epidemic-prone areas and which, after the severe epidemic of 1934-1935 in Ceylon, received the full support of the Malaria Commission of the League of Nations (League of Nations, 1938).

The search for methods of epidemic forecasting was nevertheless discontinued as during the 1950s and 1960s, it was believed that malaria could be eradicated. As a result, all the efforts of malariologists were focused on the management of mass campaigns of insecticide spraying or drug distribution, the epidemiology of declining malaria, the detection of residual foci and the prevention or elimination of resurgences of transmission.

The return to a strategy of malaria control in the 1970s occurred at a time of economic crisis, the unprecedented development of transport facilities and the globalization of the economy. These factors created population pressures and migration for economic reasons, which resulted in new and sometimes disastrous focal outbreaks of malaria. At the same time, the reduction or discontinuation of control efforts in many areas produced resurgences of sometimes dramatic proportions, while the periodic epidemic waves returned to the well-defined epidemic-prone areas. The rise in temperature over recent

WHOMAL/98.1084 Page 4

years may have been an important contributory factor in the occurrence of the numerous local epidemics in highland areas in many parts of the world.

1.2 Major determinants of malaria epidemics

An epidemic is the result of the disturbance of a previously existing equilibrium of the ecological system comprising human, parasite and vector populations in a particular environmental niche. Depending on the resilience of the system, and whether or not the disturbance has changed some of its essential components, it will either return to its previous state of equilibrium after the end of the disturbance, or tend to find a new equilibrium, with or without going through a period of oscillation. The epidemiological history of the area and a study of the current situation will indicate the stability of the possible states of equilibrium which may be reached. In this sense it is useful to distinguish epidemics which are the result o f

temporary disturbances of a stable hypoendemic equilibnum, such as those resulting from abnormal meteorological conditions; these epidemics, if left to themselves, will return to the previous hypo- or mesoendemic situation;

major changes in the eco-epidemiological system, shifting towards a new equilibrium of higher endemicity, such as those resulting from major environmental changes, e.g., the introduction of irrigation or the colonization of sparsely populated areas. These changes create the conditions for a higher level of endemicity, so that the epidemics are part of the process of finding a new state of equilibrium. In the absence of intervention, therefore, the higher endemicity will be established and maintained. Environmental modifications for purposes of economic development should not cause increased malaria transmission and, if properly designed, should actually contribute to malaria control. If this has not been the case and an epidemic has occurred, control should be aimed at correcting the defects in the design or implementation of the development project, taking into consideration the sustainability of the controlled situation expected. Somewhat similar situations result from the invasion of an area by an exotic and highly efficient vector which finds a permanently suitable environment, as happened with A. gambiae in Brazil and Egypt. In such cases, it may be possible to eliminate the invading vector before it becomes fully established, as happened with the above-mentioned A. gambiae invasions;

interruptions of antimalarial measures which have kept malaria under control, but in an unstable equilibrium, in an area with all the epidemiological characteristics of high endemicity. The resulting epidemics are the true resurgences or failures of control, the real

WHO/MAL/98.1084 Page 5

disturbance of the epidemiological equilibrium being the introduction of unsustainable control measures. The magnitude and impact of the outbreak will depend on the length of time that the unstable state of controlled transmission was maintained. Left to itself, the original endemic situation will be re-established after one epidemic-type wave or more over a few transmission seasons. Attempts to reintroduce all the previous control measures should be weighed against the risk of repeated unsustainability, as the impact in terms of human suffering and socioeconomic disturbances of repeated epidemic-type resurgences will generally be greater than that of undisturbed endemic malaria. In most cases, it will be preferable to reduce the peak of transmission so as to arrive at the new endemic situation as painlessly as possible and later to adopt a control strategy based on case management and selective vector control.

2. Epidemic Waves and Periodicity

As was stressed above, the concept of an epidemic implies the temporary increase in malaria incidence. In most cases, this is followed by a return to normality. The form of an individual epidemic is primarily determined by the species of parasite, its inoculation rate and the proportion of susceptibles in the human population.

2.1. Epidemic waves of different Plasmodium species

Gametocytes of P. falciparum do not appear in the peripheral blood of an infected case until about 10 days after it is invaded by young trophozoites (ring forms), while in P. vivax, garnetocytes and trophozoites develop simultaneously. In addition, the development in the anopheline vector takes longer for P. falciparum at any given temperature. As a result of these differences, the incubation interval, i.e., the length of time between the occurrence of infective gametocytes in a case and their appearance in an infective form in a secondary case derived from it, is longer in P. falciparum than in P. vivax, resulting in the slower buildup of an epidemic. Macdonald (1957) described the typical epidemic curves for both species, estimating the incubation intervals as 20 days for P. vivax and 35 days for P. falciparum (graph 1). These theoretical curves describe the dynamics of epidemics originating from a very small number of cases in a population of susceptibles with no constraints on dissemination. In fact, most epidemics occur in areas of low endemicity or as a result of the mixing of infected and susceptible individuals and thus originate from a rather larger reservoir in a population not fully susceptible.

Graph 1 Separate and combined curves of epidemic

0

5 10 15 20 25

Weeks

(MacDonald, 1957, by permission af Oxford Universily Press]

-P.falciparum --.P.vivax Composite curve

I

, ^{- -}

-.

. . . - - - - . . . - . . - - f - - - . - - . . l . . - - -

/ ^\ _\

. . . - . - - -

P. vivax epidemics occur mainly in areas with seasonal transmission, and represent a magnification of normal seasonal peaks in temperate and subtropical areas. The curve is typically bimodal, with a spring wave before transmission actually starts owing to long-term relapses from the previous year and infections with a long incubation period, and a summer-autumn peak which is in general much more marked (graph 2).

Graph 2 Monthly malaria morbidity, Posada, Sardinia, 1930

P. falciparum epidemics initially grow in a relatively slow, step-wise manner owing to the delay in the development of gametocytes. In fact, because of the protean clinical picture of falciparum malaria, the early cases may not be recognized in an area of low or no endemicity. Later, owing to the rapid development of the parasite, such an epidemic may appear quite explosive in character.

P. malariae epidemics are very rare. They may occur in isolated communities, but are mild and develop slowly. One example of this is the epidemic on the island of Grenada in 1978 (Tikasingh et al., 1980). No isolated epidemics of P. ovule have been reported.

Except in tropical Africa, most epidemics are not caused by a single species, but by the superposition of P. v i v a and P. falcipamm epidemics, together with limited transmission of P. malariae. These mixed epidemics can be separated into a bimodal P. vivax and a single-wave P. falciparum epidemic, which is often delayed with respect to the second P. vivax wave, as the former requires a lower temperature for similar development in the vector (graph 3). The geographical spread of an epidemic may involve several chains of transmission following different paths, so that the consolidated data for a relatively large area may appear to show a single prolonged epidemic, which may actually be a number of overlapping local epidemics in different phases.

Graph 3 Malaria cases diagnosed in Palestine by month (1922 - 1924)

0

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

2.2. Epidemic periodicity

Epidemics may last only one transmission season and, in fact, most of the dramatic regional epidemics due to abnormal meteorological events have been of this type and represent extraordinary exaggerations of the seasonal periodicity of weak malaria transmission in the dry subtropical belt. The exceptional conditions, e.g., rain, floods, and long warm and humid summers, cause local epidemics to recur more or less regularly in most of the epidemic- prone areas. However, their effect on transmission may differ in areas relatively close to one another, depending on the nature of the soil, the proximity of rivers, irrigation and agricultural practices and, of course, the distribution of the population and their herd immunity.

Most recurrent epidemics follow cycles of two to seven years, which is the paraquinquennial cycle described by Gabaldon (1946) (graph 4). These cycles reflect a similar periodicity of abnormal meteorological conditions (heavy rains, floods, draughts, etc.) which may determine the increased proliferation and survival of vectors responsible for the high transmission potential and of the human distress which aggravates the impact of the disease.

Some areas are affected by epidemics in every paraquinquennial meteorological cycle, while others may only be affected by the more intense abnormalities and follow cycles of periodicities of closer to ten years or more.

Graph 4 Paraquinquennial periodicity in the northern para-equatorial zone 1400

Malaria Death Rates

- - - . . - . . . - - . . - . . - - - - - - . - State-of Cwabobo, Venezuela

~ - - - . - . . - - . ~ ~ . - - - ~ - . - . . - - . - -

- - . . - . . - - . - - . .

~ - - . . - . . - ~ . - - . . - ~ ~ - - - . - - . - - . - ~ . - -

Years (Galbadon)

Page 9 During the interepidemic years, the vector generally survives in a few residual breeding places, such as waterlogged areas, often constituting isolated endemic localities that are important reservoirs of both vectors and parasites. In relatively flat and densely populated areas, epidemic spread can be very rapid.

Examples of such periodicity were the epidemics caused by Anopheles albimanus in the Venezuelan llanos (graph 5a), which were mainly owing to increased densities following abnormal rains and which presented the typical one-peak pattern.

Graph 5a Epidemic malaria death rate Graph 5b Epidemic malaria death rate

due to A. Albimanus due to A. Darlingi

l 1001 1

Meteorological cycles often consist of periods of two or three years of heavy rains followed by two or three of drought. In very sparsely populated areas or in hilly areas forming relatively isolated valleys, the vector reservoir may be limited to distant wet or low-lying areas; the vector may then spread during the two or three years of heavy rains and disappear during the period of drought.

The resulting epidemics may follow two- or three-peak patterns, e.g., the epidemics caused by A. darlingi in Venezuela. This highly efficient vector was able to extend its area of distribution in relatively wet years, but did so only gradually since the low population density ( 3 2 h 2 ) did not permit the infestation to reach all of the population in one season, requiring instead two or three years to do so. Then, in years of excessive drought, the vector disappeared from the invaded areas, since it was not able to survive the prolonged dry season (graph 5b).

The examples shown so far (graphs 1 to 5) all date from before the launching of the Malaria Eradication Campaign in the late 1950s and early 1960s in most of the countries of Asia and the Americas, since which time malaria morbidity has been greatly influenced by fluctuations in control activities. In these countries, the natural periodicity is o hcomplicated by the common tendency to intensify or revive control efforts following an epidemic wave, after which

Page 10

they are progressively reduced. As a result, by the time a new wave occurs, the control programmes have lost most of their effectiveness. This leads to a return to the paraquinquennial pattern, and even to an increase in its oscillations, as efforts to control transmission may have resulted in an increased proportion of susceptibles (graphs 6 & 7).

Graph 6 Malaria cases reported in Nicaragua and Honduras (1962 - 1990)

Thousands

60 X

/

-

^Honduras

i + Nlcaraoua

1

Graph 7 Malaria cases reported in Iran (1962

-

1990)

Thousands

Page I I 3. Classification of Major Epidemic Types

Epidemics differ not only in their causality but also in their form of presentation, evolution, incidence by age groups, severity and socioeconomic impact. These characteristics can usually be determined by a study of a given epidemic, since they often fall into certain patterns depending on the main determining factors involved, as well as on the ecology of the area affected, the time of occurrence and the socioeconomic development of the populations concerned, including the degree of development and peripheral coverage of the health services. It is therefore both possible and desirable to establish and develop a typology of malaria epidemics which will assist in controlling them and in designing forecasting systems.

The many variables which constitute the causal complex of an epidemic are not fully independent, in that they generally combine a number of patterns which, if recognized, may be useful in forecasting their potential evolution and impact, and therefore in guiding their control.

A preliminary classification of the various types of malaria epidemics may be based on the initial presentation and early evolution as well as on the existence of some obvious ecological or social determinants. While not pretending to be comprehensive or definitive, the following classification may be of assistance to malariologists in their field investigations (see also Annex, Tables A and B . Perhavs the most obvious distinction to be made is that between 'true epidemics' and 'resurgences', or failures of control.

3.1. True epidemics

This broad category groups together those epidemics which are the result of some disturbance of the epidemiological equilibrium, as opposed to the direct effect of interruptions or failures of control. True epidemics can be classified according to whether they are the result of a 'natural' or of a 'man-made' ecological disturbance. Nevertheless, epidemics are often due to a complexity of causes which may include both natural and man-made disturbances. The following classification, based on the form of presentation of epidemics, has therefore been chosen and it is hoped it will be more useful for the purposes of characterizing a developing epidemic.

I. Sudden or explosive malaria epidemics in areas with a high proportion of non-imrnunes in the population. This type of epidemic is generally the result of the rapid build up of an abnormally high vectorial capacity in areas where transmission has been very limited for long periods of time - -

owing to unsuitable ecological conditions, e.g., arid or semi-arid areas highly unfavourable to vector breeding and where vector longevity is

Page 12

insufficient, or highland areas where parasites cannot complete their sporogonic development because of the low temperature. When caused by P. falciparurn, as is frequently the case in the tropics, these epidemics result in high mortality and great suffering, and are complicated by the fact that the disturbances producing the epidemic (e.g., floods or droughts) often make populations more vulnerable and reduce their _{- .} capacity to control its effects. The epidemics may be triggered off either by exceptional meteorological conditions or by massive destruction as a consequence of war or natural disasters followed by large-scale population movements, as discussed below.

a) Exceptional meteorological conditions which, as shown by the historical record, occur repeatedly with a periodicity of approximately five or 10 years. These may take the form of:

i) abnormally heavy rains (early, persistent and abundant), in arid areas, such as north-west India, Pakistan or Sudan, producing the classic regional epidemics, particularly when they follow an abnormally dry year and affect an impoverished population (Mathur et al., 1992);

ii) extensive floods of large rivers crossing dry plains, such as the Nile in north-central Sudan or the Niger, Senegal, Indus, Euphrates, etc.; these floods, mainly caused by heavy rains upstream, may or may not be accompanied by heavy local rains;

iii) the abnormal extension of waterlogged areas in lowland plains, associated with extensive irrigation systems and occurring in conjunction with types i) or ii) in neighbouring areas;

iv) unusually long warm and humid summers in high-altitude valleys, such as the East Afncan or sub-Andean highland valleys, ofien preceded by an extension of the area occupied by efficient vectors, brought about by events such as the extension of agricultural activities or the building of fish ponds (Marimbu et al., 1993);

v) abnormally prolonged dry seasons in relatively humid valleys leading to the pooling of river courses, as found in the central south-west 'intermediate' area (between the dry and wet zones) of Sri Lanka, when the south-west monsoon fails (Sivaguanasundram, 1973; Wijesundera, 1988);

vi) prolonged periods of warm and relatively humid conditions in desert areas such as the Arabian peninsula. These give rise to epidemics of 'oasis fever', caused by A. sergenti and A. stephensi, which in the past have wiped out colonies of

Pane 13 settlers, while at the same time maintaining a relatively stable mesoendemicity among the local populations. The vectors breed in the springs which have produced the oases, causing seasonal epidemics with periodic exacerbations. These are particularly severe among newcomers to the area, as seen in the epidemics among Aramco personnel in 1946 and 1947 (graphs 8a & 8b). These epidemics did not depend directly on rainfall, which seldom surpassed 100 mm in 5-6 rainy days per year. It was the prolongation of the normally very short periods in spring when the combination of increasing temperature and decreasing relative humidity, and in autumn, when the combination of decreasing temperature and increasing relative humidity, permitted sufficient survival for sporogonic development (Daggy, 1959).

All these epidemics occur with quasi-regular cycles and should therefore be both predictable within a reasonable degree of precision and preventable, if the health services are adequately prepared. As the epidemics are generally of short duration, if vector control cannot be instituted immediately, preference should be given to case management and to improving the information system for forecasting so that future epidemics can be prevented with timely vector control.

b) Massive destruction followed by the displacement of large numbers of people, either as a consequence of war or natural disasters, such as earthquakes, hurricanes or cyclones, and particularly the latter, which are often followed by heavy rains.

Epidemics are the result not only of the increase in vector-breeding places but also, and to an even greater extent, of increased man-vector contact in precarious shelters in partially destroyed homes or temporary camps, and of the disruption of the capacity of both governments and populations to implement control activities.

Depending on the degree of destruction and population displacement, the acute epidemic after the disaster may be followed by a sequence of epidemic peaks, often even more serious and lasting for several transmission seasons, which are associated with the process of resettlement and reconstruction.

Graph 8a Seasonal malaria incidence in relation to temperature and humidity, Qatif Oasis, 1946

", Temperature

0 9 0 0 Relative Humidi

Graph 8b Seasonal malaria incidence in relation to temperature and humidity, Qatif Oasis, 1947

L Temperature

(Daggy, The American Journal for Tropical Medicine and Hygiene, 1959)

Graph 9 Registered malaria morbidity & mortality in Italy

150,000 -

100,000

5 I 5 25 35 45

Years [ I Q O l d O )

The magnitude of the destruction and the stability of the previous situation may be such as to:

i) produce a major epidemic which subsides after reconstruction, e.g., the epidemics which followed the Second World War in Italy (Benn, 1947; A.C.I.S., 1950; Coluzzi, 1961) (graph 9) and the Spanish civil war (Rico-Avello & Rico, 1950) (graph 10).

In both these epidemics, the recorded morbidity reached levels comparable to those attained in 19 18 and both subsided quite rapidly, the first with the help of a DDT spraying campaign, but the second without. The most important difference was that, while the Spanish epidemic resulted in high mortality, the Italian one did not, most probably owing to the more rapid and complete mobilization of diagnostic and treatment facilities in Italy;

ii) trigger the failure of existing control programmes, compounding the epidemic with a resurgence of the previous endemicity, as was the case with the epidemic which followed hurricane Flora in Haiti in 1962 (Masson & CavaliC, 1965) or cyclone Namu in the Solomon Islands in 1986 (Madeley, 1988);

iii) result in socioeconomic upheaval such that there is a tendency towards the establishment of a new epidemiological equilibrium, as happened in northern Iraq following the Gulf War and the subsequent economic embargo. Malaria epidemics occurred as a result of the destruction caused by the war, but the resettlement of previously displaced populations and the rapid intensification of agriculture with temporary concentrations of migrant workers, created new risk situations which resulted in progressively spreading epidemics from 1990 to 1994.

iv) cause mass population displacement and the establishment of refugee camps, which often receive population groups with very different malaria experiences and therefore immune status.

These camps differ from labour camps in the dramatic experience of the population's exodus and the frequently high degree of exhaustion and the poor nutrition and health status attendant on the refugees' arrival.

Graph 10 Registered malaria cases and deaths in Spain

"O

l\ 1

+Registered cases

-

^DeaUls

700

4

5 15 25 35 45

Years (1901-48)

These epidemics, though generally not cyclical in character, are predictable following the occurrence of a disaster. Malaria is only one of the many problems to which the populations affected are exposed. Normally, relief operations will be concerned primarily with meeting the most acute needs for shelter, food and water, followed by measles vaccination and the organization of care for diarrhoea, acute respiratory infections and malaria. Malaria-control measures should include emergency case management and vector control, where possible, of refugee camps and villages in the process of reconstruction. The relief operations, which are part of the health care of displaced populations, are often dependent on external support, including international collaboration, which is more likely to be available for the risks associated with the acute phase of the problem. The resulting equilibrium will depend on the extent of the damage and the more or less rapid reconstruction of villages and health and social services.

11. Progressive invasion by a succession of severe local epidemics of an area previously considered to be of low endemicity, because of the low vectorial capacity of local vectors. The epidemiological pattern is that

of a succession of more or less severe epidemics in different neighbouring areas, which often take one season or more to spread from one to another. These epidemics, each of which may be quite dramatic and lethal, often occur without any apparent catastrophic triggering event. They are often seen when:

a) an exotic, or previously eliminated, highly efficient vector invades an area providing a very suitable habitat, as was the case with the A. gambiae invasion of north-eastem Brazil in the 1930s (Soper &

Wilson, 1943) and of Upper Egypt in the 1940s (Shousha, 1948);

b) a dangerous vector periodically extends its range beyond its normal area of distnbution when temporarily suitable conditions occur in neighbouring areas, as happened with A. darlingi in the Venezuelan llanos during the two- or three-peaked epidemics previously mentioned;

c) an area is reinvaded by a previously eliminated vector, as was the case in the Madagascar highlands (1986-1990) with A. funestus (Lepers et al., 1990a & b, 1991; Razanamparany, 1989);

d) chloroquine resistance occurs in epidemic form (Bastien, 1990), particularly when the wide accessibility of chloroquine has previously kept the disease under control to a certain extent. The spread of chloroquine resistance in tropical Africa can also be considered as an epidemic, even if the incidence of malaria infection does not change (Warsame et al., 1995).

Although basically unpredictable, the progress of such epidemics is partially predictable and they can be contained or slowed down by selective vector control once detected. The eradication of the invader may be contemplated, although it should be noted that, while an exotic vector may be eradicated, leaving a fairly stable equilibrium, the elimination of a local vector fi-om an area will be much more difficult and, in most cases, somewhat unstable, as the area will remain vulnerable to reinvasion from neighbouring localities

In highly malarious areas, a similar epidemiological pattern has accompanied public works such as railway and road construction or large irrigation projects in the tropics. Even with modem control measures and the wealth of resources available to such projects, malaria continues to threaten them to this day. These epidemics affect not only the project workers, but also the local populations, as a result of the disturbed environment left behind, and the people attracted to create new

settlements. These epidemics are basically predictable and can easily be identified since they follow the path of the particular project concerned. Prevention will depend on the adoption of appropriate engineering practices and the design of new settlements in such a way as to prevent man-made malaria, as well as on vector control in affected localities. Unfortunately, even in well-designed projects which do not produce epidemics in the construction phase, the new economic opportunities created will attract large numbers of people who may, at least for some time, create unplanned settlements. Even later, when the rural economy is better organized, productive farms will attract temporary labourers, often in seasonal waves. Since these workers are often in an illegal situation, they are difficult to contact and protect.

111. Focal serious epidemics in areas of increasing stability accompanying a period of settlement in, and adaptation of non-irnrnunes to, a high-risk area. They may be preceded by a more or less apparent string of focal epidemics affecting the early stages of development projects. These include:

a) the colonization of tropical jungle areas by populations which, although originally susceptible, manage to survive and establish successful agricultural settlements, as in many areas of Brazilian Amazonia (Cruz Marques, 1987) or in the outer islands of Indonesia (Binol, 1983). It has been reported that, in both areas, the original malaria epidemics, which were one of the main early obstacles to the success of the colonies, were progressively declining in intensity in surviving settlements, eventually leading to an endemic situation. In contrast, the slow invasion of jungle areas or tropical highlands by people from highly endemic areas results in extensions of the endemic areas without obvious epidemics (Matola et al., 1987);

b) the explosive growth of tropical urban areas, where the continuous influx of newcomers creates periurban epidemic foci which may evolve to become highly endemic if large, permanent breeding places are present, or lead to very low endemicity or even non-endemicity, as the result either of urbanization and the elimination of breeding or of the development of more or less permanent slums with high organic contamination of surface waters. Control measures during the epidemic period should take into account possible changes in the situation and the need for adaptation so as to provide a feasible control strategy for the subsequent endemic situation;

c) the establishment of highly efficient forest vectors in neighbouring tree plantations, such as occurs in Myanmar (Tun-Lin et al, 1987)

Page 19 where, if feasible and sustainable, local eradication may be considered; if this is not possible, the aim should be the control of the endemicity established.

These epidemic areas are also predictable, although very difficult to control in the acute period of the epidemic as even when given official backing and included in vector-control programmes, such resettlement projects have serious local organizational problems. The most effective method of control would be to improve case management and selective vector control, while adapting to the development of the endemic situation. Such situations should be prevented by discouraging unplanned settlements and providing adequate logistical support for authorized settlements, not only to control malaria but to reduce the deleterious ecological impact as well (Sawyer, 1993; Najera, 1993).

IV. Creation of foci of high apparent endemicity:

a) in gold or gem mining areas in South American or south-east Asian forests, where a continuous flow of mainly temporary migrant groups engaged in open-cast mining for gold or gems is exposed to very high malaria transmission. These are the hotbeds of parasite drug resistance, with the highest malaria incidence outside tropical Africa, although the situation is not one of high endemicity and herd immunity is quite low. It has been described as a pattern of localized permanent epidemics (Verdrager, 1995), since each successive wave of non-immune migrants suffers in turn from epidemic malaria.

Diagnostic and treatment facilities in these areas are generally privately operated and often expensive, but the abundance of cash means that all drugs are locally available and often overused. These situations are known, rather than predictable, at the public health level. Control should include information and education, although the marginalized position

-

often not only social but legal as well

-

of the miners frequently makes this option, as well as organized vector control, difficult to implement. Malaria clinics or treatment posts at the points of entry into these areas have been used to provide some form of relief;

b) in the classic tropical aggregation of labour described in colonial times (Christophers & Bentley, 1908) and still persisting in many large labour-intensive agricultural undertakings in the tropics. A stream of temporary workers, often illegal immigrants, are employed and generally dismissed before they can acquire labour rights. The epidemiological problem is often similar to, and as difficult as, that of the miners previously mentioned, but without the intense drug

WHOLkiAU98.1084 Page 20

pressure because of the lack of cash necessary to pay for medical care or drugs.

3.2. Resurgences or failures of control

A malaria resurgence is actually the return to a state of equilibrium which has been disturbed by the efforts to interrupt transmission. Perhaps the most important characteristic of these outbreaks is the fact that they represent a serious indication of the unsustainability of previous malaria-control policies aimed at the interruption of transmission. Health authorities should therefore consider the need to change to a more conservative control policy which will not recreate similar situations of unstable equilibrium. These situations include:

I. The explosive resumption of transmission, producing an epidemic situation with an incidence much greater than that previously seen in the endemic situation. This type includes:

a) the total loss of the protective effect of control interventions, as observed in the early 1970s following the interruption of two programmes based on mass drug administration (MDA) in a) Central America, where the programme had been instituted following the recognition of multiple resistance to insecticides; and b) in Haiti, where the programme had been instituted following the resurgence after hurricane Flora in 1962. These situations were highly predictable, and could have been better controlled. However, they are now mainly of historical interest, as MDA is no longer normally recommended as a measure for large-scale malaria control.

b) the explosive epidemic return to endemicity that may occur in semi-arid areas, traditionally suffering from mesoendemicity with periodical epidemic exacerbations, which have been protected for a number of years by vector control that successfully eliminated transmission for several epidemic cycles. The interruption of vector control, or its gradual loss of effectiveness, will leave the largely non-immune population unprotected for the next epidemic cycle, which will develop into a full-blown epidemic. Such a situation occurred recently in the Gezira Blue Nile Health Project in the Sudan (graph ll), where malaria transmission was more or less completely interrupted for more than 10 years by a control programme dependent on external support. Such a highly predictable situation should be dealt with in the same way as a temporary epidemic by reducing, if possible, the peak of transmission and developing the capacity for case management, and not by the re-establishment of the

full vector-control programme, which will most probably be as unsustainable as in the past.

" Graph 11 Malaria prevalence rates in Gerzira, children 2-9 years old

II. The progressive return of endemicity in highly endemic areas where control interventions which reduced or interrupted malaria transmission could not sustain their success. Depending on whether transmission was actually interrupted and on the duration of effective protection, the return to endemicity may result in more or less severe disease manifestations and affect a larger or smaller number of age groups, but in most cases the incidence will increase slowly, often from dispersed foci. This pattern may result from: a) the loss of effectiveness of the insecticides used for indoor residual spraying because of the development of resistance; b) the deterioration in the quality of the spraying operations; or c) the complete interruption of spraying. Even the sudden interruption of an effective spraying programme, on the completion of some successful pilot projects

* in highly endemic areas of Africa, did not result in dramatic epidemics, because the insecticide deposits retained their effects, either killing or repellent, on some surfaces for quite long and variable periods. This slow decline of insecticidal effect had been studied since the early applications of DDT (Raffaele & Coluzzi, 1953). Another classic example is that of the famous and massive epidemic in Sri Lanka in 1968, which can be traced to a slow buildup, including local epidemics, since 1964, when it was considered that malaria had practically been eradicated in 1962- 1963 (graph 12).

WHOMAU98.I 084 Page 22

Graph 12 Malaria cases reported in Sri Lanka

Tburands

800 I I

In contrast to the previous type, in the areas of high potential endemicity, transmission is built up as soon, and to the same extent as, the insecticide effect is lost, while in the former the insecticide effect is probably completely lost before the epidemic risk develops, as in Sri Lanka (Pinikahana & Dixon, 1993), India (Sharma & Mehrotra, 1986) and Sao Tome and Principe (Baptists, 1996). These epidemics, which are clearly predictable, should be dealt with by improving case management, while at the same time changing the control strategy to that of managing the ensuing endemic situation.

4. Classification of Epidemic Risk

The formulation of an epidemic forecasting system will require the identification of epidemic-prone areas, based not only in the study of recent or historical epidemics, but also on the recognition of potential epidemic risk factors. For an epidemic to occur, it is necessary that a population with a high proportion of non- immunes be suddenly subjected to intense malaria transmission. Epidemic risk factors may be classified as they relate to these main determinants:

a) The sudden increase in the number of exposed non-immunes, owing to:

i) the arrival en bloc of a non-immune population into a malarious area, the classic examples of which are the deadly outbreaks affecting new settlements in tropical areas, from the European invaders of Africa to the modem colonizers of the Amazonia or the

WHOMAW98.1084 Page 23 Outer Islands of Indonesia, or the prospectors and miners in the jungles of South America or south-east Asia; similar examples are the epidemics suffered by armies operating in malarious areas, police and army camps in jungle areas, and non-immune refugees (e.g., from the Ethiopian or Rwandese highlands) in endemic areas;

ii) the introduction of a number of infected individuals into a malaria-free area, where both the Anopheles vector and the conditions of transmission are present, i.e., a receptive area;

examples of this type are the re-establishment of malaria in areas from which it had been eradicated and the small focal epidemics resulting from the return of war veterans or infected tourists to the USA or Europe. Historical examples include the introduction of P. falciparum into the Americas which contributed to the epidemic hecatomb of the Amerindian population (Naranjo, 1992), the introduction of malaria into Mauritius, Reunion and Rodrigues (Julvez et al., 1990), and more recently, into isolated populations in Papua New Guinea (Jenkins, 1988);

iii) the admixture of large numbers of immunes and non-immunes living under primitive conditions, described as 'tropical aggregation of labour', which still occurs in many temporary labour camps in agricultural exploitations and economic development projects in tropical and subtropical areas, such as the recent epidemic outbreaks associated with the expansion of banana and African palm plantations in some areas of the Atlantic coast of Central ~ m e r i c a (PAHOIWHO, 1993,1994);

b) The sudden increase in vectorial capacity:

i) the sudden increase in Anopheles densities, owing to abnormal rains, andlor their survival, caused by prolonged periods of abnormal warm weather, in hypo- or mesoendemic areas, where transmission is normally absent because of adverse meteorological conditions. In most cases, such epidemics occur with certain periodicity and may affect very large areas quasi-simultaneously, constituting the classic 'regional epidemics' of north-west India and Pakistan, as described by Christophers (Gill, 1928). Development of rural housing, changes in agricultural practices in many areas and continued control activities since the late 1950s in others have prevented the spread of regional epidemics to the extent of the great pandemics of the past, but serious epidemics continue to occur, with their quasi-cyclical pattern in many of the known epidemic-prone areas of the Indian subcontinent, Sri Lanka, the East African highlands and South America;

ii) the invasion of areas of low endemicity by a very efficient vector, owing to the poor vectorial ability of the local anopheline fauna; the classic example is the invasion by A. gamhiae of north-east Brazil in the 1930s and Upper Egypt in the 1940s.

c) Environmental modifications, which may create both increased vector densities and population movements:

i) the modification of the environment for agricultural development, creating conditions more favourable for malaria transmission, e.g., irrigation works in arid areas resulting in increased anopheline density, or tree plantations providing micro-climatic conditions favourable for anopheline survival; in the above examples, there is often a simultaneous increase in population and the demand for temporary labour force, so that the epidemics are the result of a complexity of causes;

ii) the rapid unplanned growth of cities in tropical areas, in some way similar to the labour camp situations, as they are formed by the admixture of infected and susceptible people, but which are often spread over a wider area and which, as the population density grows, are progressively urbanized or transformed into slums, where the pollution of surface water prevents the development of anophelines, which are eventually replaced by culicines.

d) Failure to maintain previously effective control. After the general introduction in the 1950s of national malaria control programmes based on vector control and the widespread use of antimalarial drugs, particularly chloroquine, two new forms of malaria epidemic outbreaks are occurring throughout the world:

i) the resurgence of malaria transmission in epidemic form following the discontinuation, weakening or loss of effect of vector-control programmes; they have been described as post-eradication epidemics (WHO, 1974), and classic examples are the massive epidemics of Sri Lanka in 1968, and of India and Pakistan in 1976- 1977;

ii) the progressive spread of chloroquine resistance, particularly in Africa, over the last two decades, which in some situations has acquired epidemic proporhons (Warsame et al., 1995); conversely, an epidemic often provides a major vehicle for the spread of drug resistant strains (Sharma et al., 1996).

Page 25

11. EARLY DETECTION AND CONTROL OF EPIDEMICS 5. Recognition and Epidemiological Investigation

Malaria epidemics seldom pose problems of recognition at the local level in epidemic-prone areas, where they are often recognized by laymen.

Nevertheless, in areas which are considered free from transmission, it is common for individual malaria cases to be misdiagnosed and for even the initial stages of an epidemic not to be recognized as being due to malaria.

Similarly, a fever outbreak not responding to common antimalarials may not be recognized, even for more than one transmission season, as exemplified by the epidemic spread of chloroquine-resistant P. falciparum when, in some areas, malaria was misdiagnosed as typhoid fever since cases responded to antibiotic treatment (Onuigbo, 1990). More often, epidemics occurring in rural areas poorly served by health care or epidemiological services are locally attributed to malaria but the health authorities often only learn about them from newspaper reports or from questions in Parliament. Any outbreak of febrile disease in a potentially malarious area is generally attributed to malaria until proven otherwise.

In any case, every fever outbreak requires an epidemiological investigation, and malaria should be suspected whenever the ecological or meteorological conditions are such that malaria transmission is possible. Particular attention should be paid to any clustering or increase in fever cases in areas with a history of malaria epidemics or where malaria has been under control for some time.

5.1. Confirmation and initial assessment

Whatever the original sources of information (reports from local antimalarial or general health services, other official sources, newspapers, etc.), the first action to be taken by the epidemiological service is to check the validity of that information by contacting the local sources of the information as well as the local health services by the fastest available means of communication. It will be necessary to decide to what extent these communications should be kept confidential in order to avoid unjustified public alarm, while at the same time ensuring their maximum speed and accuracy. If the existence of an abnormal situation is confirmed, a number of activities should be undertaken simultaneously, as follows:

I. A preliminary analysis of the local situation, which in most cases will require a visit by central and/or provincial professional and technical staff for the clinical and laboratory diagnosis of the suspected cases.

Page 26

Even in recent times, some local epidemics of brucellosis, relapsing fever or visceral leishmaniasis (de Beer et al, 1991) were originally thought to be malaria. The analysis should concentrate on:

a) the quantification of the problem, i.e., the daily or weekly incidence, the development to date and the initial trend;

b) the identification of the disease or diseases involved (in the case of malaria, the species of parasite involved and their frequency), the severity, age and sex distribution, the geographical distribution, clustering or dispersal of cases and the possible existence of certain social characteristics (occupation, place of employment, etc.);

c) an assessment of the socioeconomic complicating concomitant processes, such as crop failure, famine or displaced populations;

d) an assessment of the local resources available to cope with the problem, including:

i) the coverage by the health services and other diagnostic and treatment facilities (e.g., voluntary collaborators, community health workers, local health committees) and their use by the population;

ii) the manpower and drugs available; often one early indicator of an epidemic is the rapid exhaustion of drug supplies in some peripheral areas;

e) the logistics of case referral;

f ) community and private sector participation;

g) intersectoral collaboration; if the epidemic is not caused by malaria, it will be necessary to assess what collaborative action may need to be taken by the antimalarial programme;

h) the need for support and for strengthening intra- and inter-sectoral collaboration;

i) the identification, if possible, of the potential determinant factors (floods, heavy rains, drought with river pooling, population movements, etc.), which may have preceded the epidemic;

i) the formulation, if possible, of some hypothesis as to the origin of the

-.

outbreak.

II. The geographical delimitation of the problem by means of a telephone survey andor site visit to neighbouring areas or areas with similar ecology or which may have been subject to similar risk factors to determine possible increases or clusterings of fever cases, deaths, school absenteeisms, drug-stock ruptures, etc., supplemented when necessary by:

a) a malariometric survey including, if the human and material (laboratory) resources permit, a fever survey and spleen andtor

parasite surveys of the general population and any population group already suspected to be at high risk. If resources are available, it may be useful to use a rapid dipstick irnmunodiagnostic test in remote areas when it is necessary to obtain a rapid confirmation of a P. falciparum outbreak (Verle et al., 1996). It should be noted that these tests are quite expensive (c. US$1.60/test), are currently only specific for P. falciparum and are not quantitative;

b) a review of general geographical information on ecologically similar areas potentially at risk, to assess the presence of risk factors, and complemented if necessary by rapid geographical reconnaissance;

c) the strengthening and local adaptation of existing emergency preparedness plans and the establishment of a system of watching for the occurrence of identified or potential risk factors.

111. The identification and mobilization of the necessary support from the health services, the civil authorities, potential sources of intersectoral collaboration, NGOs and affected or neighbouring communities, particularly to cover the expected requirements for drugs, laboratory supplies and manpower for the affected area and areas at risk.

IV. The mobilization of the necessary logistical resources for any support which may be necessary or the alerting of those responsible for them.

5.2. Epidemiological investigation

The preliminary situation analysis should provide sufficient information to identify the affected areas and the areas at risk of further spread, as well as whether that risk is imminent or likely to occur in the next transmission season.

The importance of an epidemic depends not only on the magnitude it has attained but also, and perhaps even more so, on its potential development and spread. In turn, these are dependent on a number of factors, such as the distribution and density of the population, the proportion of susceptibles, the intensity of transmission, the communication facilities, the degree of development of the health services, their coverage and capacity of response, and their use by the population.

In any case, it will be necessary to plan an appropriate epidemiological investigation, namely:

I. Epidemiological analysis of all information pertaining to the affected and neighbouring areas, including:

a) the general and malaria-specific morbidity and mortality data from all sources, and in particular, any information on past epidemics and