ENTOMOLOGICAL IN THE ACTIVITIES AND THE

WORLD HEALTH ORGANIZATION AFRICAN REGION

ORGANISATION TVIONDIALE DE LA SANTE REGION DE L'AFRIQUE

ONCHOCERCIASIS CONTROL PROGRAMME IN WEST AFRICA

PROGRAMME DE LUTTE CONTRE L'ONCHOCERCOSE EN AFRIQUE DE L'OUEST B P. 549 OUAGADOUGOU, Burkrna Faso Télégr . ONCHO OUAGADOUGOU

Tél

:

34 29 53 - 34 29 59 - 3429 60 Télex. ONCHO 5241 BF Fax' 34 2875

Original. English

REPORT OF OCP ACTIVITIES AND THE ENTOMOLOGICAL ^RESULTS IN THE OUEME

BASrN (1978-2001)

Samuel A. SOWAH* and Hyacinthe AGOUA*

Under contractual

service agreement

with WHO/OCP

The

first

version of this report was written by

Mr

S.A. Sowah for the period 1978-1998.

The document was reviewed and updated by Dr. Hyacinthe

AGOUA until2001, with

the collaboration of the Programme Director and the Chiefs of Units.

N" 123|02NCUTEC/3.9

*Medical

Entomologists. Previously WHO-Oncho control Programme staff members

By

30-07 -02

Ouagadougou, June 2002

2

-) 4

CONTENTS

I. INTRODUCTION

2. OVERVIEW OF THE ONCHOCERCIASIS CONTROL PROGRAMME IN WEST AFRICA

DESCRIPTION OF THE OUEME BASIN AREA

ENTOMOLOGICAL ACTIVITIES (EVALUATION ^AND VECTOR CONTROL)

4.1. Entomologicalsurveillance.

4.1.1.

Breeding sites.

4.1.2.

Catching points.

4.1.3.

Entomological prospections.

4.2. History

on

larviciding.

4.2.1. Aerial

larviciding.

4.2.2.

Ground larviciding.

4.2.3.

Insecticides used (Insecticides rotation) 4.3.

4.3.1 4.3.2 4.3.3 4.3.4 4.3.5

Entomological

results.

Vectors concerned.

Parasites concerned.

Transmission.

Special studies on S. soubrense Beffa form.

Entomological results on 3 1 -12-2001.

5.

6.

SURVEILLANCE OF AQUATIC FAUNA EPIDEMIOLOGICAL EVALUATIONS

6.1. Epidemiological

results before

vector control.

6.2. Epidemiological

results

during vector control.

6.3. Latestepidemiologicalresults.

6.4.

Recent

ophthalmological

evaluation.

IVERMECTIN DISTRIBUTION.

SOCIOLOGICAL INVESTIGATIONS.

10. DISCUSSIONS.

7

8

9. POPULATIONS SENSITIZATION AND MOBILIZATION ACTIVITIES

I1. CONCLUSION

1. INTRODUCTION

This report is written as a result

of

contractual service agreement entered into

by

OCP

with

the authors.

The aim

of this

report is

to

give a consolidated account

of

OCP activities

in

the Oueme

river

basin as

well

as the results obtained from the beginning of OCP activities to date.

The report should cover not

only

entomological activities but also

all

aspect

of

OCP activities such

as, epidemiological

investigations

carried out (with their

corresponding

results),

Ivermectin clistribution. sociological investigations

if

^any, sensitisation carried

out by

OCP

field staff.

and any other relevant inforrnation available.

2. ^{OVERVIBW OF THE} ONCHOCERCIASIS CONTROL PROGRAMME IN WEST AFRICA

The

Onchocerciasis

Control

Programme

in West Africa

started

its activities in ¹⁹⁷⁴

^and

covered

7

countries (Benin,

Burkina

Faso, Côte

d'Ivoire,

Ghana,

Mali, Niger

and Togo).

Its

original savannah area covered 654,000 km2.

This

was

the

area where

the

severe

form of

onchocerciasis resulting in blindness prevailed leading to the depopulation

of

the valleys.

Following

the extension

of

tlre Programme activities

in

Côte

d'Ivoire in

1978-lg7g, the Programme area covered

764,000km2,

which represents about 18,000 kms of rivers treated.

In

view of the good results obtained and

in

order

to prevent reinvasion, further

extensions

were made

southwards

and westwards in

^1987-1988.

Currently, the Programme concerns 11 countries (Benin, Burkina Faso, Côte

d'Ivoire,

Ghana, Guinea, Guinea-Bisau.

Mali. Niger,

Senegal, Sierra Leone and Togo) and covers 1,300,000 ^t<m'z 1fig.

t;.

The Prograrnme aimed at

controlling

onchocerciasis to ensure that

this

disesase is no longer a

public

health issue.

or

an obstacle

to

socio-economic development, and also

that

countries involved are able to perpetuate this realisation.

On the administrative level, the

Head

Office of the

Onchocerciasis

Control

Programme ls based

in

Ouagadougou, Burkina Faso and includes the Administrative

Unit,

the Planning, Evaluation and Transfer

Unit,

and the Vector Control Unit.

On the operational level. the

Vector Control Unit is divided into two

^(Eastem and Western) zones

with

sectors, sub-sectors and operations bases

which

communicate

through a radio

network,

which allows a rapid

transmission

of

epidemiological data as

well

as

scientific

and administrative directives.

In

1990. there were

5

sectors,20 sub-sectors and one antenna

in

Bandiagata,

Mali in

the

original OCP

area

and its

southern extension.

The

western extension

counts 8 sectors

and 25 operations bases.

The Administration Board is the Joint Programme Committee (JPC) that

includes representatives

of programme countries, Sponsoring Agencies and delegates of 23

Donor Organisations

and

Countries.

The

Committee

of

Sponsoring Agencies

(CSA) is

composed

of

the

World

Health Organisation

(WHO)

that is the implementing agency

of

the programme, the Food and

Agriculture

Organisation

(FAO)

the United Nations Development Programme (LINDP) and the World Bank.

As from the

mid-9Os,

a

certain number

of

sectors, sub-sectors

and

operations bases were closed

down

due

to

the gradual reduction

of

some programme

activities in view of the

satisfactory results obtained

in

most river basins. OCP activities

will

end on

3l

December 2002.

4

3. DESCRIPTION OF THE OUEME BASIN AREA

The entire

Oueme basin

lies in the

Republic

of Benin. It

stretches

from latitude

10 degree

North

and enters

into the Gulf of Benin

at the coast

of

the Republic

of Benin (fig. l), ⁱⁿ

^{a woody}

Savannah area known as the Dahomey Gap, where the Savannah type

of

vegetation stretches from the

North

of Benin

right

to the ^coast.

There are

two

types

of climatic

conditions,

giving two

types

of

^seasons,

namely the

rainy season and the

dry

season. The

rainy

season starts

in the

South around late

March

and

in

the

North

between

mid May to mid

June. The

rainy

season ends around

rnid or

end

of

October.

This

ushers

in

the Harmattan, which marks the beginning of the dry season.

The

principal

tributaries

of

the Oueme are: the

Alpouro,

the

Yerou Maro,

the Wewe and the

Terou in

the

North. In

the middle portion are the

Odola,

the

Beffa,

and the

Adjiro. In

the South are the

Okpara

and the Zou.

Hydrolog.v

Generally, the main

river

and its tributaries

flow mainly in

the

rainy

season, namely

from

late .lune

to

the

middle of

October,

with

the peak around the month

of

September.

From

late October to the

middle of

November most

of

the tributaries stop

flowing.

The main

river,

however, continues to

flow

up to the middle of January.

The

discharge

of the

Oueme can

be more than

1000

cubic

metres

per

second.

This

was recorded in week 36

in

1998 at

Atchakpa(frg.2).

On the tributaries the discharge can be around 500 cubic metres per second. This was recorded at Atcherigbe on the Zou

in

1999 in week 35

(fig.

a).

In the dry

season,

the

entire basin dries

up with no flow at all from the North to the

area around Zagnanado where

minor flow

can be observed. Eventually these

flows

also stop a month or two before the onset of the rainy season.

The

Oueme

river and its tributaries are devided into

several stretches,

according to

the irnportance of the tributaries and the discharge, -OU

100, OU 400...etc.

(fig.

8).

4. ENTOMOLOGTCAL

ACTTVTTTES

(EVALUATTON AND VECTOR CONTROL)

4.1 Entomological

surveillance 4.1.1.

Breedins points

.

_{ln order} to make the work easer for the Aerial Operations team, nver sfrefches are coded by taking into account, the name of the river basin and adding a number from 100 to 900.

For example : OU 100 and OU 400 means: Oueme nver, stretches : 100 and 400.

Oueme

The

S. damnosum

s.l.

breeding points

on the

Oueme

itself

begins

from lat. 9

degrees

40

minutes north, a

little bit

downstream

from Affon

catching

point.

From

this

breeding

point to

the confluence of the Yerou Maro, six other important breeding points can be found

(fig.

6).

Eleven other major breeding points are found

from

Beterou

(lat.

9 degrees 13 minutes North) to the confluence

of

the Odola (lat. 8 degrees 46 minutes north). Note that these points are identified

only in

latitudes because

of

the fact that the Oueme

flows virtually from North to

South, hence, the latitudes pinpoint to a particular point on the

river.

From the confluence of the Odola to the Atchakpa breeding point, eleven other major breeding points can be identified.

The main complex breeding points on the Oueme are found between Atchakpa catching

point

and

Aguigadji

catching

point. At this

stretch

(from lat.

8 degrees north

to lat. 7

degrees 29 minutes north), the breeding points ^are

virtually

^joined together

with

very few pools in-between.

Below the Atchakpa-Aguigadji stretch apart from the complex breeding

point

at

Tamiedji

(lat.

7

degrees

25

minutes

north).

there are,

practically, no

natural breeding

points. On the

other hand occasionally man made fish traps can be found here and there

(fig.

6).

Alpouro

Very few breeding points are found on the Alpouro, which are not important for

S.damnosum s.l. breeding.

Yerou Maro

On the Yerou Maro

also

very few S.

damnosum

s.l.

breeding

points are found. The

only important breeding point is found near its confluence

with

the Oueme.

Wewe

The breeding

points

on

the

Wewe are

not

many and are

not

important

for

S. damnosum s.l breeding.

Terou

There are very

few

breeding points on the upper stretches

of

the Terou, where the

river

flows from

North to

South. The important breeding sites are located frorn the confluence

of

the

Aguimo

to the confluence of the Terou

with

the Oueme.

Odola

One important breeding

point

on

the

Odola

is

situated near

its

confluence

with the

Oueme.

Other

minor

breeding points are found upstream but these do not play important role

in

S. damnosum s.l breeding because of the short duration of

flow

of this tributary.

6

Beffa

The uppermost breeding

point

on the

Beffa is

located

lat.

8 degrees 48 minutes

North.

There

are no breeding points down to lat. 8

degrees

39 minutes north. From this point down to

the confluence

with

the Oueme twelve important breeding points can be found.

It

is important to note that S. soubrense Beffa form is named after this

river

since this specie was

first

discovered on this

river Adjiro

The

principal

breeding points about

fifteen in

nurnber,

lie

between longitudes

2

degrees East and2 degrees

l5

minutes East around latitude 8 degrees 29 minutes North.

Okpara

Among all

the tributaries

of

the Oueme, the Okpara

is

the

tributary with

the greatest number breeding

points, for, at

least, about one hundred and eighteen

(118) principal

breeding

points

can easily be identified. The

majority

of these are found between lat. 9 degrees 4 minutes

North andlat.

T

degrees

53

rninutes

North,

where

the

Okpara

forms the

demarcation

line

between

the Republic of

Benin and Nigeria. The rest of the breeding points (about

thirty-eight in

number) are entirely found

in

the Republic of Benin.

Zort

The

Zou

lies

entirely in

the Republic

of Benin

and takes

its

source around

lat. 8

degrees 27 nrinutes

North,

^near

a village

called

Pira

and enters

into the

Oueme around

lat. 7

degrees near a

village called Gongbe.

TheZou

is considered to be one of the longest tributaries of the Oueme but has

relatively fèwer

breeding

points

compared

with the

Okpara.

the

other

long tributary. The

principal breeding points, twenty-one in number, are situated between latitudes 7 degrees 35 rninutes

North

and 7 degrees 20 minutes North.

Downstream

from the

breeding

point on lat. 7

degrees

20

minutes

North. no more

natural breeding

points

occur, except one

or two

man made

fish

traps

which

constitute

artificial

breeding points.

The

river

itself

flows

very

little

at this stretch

with

the presence

of lot of

green algae

Kouffo

The

Kouffo is not

a

tributary of

the Oueme.

It is

a coastal

river that

springs

in Togo in

the legion of Dotoé on the east of the "Okéloukoulou Rocs" and enters into the Ahémé Lake in Benin.

This

river

has

two

stretches: the stretch OU801, upstream Lanta that

is

located

in

a

hilly

area

with

breeding sites, and the stretch OU802 that is downstream Lanta.

4.1.2.

Catching

^points.

4.1.2.1. Oueme

The

total

number

of

catching points on the Oueme

itself

along the years had been

six.

These are; Beterou. M'Betekoukou, Atchakpa,

Affon, Aguigadji.

and Zagnanado,

in

order

of

creation

(fig.

t).

Beterou 1819

The

first

catch was made at

this

point

by the

Sub-sector

of Natitingou in

October

1976.The

point was then passed on to the Parakou Sub-sector after the creation

of

the latter

in

1978. The

point

has therefore been under

the

Parakou Sub-sector

to

date. Beterou,

the hrst

^catching

point on

the Oueme, is regularly used as monitoring point to date.

Atchakpa2703

Atchakpa was created

in

1978

with

the creation

of

the Sub-sector

of

Parakou. The

first

catch was made at this point in the rnonth of December 1978, and is being used regularly to date.

M'Betekoukot2704

M'Betekoukou has a

similar history like

Atchakpa.

It

was created

in

1978 and the

first

catch was made

in

the month of November 7978, and

is still

being used as a monitoring point

to

date. This point was

first

under the Parakou Sub-sector but passed on to the Bohicon Sub-sector

in

1982.

Affon27l0

Affon

was created

inl979

and the

first

catch was made

in

January 1979. This

point

was used regularly

until Aplil

1986. Catching was stopped because

of

the

low

catches registered at

this point

and also because

of

economic reasons.

In

1987 OCP had

to

take strong economic measures

to

stay

within

the budget. because

of

lack

of

funds. There was therefore reduction

in VCU (Vector

Control

Unit)

activities.

This

reduction affected certain number

of

catching points

visited

at that

time. Affon

was one of them.

Aguigadji

2711

Aguigadji

was created

in

1982 and the

first

catch was made

in

September 1982. The point was

flrst

under Parakou Sub-sector

but

later on transfemed

to

the

Bohicon

Sub-sector

in

1982. Since its creation

Aguigadji

has been a regular catching point to date.

Zaenanado 3002

This point

was created

in

1982

with

the creation

of

the

Bohicon

Sub-sector.

The point

was

regularly used from

September 1982

until

September 1987. because

of similar

^reasons already rnentioned under

Affon.

4.1.2.2.

Yerou Maro

Barerou2T14

The

first

catch was made at Barerou

in

1982 and has been used regularly to date.

8 4.1.2.3.

Terou

Wari Maro 2705

The only catching point on the Terou was

created

in

¹⁹⁷⁸

and first catch was

made

in

November same year, and has been used regularly as a monitoring point to date.

1.1.2.4.

Adjiro

Banon2107

Like Wari Maro, the

Banon

point

was created

in

1978.

It is the only

catching

point

on the

Adjiro.

The

first

catch was made in November in the same year and has been used regularly to date.

4.1.2.5.

Beffa

Vossa 2701

Vossa was created

in

1978

with

the

first

catching made in November and

is still

a monitoring point to date.

4.1.2.6.

Okpara

Binassi

l9l8

Created in June 1978 and used regularly

till

¹⁹⁸⁷

Kaboua2702

Kaboua was created

in

1978. The

first

catch was made

in

November same year and has been used regularly to date.

Bassa 2706

The point was created

in

1979. The

first

catch was made in June 1978 and is

still

being used as

rnonitoring point to date.

Okeo 2713

Okeo was created

in

1982

with

its

first

catch

in

June the same year.

It

was used as a regular monitoring point

until

1990.

4.1.2.7.

Zou

Tlre Zou has three catching points, namely. Atcherigbe, Zoudli and Cove.

I

Atcherigbe 2709

The point was created

in

1978 and was under the Sub-sector

of

Parakou. but transferred to the

Bohicon

Sub-sector

in 1982 after the creation of this

Sub-sector.

The first catch was made in

November 1978 and has since been ^a regular catching point to date.

Cove 3003

Cove was created

in

1982 and was under the Bohicon Sub-sector. The

first

catch was made

in August 1982.It

was a regular catching

point until

October 1987 when catches were stopped

for

the same reasons given under

Affon.

Zotüi3994

Zoû1i

was created

in

1982 and was a regular catching point

for

only

two

years because

of it's low fly

count.

The hrst

catch was made

in August

1982;

in

1984 catches were drastically reduced.

After

that year, no catches were made

until

1989 when only three months catches were made and one month

in

1995.

4.1.2.8.

Kouffo

The

Kouffo

has

two

catching points

;Lanta

^{and Konfokpa.}

Lanta (code 3001)

Catches were made atLanta

from

1982

to

1999 Konfopka (code 3008)

This point is located upstream of Lanta. Catches were made between 1995

and200l

4.1.3.

Entomolosical

prospections

Several entomological helicopter

prospections

were carried out on the Oueme and

its tributaries

for

various reasons. Among these were the

following:

a)

Investigations

into

suspected treatment failure.

b) To

investigate the problem

of

re-invasion

of the

Oueme

valley by

S. soubrense

Beffa

form.

from neighbouring Nigerian rivers.

c)

For, purely research reasons

d)

For. Operational reasons

4.2. ^History

^on

larviciding

10

4.2.1. Aerial larvicidins

Aerial larviciding

began in the Oueme basin

in

week 12, 1987.

This

continued

until

week ^18, then treatment

was

suspended due

to lack of flow.

Vectobac*

(8.r. H-14)

was

the larvicide

used.

Treatment was resumed

in

weeli 36 and continued

for six

cycles, namely,

from

week 36 to week 41.

The larvicide used this time was temephos ^(Abate@).

It

is interesting to mention that,

only

¹³ treatment cycles had been carried out

in

1987.

In

fact the "attack phase"

of larviciding in

the Oueme basin started

in

1988 on week 9

with

B.t.

H-14;

(table

1). The

larviciding

cycles

of

1987 can be considered as treatment trials.

It is

important

to

note that the period

of

commencement

of

treatment was

in

the

dry

^season.

The

aim of this

was

to

eliminate the S. damnosum ^s.1. population at

their dry

season

foci

before the onset of the

rainy

season.

In

so doing, economy could be made

in

the

rainy

season by treating shorter stretches of the

river

or suspend treatment in view of good results, namely, absence

of

flies.

This theory was however, dashed because

of

invasion or contamination of the Oueme basin by S. soubrense Beffa form

from

neighbouring rivers in Nigeria.

Since the "attack phase"

in

1988, aerial operations have continued uninterrupted

in

the Oueme basin

to

date.

Aerial

operations are suspended

only in

the

dry

season when the entire basin dries up (see section 2 paragraph 7 of this report).

The period

for

a minirnum treatment

in

the Oueme basin

for

a given year is between weeks I

to8

The

period lbr

a maximum treatment

is

between weeks

26 to

^45,

with the

peak, occurring between, weeks 36 to 40.

These treatment cycles correspond to the pattem of discharge of the Oueme and

it's

tributaries during a given year as already mentioned.

The treatment cycles

of

the Oueme basin including the Okpara, the

Zou

and the

Kouffo from

1987

to

2001 are in the annex of this report (tables

7.2,

3, 4. 4a, 4b and 4c).

4.2.2. Ground larvrcldrug

Ground

larviciding

is not used in the Oueme basin ^as the main method

of

vector control but as

support to aerial

larviciding

in the case of treatment failure due to bad ^passage

of larvicides

or when a breeding site is missed after the ^passage of an aircraft.

It

is also used

in

the

dry

season on certain isolated breeding points

to

minimise treatment cost

of

aerial larviciding.

At

very

low

discharge in the dry season when aerial

larviciding

is

difficult

to carry out, ground

larviciding

is again ernployed.

This is usually between weeks 1 to week 8 on the tributaries

-"ir

11

Ground

larviciding is

carried out

by

the various Sub-sectors

of

ex Parakou Sector. depending on the location of the points to be treated.

4.2.3.

Insecticides used (insecticides

rotation)

From

1987

to

date

all

the insecticides employed by the Programme (Abaten',

B.t.

H-14, both Vectobac* and Teknarc',

phoxim,

chlorphoxim, pyraclofos, permethrin, carbosulfan and etofenprox) are being used on rotational basis in the Oueme basin.

The reasons for the rotation of insecticides are as

follows

1. ^To

prevent the occurrence

of

resistance,

which

may occur

if only

one insecticide

is

used, ^as occurred

with

Abatee

in

the early stages of the Programme.

In

order to avoid this, the consecutive ^use

of

insecticides belonging to the same chemical compound is discouraged to prevent cross-resistance.

2.

^The rotation is also carried out

for

the cost-effectiveness of treatment. Using B.t. H-14 at very

high

discharge

will

require a huge volume

of

insecticide.

This will

mean a requirement

of a lot of

fèrry time

for

an aircraft and loading. On the other hand an insecticide

like

permethrin which is

highly toxic

requires

very

small volume at very high discharge. thus a small amount can cover a

lot of river

stretch to be treated.

3. ^{Rotation of}

insecticide

is

carried

out

according

to

the

toxicity of the

insecticide

at

different discharges

on

non-target organisms.

For

example

B.t. H-I4 which

has a

very low toxicity on

non- target organisms is used at

low

discharges, namely

from I

cubic metre per second

to

about 35 cubic metres per _second. While permethrin and carbosulfan,

which

are

highly toxic,

are

only

used when the discharge

is

higher than 70 cubic metres per second and cannot be used

for

^{more than}

six

treatment cycles

in

a given year. Pyraclofos is used at a discharge

of

^{15 cubic} metres per second onwards, and this applies to etofenprox also. On the other hand, insecticides

like

Abate@, phoxim. and chlorphoxim, which have relatively

low toxicity.

can be used even at

low

discharge.

4. ^Finally

rotation is carried out according to the carry

of

the insecticide. B.t.

H-14, for

example,

which

has

very low

carry,

will

require

a lot of

treatment

point

at

higher

discharges.

This

therefore renders

it

unsuitable

for high

discharge treatment because the volume

of

product

to

be used

will

not be cost effective as pointed out earlier on. However an insecticide

like

Abate@,

which

has a very good

carry

coupled

with its low toxicity on

non-target organislns can be used

at almost any

discharge.

Pyraclofos

is

another insecticide, which has a very good carry but because

of

its

toxicity

as indicated earlier on,

it

is used

from

15 cubic metres per second upwards.

The practical aspect of the rotation

of

insecticides can be seen on the tables of treatment of the Oueme basin in the annex of this report (tables

I

to

4,4a

and 4b).

In the

Oueme basin, resistance

to

temephos had been

intermediary in

1981,

but in

^1993,

resistance was detected at the

Lower

Oueme area around

Agonlin

Kpahou

(OU

700), Atchakpa (OU 600). the Upper Oueme around

Affon (OUl04),

and the Upper Okpara around Bassa

(OU

710 and

OU 720). The susceptibility to

temephos became

normal in ^1998.

See

graph on

"Resistance to temephos Zone East"

(fig.

9).

4.3. Entomolosical

results

12

4.3.1.

The vectors

involved

To

determine

the vectors involved in the

Ouerne

basin, adult flies are caught by

^vector

collectors as described

by

Walsh er

al.

⁽¹⁹⁷⁸⁾ and are separated

into different

groups, through their

wing tuft

examination using

Kurtak

et

al.

(1981)

method.

These groups are classif,ted as

01, 02.03,

04. and 05.

The 01

groups are

the

ones where

all the individual

hairs

of

the

wing tuft

are pale. The 02 groups are the ones

with

the

majority of individual

hairs

of

^the

wing tuft

pale,

with

the presence

of

some

few

dark hairs (less than 50% of the total numbers of the

wing tuft

hairs). These

two

groups are considered to belong to the savannah group

of

species, namely, S. damnosum s.s. and S. sirbanum.

The 03 groups are the ones

in which

50o are pale hairs and 50o are dark hairs. These can be savamrah species

(S.

damnosum s.s)

or S.

squamosum

or can

even

be S.

soubrense. These are however rare in the Oueme basin.

The 04 groups are those

in which

the

majority of

the hairs on the

wing tuft

are dark. The 05

groups are those with 100% dark wing tuft ^hairs.

^These

two groups are the forest

species, S.

sanctipauli or S.

soubrense

falthough it

^{has been}

^{pointed out that}

some members

of adult

flies rnorphologically

looking

S. soubrense Beffa form have pale

wing tufts

(Garms 1978 and Meredith er

al

1983.). these have disappeared after the 1983 drought ( Cheke et

al

1987)1.

From available data

for

both adult population and larval population, the dominant species on the tributaries

of

the Oueme

is

S. damnosum s.s. On the other hand, on the Oueme South

of

Beterou catching

point

the dominant species during the major part

of

the

rainy

season

is

S. soubrense Beffa

form.

See graphs on

MBR (Monthly Biting

Rate) on seasonal variation

of

S. soubrense

Beffa form

at Atchakpa,

M'Betekoukou

and

Aguigadji

catching points. These

two

graphs are obtained

from

data

from

1980 to 2001

(fig.

13, 14, and 15).

No S. sanctipauli and S.

_)rahense

have ever been found in the

Oueme

basin, either

by cytotaxonomy or by adult morphological examinations (table 5).

4.3.2.

The parasite involved

Since the creation

of

the molecular

biology

laboratory.

it

has been shown that the

majority of the

parasites transmitted

in the

Oueme

basin

are

the

Savannah strains. Occasionally some forest strains are also transmitted.

It

is important to note that these Savannah strains are transmitted

by

both

the

Savannah

flies

(S. damnosum s.s. and S. sirbanum). and the forest

flies,

namely,

(S.

soubrense Beffa

form)

(table 6).

4.3.3.

Transmission

From the tables

of

the Annual Transmission Potential

(ATP)

and the graphs

of ATP for

both the Oueme and

it's

tributaries. the pre-control data showed that transmission was

very high

on both

the

Oueme as

well

as

it's

tributaries. The

ATP

^values

for all

species combined were

over

6000 at

M'Betekoukou

(Oueme), and Atcherigbe (Zou). On the Okpara at Kaboua, the

ATP

was

slightly

over 8000 (tables 7

,7a

^{and 7b).}

The other points

like

Atchakpa, Beterou and

Aguigadji

on the Oueme had pre-control

ATP of

less than 4000.

On the

other

points of the

tributaries,

like Wari Maro (Terou),

and Vossa

(Beffa) the

pre- control

ATP

was around 5000.

Tlre rest

of

the points

like

Banon

(Adjiro),

Zagnanado (Oueme),

Affon

(Oueme), and Barerou (Yerou Maro) had pre-control ATP

of

less than 1000.

Before vector control operations started, there was a severe drought

in

1983, and the situation returned to normal after 1985. This drought affected both the pre-control

ATP

and

ABR

significantly, as shown on the graphs of ATPs and ABRs

(fig.

10,

11,12,

and l2a).

The transmission during the

larviciding

period

is from

1988

to

2001. The drought mentioned above greatly helped

to bring

down transmission

from

pre-control data

to

around an

ATP of

about 1000 as early as 1986. a year before aerial

larviciding.

The

only point which

had

ATP of

more than 1000 was M'Betekoukou,

with

ATP of more than 2000

in

1992.

In

1994, after seven years of larviciding, the ATPs on all the points in the Oueme basin, except at Kaboua on the Okpara, had fallen

below

1000. These results are exceptionally good, considering

the time involved. It is

however important

to

note

that

apart

from the drought factor,

large-scale ivermectin distribution began in the Oueme basin

in

1989.

On the

Oueme

itself

greater

proportion of

transmission

is by S.

soubrense

Beffa form,

as

shown by

histograms

of ATPs at the following

^catching

points, M'Betekoukou, Aguigadji,

and Atchakpa. However, on the Upper Oueme at Beterou transmission

is mainly by

Savannah species S.

damnosum s.s and S. sirbanum

(fig.

13

to

^16).

On

the northern tributaries

like

the Terou and

Yerou Maro,

again S. damnosum s.s. and S.

sirbanum are the major transmitters;

ATP

histograms of Wari Maro and Barerou catching points show tlris clearly

(frg.2l

and22).

As

one goes down south, transmission

by

S. soubrense Beffa

form

becomes important on the tributaries

like

Beffa, Okpara, and the Zou

(fig.

I 8,

l9

and 20).

4.3.4.

Special studies

qn§. soubrenseBef for

Special studies on

the

seasonal variation

of

S. soubrense

Beffa form

had been carried

out in

tlre rainy season on the Oueme river and its tributaries

in

1991 and 1992.

These studies had shown that the M'Betecoucou area was

very highly

contaminated.

A

very

important

contamination was also observed around Kaboua

on the

Okpara

river.

Farther

from

the Nigerian border towards 'West

and

North,

less or no contamination had been observed (frg.

23.24

and

?5\

4.3.5. Entomological

Results on

3l

Décembre 2001.

In the Ouérné basin, out

of

a total

of

15 catching points regularly monitored

in 2001,

three (3) showed a corrected ATP O. volvulus above 100. These points are M'Bétékoukou

with ATP :294:.

Kaboua

with ATP :210

and Djabata

with ATP :

261.

All

these points are located near the nigerian border and are regularly re-invaded in the rainy season

by

S.soubrense Beffa form.

Considering the transmission

by

the savanna species

of similium, only the

catching

point of

Diabata (on the Okpara

river)

showed an

ATP

above 100 which was 138.

Given the very high precontrol data

(ATP

above 2,000 at most catching points and 6.000

#

8.000 at M'Bétékoukou and Kaboua)

it

can be said that the entomological situation is

relatively

good in tlre Oueme

basin

(tabl es J , ⁷

a,7b,

et frg. 26 to 3 5).

5. SURVEILLANCE OF AQUATIC FAUNA

The chemical insecticides used by OCP are degradable and do not accumulate

in

the food chain.

The

doses used

to kill simulium

larvae are

not toxic for

humans and mammals.

This is

OCP

first

requirement in the choice

of

anti-simulium larvicides.

Insecticides that meet these requirements are submitted

to the

screening procedure established by OCP Programme and its Ecological Group.

This

independant Ecological Group

which

includes international Experts scientists analyzes the results to provide guidance to OCP monitoring and aquatic environment protection action.

National teams of hydrobiologists are in charge of the surveillance of ^aquatic

^fauna

(invertebrates and fish). See figure 36 for the location of aquatic fauna surveillance sites.

Most hydrobiological

surveillance stations

in Benin are located on the

Ouémé

river

near Zagnanado,

M'Bétékoukou,

Atchakpa and Bétérou. The stations on the tributaries are; Atchérigbé on the

Zoq

Kaboua on the Okpara, Vossa on the

Beffa

and Lanta on

the Kouffo. The Bénin

national hydrobiologists team, had undertaken monitoring at these sites

from

1981

to

1987.

In

1987,

monitoring

was suspended

following

the reduction

of the

surveillance stations

by

the Ecological Group and the financial

difficulties

of the Programme.

In

1995, OCP consultants

did

some samplings at

M'Bétékoukou

and Bétérou.

Regarding

the ichtyofauna, the two

stations

regularly monitored were M'Bétékoukou

and Bétérou on the Ouémé river.

Total weight

of

CEU showed an increasing trend

fi'om

1981

to

1986

for

the Bétérou station and

frorn

1982

to

1986

for the M'Bétékoukou

station.

This

increase

in

catches was

confirmed by

the ponctual catches carried

out in

1995

at both

stations.

The

specific abundance

noted in

13 fishing sessions showed

41

species

at

Bétérou and 55 species at

M'Bétékoukou in

16 samplings.

The fish inventory carried out in ¹⁹⁹⁵ did not show the

disappearance

of any fish

species because

of larviciding. However, it

^should

^be

^noted

that

some cases

of fish mortality

related

to the

use

of

ichtyotoxic chemicals

in

low waters were reported several times in the Ouémé basin.

Regular surveillance

of

aquatic entomofauna was not organized due to unfavorable hydrological conditions. Ponctual samplings carried out at Atchakpa and

M'Bétékoukou

do

not allow to

draw any conclusion concerning the impact

of

larvicides on the fauna at these

two

stations

which

showed poor taxonomic abundance

of

saxicole fauna.

At

Bétérou, the taxonomic abundance

of

saxicole fauna is more important than

in

the

two

other stations. On the contrary, the density

of

fauna observed

in

1995 is lower compared to densities recorded

in

1985 and 1986.

On

the whole,

as the period

of

surveillance

of

aquatic fauna

on

Ouémé was short,

it

^{was not}

possible to detect any effect

of larviciding

on the fauna in this river. But

from

observations carried out on other

rivers

treated

with

the same larvicides

it

can be envisaged that there

is no

impact on non- target aquatic fauna.

4

5

6. EPIDEMIOLOGICAL EVALUATIONS

6.1. Erridemiological

results before

vector control.

The surnmary

of

the epidemiological data,

for

the Oueme basin, can be found

in

the annex

of

tlris report (tables 8. 9, 10 and figures 37

to 4l).

6.1.1.

Oueme

The pre-control data for the epidemiological survey showed villages with very

high prevalence. The notable ones are villages on the Oueme

itself,

south

of

Beterou catching

point.

The Community

Microfilarial

^Load

^(CMFL)

was also quite high in these villages, e.g.,

l. ^A village like

Abgagoule. code 3g2,hadprevalence

of

70.20Â

in March

1979 (8

years

before vector control),

with CMFL of

16.74 mf/s, and prevalence

blind

of 0.6%'

2. Agonlin Kpahou,

code 5078, had prevalence

of

72.2%

in

December 1989

(barely 2

^yearc

after aerial

larviciding), with CMFL of

13.91 mf/s. no

blind

case recorded.

3. Atchakpa, code 3881, had

prevalence

of

^68.2Yo

^{in May 1978 (9 years before}

^aerial

larviciding). with CMFL of

23.51 mf/s and prevalence

blind of

3.5Yo.

M'Betekoukou,

code

399,

had prevalence

of

78.00Â

in April

1979

(8

years

before

aerial

larviciding), CMFL of

19.59 mf/s, and prevalence

blind of

1.7%.

Koko.

code 205, had prevalence

of

64.4%

in May

1978

(9

years before aerial

larviciding), with CMFL of

36.79 mf/s. and prevalence

blind

of 2.3oÂ.

Idadjo.

code 390, had prevalence

of

73.4 %

in March l9l9

⁽⁸ years before aerial larviciding),

with CMFL

of 28.89 mf/s. and prevalence

blind of

1.3%.

Banicri,

code 315, had prevalence

of

69.3%

in

June 1977 (10 years before aerial

larviciding), with CMFL

of 24.80 mfls, and prevalence

blind

of 0.3%.

Aguigadji,

code 408/5083, had prevalence

of 8l.9yo in April

¹⁹⁷⁹

⁽⁸

^years

before

aerial

larviciding). with CMFL of

33.31 mf/s, and prevalence

blind of

3.7Yo.

Wewe,

code 316, had prevalence

of

^61.90Â

in

June 1977

(I0

years before aerial larviciding),

with CMFL of

12.72 mf/s, and prevalence

blind

of 0.3%.

6.1.2.

Zou

In

the

Zou

valley, epidemiological surveys had been carried

out at

least once

in

14 villages.

Out

of

these,

four

villages

will

be used for this report. These

four

villages are chosen because of their high pre-control prevalence.

1. ^Ferme Agbodjime,

code 5076, had prevalence

of

^68.8%

ⁱⁿ

^{December 1989 (barely}

^2years

after aerial

larviciding). with CMFL of

17.49 mf/s.

with

no record on

blind

^case.

Konkondji. code 395, had

prevalence

of

72.4oÂ

in March 1979 (8 years before

aerial

larviciding), with CMFL

of 24.56 mf/s, and prevalence blind of 2.3oÂ.

6

7

8

9

2

J

4

16

Zonto,

code 400, had prevalence

of 695% in April

1979 (S years before aerial

larviciding).

witlr CMFL of

25.82 mf/s and prevalence

blind

of 3.5%.

Avideme.

code 5249, had prevalence

of

66.4%

in

October 1990

(barely 3

^years

after

aerial

larviciding), with CMFL of

20.16 mf/s. No

blind

case recorded.

6.1.3.

Terou

01

the Terou-Oueme basin. surveys had been carried out at least once

in

seven villages. The pre-control prevalence for these seven villages has a range

of

59.7oÂto 82.6%o.

Except

for two

of these villages, namely, Igbomakoro, code 386, and Sawoundi, code 387, the rest had been visited only once. These villages therefore

will

be used

for

illustrative puposes.

Igbomakoro

had prevalence

of

78.40Â when

it

was

first visited in May

1978

(9

years before aerial

larviciding), with CMFL

of 31.89 mf/s, and prevalence

blind of

0.7%.

Sawoundi

had prevalence

of

59.7%

in May

1978

(9

years before aerial

larviciding), with CMFL of

14.72, and prevalence blind of 0.60Â.

6.L.4. Kouffo

Pre-control epidemiological

surveys

had

been

carried out in eight villages in the Kouffo valley.

The pre-control prevalence was

not very high

cornpared

to

that

of the

Oueme

or the

Terou.

Except

for

the

village

Kogbetohoue, code 401, where the pre-control prevalence was 60.4oÂ.

in April

1979, the rest of the villages had prevalence of less than 50oÂ. The range is 6.7Yo to 46.50Â; (table 8).

6.1.5 Oknara

For pre-control data of epidemiological

surveys

two villages are

chosen

for

illustrative prlrposes, and these are:

Kaboua,

code 275, had prevalence

of

69.6%

in May

1978 (9 years before aerial

larviciding), CMFL of

¹⁷ .52 mf/s and a prevalence

blind

of 0.3oÂ.

Woria,

code 396, had prevalence

of

70.4%

in April

1979

(8

years before aerial

larviciding), with CMFL

of 23.91 mf/s, and prevalence blind

of

I .5%.

6.2. Epidemiolosical

results

durins Vector control.

It is

irnportant

to

mention that

the

good epidemiological results

on the

Oueme

river

and its tributaries are due to both vector control and ivermectin distribution.

The

following

villages illustrate these results

As

already shown the pre-control prevalence

for M'Betekoukou

was 78.0%

with CMFL of

19.59

mf/s (in

1979).

In February

1987,

the

prevalence

was

unchanged

(78.8%). In

1994 during the month of December the prevalence

fell

as

low

as 18.1%

with CMFL of

0.41 mf/s.

Zoudji,

code 5396, had a prevalence

of

46.40Â

in

December 1992,

with CMFL of

4.75

mfls.

In November 1995, the prevalence

fell to

14.60Â

with CMFL

of only 0.44

mfls.

2.

2

2 I

J

Fonkpodji,

code

394,had

prevalence

of

63.10Â

in March

1979,

with CMFL of

17.45

mfls.

Two

years after ivermectin distribution, namely

in7992,

and barely

five

years after vector control started, the prevalence

fell to

50.8%,

with CMFL

o18.92

mf/s,

and three years later, namely,

in

1995, the prevalence

fell

to 22.6%

with CMFL

of only 0.88 mf/s.

6.3. ^{Latest losical}

^results

6.3.1.

Oueme

On the

Oueme

itself, the latest epidemiological

evaluations

had been carried out in

the

following

villages.

1. ^Atchakpa.

code 381/5880, which had pre-control prevalence

of l2.2yo,

as already mentioned, now has prevalence

of

3.8o in

April

1999,

with CMFL

of only 0.13 mf/s.

2. Idadjo, in

^February 1998 has prevalence

of

17.7oÂ.

with CMFL of only

0.55

mfls,

compared to 73.4%o as already mentioned.

3. Koko,

in February 1998 has prevalence

of

9.8Y" compared to 64.40Â in

May

1978.

4.

^{Tosso. in}

^April

^{1999, has} prevalence

of

5.4o compared

to

^57.3oÂ

in

October 1989.

5.

^{Wewe, in}

^April

^{1999. has} prevalence of only 0.6% compared

to 619% in

J:une 7977 .

6.3.2. Tributaries

of the Oueme

Latest results of the villages on the tributaries of the Oueme,

like,

Okpara, Zott, Terou and the

Kouffo.

showed that the prevalence has

fallen

so

low. In

some areas

it is

less than l0oÂ,

with CMFL

of less than

I

rnf/s. (tables 8, 9, 10 and

fig.

38 to 41).

1. Konkondji;

code 395;

in

March 2000 had ^a prevalence

of

¹ .9o and a

CMFL of

0.08 mf/s compared to 72.40Â of prevalence in March 1979.

2. Wokpa;

code 5038.

in

December 2000 had a prevalence

of

0.8%

with a CMFL of

0.01 mf/s. compared

to

54.8% of prevalence

in

October 1989

with

a

CMFL of

7.89 mfls.

3. ^Zouto:

^{code 400,}

ⁱⁿ

December 2001, had ^a prevalence

of

¹ .8% and a

CMFL of

0.06 mf/s.

compared

to

69.5Yo of prevalence

in

1979 as already mentioned.

6.3.3. Kouffo

1. Lanta; code423 inApril 1999,hadaprevalenceof

^0.60Â

withCMFLof 0.02mfls

compared to ^42.70Â of prevalence in

Mai

1979 wrth

CMFL of

Smf/s.

2.

Kogbetohoué; code 401; in february 1998. had a prevalence

of

13.6%o

with

a

CMFL of 0.47nfls.

compared

to

60.4oÂ

of

prevalence

with

a

CMFL of

13.97mfls.

and

1.26%

of

blindness

in April

1979.

(fre.42).

6.4. Recentophtalmologicalevaluation.

In

December 2001, an ophthalmological evaluation was carried

out in 06 villages

in the Oueme basin

in

Benin.

18

Out of

a

total

population

of

1987 people, 1365 were present.

Skin

snip was made on 1362 people and 1105 people aged 05 years and above underwent ophthalmological tests. This test concerned the anterior and posterior segments of the eye.

Results analysis showed

that skin

and eye parasitisms had

strongly

reduced

both in

terms

of

rnicrofilariae load and number

of

cases. Indeed, the highest

microfilariae

loads were 377 at Zouto

in

1987;66 at Okpa and 64 at Madengbé

in

1995 compared to 22 at Okpa and I ¹ at Madengbé

in

2001.

Concerning

the

eye lesions, the

microhlariae

loads were

63 at Zouto in

1987; 19 at Madengbé

andT

at Okpa

in

1995 against

4

at Okpa

in

2001.

A

clear regression

in

cases

of

serious onchocerca eye lesions was noticed.

All

inflamatory lesions had regressed except two cases at Okpa. The highest microfilariae loads were recorded in this village (tables

l1

and 12).

Indeed, the

village of

Okpa

is

located at the confluence

of

Oueme and Okpara rivers close

to the border

between

Nigeria

and

Benin. It is known that the medium

Ouémé and Okpara are re-invaded in the rainy season

by

S. soubrense Beffa form coming from Nigeria.

In

conclusion, the ophtalmological survey carried out

in

December 2001 revealed that there was a clear regression of eye onchocerciasis in the 06 villages surveyed.

7. IVERMECTINDISTRIBUTION

Large-scale ivermectin

distribution

began

in

the Oueme basin and the

Kouffo in

1989. Nine villages were covered.

In

¹ 990 in Zone 3, eleven ^{( 1 1} ) villages were covered

In

1991 the Oueme area was again visited, this tirne covering 36 villages in Zone 3

In

1992 the Lower and the Upper Okpara, as

well

as the

Middle

Oueme were visited, covering again 36 villages. also on the Agbado, the middle

Zot(Zote

4), where 104 villages were covered.

In

1993 the areas, where large-scale ivermectin

distribution took

place were

Kouffo,

Oueme and Okpara, where 415 villages were covered.

In

the same year (1993) the Oueme and the

Zou

were visited. covering 347 vlllages. and on the

Kouffo

187 villages were covered.

ht

1994 the Agbado, the

Middle

Zou had large-scale ivermectin distribution

which

covered 14

villages; (Zone 4).

In

1995, Zones 3 and 4 were visited again covering 349 vlllages.

In

the same year, the Upper and

the Middle

Okpara (Zone

5)

were visited, where 312 vrllages were covered.

In the

same year, Terou (Zone 6) was also visited covering 305 villages.

It

should be noted here that,

from

1990

to

end

of

1996, the national mobile teams. headed by the national Co-ordinator carried

out

large-scale ivermectin distributions,

involving

the

local

district health units.

The

details

of

ivermectin

distribution. from

1989

to

2001. can be

found in table

13

in

the annex of this report.

In January

1997, the

village

communities were trained

to carry out

large-scale ivermectin distribution themselves, this had therefore ^been the case since then.

8. SOCIOLOGICAL INVESTIGATIONS.

At

the time of

writing,

no sociological investigation had been carried out in the Oueme basin.

9. POPULATIONS SENSITIZATION ^AND MOBILIZATION ACTIVITIES

It is irnportant to recall that since 1974, OCP

Programme management

have

constantly expressed

the need to ^get various community social elements involved in sensitization

and

mobilization

activities whenever this was feasable.

As from

1975 an information campaign through the media and health education services was launched

to

sensitize people about the dangers relating

to

onchocerciasis. Advertisement

on

posters and in the rnedia was used. The communities were rapidly able to understand the relationship between the black flies and blindness as

well

as the objectives of the programme. They understood the weekly intervention of airplanes and helicopters that carried out aerial larviciding.

The efforts

to

provide information

did

not target populations at

risk only. They

also targeted

administrative and health authorities to

ensure

their full

adherence.

They also targeted

donors countries which were concerned

with

the monitoring

of

an action they were supporting.

Population sensitization and mobilization activities are

^ensured

by VCU teams

during entomological prospections, and by national teams before epidemiological evaluations and ivermectin distribution.

All the social

segments as

well

as

the political

and administrative authorities

in the

areas visited by the teams were targeted by the sensitization (teachers, traditional healers, health agents and religious leaders, etc.).

Talks were organized and leaflets on OCP activities were distributed

in

the villages; and

films

were shown anytime that was possible.

The

following

themes were discussed during the talks

*

Onchocerciasis transmission; relation between the

river.

the bites

of

^the

black flies

and the disease

*The

clinical

signs of the disease: itching, acute skin lesions (onchocerciasis dermatitis), nodules, chronic cutaneous symptoms (depigmentation), dimsight (progressive visual impairment, blindness).

*

The socio-economic consequences; disabilities (blindness), abandonment

of first

line villages (closest to the

river)

and most fertile valleys, which is an obstacle

to

socio- economic development.

*

_The control of the disease, vector control, ground

larviciding,

chemiotherapy (treatment

with

ivermectin).

20

Regarding vector control, the involvement of the populations is

requested

to look

after kerosene and insecticde stocks to prevent theft.

Concerning

ivermectin

treatment,

a particular

emphasis

is laid on the

doses

to

administer depending

on patients'

size, contra-indications,

periodicity, drug

intake

regularity,

side effects and appropriate behavior.

The progressive intensification

of

sensitization resulted in the gradual increase in the coverage

of ivermectin distribution. It also resulted in the easier

acceptance

of skin snip

during epidemiological surveys.

As far

as

vector control is

concerned,

the

sensitization

of

populations

led to a

significant reduction of cases of theft of kerosene and insecticides in the depots in the field.

10. DISCUSSIONS.

I 0.1

. Fly

catching data

from

1980

to

2001 showed a particular pattern

of

seasonal

variation in fly

snecies oooulation. The seasonal variation is between S. datnnosuln s.s and S. sirbanum on one hand and S. soubrense

Beffa form

on the other. From the analysis

of

data

from

1980

to

2001,

it is

clearly shown that S. soubrense Beffa form becomes the major component of the

fly

population

in

the Oueme basin

during

the

rainy

season and completely absent during

the dry

season.

This is true only

at the stronghold

of

S. soubrense Beffa

form

areas (the Oueme

valley from

Atchakpa

to Aguigadji,

and on the Okpara from Kaboua to the confluence

with

the Oueme). ^See radar graphs on seasonal variation

of

S. soubrense Beffa form at Atchakpa and M'Betekoukou

(frg.24

and 25).

10.2. From

1988

the first

year

of the

"attack phase"

of aerial larviciding to

date,

vector

control operations have produced very good results, as can be seen in the various

ATP

graphs

in

the annex

of

this report;

(fig. l2

and 13).

In

some places the ATP had rapidly fallen to very

low

levels. The

ATP of

less than 100 is recorded

in

some places. after

only

9 years

of

aerial

larviciding.

The drop

in

ATPs

in

the Oueme basin

is very rapid

compared

to

what

normally

occurs

during

vector

control. This

rapid drop in ATPs can be attributed to three factors:

i.)

^the

^first

factor, being the severe drought,

which

occurred

in

the sub-region

in

1983. From the graphs

of ATPs

on

the

Oueme and the tributaries,

it is

shown that after

the

drought, and even four years before aerial

larviciding,

the ATPs had started dropping already. These ATPs

did

not come up to their

original

values after the end of the drought, which was marked

with

the beginning of the 1985 rainy season.

In

fact. the ATPs remained low

till

the beginning of aerial

larviciding in

1988.

ii.) The

second

factor

being,

the

beginning

of

large-scale

ivermectin distribution in the

Oueme basin

in

1989.

It

has been shown that the intake

of

ivermectin by the human population

in

an area can reduce the rnean microfilariae load (mf1 in the skin of treated person by 96%,

- 99%. After

¹² months, tl-re subsequent mf-repopulation

in

the skin

is

about ^40oÂ

of its original level

(Remme J. et

al

^1990).

The reduction of microfilariae load in the

human population therefore results

in the reduction of infective

larvae

in

the

fly's

head

(L3H).

The

ATP

calculation

is

based on the presence

of

infective larvae

(L3H)

in the head of the

fly.

The ATP

will

theretbre be

low if

few L3Hs are obtained.

iii.)

^The

^third

factor is vector control

itself.

From the graph

of

the Annual

Biting

Rate

(ABR). it

can be seen that. although the basin is contaminated each year

by

S. soubrense Beffa

form flies

from Nigeria, there is progressive downward trend. This means, there is gradual reduction

in fly

population each year. indicating good control strategy

(fig.

10 and 1 1 ).

10.3

It

is irnportant also to comment on the vectorial role of the S. soubrense Beffa form in the Oueme bassin.

As already mentioned earlier on in this report, S. soubrense Beffa form is the major transmitter

i1

the Oueme

valley

itself. Transmission

by

S. soubrense Beffa

form

is

in

the

rainy

season

only ( fig.

24

and

25).

There

is no

transmission

in the dry

seasou as

it ^is in the

case

of S. squamosum in

perennial

rivers

(Cheke et

al

1991). On the other hand.

it

has been shown

that

S. yahense does not sliow any major seasonal variation in transmission (Cheke el

ul

^{1992). This} therefore, means that

if

S.

soubrense

Beffa form

were not to be a contaminant

of

the Oueme. the

ATPs would

have been lower tlran what they

actually

^are. This can be seen in the parts of the basin, which are not the strongholds

of

S. soubrense Beffa

form (frg.23)

If

one compares the ATPs

from

the

Beffa

strongholds

with

the ATPs

from

other parts

of

the basin.

it

can be seen that, the values of ATPs in the strongholds are 3 to 4 times higher.

10.4. It is

important

to

note

that

S. soubrense

Beffa form

spreads

very little from their points of arrival in the

Oueme basin, namely,

the Lower

Okpara and

the Lower

Oueme

(their

strongholds).

They are therefore rarely found

to

the west

or to

the north

of

the strongholds.

Although it

has been shown in the laboratory that S. soubrense Beffa form can

fly

long distances (Cooter 1983), they seem to restrict themselves to the strongholds in the Oueme basin

(

frg.23)-

10.5.

The pre-control epidemiological data showed

high

prevalence values

for the

Oueme basin, around the strongholds

for

S. soubrense Beffa

form

(68%

-

80%). On the other hand prevalence

blind

values

for the

same areas were

quite low

⁽¹ .70%

-

3.5%).

This

shows

that,

although

S.

soubrense

Beffa form, are good

transmitters

of the

Savannah

strain of the

Onchocerca parasite

(DNA

lab results), the transmission does not result in the blinding type of Onchocerciasis.

This

situation

is worth looking into for clarification.

^{The question}

to

be answered

is

this.

"Is the

Savannah strain

of

Onchocerca parasite transmitted

by S.

soubrense

Beffa form different from

other

blinding

Savannah strains found elsewhere?"

11. CONCLUSION.

The latest entornological and the epidemiological results show that the onchocerciasis situation

in

the Oueme basin is good. There is progressive regression of onchocerciasis by way of transmission by the

fly (ATPs)

and the prevalence

in

the human

population. This

means the

control

strategy

in

use (cornbination of vector control and ivermectin distribution) is a good one. Especially,

if

one takes into consideration the duration of vector control in the Oueme basin.

The composition

of

the

fly

population

in

the Oueme

valley

varies according

to

the season.

In

the

rainy

season,

S.

soubrense

Beffa form

constitute

the majority of the fly population on the

Oueme itself.

In

the

dry

season and late rainy season the population composition changes

to

S. damnosum ^s.s and S. sirbanum.

The data available show that S. soubrense Beffa

form is

a good and important transmitter during the

rainy

season, but not a good transmitter

if

present

in

the

dry

season. This means that

if

there was no contamination phenomenon by S. soubrense Beffa form, the onchocerciasis situation

in

the Oueme basin would have been better.

22

ACKNOWLEDGEMENTS

Our

sincere thanks

go to

^the Programme

Director of

OCP and

the Chief of Vector

Control

Unit (VCU) for giving

us

the

singular honour

by inviting

us

to

come

to

Ouagadougou

to write

this report.

The

writing of

this report

would

not have been possible

without

the help

of all

the

VCU staff

in both Ouagadougou and in the

field.

This is to say a big thank you to them all.

Our

special thanks go

to Mr. Aké

Flavien,

Mr.

Pierre

Nikiéma

and Robert Toé

for

the

vital tips

provided

in

the use

of the

computer

in

the

writing of this

report,

in

the

making of

the various graphs and the maps of this report.

Many

thanks

to Mr

S. N'Gadjaga and

Mr. D.

Somé

for all

the computer installations which made

it

easier to write this report.

We

would

also

like to

express

our

sincere gratitude

to

the PET

Unit for

the epidemiological infbrmation provided as

well

as the data on the large-scale ivermectin distribution in the Oueme basin.