1975. It is

) 7 I

ONCHOCERCIASIS CONTROL PROCRAMME IN THE VOLTA RTVER BASIN AREA PROCRAMME DE LUTTE CONTRE L'ONCHOCERCOSE DANS LA RECION DU BASSIN DE LA VOLTA

Til.: 333 l2 ^- 329.10 - 338.57 - ^{Tdl6x ONCHO 5241 UV}

EXPERT ADVISORY COMMITTEE

Third Meeting

Geneva 13-17 ^S tember 1982

A COMPARISON OF REINVASION BY SIMULIIJ}T DA}INOSUM

s.I.

FEMALES IN 1982 AI.ID IN THE TTTREE PRECEDING YEARS CONTROL PROSPECTS

1.

Introduction

Reinvasion

is

a well-known phenomenon

in

the Progranune, having reappeared regularly

Eowards every rainy season since

1975. It is

essenEially certain regions adjacenE t.o

unEreated zones thaE are affecced.

Reinvasion affeccs parEicular reaches

of

cerEain wat.ercourses, and the poinEs

of

^impacc

on Ehe affected

rivers

^{are now}

well

^known.

Scudies

of

the phenomenon carried ouE since 1975 have shown EhaE Ehree species

of

^che

S damnosum complex are more parEicularly involved

in reinvasion:

two savanna species, S. damnosum

s.s.

^and

S.

sirbanumrPlus S. squamosum , ^whose t'migratory capaciEy" is, however,

less Ehan

that of

the

first

^Ewo species

in

^terms

of

the distances

covered. In

^some insEances

the same research has revealed the

starting

points

of

the invading blackfly populaeions, Lhus

enabling che problem Eo be tackled by prompt

larviciding. It

has also revealed the localized behaviour

of

the reinvading females, which were found Eo be confined

to

the gallery forest along che affected waEercourses, noE radiating out

much.

Only che approaches Eo the waEer- courses

in

a sErip no more chan 3.5 km wide would be affected wtren che level

of

Eransmission on the

river

bank was hyperendemic.

However, now EhaE resisEance Eo organophosphorus insecticides

is

^{becoming general}

in

^rhe forest, and sub-forest

foci in

the south-west.

of

the Programne area, wtrich may be breeding grcunds

for

reinvading

blackflies,

reinvasion control st,rat,egy

is

becoming complicated.

2.

AspecEs

of

reinvasion ^beEween 1979 and 1982

Before^1979, some watercourses

within

the limics

of

the

initial

Prograrrne area

(654 OO0 kmz) were heavily reinvaded, and

in

^the lighc

of

our present knowledge these could ^be grouped geographically

into five

main rrreceptionrr

foci of

reinvading blackflies:

Ehe western focus: the $ratercourses affect,ed are the Wassouloubal6, the Bago6, che

Faya and the upper Sassandra basin;

Ehe western cent,ral

focus:

with the L6raba, ^{Ehe WhiEe Bandama and} ics Eribur.ary, ^Ehe

Bou and the ^{upper Marahou6;}

che southern

focus:

consisEi

WORLD

HEALTH

ORGANIZATION

AFRICAN RECION

ORCANISATION MONDIALE DE LA SANTE

RECION DE L'AFRIQUE

ocP/EAC3 .4 (ocP82. 10) ORIGINAL: FRENCH

between Tagadi and Bui;

ng

of

a stretch

of

some IOO km

of

che lower Black Volca

ocP/EAc3.4 (ocP82. ro) Page ²

Ehe south-eastern focus: comprising Ehe M6 and the upper course

of

some

of

the

Eribucaries

of

che Oti;

the eastern

focus:

inc orporaEing the

tributaries of

the

river

Niger

in

Benin, the middle reaches

of

which are parEicularly affected.

In

1979 and the years

that

followed, wtren insecticides were used Eo

treat

the phase IV

rivers

(onchocerciasis

foci in

the south-rrest and south

of

Ivory Coast), such watercourses as

the upper Bago6, che Wtrite Bandama, the L€raba and the lower Black Volta were practically

unaffect.ed by reinvasion

or

only very

slightly affected.

VecEor denslEies were very Low and

cransmission was

int.errupted.

Consequently, Ehe reinvasion

of

the wesEern cenEre and Ehe

south

of

the

inicial

Prograrme area appeared Eo have been brought under control

in

three con-

secutive rainy seasons (L979, l98o and

1981).

On the other hand, reinvasion continued

in

^che west, the south-east and the easE.

Ihese

latter

reinvasions rrere used

for

pin-point studies, as indicaEed

in

the intro- duction,

to arrive

ac a betEer undersEanding

of

the phenomenon. However, r"rhat.

is

remarkable

is

the suppression

of

the $resEern central and southern reinvasion

foci

following the treatment

of

potential sources

of

deparEure.

3.

Features of reinvasion

in

1982

3.1

Reinvasion appeared

in

^1982, as every year, true Eo the st.arE

of

the rainy season.

However, wtten compared

with

the three previous years,

ttris

reinvasion had suffered the con- sequences

of

double resistance

to

^Eemephos and chlorphoxim, the only two larvicides

for

use in simuliid conErol

in aIl

seasons available to OCp.

Because

of this,

^Ewo sit.uations arose, at. least

in

the lead-up

to

the rainy season in April-May and

in

Ehe hrestern

zone.

For the momenE there has been no change

in

reinvasion in

the eastern zone by comparison wiEh reinvasions

in

the previous years, unless

it

be in

intensity,

r.rtrich appears Eo be lower

this

^year than

in

past years.

Ihe

first of

the situations

in

che western zone

is like

thae wtrich prevailed before che

emergence

of

double resistance

to

the organophosphorus insecticides used by the prograrmre.

Ihe valleys

of

the waEercourses habicually preferred by

blackfly

populations developing in places outside the Prograntrne area have noc been

spared.

The

rivers

concerned are tributaries

of

che Niger

in

Mali ptus the upper basin

of

che Sassandra, Eogether consEiEuc,ing Ehe wesEern

reinvasion

focus. Ihis is

a sustained reinvasion.

The second situation

is

a consequence

of

the double resisEance and

of

the experimental use

of

Teknar (R) as a substicute simuliid

larvicide in

^place

of

cemephos and chlorphoxim.

As

this

Larvicide cannot be used during the high-waEer period, iE has been

difficulc

Eo deal

with

the breeding

sites of

onchocerciasis vectors

in

those ^phase IV regions recognized as

sources

of

the reinvasion

of

the upper Bago6, Ehe white Bandama, the LEraba and the lower Black Volta,

or in

other words che wesEern central and southern

foci.

As a

result,

Ehese

watercourses _srere affected by reinvasion

in

the run-up Eo Ehe 1982 rainy season, Ehe earliness

of

which was resPonsible

for

equally early reinvasion cornnrencing

in

the

first.

and second weeks

of April-

However, the _{reEurn co} the use

of

Abate

for

che Ereatment

of

pot.ential sources of departure

in

Phase

lV

^enabled

this

reinvasion, rrtrich $ras on the wane

in

June on the L6raba and

the wtrite Bandama, to be brought under

cont,roI.

The

fact

Ehat

iE

persisted on the lower B1ack Volta

in

June may possibly be a consequence

of faulty larviciding of

che lower Como6,

which

is

^regarded as a possible source

of

reinvasion.

3.2

A new situation as regards Ehe reinvasion

of

Ehe

river

l,lassouloubal6 arose

in

19g2.

simuriid density remained asconishingly low

at

the Yanfolila sit,e throughouE the reinvasion period,

in

contrast

to

previous years (since

L977). Itris site

appears

to

have escaped

reinvasion

in

1982, wttereas

ordinarily iE is

affected

at

the same Eime as Madina-Diassa on the Baou16 and Kankela on

a

tribuEary

of

the

Bago6.

There has been an ecological change

at

^Ehe level

of

the Yanfolila breeding

site,

rrhich was submerged beneath the wat,ers

of

the new

I

-l T

(

ocPf1 c3.4 (0cP82.10) Page ³ Selingu6 hydroelecEric ^dam

in

the 1982 rainy season, rotrereas

in

^previous years iE had noE been

submerged.

ftris

observation would appear t,o be a

verification

ad absurdum

of

^Ehe hypochesis Ehat reinvading blackflies would be atEracted by the breeding

sites of

the affected water-

courses.

Itre ^use

of

Bellec Eraps aE chis previously reinvaded

site

has yielded results

lit,tle

beEEer than catches on

man.

A detailed scudy

of

the macter has been undertaken on ^Ehe Baould (see section 4).

4.

Study

of

linear dis oersion

in

1982

A detailed study

is in

progress on the Baou[6 and

will

be continued throughout the

reinvasion period,

i.e. for

^{four monEhs, from May Eo}

August.

Ten sites have been selecE,ed on

the Baould and Ewo on iEs tribuEary, Ehe

D6gou. In

so

far

as possible, catches

will

^{be made} on Ehe same days ac siEes more

or

less equidist,ant from Madina-Diassa, Eaken as a reference

site.

IE

is

also envisaged

that

there

will

be periodic rnapping

of

potenEial breeding siEes

at

^periods when Ehere are significanE modifications

in

t}re discharge

of

watercourses.

Itre

first

results

of

the study indicate Ehat Ehe upper course

of

the Baoul6 was Ehe firsE

to

be affected by invading

blackflies in 1982.

ltrereafter there rcas a gradual

shift of

^che

blackflies

Eowards the middle reach

in

the Madina-Diassa

regi.on.

Ttre stretch affected extends from l,l.ar6na upsEream Eo Madina-Diassa and Niako dorensEream.

It is

on

this

sEretch beEween Mar6na and Niako that blackfly densities were highest EhroughouE Ehe reinvasion period.

Higher upsEream and lower doqmstream the densities decline

rapidly

t.o the point of disappearance

in

Ehe Logognasso region, towards Odienn6 and

in

the Niagal6 region, norEh of

Bougouni.

Ttre

first

mapping

of

the breeding sites was carried out concurrently

with

Ehe

arrival

of the

first

wave

of

^Ehe

reinvasionl it

indicates

that

che disEribution

of

breeding

sites

is

more

or

less uniform on the Baoul6, but

that

they are closer together

in

che middle course

affected by reinvasion.

5 Possible straEeeies

for

the control

of

reinvasion

in

the ProAramme area

SErategic planning

is

^dependenE upon whether

or

noE the potenEial sources

of

deparEure of reinvading blackflies are

accessible.

IE should be not,ed chat since the onchocerciasis

vecEors involved

in

the reinvasion are species sensitive co organophosphorus insecEicides they can be aEEacked

in

^Ehe

larval

stage

in their

breeding sites.

A solut,ion had already been found

for

the western cenEral and southern

foci at

the Eime

of

che inclusion

of

Phase IV regions

in

the onchocerciasis control operaEions. Although these regions are

in

^zones

in

wtrich forest. species have become resiscanE t.o insecEicides, the use

of

Abate has been effective againsE the reinvasion

of

Ehese

foci.

Studies carried ouE

in

1981 on pot.enEial sources

of

departure

for

Ehe reinvasion

of

the south-easEern focus would seem

to

indicace Ehat these sources are

in

^Ehe basins

of

the Asukawkaw and

of

the cributaries

of

the upper Mono

in

Ehe souEhern extension

zone. Ic

is considered Ehat appropriate

larvicidal

control could neutralize Ehe south-east.ern reinvasion focus

in

these

regions.

^New

feasibility

studies are planned

for

1983.

As regards the wesEern and easEern

foci,

Ehe sources

of

departure remain unknown,

although they are certainly out.side Ehe Progranrne

limits.

On Ehe other handr given Ehat, Ehe

behaviour

of

reinvading females

in

the affected reaches

is

increasingly

well

known, and Ehat

iE is

impossible

to

reach the sources

of

departure, the preferred tact,ic

in

Ehese

foci

could be some form

of adulticidal

control

sEill

to be defined

in

the

light of

the conclusions from

the linear dispersion scudy now

in

^progress.

I

t

l-

tk

C)

U ooan r.r{ aO

o ^.F{

rJ oa o

#

d

ct)

U)

tHoOE

+J E^(U

.dkEooo

..{co

'iklr

. ao

trk

c.,

!J

ocPf FAc3.4 (ocP82 . ro) page 4

H oah

(,

Pr dtr ro{JAoo0

oc..{

f+{ ooor{ , r,H(/),o

6(u

>k

0

(/)c,o.

X"{

dro o dkodd(u>'O

>E N

t

,a

J(i"Cil

\:-irl

_ -^,)!:

)_

't

.r

,il

) ,

_o

6

=

@

= a z9 o G 2 o o o

-

oz IJ e

=o

FF

= o

==

eo oG G

z

EA

@ G

t) o

-

F

= E

=e oo o

F2 o a 6e oL zo

I

\L

\.

,\ _,, /

.)t N

g,' \ i o\ \ _|

.I

(t i. I

)

:

I l.r.

f _{\_.}

,\

1 _/\

^{I ';,}

t! .:

--I) n6

ii,!il l d.

it .I

) t.'

:,

I I

!.

/;

/<

j

)

\

r-

t.

i\

(

I (

!'

I ii

t\

I I

e*

t\

't

(

H

rnt 4o

o- +J

e

ak Eo

fJt{

zo

a

_col

orl

:l

(Bl

TI

;I

€l

z 9 a Z

LU

E.

o

It

^r---'

.:l

o

t9 t6tOu.a

i

"' 1

)

t

+

't

\

t

I

I

)

t

t

I

+

.I

--t

!.

)

ocPf1 c3.4 (ocP82. ro) Page ⁵

ac oo

.r{ ..1

o ^at)

(E(u

.'{

aa

..1

ooKH

xu >-c

O (E00_'.{

!E ^FI

Et t5t OHd

!

h

rl lr

,,I

.ll

/^ra;\l

,A\r

\3Zi

->>7r'

\'

I

v

5o

J

a

=

6

= a oo

E J

=a

g

oo

G oo az IJ Ets

zo o

FF

= a

==

Go oE 4

\i

j I

I

fl.i _I

t.

t

l

il

)

6z

@ G 4

,

o

-

F

= E

=e 0o oG

F

= o 6s

G o

-

_z

o

I

I

{

I

la

,

( I

It t

'J .t .o

(\

(

)

Srorl

:l

rl

st

z o a

=

tr,E

GI

siIL

(

,. t^'- - -

a

I

I

.l t

o

.t,

.t