Cliff, B.

Davies, J.B.

Le Berre, René

Walsh, J.F.

Horizon / Pleins textes

Reprinted from MOSQUITO NEWS, Vol. 38, No. 4, December, 1978 
466 MOSQUITO NEWS VOL. 38, No. 4 
ONCHOCERCIASIS AND SIMULIUM CONTROL IN T H E  
VOLTA RIVER BASIN 
JOHN B. DAVIES, R. Le BERRE, J. F. WALSH, B. CLIFF 
Onchocerciasis Control Programme, World Health Organization, 
Ouagadougou, Haute Volta 
ABSTRACT. The World Health Organiza- 
tion Onchocerciasis Control Programme in the 
Volta River Basin of West Africa aims at reduc- 
ing the transmission of Onchocerca volvulus 
Leukart, the cause of river blindness, to a level 
no longer dangerous to health by attacking the 
larval stage of the vector Simulium damnosum 
Theo. s. 1. at its breeding sites in the rivers. The 
Programme covers an area of 700,000 sq. km. 
embracing 7 countries in which an estimated 1 
million people are affected. Up to 14,000 km 
of rivers are treated with temephos by air using 
a fleet of 8 helicopters and 2 fixed-wing air- 
River Blindness or onchocerciasis is a fi- 
larial disease transmitted to man in West 
Africa by black flies of the Simulium dam- 
nosum Theo. complex. Light infectiom by 
the worm Onchocerca voluulus Leukart do 
not normally cause any grave symptoms, 
but heavy and repeated infections may 
lead to blindness caused by physical dam- 
age and the reaction of the eye to dead 
microfilariae. 
The disease is widespread in Africa, but 
appears to be clinically more serious in 
the savanna areas (Anderson et al. 1974). 
In heavily infected communities about 
20% of persons, particularly males, over 
the age of 20 may have seriously impaired 
vision (Rolland & Balay 1969). These per- 
sons become a burden to communities al- 
ready existing at subsistence level. As the 
productive output of the village falls the 
younger people and sometimes whole 
communities move away from rivers 
which they associate with blindness to 
more distant and less fertile lands 
(Hunter 1966, Rolland & Balay 1969). 
The objective of the Onchocerciasis 
Control Programme is to reduce the risk 
of blindness in these river basins by lower- 
int the intensity of transmission to a level 
that does not lead to impaired vision, and 
to maintain this level for up to 20 years, 
that is, longer than the maximum life 
craft. The insecticide is applied at a dosage of 
0.05 ppm/lO mins of river discharge. The ef- 
fect of the applications is monitored on the 
ground by inspection of the rivers for aquatic 
stages and by assessing biting rates of black 
flies by making full day catches at some 270 
points each week. After the 1st year of full 
scale control transmission rates in the center of 
the area are deemed satisfactory, but on the 
perimeters dangerous transmission continues. 
All evidence points to this being caused by flies 
invading the area from the south and west. 
span of the adult worm (Anonymous 
1973). 
The Programme is therefore not just 
an insect abatement programme, but is 
linked with environmental, demographic 
and development projects whose com- 
bined objective is to make these poorly 
developed savanna communities more 
productive. 
The main aspects of the Programme 
are thus: 
1. Vector Control to reduce transmis- 
sion, 
2. Entomological Evaluation to assess 
the level of transmission, 
3. Environmental Monitoring to assess 
effects on non-target organisms and the 
environ ment, 
4. Epidemiological Evaluation to moni- 
tor the prevalence and intensity of the 
disease, 
5. Coordination of Development Proj- 
ects, 
6. Research to find better methods of 
attack and possible chemotherapy. 
This account deals mainly with the first 
2 aspects as being of greatest interest to 
entomologists. 
The present Programme covers an area 
of 700,000 km2 (273,000 sq. miles) which 
is about eauivalent to the area covered bv 
-- -I.-__-... "_-__I_- _  ---- -.--- 
Figure I.  West Africa showing the position of the Onchocerciasis Control Programme Area in 
relation to the seven participating countries, vegetation zones and the northern limit of distribu- 
tion of S. damnosum. 
MOSQUITO NEWS VOL. 38, No. 4 468 
Upper Volta, Mali, Ivory Coast, Ghana, 
Togo, Benin and Niger. It is bounded on 
the east and west by the R.Niger and 
political frontiers, to the north by the 
natural northern limit of the vector, and 
to the south by tropical forest and the 
huge Volta Lake. (Figs. 1 and 2). 
In this area live about 10 million people 
of whom it was originally estimated about 
1 million have onchocerciasis and 70,000 
have impaired vision or are blind from 
the disease. Because there is no effective 
mass chemotherapeut ic  cure  o r  
prophylaxis the disease must be attacked 
through its vector. 
Simulium damnosum s.1. breeds in fast- 
flowing water and rapids of large streams 
and rivers. These rivers flow from one 
country to another and often form the 
frontiers between them. I t  has been 
known for some time that the fly may at 
times disperse up to 150 km from its 
breeding sites (Le Berre 1970, Walsh 
1970), and it became obvious that the only 
way to attempt to control the fly would be 
to attack on an international scale. In this 
a o o k m .  , 
Figure 2. The Onchocerciasis Control Programme area showing the principal river systems, the 
phases of development (roman numerals) and the operations bases of Bobo Dioulasso and 
Tamale. 
DECEMBER, 1978 M~SQUITO NEWS 469 
respect the O.C.P. is probably unique 
amongst vector control projects since it is 
truly international in concept. The Pro- 
gramme accepts complete responsibility 
for carrying out Simulium control in its 
area. All ground operations are carried 
out by staff directly employed by the Pro- 
gramme. All equipment, insecticides and 
vehicles are purchased by the Pro- 
gramme. The aerial operations are car- 
ried out by a contractor who provides air- 
craft and staff. Staff and vehicles have 
complete freedom of movement across 
frontiers just as the fly they are combat- 
ting. 
The Project is financed by a special 
fund set up and administered by the 
World Bank. Funds are provided by a 
number of donor countries, which in- 
clude the participating countries, as well 
as by UNDP, FAO and the World Bank 
itself. The executing agency is the World 
Health Organization. 
The principles of attacking Simulium 
larvae by adding insecticide to river water 
are well known (Davies et al. 1968, Le 
Berre 1970, Walsh 1970) but when appli- 
cations are to be made to thousands of 
kilometers of river in underdeveloped 
areas it becomes a fascinating en- 
tomological and logistical problem. First, 
all rivers had to be surveyed by helicopter 
in both dry and wet seasons, and all po- 
tential breeding sites verified by sampling 
and their position plotted on 1:200,000 
scale maps. Because of the size of the area 
it was divided into 3 operational phases to 
enable work to be extended as trained 
staff and experience accumulated (Fig 2). 
Phase I commenced with preparations 
and surveys in 1974 and larviciding in 
February 1975. Phase II with surveys in 
1975 and larviciding in May 1976. Phase 
III with surveys in 1976 and larviciding in 
March 1977. 
Many of the treated rivers flow into 
man-made lakes. This, coupled with the 
use of river water for drinking and the 
very important fish industries connected 
with them make it imperative that no in- 
secticidal build-up could be allowed to 
take place. A special formulation of 
temephos (Abate@) 20% emulsifiable 
concentrate was chosen because it gave a 
stable emulsion in river water under local 
conditions, was very effective and was 
biodegradable, breaking down within 3-4 
days. Larviciding is carried out by an 
aerial contractor currently employing a 
fleet of 8 helicopters and 2 Pilatus Porter 
aircraft, operating from 2 bases at Bobo 
Dioulasso and Tamale (Fig. 2). Because 
the minimum time required for larval de- 
velopment is about 8 days the insecticide 
is applied weekly to all potential breeding 
sites at a dosage calculated to give a con- 
centration 0.05 ppm . temephos in 10 
minutes of river discharge. Resulting 
from the breeding surveys which are con- 
tinually being up-dated, the pilots are 
briefed each week and provided with 
maps on which each drop point is indi- 
cated, together with the quantity of 
temephos to be applied. This is recalcu- 
lated each week from river gauge 
readings which are radioed to base prior 
to the briefing. Drops are made upstream 
of breeding sites either by an almost sta- 
tionary helicopter, so that the insecticide 
can penetrate gaps in the tree canopy, or 
by moving at moderate speed to leave a 
line across the width of the river. 
Large rivers are treated by Porter air- 
craft. The insecticide is projected back- 
wards under pressure at a velocity equiv- 
alent to the forward speed of the aircraft 
so that it falls on the water in a compact 
mass. In 1977 the aircraft flew 6374 hr 
and dispensed 155,615 liters. (41,059 
gal.) of insecticide covering a maximum 
of 14,000 km. of river per week. The area 
experiences a severe dry season of 4-7 
months duration depending on latitude. 
This results in enormous variations in 
river discharge. At the height of the wet 
season, Sept-Oct., there may be 14,000 
km. to be treated while in the dry season, 
Dec-April, this may be reduced to about 
one third. To provide fuel and insecticide 
for the aircraft 121 dumps are main- 
tained at strategic points along the rivers 
and at bush airstrips to reduce ferry- 
flying to a minimum. The dumps are ser- 
viced by a fleet of 21 trucks. 
470 MOSQUITO NEWS VOL. 38, No. 4 
The effect of the larviciding on non- 
target organisms is monitored by 3 teams 
based in the Ivory Coast, Upper Volta 
and Ghana. So far they report about 25% 
reduction in river invertebrates, but the 
community structure remains good. 
There  are  even several species of 
Simulium which survive the treatments. 
No adverse effect has been so far noticed 
with fish populations. 
The efficacy of larviciding is assessed 
by estimating the biting densities of S .  
damnosum and by checking breeding sites 
and is carried out by the entomolo&al 
evaluation teams. The area is divided into 
7 sectors each supervised by an en- 
tomologist; and 24 sub-sectors, each 
supervised by a technician who also 
supervises 3-4 catching teams. (There is 
still no effective substitute for human 
bait.) They dissect a large proportion of 
the flies caught for age-grading and the 
presence of Onchocerca larvae. They make 
physical checks for larvae in the rivers 
and are responsible for reading the water 
gauges. These data are then transmitted 
to the operations bases by radio at the end 
of each week when they are analyzed and 
plotted on weekly maps from which the 
following week's larviciding treatmentzy- 
cles are modified if necessary. To main- 
tain this surveillance requires a staff of 9 
entomologists, 29 technicians, 190 vector 
collectors, 170 drivers, 120 Land Rovers, 
40 V.W. buses and other vehicles. In 1977 
they travelled 6.8 million km.(4.3 million 
miles) and were repaired at 7 workshops. 
Fly-catches are made at about 350 
points, mostly associated with breeding 
sites. Of these about 270 are regularly 
visited. Catches are made for 11 hr'per 
day (7am to 6pm). Flies are recorded by 
hr of capture and sent alive to the Sub- 
Sector laboratory for dissection within 24 
hr of capture. As an example of the effort 
expended we can consider the statistics 
for a typical month. In May 1977 catches 
were made at 271 points by 85 teams 
working a total of 1653 man-days; 10,537 
S. damnosum were caught, of which 8065 
were dissected to find 6816 parous and 
253 infective Onchocerca larvae. 
Two broad criteria are being used to 
evaluate the progress of the control ef- 
forts. These are the Annual Biting Rate 
(estimated number of bites received by a 
person exposed for 1 1  hr per day for 365 
days), and the Annual Transmission Po- 
tential (number of infective Onchocerca 
larvae carried by the same flies) (Duke 
1968, Walsh et al 1978). Both can be eas- 
ily calculated from the fly-catch data. An 
expert group meeting in Geneva has 
provisionally set safe levels of transmis- 
sion at less than 100 3rd stage O. volvulus 
larvae per year or a biting rate of less than 
1,000 per year. 
A comparison of estimated annual 
transmission potentials at representative 
points pre-control and in the first year of 
complete control, 1977 (Fig. 3) shows that 
very few places were free from a danger- 
ous level of transmission before control, 
but now most of the central area could be 
classed as habitable. 
Of particular interest is the band of 
transmission persisting along the western, 
southern and eastern perimeters. Exten- 
sive field and laboratory research involv- 
ing the morphology, physiology and 
cytotaxonomy of the flies caught in these 
areas now indicate very strongly that they 
originate from savanna habitats outside 
the programme area and invade during 
the wet season aided by the prevailing 
winds. Experimental larviciding of rivers 
to the southwest during 1976 and 1977 
indicated 2 river systems as potential 
sources in this sector. We hope to confirm 
these findings in 1978. At present it looks 
as if flies are invading over distances up to 
250 km, and for this reason we hope to 
extend the southern boundary of the pro- 
gramme to include all savanna habitats. 
This extension will commence in the 
Ivory Coast in 1978, and we hope to ex- 
tend to the other countries later. 
We wish to thank Marc Bazin, Director 
of O.C.P. for permission to present this 
paper, and acknowledge the contribution 
made by all 700 employees of the Pro- 
gramme and the several consultants em- 
ployed on specific projects. 
Figure 3. Annual transmission potentials of Onchocerca volvulus 3rd stage larvae pre-control (left 
half of circle) and in 1977 (right half of circle), at 147 representative catching stations. Black 
indicates a transmission potential of more than 100 O. volvulus larvae per year. 
472 MOSQUITO NEWS VOL. 38, NO. 4 
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, 


Onchocerciasis and Simulium control in the Volta river basin

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