The current status of onchocerciasis in the forest/savanna transition zone of Côte d'Ivoire.

Onchocerca volvulus exists in at least two strains in West Africa, while its black-fly vectors consist of sibling species, dwelling in the savanna and forest/transition zones. In transition and degraded forest zones both parasite strains and different sibling species of the vector can be sympatric. The strain of parasite in infected humans and in vector black-flies was determined in two bioclimes along the Bandama river of Côte d'Ivoire. The upper Bandama is located in the savanna bioclime while the Middle Bandama is located in a degraded forest zone. At both sites, savanna-dwelling sibling species of the Simulium damnosum sensu lato species complex predominated. The severe-strain of O. volvulus was the predominant strain at both sites. However, severe-strain parasites represented a significantly larger proportion of those found in the vector population than in the human population in the degraded forest of the Middle Bandama. These data suggest that in degraded forest areas recently invaded by savanna-dwelling species of S. damnosunz s.l. transmission of the severe-strain of the parasite might be more efficient than transmission of the mild-strain.

[A case of cross infection by Onchocerca volvulus and Onchocerca ochengi in Simulium damnosum S.L]. / Un cas d'infectivite mixte par onchocerca volvulus et Onchocerca ochengi chez Simulium damnosum S.L.

During a routine entomological survey conducted within the framework of the Program to Control Onchocerciasis in West Africa, a female simulium forest fly was found to be contaminated by 13 Onchocerca volvulus larvae and 7 Onchocerca ochengi larvae. The two Onchocerca species were identified using specific DNA probes. We speculate that cross infection could be related either to behavioral factors, e.g. interruption of blood meals on two different hosts, or developmental factors, e.g. asynchronous development of parasites of the same species or specific differences in the duration of parasite cycles. Further study will be needed to determine the incidence and scope of cross infection in areas where accurate assessment of the impact of vector control on transmission of onchocerciasis in man is required.

Drastic reduction of populations of Simulium sirbanum (Diptera: Simuliidae) in central Sierra Leone after 5 years of larviciding operations by the Onchocerciasis Control Programme.

The major vectors of the blinding form of human onchocerciasis in West Africa are two blackfly species, Simulium sirbanum and Simulium damnosum s.s. (Diptera: Simuliidae), identified at the adult stage as the 'savanna group' of the Simulium damnosum complex. In 1988, in the central part of Sierra Leone, the average daily biting rate (females/man/day) by savanna blackflies (mostly S. sirbanum) during the peak of the dry season (April-May) was 59.9, making up 69.1% of total captures on average. There was evidence of a strong long-range immigration of adult females of S. sirbanum through eastern Guinea in the dry season, with a reverse movement towards Guinea in the rainy season. Therefore, in 1989, the World Health Organization's Onchocerciasis Control Programme (OCP) extended its vector control operations from central West Africa to rivers of central and northern Sierra Leone, and to rivers of eastern Guinea. Four years of efficient larviciding drastically reduced adult populations of S. sirbanum in Sierra Leone. In the peak of the dry seasons of 1993 and 1994, the average biting rate by savanna blackflies in central Sierra Leone had dropped to 1.0, making up only 4.3% of total captures on average. Yearly biting rates by S. sirbanum in central Sierra Leone were therefore reduced to 2% of their pre-intervention levels. Based on larval samples, the S. sirbanum has been replaced by two forest species, S. leonense in the south and S. squamosum in the north. Since 1992, it has been possible to calculate accurate transmission rates for blinding onchocerciasis, based on DNA-probe identifications. From 1993, the risk of transmission has not only been reduced by vector control but also by mass distribution of ivermectin to rural communities. In terms of control strategy, the authors conclude that larviciding operations could be alleviated in central Sierra Leone without increasing the risk of blinding onchocerciasis transmission, as long as the migration of S. sirbanum through eastern Guinea and northern Sierra Leone is prevented.

Impact of combined large-scale ivermectin distribution and vector control on transmission of Onchocerca volvulus in the Niger basin, Guinea.

As part of the WHO Onchocerciasis Control Programme in West Africa (OCP), the attack phase of operations in the Niger basin in Guinea began in 1989 with the simultaneous use of ivermectin and vector control. Larvicide applications coupled with annual large-scale ivermectin distribution have greatly reduced blackfly infectivity (by 78.8% for the number of infective larvae per 1000 parous flies). The combination of vector control and ivermectin has permitted excellent control of transmission. In the original OCP area, it took 6-8 years of vector control alone to obtain an equivalent decrease in blackfly infectivity. For the same number of flies caught, transmission was much higher in areas where ivermectin had not been distributed. The combined use of ivermectin and vector control has opened up new prospects for carrying out OCP operations with, notably, the possibility of reducing larviciding operations.

PIP: As part of the World Health Organization Onchocerciasis Control Programme in West Africa (OCP), the attack phase of operations in the Niger basin in Guinea began in 1989 with the simultaneous use of ivermectin and vector control. All the 16 catching points were in holoendemic foci: 8 in the Niger basin in Guinea and 8 in the original OCP area (Mali, Ivory Coast, Ghana, and Burkina Faso). The data were analyzed according to prevalence of microfilariae in the skin and the mean community microfilarial load (CMFL). Between 1990 and 1992 the number of people in the villages treated increased by a factor of 6. In 1992 a total of 91,840 persons were treated in 550 villages. The study covered 10 years, during which 34,492 blackflies were caught at the 8 sites, 87.8% of which were parous. Larvicide applications coupled with annual large-scale ivermectin distribution had greatly reduced blackfly infectivity (by 78.8% for the number of infective larvae per 1000 parous flies; the number infective larvae in the head fell by 75.7% compared with the 1986-87 data before treatment began). After 2 years of large-scale ivermectin treatment, the reduction was 64.6%. In February and March of 1992 a defective larvicide worsened the situation. The average transmission potential during this period in Guinea was 7.3 compared with 93.7 for the original area. For the same number of blackflies caught, transmission in the original area was 5.6 times higher. The combination of vector control and ivermectin permitted excellent control of transmission. In the original OCP area, it took 6-8 years of vector control alone to obtain an equivalent decrease in blackfly infectivity. For the same number of flies caught, transmission was much higher in areas where ivermectin had not been distributed. The combined use of ivermectin and vector control has opened up new prospects for carrying out OCP operations with the possibility of reducing larviciding operations.

[ Onchocerciasis control program in West Africa: socioeconomic development and risk of recrudescence of transmission. 1. Experimental study of the transmission of Onchocerca volvulus strains from Southwestern Sierra Leone by Simulium sirbanum]. / Le programme de lutte contre l'onchocercose en Afrique de l'ouest: développement socio-économique et risque de recrudescence de la transmission. 1. Etude éxperimentale de la transmission des souches d'Onchocerca volvulus du sud-ouest de la Sierra Leone par Simulium sirbanum.

As part of the return of savanna migrants installed since a long time in forest regions, in the south of Sierra Leone, we carried out an experimental study about a cross-transmission between Simulium sirbanum from Missira (West-Mali) and the forest strain of Onchocerca volvulus in the south-west of Sierra Leone. This study will allow to know if there is a risk of onchocerciasis transmission recrudescence in relation to the reinstallation of these migrants in their native region. Because of the very high limitation to the forest strain of O. volvulus microfilariae output of the peritrophic membrane reduction with savanna black-flies and according to the very low mature parasite out put of S. sirbanum with this strain observed along this experimentation, the forest strain of O. volvulus from the south Sierra Leone appears maladjusted to S. sirbanum, the main vector of onchocerciasis in savanna regions. This observation implicates a very low intensity of transmission for this forest strain by savanna onchocerciasis vectors. The return of savanna migrants in their native region, installed in the south Sierra Leone since several decades, could not be, in a short time, an origin of onchocerciasis recrudescence in savanna regions of the Onchocerciasis Control Programme area cleaned by an effective vector control carried out since 1975 sustained now by a chemotherapeutic treatment reducing the human parasite reservoir. However, the preservation of this acquired necessitates an epidemiological supervision increased, because the interactions between the vector and the parasite for a long time could carry away a mutual adaptation and a sickness recrudescence.

[Onchocerciasis control program in West Africa: socioeconomic development and risk of recrudescence of transmission. 2. Experimental study of the transmission of Onchocerca volvulus strains from Southwestern Sierra Leone by Simulium yahense and Simulium squamosum]. / Le programme de lutte contre l'onchocercose en Afrique de l'ouest: développement socio-économique et risque de recrudescence de la transmission. 2. Etude éxpérimentale de la transmission des souches d'Onchocerca volvulus du sud-ouest de la Sierra Leone par Simulium yahense et Simulium squamosum.

The movements of human populations towards the mining wealth of the northern parts of Sierra Leone are favorable to a high contact rate between onchocerciasis patients coming from the south-western area of this country and the vector species Simulium yahense and Simulium squamosum which assume the essential of onchocerciasis transmission in the above-mentioned mining area. In fact, the Onchocerca volvulus strains concerned by this contact seem to be more pathogenic than those locally transmitted. In order to assess the danger it could represent for the Onchocerciasis Control Programme in West Africa, we carried out the experimental study of transmission which may result from this contact when more or less infected onchocerciasis patients are involved. The results indicated that this transmission by S. yahense may reach high proportions only when heavily infected onchocerciasis patients are implicated. We took also notice of the low capacity of S. squamosum to transmit the O. volvulus strains from the south-western Sierra Leone, irrespective of the microfilarial load of patients. Thus, in the most favorable conditions of a high parasite-vector contact of the study, involvement of S. yahense and onchocerciasis patients with high skin microfilarial loads is the only occurrence to which a high risk of intensive transmission may be related. The authors consider that the probability of such a risk occurring will be drastically reduced, due to the considerable decrease of skin microfilarial loads in human communities which regularly have the advantage of ivermectin (Mectizan) mass treatments.

[Tolerance of ivermectin treatment of rural communities infected by savannah onchocerciasis in Mali]. / Tolérance d'un traitement de collectivités rurales atteintes d'onchocercose de savane par l'ivermectine au Mali.

In an open clinical trial in phase IV, 856 onchocerciasis infected subjects received 150 micrograms/kg of ivermectin in May 1987. While 607 were included as witness. This cohort was revisited 7 and 12 months after. In June 1988, the same treatment was administrated to the previously treated subjects, and the witnesses received their first ivermectin' dose. The clinical tolerance of the treatment appears good and, even improved during the second dose one year after. Among the subjects treated in May 1987, 15.2% of them showed secondary reactions mostly discrete or moderate, precocious and quickly reversible after a second dose. Only 8 of them were incommodated in their daily occupations. A second treatment of these same subjects one year later, caused reactions of feeble intensity 3.7% only. The research of intolerance risk factors, incriminated the high density of microfilaremia. This incite to be careful in mass treatment of hyperendemic area.

Progress in controlling the reinvasion of windborne vectors into the western area of the Onchocerciasis Control Programme in West Africa.

Since vector control began in 1975, waves of Simulium sirbanum and S. damnosum s.str., the principal vectors of severe blinding onchocerciasis in the West African savannas, have reinvaded treated rivers inside the original boundaries of the Onchocerciasis Control Programme in West Africa. Larviciding of potential source breeding sites has shown that these 'savanna' species are capable of travelling and carrying Onchocerca infection for at least 500 km northeastwards with the monsoon winds in the early rainy season. Vector control has, therefore, been extended progressively westwards. In 1984 the Programme embarked on a major western extension into Guinea, Sierra Leone, western Mali, Senegal and Guinea-Bissau. The transmission resulting from the reinvasion of northern Côte d'Ivoire and Burkina Faso has been reduced by over 95%, but eastern Mali has proved more difficult to protect because of sources in both Guinea and Sierra Leone. Rivers in Sierra Leone were treated for the first time in 1989 and biting and transmission rates in Sierra Leone and Guinea fell by over 90%. Because of treatment problems in some complex rapids and mountainous areas, flies still reinvaded Mali, though biting rates were approximately 70% lower than those recorded before anti-reinvasion treatments started. It was concluded that transmission in eastern Mali has now been reduced to the levels required to control onchocerciasis.

[Ivermectin and human onchocerciasis. A study of 234 onchocerciasis patients in the Republic of Mali]. / Ivermectine et onchocercose humaine. A propos d'une étude portant sur 234 onchocerquiens en République du Mali.

In a double blind study Ivermectin has been compared to a placebo in 234 male and female with onchocerciasis who had more than 20 microfilariae per milligram of skin and moderate ocular involvement. Patient was randomized to receive a simple oral dose of Ivermectin 100, 150 and 200 micrograms/kg or placebo. The following was 12 months. The decrease of microfilarodermia since to the 3rd day was from 72.8 to 79.3% of initial rate. At six months it was more than 91% and more than 87% in 12 months. Ocular microfilariae, initially between 12 and 23 stay lower than 2 at 12 months. Punctuated keratitis disappear and did not recidive still 6 months in patients with persistent microfilariae. Ivermectin produce only few side effects. Negative waves have been observed on ECG but without any clinical signs. The Power efficient dose seen to be 100 micrograms/kg.