Pool screen polymerase chain reaction for estimating the prevalence of Onchocerca volvulus infection in Simulium damnosum sensu lato : results of a field trial in an area subject to successful vector control

Detection of infective parasites in the vector population can be an early indicator of recrudescence in areas freed of new cases of onchocerciasis. However, dissection of vector black flies is inefficient in areas subject to effective control. Recently, a polymerase chain reaction (PCR)-based assay has been used to detect a single #Onchocerca volvulus-infected black fly in pools containing large numbers of uninfected flies. This method had not been validated on wild-caught black flies in an area subject to effective vector control. Here, we report a method of restricting the pool screen PCR assay to infectious parasites and the results of a field test in an area subject to long-term vector control. The prevalence of infection determined by dissection did not differ from that determined by pool screen PCR. The results suggest that the PCR assay may be a useful tool for epidemiologic surveillance for #O. volvulus infection. (Résumé d'auteur

Can ivermectin mass treatments eliminate onchocerciasis in Africa?

OBJECTIVE: To elucidate the conditions in which mass treatment with ivermectin reduces the transmission of Onchocerca volvulus sufficiently to eliminate infection from an African community. METHODS: ONCHOSIM, a microsimulation model for onchocerciasis transmission, was used to explore the implications of different treatment intervals, coverage levels and precontrol endemicities for the likelihood of elimination. FINDINGS: Simulations suggested that control strategies based exclusively on ivermectin mass treatments could eliminate onchocerciasis. The duration of treatment required to eliminate infection depended heavily on the treatment programme and precontrol endemicity. In areas with medium to high levels of infection, annual mass treatments with 65% coverage for at least 25 years were necessary. Model predictions suggested that durations exceeding 35 years would be required if there were much heterogeneity in exposure to vector bites and, consequently, wide individual variation in microfilaria counts. If the treatment interval were reduced from 12 to 6 months the time for completion of the programme could be more than halved and elimination could be accomplished in areas of hyperendemicity, provided that the effects of each treatment would be the same as with annual treatments. However, it was doubtful whether high coverage levels could be sustained long enough to achieve worldwide eradication. CONCLUSION: Elimination of onchocerciasis from most endemic foci in Africa appears to be possible. However, the requirements in terms of duration, coverage, and frequency of treatment may be prohibitive in highly endemic areas

Can ivermectin mass treatments eliminate onchocerciasis in Africa?

OBJECTIVE: To elucidate the conditions in which mass treatment with ivermectin reduces the transmission of Onchocerca volvulus sufficiently to eliminate infection from an African community. METHODS: ONCHOSIM, a microsimulation model for onchocerciasis transmission, was used to explore the implications of different treatment intervals, coverage levels and precontrol endemicities for the likelihood of elimination. FINDINGS: Simulations suggested that control strategies based exclusively on ivermectin mass treatments could eliminate onchocerciasis. The duration of treatment required to eliminate infection depended heavily on the treatment programme and precontrol endemicity. In areas with medium to high levels of infection, annual mass treatments with 65% coverage for at least 25 years were necessary. Model predictions suggested that durations exceeding 35 years would be required if there were much heterogeneity in exposure to vector bites and, consequently, wide individual variation in microfilaria counts. If the treatment interval were reduced from 12 to 6 months the time for completion of the programme could be more than halved and elimination could be accomplished in areas of hyperendemicity, provided that the effects of each treatment would be the same as with annual treatments. However, it was doubtful whether high coverage levels could be sustained long enough to achieve worldwide eradication. CONCLUSION: Elimination of onchocerciasis from most endemic foci in Africa appears to be possible. However, the requirements in terms of duration, coverage, and frequency of treatment may be prohibitive in highly endemic areas

Mectizan and onchocerciasis : a decade of accomplishment and prospects for the future : the evolution of a drug into a development concept

For many years there was no suitable drug available for the control of onchocerciasis. The advent of Mectizan (ivermectin, MSD ; an effective microfilaricide), its registration in October 1987 for the treatment of human onchocerciasis, and its suitability for large-scale application were major break-throughs in the control of human onchocerciasis via chemotherapy. Several studies, both fly-feeding experiments and community trials, have established that Mectizan treatment causes a significant reduction in the transmission of infection. Although long-term treatment in some isolated foci (such as occur in the New World and in some hypo- and meso-endemic areas elsewhere) appears to interrupt transmission, more prolonged treatement is required to prove if transmission can be stopped. Advantage could be taken of the significant impact of Mectizan on transmission by giving treatement while or just before transmission by blackflies is most intense. (Résumé d'auteur

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 atone 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.Dans le cadre du Programme OMS de lutte contre l’onchocercose en Afrique de l’Ouest, la phase d’attaque des opérations dans le bassin du Niger en Guinée a démarré en 1989 avec l’utilisation simultanée de l’ivermectine et de la lutte antivectorielle. Parmi les seize points retenus pour cette étude, tous situés dans des foyers holoendémiques de savane, la moitié sont répartis dans le bassin du Haut Niger en Guinée et les autres dans l’aire initiale du Programme.Après trois à quatre années d’opérations avec une distribution annuelle d’ivermectine à grande échelle, on note en Guinée une forte réduction de l’infestation des simulies (78,8 % pour la proportion de mouches infestantes et 82,9 % pour le nombre de larves infestantes pour 1 000 femelles pares). La prévalence des microfilaires a peu diminué; en revanche, les charges microfilariennes des communautés qui mesurent l’intensité de l’infestation ont chuté de 60 à 80 % par rapport aux données de prétraitement. La combinaison de la lutte antivectorielle et de la distribution de masse de l’ivermectine a permis un excellent contrôle de la transmission. Par contre, dans l’aire initiale du programme, il a fallu 6 à 8 ans de lutte antivectorielle seule pour obtenir une réduction équivalente de l’infestation des simulies. A nombre égal de mouches capturées, la transmission est beaucoup plus forte dans les zones où l’ivermectine n’est pas distribuée, même après plusieurs armées de lutte antivectorielle efficace et ininterrompue.L’utilisation combinée de l’ivermectine et de la lutte antivectorielle ouvre des perspectives nouvelles dans la conduite des opérations avec notamment une possibilité d’allégement de la lutte antivectorielle