Population biology of human onchocerciasis.
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Human onchocerciasis (river blindness) is the filarial infection caused by Onchocerca volvulus and transmitted among people through the bites of the Simulium vector. Some 86 million people around the world are at risk of acquiring the nematode, with 18 million people infected and 600,000 visually impaired, half of them partially or totally blind. 99% of cases occur in tropical Africa; scattered foci exist in Latin America. Until recently control programmes, in operation since 1975, have consisted of antivectorial measures. With the introduction of ivermectin in 1988, safe and effective chemotherapy is now available. With the original Onchocerciasis Control Programme of West Africa coming to an end, both the new African Programme for Onchocerciasis Control and the Onchocerciasis Elimination Programme for the Americas, rely heavily on ivermectin self-sustained mass delivery. In consequence, the need for understanding the processes regulating parasite abundance in human and simuliid populations is of utmost importance. We present a simple mathematical framework built around recent analyses of exposure- and density-dependent processes operating, respectively, within the human and vector hosts. An expression for the basic reproductive ratio, R0, is derived and related to the minimum vector density required for parasite persistence in localities of West Africa in general and northern Cameroon in particular. Model outputs suggest that constraints acting against parasite establishment in both humans and vectors are necessary to reproduce field observations, but those in humans may not fully protect against reinfection. Analyses of host age-profiles of infection prevalence, intensity, and aggregation for increasing levels of endemicity and intensity of transmission in the Vina valley of northern Cameroon are in agreement with these results and discussed in light of novel work on onchocerciasis immunology. (+info)
The Simulium damnosum complex in western Uganda and its role as a vector of Onchocerca volvulus.
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The status of onchocerciasis vectors in the former Ruwenzori focus in western Uganda was re-examined some 15 years after control measures against Simulium damnosum s.l. were suspended. The four cytoforms S. kilibanum, 'Sebwe', 'Nkusi' and S. pandanophilum were found. While the nonanthropophilic 'Sebwe' was still widely distributed in rivers north, east and south of the Ruwenzori, the only anthropophilic species and vector, S. kilibanum, had disappeared from most of its former habitats and was now restricted to two limited foci, where high biting densities were encountered. It was still a vector south of the Ruwenzori (Kasese focus), where 15.4% of the parous flies were infected with larval stages of Onchocerca volvulus and 34 infective larvae were found in the heads of 1000 parous flies. In the second focus along the Mahoma and Nsonge rivers, a chromosomally highly polymorphic population of S. kilibanum had replaced the former vector S. neavei, but does not act as a vector. Only 2.3% of the parous females were infected and just 1 infective larva was found in the heads of 1000 parous flies. (+info)
Transmission of vesicular stomatitis virus from infected to noninfected black flies co-feeding on nonviremic deer mice.
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Vesicular stomatitis is an economically important arboviral disease of livestock. Viremia is absent in infected mammalian hosts, and the mechanism by which insects become infected with the causative agents, vesicular stomatitis viruses, remains unknown. Because infected and noninfected insects potentially feed on the same host in nature, infected and noninfected black flies were allowed to feed on the same host. Viremia was not detected in the host after infection by a black fly bite, but because noninfected black flies acquired the virus while co-feeding on the same host with infected black flies, it is concluded that a viremic host is not necessary for an insect to be infected with the virus. Thus co-feeding is a mechanism of infection for an insect-transmitted virus. (+info)
New species records for the blackfly (Diptera-simuliidae) fauna of Argentina with description of adults, pupa and larva of Simulium oyapockense s. l. And S. seriatum.
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Two blackfly species Simulium (Cerqueirellum) oyapockense Floch & Abonnenc and S. (Hemicnetha) seriatum Knab are recorded from Argentina, representing the most southern register for both species. S. oyapockense is a species epidemiologically very important, as a vector of onchocerciasis in the Amazonian focus. Both species are described and illustrated and their distribution are reported, in similarity to others like S. roraimense Nunes de Mello and S. ganalesense Vargas et al. in reference to S. oyapockense and S. mexicanum Bellardi similar to S. seriatum are discussed. (+info)
Simulium (Psaroniocompsa) tergospinosum new species (Diptera: Simuliidae) in siolii group from the southern part of the State of Amazonas, Brazil.
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The larva, pupa, male and female of Simulium tergospinosum n. sp. are described and illustrated. The adults of this new species share many characters with species in the subgenus Psaroniocompsa, where it is placed. The larva of this species bears dorsal and lateral triangular tubercles on the abdomen and multiply branched scale-like setae on the body, suggesting that it belongs to the S. siolii species group. S. tergospinosum n.sp. was collected along the Juma River, Apui county, in the southern part of the State of Amazonas, Brazil. The Juma, a black-water river, is a tributary of the Aripuana River in the Madeira River hydrographic basin, on the southern part of Amazonas. Females were collected biting humans along the water courses during the sampling period (in the dry season). (+info)
Onchocerca ochengi transmission dynamics and the correlation of O. ochengi microfilaria density in cattle with the transmission potential.
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The intensity of natural transmission of Onchocerca ochengi and Onchocerco volvulus by anthropo-boophilic Simulium damnosum s.l. was studied longitudinally in two cattle watering sites of a cattle ranch within a predominantly cattle populated area of the Guinea savanna of Cameroon and related to cattle O. ochengi skin microfilaria abundance. During the 12 months study period, a total of 4696 flies was individually dissected to examine the monthly transmission potential (MTP) of O. ochengi and O. volvulus. The estimated Simulium damnosum s.l. annual biting rates (ABR) on human baits were 47529 flies at the bank of the Vina "du sud" river. The ABR at the lake, which was situated at about 2 km upland from the perennial river, was 8579. The monthly parous rate was highly correlated with monthly biting rate. The annual transmission potentials (ATP) of O. ochengi were calculated to be 7732 and 1669 at the riverbank and the lake, respectively. Transmission occurred mainly in the dry season, peaking in the months of January to mid-March when dermal microfilaria density in the animals was also the highest. The O. ochengi microfilaria uptake by the fly vectors was host microfilaria density-dependent. The MTP of O. ochengi was positively correlated with dermal microfilaria density. The mean number of microfilariae per fly taken up during a blood meal was high during the dry season as was the mean number of infective larvae per fly but declined significantly with the onset of the early rains. A similar seasonality of transmission was also observed for O. volvulus that was concurrently transmitted by the same vector flies, but its ATP was comparatively much lower: 1332 infective larvae per man per year at the riverbank and 107 around the lake. The population dynamics of cattle microfilariae therefore plays an important role in the regulation of O. ochengi transmission. (+info)
Chromosomal comparisons among and within populations of Simulium (Chirostilbia) pertinax (Diptera, Simuliidae).
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Chromosomal studies were carried on six larval populations of Simulium (Chirostilbia) pertinax from different locations in Brazil. Larvae were collected in the states of Parana, Rio Grande do Sul, Rio de Janeiro and Sao Paulo. Polytene chromosome map comparisons within and among populations showed no differences in banding pattern, except for some limited polymorphism (secondary NOR and four band polymorphisms). There were no chromosomal variations associated with the resistance or susceptibility of the larvae to temephos. The chromosomal homosequentiality found among the six populations suggests that S. pertinax may be a monomorphic species. (+info)
Simuliid blackflies (Diptera: Simuliidae) and ceratopogonid midges (Diptera: Ceratopogonidae) as vectors of Mansonella ozzardi (Nematoda: Onchocercidae) in northern Argentina.
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Mansonella ozzardi, a relatively nonpathogenic filarial parasite of man in Latin America, is transmitted by either ceratopogonid midges or simuliid blackflies. In the only known focus of the disease in north-western Argentina the vectors have never been incriminated. This study investigated the potential vectors of M. ozzardi in this area. The only anthropophilic species of these Diptera families biting man at the time of the investigation were Simulium exiguum, S. dinellii, Culicoides lahillei and C. paraensis. Using experimentally infected flies S. exiguum and both species of Culicoides allowed full development of microfilariae to the infective stage, with C. lahillei being a more competent host than S. exiguum. Based on these data, biting rates and natural infectivity rates it is probable that at the begininning of the wet season C. lahillei is the main vector of M. ozzardi and both C. paraensis and S. exiguum secondary vectors. Additionally, it was found that a single dose of ivermectin was ineffectual in eradicating M. ozzardi from infected individuals in this area. (+info)