Phosphoglucomutase and trehalase isoenzymes of Venezuelan simulium vectors of Onchocerca volvulus.

Phosphoglucomutase (PGM) and trehalase (Tre) isoenzymes of five species of Simulium blackflies (Diptera: Simuliidae), vectors of onchocerciasis in Venezuela, were investigated by means of a portable electrophoresis field kit. Tre differed between S. incrustatum and S. oyapockense s.l. Electrophoretic variation of Tre in other members of the S. amazonicum and S. incrustatum groups merit further investigation. PGM appears to be more useful for separating populations within species complexes. Multiple populations and/or seasonal changes in population structure of S. guianense s.l., S. exiguum s.l. and S. metallicum s.l. were inferred from elecrophoretic variation of PGM.

Cytogenetical analysis of the Simulium damnosum complex (Diptera: Simuliidae) in Guinea Bissau.

In a 3-year study during 1990-92, larval collections of the Simulium damnosum complex from the River Corubal system in Guinea Bissau revealed that the only sibling species present were S. sirbanum and the Konkouré form of S.konkourense, but not S.damnosum s.s. which had been found at some of the localities (Saltinho and Cusselinta) in a previous survey by Quillévéré et al. (1981). Their differential distributions were seasonally consistent between years. S. sirbanum was concentrated in the upper reaches of the Corubal, moving downstream at the end of the dry season to exploit new breeding grounds, whereas S. konkourense seemed to be concentrated in the lower reaches and moved upstream in the middle of the dry season. Since the previous survey, it appears that S. konkourense has largely replaced S. sirbanum and S. damnosum s.s. in the lower reaches of the Corubal. S. sirbanum was consistently associated with man-biting behaviour and, although infection studies were not performed, it is likely that S. sirbanum is the main vector of onchocerciasis in Guinea Bissau as S. konkourense does not seem to be anthropophilic. Among five polymorphic inversions in S. sirbanum, the frequency of inversion IL-B increased during the dry season; IL-2 and IIL-7 showed homozygous disadvantage whereas IIL-3 and IIIL-6 did not; IS-2 tended to be fixed in all populations. Simulium konkourense populations in the River Corubal differ from those found upstream in its tributary the River Koumba, in the Fouta Djallon of Guinea, by their sex chromosomes (having no sex-linked inversions) and biting preferences, indicating that the populations are not freely interbreeding.(ABSTRACT TRUNCATED AT 250 WORDS)

Population decline of Simulium woodi and reduced onchocerciasis transmission at Amani, Tanzania, in relation to deforestation.

To investigate an apparent decline of the onchocerciasis vector Simulium woodi, in the Simulium neavei group, weekly 12-hour biting catches on man were carried out for 13 months near Amani and compared with those obtained 22 years earlier. The results showed a marked fall in biting S. neavei s.l. females and supported earlier findings on the aquatic stages. The mean biting-catch in 1985/6 was only 13% of that in 1963/4 and comprised 91% S. woodi and 9% the more common, but largely zoophilic, Simulium nyasalandicum (Amani form). The infection rate of S. woodi with Onchocerca volvulus larvae had also fallen markedly from 17% to 3% of parous flies; no S. nyasalandicum females were infected. Recent onchocerciasis surveys indicate that reduced transmission has resulted in lower infection rates in man. Exposure by deforestation of the breeding sites of S. woodi in small shaded streams seems to have caused the simuliid's population decline. Reduced populations of S. neavei group vector species have been reported in other deforested parts of eastern Africa. Our long-term quantitative data on declining numbers of biting females without control measures, help to corroborate such reports. Control may no longer be necessary in some areas allowing a greater concentration of limited resources.

Experimental hybridization between members of the Simulium damnosum complex.

Males of the Beffa form of Simulium soubrense Vajime and Dunbar were successfully crossed with female S.squamosum V. & D. and S.sanctipauli V. & D. Fertile eggs from both crosses were reared to larvae and, in the case of the Beffa x squamosum cross, to adults. Larval chromosomes from the Beffa x sanctipauli cross had normally paired homologues but those of the Beffa x squamosum cross showed extensive asynapsis and large heterozygous loops. The morphology of the larval and adult hybrids from the Beffa x squamosum cross were more characteristic of squamosum, but the adult males had scutal patterns typical of the Beffa form and retained this form's laboratory-mating propensity.

Artificial feeding of west African Simulium damnosum Theobald s.l. (Diptera: Simuliidae) on man, domestic mammals and birds.

A population of known species composition (mainly Simulium damnosum s.s. with some S. sirbanum), was fed on chicken, guinea-fowl, rabbit, goat, sheep, and man, using simple techniques under similar conditions. Significant differences in feeding rates were found between the different host species, on different parts of the body, at times also between different individuals of the same species, and between techniques employed. On less favoured hosts wild-caught flies usually gave better results than laboratory emerged females. Taking both wild-caught and laboratory emerged flies into consideration, rabbit, man and guinea-fowl gave the best results, with the latter being particularly suitable.

Artificial feeding of West African Simulium damnosum Theobald s.l. (Diptera: Simuliidae) through membranes and their subsequent fecundity.

Simulium damnosum s.l. females were offered blood through artificial membranes using a feeding apparatus in which the blood is kept warm and stirred. Feeding rates and egg production were compared using a synthetic silicone or a latex membrane and ten different kinds of blood with and without added adenosine-tri-phosphate (ATP). Feeding rates were usually higher with silicone than with latex membranes. Wild flies captured just before starting to feed on man, usually fed better than females that had emerged from pupae in the laboratory. Wild-caught flies showed best results when deep-frozen human blood with EDTA or fresh human blood from a blood bank was offered through a silicone membrane. Using a latex membrane they fed best on "instant" and fresh human blood from a blood bank. Laboratory-emerged females which were offered blood only through the silicone membrane, fed best on freeze-dried porcine blood or on human blood with EDTA, both with ATP. Optimal feeding rates were obtained with one day old flies and membrane temperatures between 37.0 and 39.5 degrees C. Real fecundity depended on blood-meal volume. Females that took a complete meal on man produced 377 eggs per fly. Since a higher percentage of flies fed through membranes took small or medium sized blood meals, their fecundity was correspondingly lower (295 eggs per fly).

Temporary small-scale colonization of the Beffa form of the Simulium damnosum theobald complex (Diptera: Simuliidae) in Africa.

Two populations of the Beffa form of Simulium damnosum s.l. were colonized on a small scale for the first time in Africa. The colonies survived for three and five generations, respectively. Information gained led to improvements in adult handling techniques which proved adequate for colony maintenance despite the small numbers reared. Nevertheless, large scale rearings are required to avoid inbreeding and to produce sufficient numbers for experimental purposes.

Identification of a laboratory mating form of the Simulium damnosum Theobald complex in West Africa.

Larvae and pupae of Simulium damnosum s.l. were collected from the Begba rapids on the Mono river. Emerging adults were induced to mate, blood feed and oviposit. Six out of 13 discrete egg batches counted were fertile. The F1 females did not blood feed and thus no F2 eggs were obtained. Larvae from the breeding site and the F1 generation were identified as being the Beffa form of the S. sanctipauli/S. soubrense group. This form appears to be the only member of the S. damnosum complex in West Africa which mates in the laboratory and is almost certainly the same form as that colonized by Simmons and Edman (1982). The mating behaviour of West African S. damnosum s.l. described by several authors is similar to that observed in the Beffa form, and the possibility that earlier reports referred to the Beffa form is discussed.

A new simple technique for rearing F1 progeny from females of the Simulium damnosum Theobald complex.

Laboratory rearing of Simulium damnosum complex species enables female adults to be indirectly identified from their larval progeny. A simple, readily transportable rearing apparatus was developed for investigations by the Onchocerciasis Control programme by using unbreakable, easily packed and reassembled parts and by feeding the larvae at least partly with living green algae (Chlorophyceae) cultured in the laboratory. Although the larvae are sensitive to excess nitrogen compounds, the use of algae and a large volume of water obviated the need for a separate purification system in which water was circulated by an A/C electrical centrifugal pump between a container and an inclined trough in which most of the larvae developed. The rate of development was rapid giving rise to full size larvae (suitable for cytotaxonomic determination), pupae and adults. The adults were attracted into a collecting device by light. Developments to protect the larvae during electrical power failures included a slow-drying wooden rearing trough and an automatic switch to an alternative D/C power supply and pump. This technique has been in general use in O.C.P. laboratories in Benin, Ghana, Ivory Coast and Upper Volta and many thousands of larvae have been reared.