Rolf Garms

The elimination of the vector Simulium neavei from the Itwara onchocerciasis focus in Uganda by ground larviciding

The Itwara focus of onchocerciasis covers an area of approximately 600 km(2) in western Uganda about 20 km north of Fort Portal. The vector is Simulium neavei, whose larvae and pupae live in a phoretic association on freshwater crabs. The phoretic host in the Itwara focus is the crab Potamonautes aloysiisabaudiae. Before any onchocerciasis control, ATPs were estimated to reach between 4500 and 6500 infective larvae per person per year. S. neavei was found to be a very efficient vector with 40% of parous flies harbouring developing larvae of Onchocerca volvulus. After 4 years of community-based distribution of ivermectin transmission was still considerable and in 1995 monthly treatment of streams with the larvicide temephos commenced in the first of three sub-foci, and was gradually extended to the whole focus. Biting S. neavei disappeared from the first sub-focus (Itwara main) in June 1996, and the last infested crab was caught in November 1996. In the second sub-focus (Siisa) treatment commenced towards the end of 1995, and the last biting fly was caught in March 1997, but a deterioration in the security situation interrupted the programme (after only three treatments in the third sub-focus). Monthly treatments restarted in the second and third sub-foci (Aswa) in September 1998, and when the situation was reassessed in 2003 no biting flies were found anywhere, and the flies had not reinvaded the first sub-focus, but infected crabs were found in the second and third sub-foci. The last treatments were carried out in April-June 2003, and since then no infested crabs have been found. In summary, no S. neavei-infested crabs have been found anywhere in the focus since June 2003 and the vector is considered eliminated from that date. However, transmission had already been halted since February 2001, when the last biting flies had been collected. The parasite reservoir should die out in the human population by 2016.

The fecundity of Simulium damnosum s.l. in northern Togo and infections with Onchocerca spp.

The numbers of maturing oocytes in gravid Simulium damnosum s.l. were related to the sizes of the flies and when this was taken into account the numbers fell into two groups. It was concluded that the group with the highest oocyte counts were nulliparous and that the remainder were parous flies. No evidence was found to suggest that flies in their third or subsequent gonotrophic cycles matured fewer oocytes than those in their second cycles. However, the results did suggest reduced fecundity of those flies infected with Onchocerca spp. The frequency distributions of the Onchocerca spp. infections for each of three different stages can be fitted to negative binomial distributions with similar values (0·04–0·07) for the parameter k.

The disappearance of onchocerciasis from the Itwara focus, western Uganda after elimination of the vector Simulium neavei and 19 years of annual ivermectin treatments

The Itwara onchocerciasis focus is located around the Itwara forest reserve in western Uganda. In 1991, annual treatments with ivermectin started in the focus. They were supplemented in 1995 by the control of the vector Simulium neavei, which was subsequently eliminated from the focus. The impact of the two interventions on the disease was assessed in 2010 by nodule palpations, examinations of skin snips by microscopy and PCR, and Ov16 recombinant ELISA. There was no evidence of any microfilaria in 688 skin snips and only 2 (0.06%) of 3316 children examined for IgG4 were slightly above the arbitrary cut off of 40. A follow up of the same children 21 months later in 2012 confirmed that both were negative for diagnostic antigen Ov-16, skin snip microscopy and PCR. Based on the World Health Organization (WHO) elimination criteria of 2001 and the Uganda onchocerciasis certification guidelines, it was concluded that the disease has disappeared from the Itwara focus after 19 years of ivermectin treatments and the elimination of the vector around 2001. Ivermectin treatments were recommended to be halted.

Field evaluation of four widely used mosquito traps in Central Europe.

BackgroundTo monitor adult mosquitoes several trapping devices are available. These are differently constructed and use various mechanisms for mosquito attraction, thus resulting in different trapping sensitivities and efficacies for the various species. Mosquito monitoring and surveillance programs in Europe use various types of mosquito traps, but only a few comparisons have been conducted so far. This study compared the performance of four commercial trapping devices, which are commonly used in Europe.MethodsFour different traps, Biogents Sentinel trap (BG trap), Heavy Duty Encephalitis Vector Survey trap (EVS trap), Centres for Disease Control miniature light trap (CDC trap) and Mosquito Magnet Patriot Mosquito trap (MM trap) were compared in a 4 × 4 latin square study. In the years 2012 and 2013, more than seventy 24-hour trap comparisons were conducted at ten different locations in northern and southern Germany, representing urban, forest and floodplain biotopes.ResultsPer 24-hour trapping period, the BG trap caught the widest range of mosquito species, the highest number of individuals of the genus Culex as well as the highest number of individuals of the species Ochlerotatus cantans, Aedes cinereus/geminus, Oc. communis and Culex pipiens/torrentium. The CDC trap revealed best performance for Aedes vexans, whereas the MM trap was most efficient for mosquitoes of the genus Anopheles and the species Oc. geniculatus. The EVS trap did not catch more individuals of any genus or species compared to the other three trapping devices. The BG trap caught the highest number of individuals per trapping period in urban environments as well as in wet forest, while the CDC trap caught the highest number of individuals in the floodplain biotopes. Additionally, the BG trap was most efficient for the number of mosquito species in urban locations.ConclusionThe BG trap showed a significantly better or similar performance compared to the CDC, EVS or MM trap with regard to trapping efficacy for most common mosquito species in Germany, including diversity of mosquito species and number of mosquitoes per trapping period. Thus, the BG trap is probably the best solution for general monitoring or surveillance programs of adult mosquitoes in Central Europe.

The impact of ivermectin treatment alone and when in parallel with Simulium neavei elimination on onchocerciasis in Uganda.

The study was conducted in August 2002 in Kigoyera parish in Kyenjojo district, where ivermectin treatment had been the strategy to control onchocerciasis since 1991 and was later supplemented with Simulium neavei control in 1995 and subsequent elimination; and in July 2003 in Kicece parish in Kamwenge district, where ivermectin treatment alone had been the strategy used to control onchocerciasis since 1991. Our objective was to examine and compare the impact of ivermectin treatment alone and when in parallel with S. neavei elimination on nodule and microfilariae carrier rates and on onchocercal dermatitis to provide baseline data that could be used to monitor the trends of microfilariae carrier rates for decision‐making on when to discontinue ivermectin treatment. The combined interventions had significantly reduced onchocercal dermatitis from 34% pre‐control to 2.9% (P < 0.001), microfilariae carrier rate from 88 to 7.5% (P < 0.001) and nodule prevalence from 49 to 19.2% (P < 0.001). Ivermectin treatment alone had also reduced onchocercal dermatitis prevalence from 34.2% pre‐control to 9.5% (P < 0.001), the microfilariae carrier rate from 78 to 27.8% (P < 0.001) and nodule prevalence from 49.1 to 14.2% (P < 0.001). None of the children under 10 years were nodule or microfilariae carriers in both study areas. Histological examination of eight nodules extirpated from patients in Kigoyera identified five male and 12 female adult worms that were all old and alive. Five live and one dead female worms and one live male worm were identified from nodules extirpated from patients in Kicece. There was no female worm with embryogenesis from the nodules obtained from Kigoyera while two female worms from five nodules obtained from Kicece still showed a few embryos. Two female worms in each of the study area had sperms in uteri indicating that male worms were still active. Ivermectin treatment in parallel with vector elimination had a greater impact on onchocercal dermatitis and microfilariae carrier rates than ivermectin treatment alone. It would be worthwhile considering supplementation of ivermectin treatment with vector elimination in all isolated foci with S. neavei transmission to hasten the elimination of onchocerciasis as a public health and socio‐economic problem in those foci.

Efficiency of Simulium sanctipauli as a vector of Onchocerca volvulus in the forest zone of Ghana.

Abstract.  The role of Simulium sanctipauli Vajime & Dunbar (Diptera: Simuliidae) as a vector of Onchocerca volvulus (Leuckart) (Spirurida: Onchocercidae) in the forest zone of central Ghana was studied in the Upper Denkyira district, where onchocerciasis is hyperendemic. Simulium sanctipauli was found to be a highly efficient vector, with a mean of 377 infective (L3) larvae in the heads of 1000 parous and 122 in the heads of 1000 biting flies. The overall infection rate of 44% of the parous flies with L1, L2 and L3 stages of O. volvulus (identity confirmed by polymerase chain reaction) demonstrates marked anthropophily. Female flies dispersed over a wide area and can transmit onchocerciasis up to at least 10 km away from their breeding sites. Annual community‐directed treatments with ivermectin did not have a noticeable effect on the infection rates and parasitic loads of fly populations, which were as high 2 months after as 3 months before the distribution of ivermectin. This failure can be attributed to poor coverage, with treatment taken by only 24.4% of the population of the six study villages.

Potential effects of warmer worms and vectors on onchocerciasis transmission in West Africa

Development times of eggs, larvae and pupae of vectors of onchocerciasis (Simulium spp.) and of Onchocerca volvulus larvae within the adult females of the vectors decrease with increasing temperature. At and above 25°C, the parasite could reach its infective stage in less than 7 days when vectors could transmit after only two gonotrophic cycles. After incorporating exponential functions for vector development into a novel blackfly population model, it was predicted that fly numbers in Liberia and Ghana would peak at air temperatures of 29°C and 34°C, about 3°C and 7°C above current monthly averages, respectively; parous rates of forest flies (Liberia) would peak at 29°C and of savannah flies (Ghana) at 30°C. Small temperature increases (less than 2°C) might lead to changes in geographical distributions of different vector taxa. When the new model was linked to an existing framework for the population dynamics of onchocerciasis in humans and vectors, transmission rates and worm loads were projected to increase with temperature to at least 33°C. By contrast, analyses of field data on forest flies in Liberia and savannah flies in Ghana, in relation to regional climate change predictions, suggested, on the basis of simple regressions, that 13–41% decreases in fly numbers would be expected between the present and before 2040. Further research is needed to reconcile these conflicting conclusions.

Indices of onchocerciasis transmission by different members of the Simulium damnosum complex conflict with the paradigm of forest and savanna parasite strains

Onchocerciasis in savanna zones is generally more severe than in the forest and pathologies also differ geographically, differences often ascribed to the existence of two or more strains and incompatibilities between vectors and strains. However, flies in the forest transmit more infective larvae than their savanna counterparts, even in sympatry, contradicting expectations based on the forest and savanna strains paradigm. We analysed data on the numbers of Onchocerca volvulus larvae of different stages found in 10 different taxonomic categories of the Simulium damnosum complex derived from more than 48,800 dissections of flies from Sierra Leone in the west of Africa to Uganda in the east. The samples were collected before widespread ivermectin distribution and thus provide a baseline for evaluating control measures. Savanna species contained fewer larvae per infected or per infective fly than the forest species, even when biting and parous rates were accounted for. The highest transmission indices were found in the forest-dwelling Pra form of Simulium sanctipauli (616 L3/1000 parous flies) and the lowest in the savanna-inhabiting species S. damnosum/S. sirbanum (135) and S. kilibanum (65). Frequency distributions of numbers of L1-2 and L3 larvae found in parous S. damnosum/S. sirbanum, S. kilibanum, S. squamosum, S. yahense, S. sanctipauli, S. leonense and S. soubrense all conformed to the negative binomial distribution, with the mainly savanna-dwelling species (S. damnosum/S. sirbanum) having less overdispersed distributions than the mainly forest-dwelling species. These infection patterns were maintained even when forest and savanna forms were sympatric and biting the same human population. Furthermore, for the first time, levels of blindness were positively correlated with infection intensities of the forest vector S. yahense, consistent with relations previously reported for savanna zones. Another novel result was that conversion rates of L1-2 larvae to L3s were equivalent for both forest and savanna vectors. We suggest that either a multiplicity of factors are contributing to the observed disease patterns or that many parasite strains exist within a continuum.

The Simulium damnosum complex in western Uganda and its role as a vector of Onchocerca volvulus.

Summary 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.

The Effect of Deltamethrin-treated Net Fencing around Cattle Enclosures on Outdoor-biting Mosquitoes in Kumasi, Ghana

Classic vector control strategies target mosquitoes indoors as the main transmitters of malaria are indoor-biting and –resting mosquitoes. However, the intensive use of insecticide-treated bed-nets (ITNs) and indoor residual spraying have put selective pressure on mosquitoes to adapt in order to obtain human blood meals. Thus, early-evening and outdoor vector activity is becoming an increasing concern. This study assessed the effect of a deltamethrin-treated net (100 mg/m2) attached to a one-meter high fence around outdoor cattle enclosures on the number of mosquitoes landing on humans. Mosquitoes were collected from four cattle enclosures: Pen A – with cattle and no net; B – with cattle and protected by an untreated net; C – with cattle and protected by a deltamethrin-treated net; D – no cattle and no net. A total of 3217 culicines and 1017 anophelines were collected, of which 388 were Anopheles gambiae and 629 An. ziemanni. In the absence of cattle nearly 3 times more An. gambiae (p<0.0001) landed on humans. The deltamethrin-treated net significantly reduced (nearly three-fold, p<0.0001) culicine landings inside enclosures. The sporozoite rate of the zoophilic An. ziemanni, known to be a secondary malaria vector, was as high as that of the most competent vector An. gambiae; raising the potential of zoophilic species as secondary malaria vectors. After deployment of the ITNs a deltamethrin persistence of 9 months was observed despite exposure to African weather conditions. The outdoor use of ITNs resulted in a significant reduction of host-seeking culicines inside enclosures. Further studies investigating the effectiveness and spatial repellence of ITNs around other outdoor sites, such as bars and cooking areas, as well as their direct effect on vector-borne disease transmission are needed to evaluate its potential as an appropriate outdoor vector control tool for rural Africa.