Trends in DDT and pyrethroid resistance in Anopheles gambiae s.s. populations from urban and agro-industrial settings in southern Cameroon

Background: Pyrethroid insecticides are widely used for insect pest control in Cameroon. In certain insect species, particularly the malaria vector Anopheles gambiae, resistance to this class of insecticides is a source of great concern and needs to be monitored in order to sustain the efficacy of vector control operations in the fields. This study highlights trends in DDT and pyrethroid resistance in wild An. gambiae populations from South Cameroon. Methods: Mosquitoes were collected between 2001 and 2007 in four sites in South Cameroon, where insecticides are used for agricultural or personal protection purposes. Insecticide use was documented in each site by interviewing residents. Batches of 2-4 days old adult female mosquitoes reared from larval collections were tested for susceptibility to DDT, permethrin and deltamethrin using standard WHO procedures. Control, dead and survivors mosquitoes from bioassays were identified by PCR-RFLP and characterized for the kdr mutations using either the AS-PCR or the HOLA method. Results: Four chemical insecticide groups were cited in the study sites: organochlorines, organophosphates, carbamates and pyrethroids. These chemicals were used for personal, crop or wood protection. In the four An. gambiae populations tested, significant variation in resistance levels, molecular forms composition and kdr frequencies were recorded in the time span of the study. Increases in DDT and pyrethroid resistance, as observed in most areas, were generally associated with an increase in the relative frequency of the S molecular form carrying the kdr mutations at higher frequencies. In Mangoum, however, where only the S form was present, a significant increase in the frequency of kdr alleles between 2003 to 2007 diverged with a decrease of the level of resistance to DDT and pyrethroids. Analyses of the kdr frequencies in dead and surviving mosquitoes showed partial correlation between the kdr genotypes and resistance phenotypes, suggesting that the kdr mechanism may act with certain cofactors to be identified. Conclusion: These results demonstrate the ongoing spread of kdr alleles in An. gambiae in Central Africa. The rapid evolution of insecticide resistance in this highly dynamic and genetically polymorphic species remains a challenge for its control

Kdr-based insecticide resistance in Anopheles gambiae s.s populations in

<p>Abstract</p> <p>Background</p> <p>The spread of insecticide resistance in the malaria mosquito, <it>Anopheles gambiae </it>is a serious threat for current vector control strategies which rely on the use of insecticides. Two mutations at position 1014 of the S₆transmembrane segment of domain II in the voltage gated sodium channel, known as <it>kdr </it>(<it>knockdown resistance</it>) mutations leading to a change of a Leucine to a Phenylalanine (L1014F) or to a Serine (L1014S) confer resistance to DDT and pyrethroid insecticides in the insect. This paper presents the current distribution of the <it>kdr </it>alleles in wild <it>Anopheles gambiae </it>populations in Cameroon.</p> <p>Results</p> <p>A total of 1,405 anopheline mosquitoes were collected from 21 localities throughout Cameroon and identified as <it>An. gambiae </it>(N = 1,248; 88.8%), <it>An. arabiensis </it>(N = 120; 8.5%) and <it>An. melas </it>(N = 37; 2.6%). Both <it>kdr </it>alleles 1014F and 1014S were identified in the M and S molecular forms of <it>An. gambiae </it>s.s. The frequency of the 1014F allele ranged from 1.7 to 18% in the M-form, and from 2 to 90% in the S-form. The 1014S allele ranged from 3-15% in the S-form and in the M-form its value was below 3%. Some specimens were found to carry both resistant <it>kdr </it>alleles.</p> <p>Conclusion</p> <p>This study provides an updated distribution map of the <it>kdr </it>alleles in wild <it>An. gambiae </it>populations in Cameroon. The co-occurrence of both alleles in malaria mosquito vectors in diverse ecological zones of the country may be critical for the planning and implementation of malaria vector control interventions based on IRS and ITNs, as currently ongoing in Cameroon.</p

The Mollusca inhabiting rice fields in northern Cameroon and their role as intermediate hosts for schistosomes

A systematic account is given of the freshwater gastropod and bivalve fauna of Ouro-Doukoudje, northern Cameroon, based on a survey conducted from July 2000 to June 2001 at 12 sites in rice fields spanning 800 ha. In total, 14 species were encountered, one of them (Burnupia sp.) being a first record for the area. Thirteen of the species are widely distributed in the Sahelo-Sudanese or Afrotropical regions. Bulinus senegalensis, Bulinus forskalii, Bulinus globosus, Biomphalaria pfeifferi and Lymnaea natalensis occurred throughout the study area and are potential intermediate hosts for schistosomes. Only Biomphalaria pfeifferi was found to be naturally infected with Schistosoma mansoni at nearly all sites in the rice fields. Infection rates were high and varied among sites (21.0–68.6%) and months (14.8–76.6%). Bulinus forskalii was scarce in, and Bulinus truncatus absent from, the study area. Our data are compared with published data for this area as well as for northern Cameroon.Keywords: Biomphalaria, Bulinus, Burnupia, Cameroon, population dynamic

Morphological studies on Bulinus sp. (Gastropoda: Planorbidae) from Nchout Monoun, Cameroon

Since 1990, diploid Bulinus sp. populations have been reported from many sites in the volcanic western Bamileke and Bamoun plateau in Cameroon; starch gel electrophoresis of enzymes and chromosome analyses have revealed a single band in hydroxybutyrate dehydrogenase (HBDH), isocitrate dehydrogenase (IDH), and a diploid chromosome number of 2n = 36. Several samples have been tentatively identified as B. natalensis (Küster, 1841) on the basis of observations on the shell and radula. The present study is a preliminary analysis of the characteristics of the shell and the radula of the snails from Nchout Monoun Crater Lake (Monoun Koumogoum) for adequate comparisons with the widely studied B.  natalensis/tropicus complex from East and South Africa. Comparison of the ratio of shell length to aperture length (L/AL), plotted against shell length (L) in a scatter diagram,  showed a 47% overlap between the dimensions of the Nchout Monoun sample and those from KwaZulu-Natal. From previous observations on the diploid chromosome number, single electrophoretic bands on HBDH and IDH enzyme  systems, euphallic genital system, arrowhead-shaped mesocones and the   characteristic features of the shell, we conclude that Bulinus sp. from Nchout  Monoun is closely related to B. natalensis.Key words: Bulinus, Cameroon, South Africa, shell morphology

Transmission of human African trypanosomiasis in the Komo-Mondah focus, Gabon

Background: Knowledge about transmission of sleeping sickness in a given focus is of a great importance since it governs the efficacy and the cost-effectiveness of control strategy. The Komo-Mondah focus is the most endemic sleeping sickness focus of Gabon. This focus has hardly been investigated and available publications are more than thirty years old. In order to update transmission features of sleeping sickness in that focus, we have conducted epidemiological and entomological surveys in March-April 2008. Methods: Epidemiological investigation relied on a casecontrol study using a quantitative and qualitative methodology (a structured questionnaire). Cases were affected people (parasitological positive) diagnosed by the national control program from 2004 to 2007, controls were those found disease-free after clinical examination and biological tests in the same period. They were asked to respond to a standard questionnaire concerning their activities after having signed a written consent. An unvaried analysis was first performed and then a multivariate analysis using the conditional logistic regression for matching method. Traps were then set out for four days in areas where people were working. Tsetse flies captured were identified and dissected; their density and human-fly contact points were determined. Results: A risk of infection was associated with fishing activities (Odds-ratio: 5.69; CI95%: 3.38-9.57). Three species of Glossina were captured: Glossina palpalis palpalis, Glossina fuscipes fuscipes and G. Caliginea. Human-fly contact points were mainly landing stages. Conclusion: A combined strategy of case-detection and vector control targeted at landing stages should be efficient against the disease.Key words: Epidemiology, entomology, transmission, trypanosomiasis, Glossina, Gabo

Entomological patterns in the human African trypanosomiasis focus of Komo Mondah, Gabon

Background: The incidence of sleeping sickness is still considerable in the Komo Mondah focus, in spite of case-detection strategy. A combined strategy that associated both mass screening and vector control is effective for the control of the disease. In the perspective of a targeted vector control in main transmission sites, we have carried out an entomological survey in the epicentre of the focus. Objectives: To determine tsetse flies distribution, human-fly contact point and eventually risk factors for acquisition of the disease. Methods: "Vavoua" traps were set for Glossina in four biotopes selected after an interview of HAT patients concerning their working places. Tsetse were captured and dissected. DNA from organs was analysed by PCR for trypanosome infections. The origin of blood meals was determined by ELISA. Results: The focus is infested by three species of Glossina: G. palpalis palpalis (1149: 91.85%) found in all biotopes; G. fuscipes fuscipes (85: 6.79%) and G. caliginea (17: 1.36%) found in water spots and landing stages. They are infected by three subgenera of trypanosomes and only G. palpalis palpalis is infected by human trypanosomes. G. fuscipes fuscipes is infected by T. brucei sl and G. caliginea is not infected. Flies are absent at the periphery of houses except in one village. Only 29.20% of blood meals were from humans. Landing stages built in swamp mangrove are presenting the higher index of epidemiological risk and populations are exposed to the disease when they go to the area for taking their fishing boats. Conclusion: Swamp mangrove would be targeted in priority during a vector control campaign

Enquête entomologique dans le foyer historique de trypanosomose humaine africaine de Bendjé (Gabon)

La situation de la maladie du sommeil est très peu connue au Gabon. De nombreux foyers historiques n’ont pas été prospectés depuis plus de 15 ans. Le foyer historique de Bendjé fournit régulièrement quelques cas, dépistés passivement, qui concernent le plus souvent des pêcheurs dont il est a priori difficile de déterminer le lieu probable de contamination du fait de leur grande mobilité au cours de leurs activités. La présence des hommes infectés dans ce foyer historique pourrait favoriser son réveil s’il existe un contact étroit entre les différents éléments potentiellement présents du cycle épidémiologique (homme, vecteur, trypanosome). Afin de vérifier l’existence éventuelle d’un risque trypanosomien dans ce foyer, nous y avons mené une enquête entomologique. Des pièges ont été posés dans des biotopes fréquentés par l’homme et laissés en place pendant quatre jours. Trois espèces de glossines (Glossina palpalis palpalis, G. pallicera newsteadi et G. caliginea) ont été capturées et deux espèces de trypanosomes (Trypanosoma vivax et T. brucei s.l.) identifiées par PCR. Ces résultats suggèrent l’existence d’un cycle de transmission animal. Le contact entre les hommes et les glossines est particulièrement étroit dans tous les types de site prospectés, à l’exception de la mangrove

Multiple insecticide resistance mechanisms in Anopheles gambiae s.l. populations from Cameroon, Central Africa

Background: Increasing incidence of DDT and pyrethroid resistance in Anopheles mosquitoes is seen as a limiting factor for malaria vector control. The current study aimed at an in-depth characterization of An. gambiae s.l. resistance to insecticides in Cameroon, in order to guide malaria vector control interventions. Methods: Anopheles gambiae s.l. mosquitoes were collected as larvae and pupae from six localities spread throughout the four main biogeographical domains of Cameroon and reared to adults in insectaries. Standard WHO insecticide susceptibility tests were carried out with 4% DDT, 0.75% permethrin and 0.05% deltamethrin. Mortality rates and knockdown times (kdt(50) and kdt(95)) were determined and the effect of pre-exposure to the synergists DEF, DEM and PBO was assessed. Tested mosquitoes were identified to species and molecular forms (M or S) using PCR-RFLP. The hot ligation method was used to depict kdr mutations and biochemical assays were conducted to assess detoxifying enzyme activities. Results: The An. arabiensis population from Pitoa was fully susceptible to DDT and permethrin (mortality rates > 98%) and showed reduced susceptibility to deltamethrin. Resistance to DDT was widespread in An. gambiae s.s. populations and heterogeneous levels of susceptibility to permethrin and deltamethrin were observed. In many cases, prior exposure to synergists partially restored insecticide knockdown effect and increased mortality rates, suggesting a role of detoxifying enzymes in increasing mosquito survival upon challenge by pyrethroids and, to a lower extent DDT. The distribution of kdr alleles suggested a major role of kdr-based resistance in the S form of An. gambiae. In biochemical tests, all but one mosquito population overexpressed P450 activity, whereas baseline GST activity was low and similar in all field mosquito populations and in the control. Conclusion: In Cameroon, multiple resistance mechanisms segregate in the S form of An. gambiae resulting in heterogeneous resistance profiles, whereas in the M form and An. arabiensis insecticide tolerance seems to be essentially mediated by enzyme-based detoxification. Synergists partially restored susceptibility to pyrethroid insecticides, and might help mitigate the impact of vector resistance in the field. However, additional vector control tools are needed to further impact on malaria transmission in such settings