High efficacy of chlorfenapyr-based net Interceptor® G2 against pyrethroid-resistant malaria vectors from Cameroon.

BACKGROUND: The increasing reports of resistance to pyrethroid insecticides associated with reduced efficacy of pyrethroid-only interventions highlight the urgency of introducing new non-pyrethroid-only control tools. Here, we investigated the performance of piperonyl-butoxide (PBO)-pyrethroid [Permanet 3.0 (P3.0)] and dual active ingredients (AI) nets [Interceptor G2 (IG2): containing pyrethroids and chlorfenapyr and Royal Guard (RG): containing pyrethroids and pyriproxyfen] compared to pyrethroid-only net Royal Sentry (RS) against pyrethroid-resistant malaria vectors in Cameroon. METHODS: The efficacy of these tools was firstly evaluated on Anopheles gambiae s.l. and Anopheles funestus s.l. from Gounougou, Mibellon, Mangoum, Nkolondom, and Elende using cone/tunnel assays. In addition, experimental hut trials (EHT) were performed to evaluate the performance of unwashed and 20 times washed nets in semi-field conditions. Furthermore, pyrethroid-resistant markers were genotyped in dead vs alive, blood-fed vs unfed mosquitoes after exposure to the nets to evaluate the impact of these markers on net performance. The XLSTAT software was used to calculate the various entomological outcomes and the Chi-square test was used to compare the efficacy of various nets. The odds ratio and Fisher exact test were then used to establish the statistical significance of any association between insecticide resistance markers and bed net efficacy. RESULTS: Interceptor G2 was the most effective net against wild pyrethroid-resistant An. funestus followed by Permanet 3.0. In EHT, this net induced up to 87.8% mortality [95% confidence interval (CI): 83.5-92.1%) and 55.6% (95% CI: 48.5-62.7%) after 20 washes whilst unwashed pyrethroid-only net (Royal Sentry) killed just 18.2% (95% CI: 13.4-22.9%) of host-seeking An. funestus. The unwashed Permanet 3.0 killed up to 53.8% (95% CI: 44.3-63.4%) of field-resistant mosquitoes and 47.2% (95% CI: 37.7-56.7%) when washed 20 times, and the Royal Guard 13.2% (95% CI: 9.0-17.3%) for unwashed net and 8.5% (95% CI: 5.7-11.4%) for the 20 washed net. Interceptor G2, Permanet 3.0, and Royal Guard provided better personal protection (blood-feeding inhibition 66.2%, 77.8%, and 92.8%, respectively) compared to pyrethroid-only net Royal Sentry (8.4%). Interestingly, a negative association was found between kdrw and the chlorfenapyr-based net Interceptor G2 (χ2 = 138; P < 0.0001) with homozygote-resistant mosquitoes predominantly found in the dead ones. CONCLUSIONS: The high mortality recorded with Interceptor G2 against pyrethroid-resistant malaria vectors in this study provides first semi-field evidence of high efficacy against these major malaria vectors in Cameroon encouraging the implementation of this novel net for malaria control in the country. However, the performance of this net should be established in other locations and on other major malaria vectors before implementation at a large scale.

Prevalence and risk factors determinants of the non-use of insecticide-treated nets in an endemic area for malaria: analysis of data from Cameroon.

BACKGROUND: Malaria is the main cause of morbidity and mortality in Cameroon. Insecticide-treated nets (ITNs) significantly reduce malaria transmission, but their use is not common in the population. This study aimed to estimate the nationwide prevalence of the non-use of ITNs and identify its major determinants. METHODS: A cross-sectional study was conducted on interview data collected in households selected across all the regions of Cameroon through a non-probabilistic, random, 2-stage stratified sampling process. Descriptive statistics were used to describe the distribution of baseline characteristics across the households, and statistical tests assessed if the distribution of these characteristics differed significantly based on the non-use of ITNs, with 0.05 serving as a threshold of the p-value for statistical significance. The prevalence of the non-use of ITNs was estimated, and logistic regression models were used to tally the odds ratios of the associations between various factors and the non-use of ITNs, along with their 95% confidence intervals. The sensitivity, specificity, and area under the receiver operating characteristic (ROC) curve (AUC) were determined, and the Hosmer Lemeshow test was used to measure the goodness of fit of each statistical model. RESULTS: Of the 7593 households interviewed, 77% had at least one ITN and 59% of the population used ITNs. Only 72% of the population with at least one ITN used it. The logistic model of the multivariate analysis was significant at a 5% threshold. The AUC was 0.7087 and the error rate was 18.01%. The sensitivity and specificity of the model were 97.56% and 13.70%, respectively. The factors that were associated with ITN use were the presence of sufficient nets in the household (p < 0.0001), the region of residence (p < 0.0001), the level of education of the respondent (p < 0.0001), and the standard of living (p = 0.0286). Sex, age, colour preferences, as well as the shape and size of the nets were not associated with ITN use. CONCLUSIONS: The use of ITNs in Cameroon was low and varied according to specific factors. These identified factors could be used as the foundations of effective sensitization campaigns on the importance of ITNs.

Case Definitions of Clinical Malaria in Children from Three Health Districts in the North Region of Cameroon.

Malaria endemicity in Cameroon greatly varies according to ecological environment. In such conditions, parasitaemia, which is associated with fever, may not always suffice to define an episode of clinical malaria. The evaluation of malaria control intervention strategies mostly consists of identifying cases of clinical malaria and is crucial to promote better diagnosis for accurate measurement of the impact of the intervention. We sought out to define and quantify clinical malaria cases in children from three health districts in the Northern region of Cameroon. A cohort study of 6,195 children aged between 6 and 120 months was carried out during the raining season (July to October) between 2013 and 2014. Differential diagnosis of clinical malaria was performed using the parasite density and axillary temperature. At recruitment, patients with malaria-related symptoms (fever [axillary temperature ≥ 37.5°C], chills, severe malaise, headache, or vomiting) and a malaria positive blood smear were classified under clinical malaria group. The malaria attributable fraction was calculated using logistic regression models. Plasmodium falciparum was responsible for over 91% of infections. Children from Pitoa health district had the highest number of asymptomatic infections (45.60%) compared to those from Garoua and Mayo Oulo. The most suitable cut-off for the association between parasite densities and fever was found among children less than 24 months. Overall, parasite densities that ranged above 3,200 parasites per µl of blood could be used to define the malaria attributable fever cases. In groups of children aged between 24 and 59 months and 60 and 94 months, the optimum cut-off parasite density was 6,400 parasites per µl of blood, while children aged between 95 and 120 months had a cut-off of 800 parasites per µl of blood. In the same ecoepidemiological zone, clinical malaria case definitions are influenced by age and location (health district) and this could be considered when evaluating malaria intervention strategies in endemic areas.

Role of Anopheles (Cellia) rufipes (Gough, 1910) and other local anophelines in human malaria transmission in the northern savannah of Cameroon: a cross-sectional survey.

BACKGROUND: As part of a study to determine the impact of insecticide resistance on the effectiveness of long-lasting insecticide treated nets (LLINs) in the north of Cameroon, the unexpectedly high density and anthropophilic behaviour of Anopheles rufipes lead us to investigate this species bionomics and role in human malaria parasite transmission. METHODS: For four consecutive years (2011-2014), annual cross-sectional sampling of adult mosquitoes was conducted during the peak malaria season (September-October) in three health districts in northern Cameroon. Mosquitoes sampled by human landing catch and pyrethrum spray catch methods were morphologically identified, their ovaries dissected for parity determination and Anopheles gambiae siblings were identified by molecular assay. Infection with P. falciparum and blood meal source in residual fauna of indoor resting anopheline mosquitoes were determined by enzyme-linked-immunosorbent assays. RESULTS: Anopheles gambiae (sensu lato) (s.l.) comprised 18.4% of mosquitoes collected with An. arabiensis representing 66.27% of the sibling species. The proportion of An. rufipes (2.7%) collected was high with a human-biting rate ranging between 0.441 and 11.083 bites/person/night (b/p/n) and an anthropophagic rate of 15.36%. Although overall the members of An. gambiae complex were responsible for most of the transmission with entomological inoculation rates (EIR) reaching 1.221 infective bites/person/night (ib/p/n), An. arabiensis and An. coluzzii were the most implicated. The roles of An. funestus, An. pharoensis and An. paludis were minor. Plasmodium falciparum circumsporozoite protein rate in Anopheles rufipes varied from 0.6 to 5.7% with EIR values between 0.010 and 0.481 ib/p/n. CONCLUSIONS: The study highlights the epidemiological role of An. rufipes alongside the members of the An. gambiae complex, and several other sympatric species in human malaria transmission during the wet season in northern Cameroon. For the first time in Cameroon, An. rufipes has been shown to be an important local malaria vector, emphasising the need to review the malaria entomological profile across the country as pre-requisite to effective vector management strategies.

Anopheles ziemanni a locally important malaria vector in Ndop health district, north west region of Cameroon.

BACKGROUND: Malaria transmission in Cameroon is mediated by a plethora of vectors that are heterogeneously distributed across the country depending on the biotope. To effectively guide malaria control operations, regular update on the role of local Anopheles species is essential. Therefore, an entomological survey was conducted between August 2010 and May 2011 to evaluate the role of the local anopheline population in malaria transmission in three villages of the Ndop health district in the northwest region of Cameroon where malaria is holoendemic, as a means to acquiring evidence based data for improved vector intervention. METHODS: Mosquitoes were sampled both indoor and outdoor for four consecutive nights in each locality during each month of survey. Sampling was done by the human landing catch method on volunteers. Anopheles species were identified morphologically and their ovaries randomly dissected for parity determination. Infection with Plasmodium falciparum was detected by Circumsporozoite protein ELISA. Members of An. gambiae complex were further identified to molecular level by PCR and RFLP PCR. RESULTS: An. ziemanni was the main malaria vector and whether outdoor or indoor. The man biting rate for the vectors ranged from 6.75 to 8.29 bites per person per night (b/p/n). The entomological inoculation rate for this vector species was 0.0278 infectious bites per person per night (ib/p/n) in Mbapishi, 0.034 ib/p/n in Mbafuh, and 0.063 ib/p/n in Backyit. These were by far greater than that for An. gambiae. No difference was observed in the parity rate of these two vectors. PCR analysis revealed the presence of only An. colluzzi (M- form). CONCLUSIONS: An. ziemanni is an important local malaria vector in Ndop health district. The findings provide useful baseline information on the anopheles species composition, their distribution and role in malaria transmission that would guide the implementation of integrated vector management strategies in the locality.

Anopheline fauna and malaria transmission in four ecologically distinct zones in Cameroon.

Knowledge of baseline malaria transmission intensity in a given environment is important to guide malaria control interventions. In Cameroon, recent information on malaria transmission intensity is insufficient. Therefore, an entomological study was conducted in four ecologically different sites throughout the country to assess the seasonal patterns in malaria transmission intensity. Anopheles arabiensis was the main vector in six of the nine study sites, while An. gambiae sensu stricto was the most important vector in the other three sites. Clear differences in entomological inoculation rates (EIR) were observed between the study sites, ranging from 0.1 infective bites per person per night in the sahelian zone of the country to 5.5 infective bites per person per night in the forest zone. Based on the observed behaviour of the vectors, insecticide-treated bed nets will be highly effective in controlling malaria. However, in the high transmission areas, additional measures will be needed to reduce the malaria burden to acceptable levels.