Behaviour of Anopheles albimanus in relation to pyrethroid‐treated bednets

Abstract

Responses of the malaria vector Anopheles albimanus to pyrethroid impregnated bednets made of cotton or nylon, compared with untreated nets, were investigated in houses occupied by two people inside and/or outside two bednets, in coastal Chiapas, México. The pyrethroid used was lambdacyhalothrin 30 mg a.i./m2. Bioassay mortality rates of An.albimanus exposed to treated nets for 3 or 15 min, rose from 40-55% to 90-100% for nylon nets 3-19 weeks post-treatment, but were consistently lower for treated cotton nets. An.albimanus females (collected unfed on human bait) were released in houses surrounded by curtains for trapping mosquitoes that exited from the house. Floor sheets were used in and around each experimental house for retrieving any mosquitoes knocked-down and/or killed. During post-treatment assessment for 17 weeks, An.albimanus blood-feeding success rates were 23-24% with untreated nets, 14-18% with treated cotton nets and 8-15% with treated nylon nets, significantly reduced when both human baits were inside the treated bednets, but not when one or more baits were outside the treated bednet(s) within the house. Proportions of mosquitoes leaving houses < 3 h post-release were 53-59% from houses with untreated bednets versus 65-78% with treated bednets. Except in one case (when both humans were outside treated cotton nets), these increased early exit rates were significant, whether or not the human baits were inside the treated bednets indoors. Mortality rates of An.albimanus females exiting overnight (22.00-06.00 hours) averaged 15-39% from houses with treated cotton and 16-46% with treated nylon nets, very significantly greater than the control mean rates of 6-8% mortality with untreated nets. Observations on wild-caught An.albimanus females marked with fluorescent powder and released indoors revealed that few mosquitoes (3-11%) actually contacted the bednets unless both human baits remained under them - when contact rates were 22% on treated nylon, 23% on treated cotton and 42% on untreated nets (P approximately 0.05). The mean resting time was significantly longer on untreated (14.4 min) than on treated nylon (5.8 min) or cotton (9.5 min) bednets, whereas mean resting times on other surfaces indoors were 16.5-19.8 min. Proportions exiting within 2h of release were significantly more from houses with treated houses (33-35%) than with untreated nets (8%). However, mortality rates of mosquitoes that landed on treated nets were very significantly greater (90-100%) than after landing on untreated nets (10%). Thus, despite some excito-repellency, lambdacyhalothrin-impregnated bednets (especially made of nylon) proved to be effective as an alternative to house-spraying against An.albimanus.

An elaborated feeding cycle model for reductions in vectorial capacity of night-biting mosquitoes by insecticide-treated nets

“…In this model, decreases in daily survival rate range from 9 to 26%, similar to the 10 to 40% decreases observed in field surveys [26]. To mimic real contexts, the input and model parameters used are developed from studies carried out in Africa, in Papua New Guinea [16,17] or in Latin America [15]. Finally, the model provides simple formulae for mosquito survival rate, duration of the feeding cycle and proportion of bites taken on humans.…”

“…Sample parameters describing the baseline feeding cycle process result from epidemiological surveys [15-17] and are listed in Table 1.…”

Behavioural responses of females of two anopheline mosquito species to human-occupied, insecticide-treated and untreated bed nets

“…The misleadingly named ‘excito-repellent’ effect [13,29], which is more correctly referred to as the ‘locomotive stimulant’ effect [30], is often cited as an important protective quality of treated bed nets since it appears to reduce mosquito entry rates into damaged bed nets [3] and to drive mosquitoes out of houses or prevent them from entering in the first place [3,31]. It was hypothesized that this apparently highly disruptive behavioural effect, combined with the insecticidal effect, would (in comparison to its untreated counterparts) 1) reduce catch and, 2) change the distribution patterns of mosquitoes on an occupied treated bed net.…”

The risk of incomplete personal protection coverage in vector-borne disease

“…We have chosen a range of h between 0 (bed nets without killing effect) and 0.3. When h ¼ 0.3, the vector equilibrial population, r/d (equation (2.5)) decreases by 23%, an upper bound for a population decrease due to ITN according to a field study on Anopheles albimanus [33]. Figure 3 is thus intended to point out on general principles, not to provide quantitative information.…”