The permethrin tolerance (PT) of a population of the mosquito Anopheles gambiae (Diptera: Culicidae) increased following the introduction of permethrin-impregnated nets for malaria control in certain villages near Kisumu, western Kenya. Using a biochemical test that indirectly measures oxidases associated with permethrin resistance, we found that this population had higher oxidase levels than a comparison population from villages without impregnated nets. Mosquitoes from a colony of An. gambiae selected for PT, the RSP (reduced susceptibility to permethrin) strain, were exposed to permethrin with or without the oxidase inhibitor piperonyl butoxide (PB). Significantly higher mortality rates occurred when permethrin was synergized by PB, presumably by suppression of oxidases responsible for PT. An unselected (UNS) colony of An. gambiae that was more susceptible than RSP in a permethrin-susceptibility bioassay (i.e. LT50 22 min for UNS, vs. 42min for RSP) was compared with the RSP colony for levels of oxidases and esterases. The levels of both enzymes were very significantly higher in the RSP strain (P<0.0001). We speculate that use of impregnated nets selected for higher oxidase and esterase levels in An. gambiae to metabolize permethrin acquired from the nets. Both oxidase and esterase mechanisms could confer cross-resistance to other pyrethroids.

Susceptibility of the malaria vector Anopheles gambiae to permethrin decreased following the installation of mosquito nets impregnated with 0.5 g permethrin per square metre in four villages near Kisumu, Kenya. During the first year that permethrin-impregnated bednets and curtains were in place, the exposure time to 50% mortality (LT50) increased 2.5-fold from 13 to 33 min, while the LT50 for An.gambiae was unchanged in two other villages where no intervention measures were used. Two years after permethrin-impregnated mosquito nets were distributed the LT50s for An.gambiae were 28, 28 and 16 min, respectively, in the villages with bednets, curtains and with no such intervention. Using a colony of An.gambiae derived from females collected in the villages using permethrin-impregnated mosquito nets, we lengthened the LT50 from 28 to 41 min in two generations by exposing all females to permethrin-treated papers for 60 min and rearing offspring of the survivors. Permethrin-impregnated bednets and curtains are intended to reduce vectorial capacity. Reduced susceptibility to permethrin could counter this beneficial effect.

Feeding behavior was compared between infected and uninfected field-collected groups of Anopheles gambiae sensu lato and An. funestus from western Kenya. A significantly greater percentage (81%) of Plasmodium falciparum-infected An. gambiae s.l. females probed on experimental hosts (hamsters) than did uninfected females (38%). Among those females that initiated probing, there was no effect of infection status on the ability to take a bloodmeal. Plasmodium falciparum-infected An. gambiae s.l. probed more often (mean = 4.0) and for a longer time (mean = 277 sec) than did their uninfected counterparts (mean = 2.4 probes and mean probing time = 214 sec). Results for the small number of An. funestus that fed followed the same trend. Among infected An. gambiae s.l. females, there was no effect of sporozoite density on either the number of probes made or the total probing time. Among uninfected females, there was no difference in feeding behavior between nulliparous and parous females. In laboratory experiments, female age had no effect on blood-feeding behavior. Our findings provide evidence that natural malaria infection modifies the feeding behavior of Anopheles females.

No abstract

1. Anopheles arabiensis Patton and An. funestus Giles were identified as vectors of Plasmodium falciparum malaria in the Mwea-Tebere irrigation scheme, Kenya. An. arabiensis was the only member of the An. gambiae complex identified from chromosome characteristics. Other Anopheles species found included An. pharoensis Theobald, An. rufipes Gough and An. coustani Laveran. Survival rates per gonotrophic cycle for An. arabiensis averaged 0.37 during the short rains (October-November), 0.49 during the dry season (February) and 0.78 during the long rains (May-June). Vectorial capacities were correspondingly low due to low survival rates and a high degree of zoophily. The average duration of infective life for P. falciparum was 0.2 days for both An. arabiensis and An. funestus. In contrast, entomological inoculation rates were comparatively high: 6-8 infective bites/man/month. An. pharoensis averaged 110 bites/man/night during the short rains; 1/999 (0.1%) was positive by ELISA for P. falciparum circumsporozoite antigen, but the ELISA evidence is not conclusive for vector incrimination. In correspondence with clinical observations, the transmission of P. malariae and P. ovale is unlikely due to the low vector survival rates. The observed anomaly between low vectorial capacities and high entomological inoculation rates demonstrates the importance of accurately estimating vector sporozoite rates to monitor unstable malaria transmission in irrigated areas.

Previous use of permethrin-impregnated bednets (mosquito nets) and curtains in four Kenyan villages for one year, 1990-91, raised the permethrin LT50 of Anopheles gambiae to 2.4-fold above its baseline value, designated permethrin tolerance (PT), as measured by exposure to 0.25% permethrin-impregnated papers in W.H.O. test-kits. During 1992-93, with ongoing use of permethrin-impregnated nets and curtains, PT regressed slightly compared with the contemporary susceptibility level of An.gambiae from non-intervention villages, to 1.8-fold in 1992 and only 1.6-fold in 1993. Thus the selection pressure of impregnated nets for PT in An.gambiae appears to be minimal in our study villages, although the impact of permethrin was demonstrated by a significantly lower parous-rate of An.gambiae females in the intervention (63-66%) than in non-intervention (79%) villages, and by reduced malaria transmission (reported elsewhere). In a selected stock of An.gambiae from the study area, PT did not affect the susceptibility to deltamethrin, fenitrothion, propoxur or DDT. Bioassays described herein provide easy procedures for field-monitoring of mosquito susceptibility/tolerance/resistance to insecticides used for net impregnation in operational programmes.

Freshly deposited third instar Glossina morsitans centralis larvae were infected with the tsetse DNA virus by microinjection, and at emergence adult males were separated from the females and fed on rabbit blood every second day for 8 days. A control group treated with sterile saline were handled similarly. They were dissected, and comparative observations made on the appearance and size of the accessory reproductive glands (ARG) in infected and control males. Regularly fed 8-day-old males from infected and control groups were mated to 2-day-old normal females obtained from the insectay. After separation from copula, the females were dissected and the uteri examined for the presence and quality of the spermatophore. The spermathecae were also examined for insemination. ARG tissues from the control and virus infected regularly fed 8-day-old male flies were fixed and processed for electron microscopic studies. The ARGs from control flies were found to be milky in appearance, whereas those from virus-infected flies were transparent in most parts. The ARGs from virus-infected males were significantly smaller in diameters (F = 42.26, p < 0.0001) and shorter (F = 200.4, p < 0. 0001) than those of the controls. Most of the virus-infected males failed to form a complete spermatophore, whereas almost all the controls formed complete spermatophore as observed in the uteri of the female mates (Chi2 = 111.661, p < 0.0001). The infected males that formed partial spermatophores and those that did not form any at all failed to inseminate their female mates. Histological studies of the ARGs revealed some lesions in the epithelial cells characterized by degeneration of cytoplasmic organelles and detachment of the muscle layer from the basal plasma membrane. However, no virus particles were observed in the affected cells.

No abstract