R. W. Mwangi

Elevated oxidase and esterase levels associated with permethrin tolerance in Anopheles gambiae from Kenyan villages using permethrin-impregnated nets

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. 42 min 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.

Reduced susceptibility of Anopheles gambiae to permethrin associated with the use of permethrin-impregnated bednets and curtains in Kenya

Abstract. 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 33min, while the LT5() 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 16min, respectively, in the villages with bednets, curtains and with no such intervention.

Malaria transmission potential of Anopheles mosquitoes in the Mwea‐Tebere irrigation scheme, Kenya

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.

Age-related changes in the response to adipokinetic hormone in Locusta migratoria

ABSTRACT. In fifth instar nymphs of Locusta there is only a feeble adipokinetic response to extracts of corpora cardiaca. In fledglings, this poor response persists for a few days but then increases dramatically to reach a plateau by day 8. The response declines to almost zero as the locusts age beyond 35 days of adult life. This pattern of change in response is similar in both males and females but there are some differences in magnitude depending upon whether the response is measured as changes in haemolymph total lipid (vanillin‐positive material) or total diglyceride (gas liquid chromatography). The poor response to adipokinetic hormone in nymphs and newly fledged locusts is not a result of shortage of stored lipid in the fat body and cannot be improved by injection of extra hormone.

Diacylglycerol-transporting lipoproteins and flight in Locusta

Abstract Evidence from chromatographic and heparin precipitation studies shows that the ‘heparin-soluble’ lipoprotein, A + , forms in the haemolymph during flight. In locusts flown continuously for 60 min, lipoprotein A + occurs in the haemolymph at low concentrations but accumulates during a short rest period following flight. After injections of tissue extracts containing adipokinetic hormone (AKH), A + accumulates in the haemolymph but disappears more rapidly in flying locusts than in resting locusts. This difference in the rate of disappearance of diacylglycerol from the lipoprotein A + can be used to estimate its rate of utilization during sustained flight (approx. 100μg. min −1 from 45–90 min of flight). It is suggested that lipoprotein A + is the major carrier of diacylglycerol from the fat body to the flight muscles during prolonged flight. The steady state concentrations of total diacylglycerol and ‘heparin-soluble’ diacylglycerol during continuous flight are unaffected when tissue extracts containing AKH are injected before flight. This suggests that there is a close homeostatic control over the steady state concentration of haemolymph lipid during flight.

Long‐term use of permethrin‐impregnated nets does not increase Anopheles gambiae permethrin tolerance

Abstract. 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 Angambiae appears to be minimal in our study villages, although the impact of permethrin was demonstrated by a significantly lower parous‐rate of Angambiae females in the intervention (63–66%) than in non‐intervention (79%) villages, and by reduced malaria transmission (reported elsewhere).

Interrelationships between haemolymph lipid and carbohydrate during starvation in Locusta

Abstract During starvation in adult female Locusta , the haemolymph total lipid concentration increases markedly while that of the total carbohydrate decreases. The majority of the increased haemolymph lipid is diglyceride and 75% of this is associated with a high molecular weight lipoprotein (A + ) which disappears rapidly after feeding when the total lipid concentration is restored to the normal resting value. The effect of feeding can be mimicked by injecting or feeding starved locusts with sugars but not with protein. The lowering of the haemolymph total diglyceride concentration in starved locusts by injection of carbohydrates is dose-related and, at doses in excess of 4 mg per locust, almost normal values (for fed locusts) are obtained within 6 hr. It is suggested that in the haemolymph there is an inverse relationship between the concentration of diglyceride and that of trehalose.

The effects of allatectomy and ovariectomy on haemolymph proteins and lipid mobilization in Locusta

Abstract After ovariectomy the concentrations of diacylglycerol and protein in the haemolymph increase markedly. The increased diacylglycerol is associated with increased quantities of the ‘heparin-precipitable’ protein (lipoprotein A) that carries diacylglycerol in the blood of normal resting locusts. After the injection of adipokinetic hormone (AKH), the blood of ovariectomized locusts contains only slight quantities of the ‘heparin-soluble’ lipoprotein A + whereas this forms in large amounts in the blood of sham-operated locusts after AKH injection. After allatectomy, the increase in the adipokinetic response is slower and the full level of responsiveness observed in sham-operated locusts is never attained. Nevertheless, allatectomized locusts develop a marked adipokinetic response which tends to stabilize as they age; it does not deteriorate as it does in aged sham-operated locusts. The effects of ovariectomy on blood metabolites can be prevented completely by allatectomy, but only partially by cautery of the cerebral neurosecretory cells. Treatment with a juvenile hormone analogue (JHA R-20458) counteracts the effects of allatectomy in ovariectomized locusts.

Irrigation scheme or mosquito hazard : a case study in Mwea irrigation scheme

A survey of the mosquito fauna in the Mwea-Tebere Irrigation Scheme in Kenya was conducted between 1984 and 1985. Two genera, Anopheles and Culex were found indoors in large numbers. The major species were Anopheles gambiae s.l., Anopheles pharoensis, and Culex quinquefasciatus, comprising 66% and 21% of the total catch, respectively. The large numbers of mosquitoes found resting indoors showed seasonal population fluctuations. The seasonal population increases observed were due to the availability of larval breeding conditions over a long period of time and to man-made environmental changes. These changes included irrigation canals and rice paddies. The irrigation canals and rice paddies were flooded from August till December, thus linking the flooding effects of the two rainy seasons. This enabled mosquitoes to breed continuously for up to 9 months per year.

Hypertrehalosaemic activity in corpus cardiacum‐corpus allatum‐aorta complex and adipokinetic response of Glossina morsitans

Abstract. Extracts from the corpus cardiacum‐corpus allatum‐aorta (CC‐CA‐A) complex of Glossina morsitans morsitans Westwood contain a hypertrehalosaemic factor when assayed in Periplaneta americana L. and in G.morsitans. A slight though significant decrease, followed by an increase, in haemolymph total carbohydrate occurs when tsetse are flown for 1 h. When assayed in Locusta migratoria L., the extracts have no adipokinetic activity, but L.migratoria corpus cardiacum extract produces an adipokinetic response in the female tsetse. It is suggested that the neurosecretions contained in the tsetse CC‐CA‐A complex contain a hypertrehalosaemic factor whose role is to mobilize glycogen.