Susceptibility of Anopheles gambiae complex mosquitoes to microbial larvicides in diverse ecological settings in western Kenya

The microbial larvicides Bacillus thuringiensis var. israelensis (Bti) and Bacillus sphaericus (Bs) (Bacillales: Bacillaceae) are well known for their efficacy and safety in mosquito control. In order to assess their potential value in future mosquito control strategies in western Kenya, the current study tested the susceptibility of five populations of Anopheles gambiae complex mosquitoes (Diptera: Culicidae), collected from five diverse ecological sites in this area, to Bti and Bs under laboratory conditions. In each population, bioassays were conducted with eight concentrations of larvicide (Bti/Bs) in four replicates and were repeated on three separate days. Larval mortality was recorded at 24 h or 48 h after the application of larvicide and subjected to probit analysis. A total of 2400 An. gambiae complex larvae from each population were tested for their susceptibility to Bti and Bs. The mean (± standard error of the mean, SEM) lethal concentration values of Bti required to achieve 50% and 95% larval mortality (LC₅₀ and LC₉₅ ) across the five populations were 0.062  (± 0.005) mg/L and 0.797 (± 0.087) mg/L, respectively. Corresponding mean  (± SEM) values for Bs were 0.058 (± 0.005) mg/L and 0.451 (± 0.053) mg/L, respectively. Statistical analysis indicated that the five populations of An. gambiae complex mosquitoes tested were fully susceptible to Bti and Bs, and there was no significant variation in susceptibility among the tested populations.

Modified veranda-trap hut for improved evaluation of vector control interventions

Experimental huts with veranda traps have been used in Tanzania since 1963 for the study of residual insecticides for use with insecticide-treated nets and indoor residual spraying. Mosquitoes are allowed unrestricted entry through the eaves to facilitate the collection of an estimable proportion of mosquitoes that attempt to exit through the eave gaps, which are left open on two sides of the hut. This study was designed to validate the use of eave baffles to funnel entry and to prevent mosquito escape, and to determine biting times of Anopheles arabiensis (Patton) (Diptera: Culicidae). Anopheles arabiensis and Culex quinquefasciatus (Say) (Diptera: Culicidae) were released into the room at 20.30 hours and collected the following morning from veranda traps, window traps and the room. Centers for Disease Control light traps hung overnight next to volunteers were emptied every 2 h to determine peak biting times. A total of 55% of An. arabiensis were trapped before 22.30 hours and the highest peak in 'biting' was recorded during 18.30-20.30 hours. Of the released An. arabiensis that exited into veranda traps, 7% were captured in veranda traps entered through baffles and 93% were captured in traps entered through unmodified eaves. When veranda screens were left open to allow for escape outdoors, recapture rates were 68% for huts with eave baffles and 39% for huts with unmodified eaves. The comparison of open eaves with baffled eaves validated the assumption that in huts of the traditional non-baffled design, 50% of mosquitoes escape through open eaves. Eave baffles succeeded in reducing the potential for mosquito exit and produced more precise estimates of effect.

Trends in chloroquine resistance marker, Pfcrt-K76T mutation ten years after chloroquine withdrawal in Tanzania

Background

Plasmodium falciparum resistance to anti-malarial drugs remains a major obstacle to the control of malaria. In 2001 Tanzania replaced chloroquine (CQ) with sulphadoxine-pyrimethamine (SP) as first-line drug, which in turn was replaced by artemisinin combination therapy in 2006. SP has however, continued to be used in intermittent preventive treatment of malaria in pregnancy (IPTp) despite reports of high levels of resistance to SP due to the lack of alternatives to SP for IPTp. Recent reports have indicated recovery of CQ-susceptibility in Malawi, Kenya, Mozambique, and Tanzania based on the prevalence of wild types at codon 76 of the Pfcrt gene in indigenous P. falciparum populations. The current prevalence of this Pfcrt-76 CQ resistance marker from six regions of Tanzania mainland is hereby reported.

Methods

DNA extracted from filter-paper dried blood spots and rapid diagnostics kit strips collected from finger-prick blood were used to genotype the Pfcrt-76 resistance marker using PCR-RFLP. Data from previously published studies were used to generate CQ susceptibility recovery trends using logistic regression model.

Results

Seven hundred and forty one (741) samples were genotyped. The current frequency of the CQ-susceptible Pfcrt-K76 was above 92% and did not differ between regions in Tanzania (χ2 = 2.37; p = 0.795). The K76 allelic prevalence was between 85.7 and 93% in regions (χ2 = 7.88, p = 0.163). The CQ resistance recovery trends showed regional variability that may be caused by differences in malaria transmission intensity, but overall the trends converge as the susceptibility levels in all regions approach >90%.

Conclusions

CQ withdrawal in Tanzania has resulted into >90% recovery of susceptibility in ten years of withdrawal. These findings are in support of the search for CQ-based combination drugs as a possible future alternative to SP for IPTp in places where full recovery of CQ-susceptibility will be evident.

Evaluation of indoor residual spraying with the pyrrole insecticide chlorfenapyr against pyrethroid-susceptible Anopheles arabiensis and pyrethroid-resistant Culex quinquefasciatus mosquitoes

Chlorfenapyr is a pyrrole insecticide with a unique non-neurological mode of action. Laboratory bioassays of chlorfenapyr comparing the mortality of pyrethroid-susceptible and -resistant Anopheles gambiae s.s. and Culex quinquefasciatus mosquitoes indicated that operational cross-resistance is unlikely to occur (resistance ratio ranged between 0 and 2.1). Three trials of chlorfenapyr indoor residual spraying were undertaken in experimental huts in an area of rice irrigation in northern Tanzania that supports breeding of A. arabiensis. Daily mosquito collections were undertaken to assess product performance primarily in terms of mortality. In the second trial, 250mg/m(2) and 500mg/m(2) chlorfenapyr were tested for residual efficacy over 6 months. Both dosages killed 54% of C. quinquefasciatus, whilst for A. arabiensis 250mg/m(2) killed 48% compared with 41% for 500mg/m(2); mortality was as high at the end of the trial as at the beginning. In the third trial, 250mg/m(2) chlorfenapyr was compared with the pyrethroid alpha-cypermethrin dosed at 30mg/m(2). Chlorfenapyr performance was equivalent to the pyrethroid against A. arabiensis, with both insecticides killing 50% of mosquitoes. Chlorfenapyr killed a significantly higher proportion of pyrethroid-resistant C. quinquefasciatus (56%) compared with alpha-cypermethrin (17%). Chlorfenapyr has the potential to be an important addition to the limited arsenal of public health insecticides for indoor residual control of A. arabiensis and pyrethroid-resistant species of mosquito.

Case studies in public-private-partnership in health with the focus of enhancing the accessibility of health interventions

Various definitions have been framed for public-private partnerships (PPPs) in health depending on the desired relationship and the characteristics of the respective sectors. These relationships span from a continuum of loose relationships with narrow objectives, lack of a legal status and an absence of a formalized membership or governing body to high level institutionalization. The latter includes concrete objectives, the presence of a legal status and permanent multi-sectoral membership. The study used qualitative research methods including case studies, literature review and interview with key informants. The research undertakes an extensive literature review of various PPP models in health in scale and in scope which are aimed at advancing public health goals in developing countries. The major emphasis is on a qualitative description of some of the PPPs in the planning and implementation phases, including the challenges encountered. This background is used to analyse in-depth two case studies which are both health oriented; the first one is a national level NGO consortium with a focus on malaria and the second one is an international advocacy group with an overarching goal of protecting children against malaria through an innovative mechanism. The case study approach is used to analyze why the PPP approach was used to address malaria control and how it was implemented. Both PPPs demonstrated that relationships between the public and private sector may begin from very humble and loose beginnings. However, with perseverance from committed individuals, a vision and trustworthiness may become powerful advocates for meeting prescribed health agendas. In conclusion, three key themes (trust, sacrifice and championship) run vividly through the case studies and are significant for developing countries to emulate.

Duration of protection against clinical malaria provided by three regimens of intermittent preventive treatment in Tanzanian infants

BACKGROUND

Intermittent preventive treatment in infants (IPTi) is a new malaria control tool. However, it is uncertain whether IPTi works mainly through chemoprophylaxis or treatment of existing infections. Understanding the mechanism is essential for development of replacements for sulfadoxine-pyrimethamine (SP) where it is no longer effective. This study investigated how protection against malaria given by SP, chlorproguanil-dapsone (CD) and mefloquine (MQ), varied with time since administration of IPTi.

METHODS AND FINDINGS

A secondary analysis of data from a randomised, placebo-controlled trial in an area of high antifolate resistance in Tanzania was conducted. IPTi using SP, CD, MQ or placebo was given to 1280 infants at 2, 3 and 9 months of age. Poisson regression with random effects to adjust for potential clustering of malaria episodes within children was used to calculate incidence rate ratios for clinical malaria in defined time strata following IPTi. The short-acting antimalarial CD gave no protection against clinical malaria, whereas long-acting MQ gave two months of substantial protection (protective efficacy (PE) 73.1% (95% CI: 23.9, 90.5) and 73.3% (95% CI: 0, 92.9) in the first and second month respectively). SP gave some protection in the first month after treatment (PE 64.5% (95% CI: 10.6, 85.9)) although it did not reduce the incidence of malaria up to 12 months of age. There was no evidence of either long-term protection or increased risk of malaria for any of the regimens.

CONCLUSION

Post-treatment chemoprophylaxis appears to be the main mechanism by which IPTi protects children against malaria. Long-acting antimalarials are therefore likely to be the most effective drugs for IPTi, but as monotherapies could be vulnerable to development of drug resistance. Due to concerns about tolerability, the mefloquine formulation used in this study is not suitable for IPTi. Further investigation of combinations of long-acting antimalarials for IPTi is needed.

TRIAL REGISTRATION

Clinicaltrials.gov NCT00158574.

Behavioural and insecticidal effects of organophosphate-, carbamate- and pyrethroid-treated mosquito nets against African malaria vectors

Three insecticides - the pyrethroid deltamethrin, the carbamate carbosulfan and the organophosphate chlorpyrifos-methyl - were tested on mosquito nets in experimental huts to determine their potential for introduction as malaria control measures. Their behavioural effects and efficacy were examined in Anopheles gambiae Giles s.s. (Diptera: Culicidae) and Anopheles funestus Giles s.s. in Muheza, Tanzania, and in Anopheles arabiensis Patton and Culex quinquefasciatus Say in Moshi, Tanzania. A standardized dosage of 25 mg/m(2) plus high dosages of carbosulfan (50 mg/m(2), 100 mg/m(2) and 200 mg/m(2)) and chlorpyrifos-methyl (100 mg/m(2)) were used to compare the three types of insecticide. At 25 mg/m(2), the rank order of the insecticides for insecticide-induced mortality in wild An. gambiae and An. funestus was, respectively, carbosulfan (88%, 86%) > deltamethrin (79%, 78%) > chlorpyrifos-methyl (35%, 53%). The rank order of the insecticides for blood-feeding inhibition (reduction in the number of blood-fed mosquitoes compared with control) in wild An. gambiae and An. funestus was deltamethrin > chlorpyrifos-methyl > carbosulfan. Carbosulfan was particularly toxic to endophilic anophelines at 200 mg/m(2), killing 100% of An. gambiae and 98% of An. funestus that entered the huts. It was less effective against the more exophilic An. arabiensis (67% mortality) and carbamate-resistant Cx quinquefasciatus (36% mortality). Carbosulfan deterred anophelines from entering huts, but did not deter carbamate-resistant Cx quinquefasciatus. Deltamethrin reduced the proportion of insects engaged in blood-feeding, probably as a consequence of contact irritancy, whereas carbosulfan seemed to provide personal protection through deterred entry or perhaps a spatial repellent action. Any deployment of carbosulfan as an individual treatment on nets should be carried out on a large scale to reduce the risk of diverting mosquitoes to unprotected individuals. Chlorpyrifos-methyl was inferior to deltamethrin in terms of mortality and blood-feeding inhibition and would be better deployed on a net in combination with a pyrethroid to control insecticide-resistant mosquitoes.

Protective efficacy and safety of three antimalarial regimens for intermittent preventive treatment for malaria in infants: a randomised, double-blind, placebo-controlled trial

BACKGROUND

Administration of sulfadoxine-pyrimethamine at times of vaccination-intermittent preventive treatment in infants (IPTi)-is a promising strategy to prevent malaria. However, rising resistance to this combination is a concern. We investigated a shortacting and longacting antimalarial drug as alternative regimens for IPTi.

METHODS

We undertook a double-blind, placebo-controlled trial of IPTi in an area of high resistance to sulfadoxine-pyrimethamine at sites of moderate (n=1280 infants enrolled) and low (n=1139) intensity of malaria transmission in Tanzania. Infants aged 8-16 weeks were randomly assigned in blocks of 16 to sulfadoxine (250 mg) plus pyrimethamine (12.5 mg; n=319 in moderate-transmission and 283 in low-transmission sites), chlorproguanil (15 mg) plus dapsone (18.75 mg; n=317 and 285), mefloquine (125 mg; n=320 and 284), or placebo (n=320 and 284), given at the second and third immunisations for diphtheria, pertussis, and tetanus, and for measles. Research team and child were masked to treatment. Recruitment was stopped early at the low-transmission site because of low malaria incidence. The primary endpoint was protective efficacy against all episodes of clinical malaria at 2-11 months of age. Analysis was by intention to treat. This study is registered with ClinicalTrials.gov, number NCT00158574.

FINDINGS

All randomly assigned infants were analysed. At the moderate-transmission site, mefloquine had a protective efficacy of 38.1% (95% CI 11.8-56.5, p=0.008) against clinical malaria in infants aged 2-11 months, but neither sulfadoxine-pyrimethamine (-6.7%, -45.9 to 22.0) nor chlorproguanil-dapsone (10.8%, -24.6 to 36.1) had a protective effect. No regimen had any protective efficacy against anaemia or hospital admission. Mefloquine caused vomiting in 141 of 1731 (8%) doses given on day 1 (odds ratio vs placebo 5.50, 95% CI 3.56-8.46). More infants died in the chlorproguanil-dapsone and mefloquine groups (18 and 15, respectively) than in the sulfadoxine-pyrimethamine or placebo groups (eight deaths per group; p=0.05 for difference between chlorproguanil-dapsone and placebo).

INTERPRETATION

IPTi with a longacting, efficacious drug such as mefloquine can reduce episodes of malaria in infants in a moderate-transmission setting. IPTi with sulfadoxine-pyrimethamine has no benefit in areas of very high resistance to this combination. The appropriateness of IPTi should be measured by the expected incidence of malaria and the efficacy, tolerability, and safety of the drug.

FUNDING

IPTi Consortium and the Gates Malaria Partnership.

Common genotypic polymorphisms in glutathione S-transferases in mild and severe falciparum malaria in Tanzanian children

Malaria infection induces oxidative stress in the host cells. Antioxidant enzymes such as glutathione S-transferases (GSTs) are responsible for fighting reactive oxygen species and reduction of oxidative stress. Common GST polymorphisms have been associated with susceptibility to different diseases whose pathologies involve oxidative stress. In this study, we tested the hypothesis that GST polymorphisms that lead to reduced or lack of enzyme activity are associated with severe Plasmodium falciparum malarial anemia. We studied the genotypic distribution of GSTM1, GSTT1, and GSTP1 polymorphisms between mild malaria (N = 107) and severe malarial anemia (N = 50) in Tanzanian children. We did not find a significant relationship with the GSTT1 polymorphism. GSTM1-null was higher in the severe malaria anemia group but the difference was not significant (P = 0.08). However, a significant association of GSTP1 I105V genotype with severe malarial anemia was discovered (26.0% against 10.3% mild malaria, P = 0.004). We concluded that GSTP1 and possibly GSTM1 may protect against severe falciparum malaria in children.

Mosquitoes and bednets: testing the spatial positioning of insecticide on nets and the rationale behind combination insecticide treatments

The recent development of pyrethroid resistance of operational significance in Anopheles gambiae s.l. is a major threat to the control of malaria in West Africa. The so-called '2-in-1' bednet, in which the top of the net is treated with a non-pyrethroid insecticide and the sides with pyrethroid, has been proposed as a way of maintaining efficacy in the wake of such resistance. A host-seeking female Anopheles mosquito must contact both the top and sides of a '2-in-1' net, however, for such nets to be useful in resistance management. In the present study, the interaction between mosquitoes and insecticide-treated bednets (ITN) was explored by restricting the insecticide to particular surfaces of the nets (top only or sides only) and then testing these nets, untreated nets and nets treated on all their surfaces in experimental huts, under simulated field conditions. Over the 6-week trial, there was no significant difference in An. arabiensis mortality between nets treated with pyrethroid on the top only (39.2%), sides only (39.6%) and all surfaces (39.7%), thus indicating that a female An. arabiensis usually contacts both the top and sides of a bednet during its host-seeking behaviour. The data on blood-feeding indicated, however, that the insecticide used on the sides of the net may be more important in preventing mosquito biting than that on the top. These results support the rationale behind the '2-in-1' nets. Such nets may have advantages over the use of nets treated on all surfaces with a mixture of insecticides that includes a non-pyrethroid component. With the '2-in-1', the more toxic component can be deployed on the top of the net, away from human contact, while the more repellent pyrethroid can be restricted to the sides, to prevent blood-feeding. With the scaling-up of ITN coverage and the need to preserve pyrethroid efficacy, more consideration should be given to switching from pyrethroid-only nets to 'combination' nets that have been treated with a pyrethroid and another insecticide. Since the mosquitoes that act as malarial vectors may contact all surfaces of a bednet during their host-seeking, spatial heterogeneity in insecticide levels over the surface of a net may not reduce that net's overall efficacy. Nets with a rather uneven distribution of insecticide (such as those that might be produced using home-treatment insecticide kits) may therefore be no less effective, prior to washing, than nets with a more even distribution of insecticide (such as long-lasting insecticidal nets produced under factory conditions).

Comparative efficacies of permethrin-, deltamethrin- and alpha-cypermethrin-treated nets, against Anopheles arabiensis and Culex quinquefasciatus in northern Tanzania

Mosquito nets treated with permethrin, deltamethrin or alpha-cypermethrin at 25 mg/m(2) were evaluated in experimental huts in an area of rice irrigation near Moshi, in northern Tanzania. The nets were deliberately holed to resemble worn nets. The nets treated with permethrin offered the highest personal protection against Anopheles arabiensis (61.6% reduction in fed mosquitoes) and Culex quinquefasciatus (25.0%). Deltamethrin and alpha-cypermethrin provided lower personal protection against An. arabiensis (46.4% and 45.6%, respectively) and no such protection against Cx. quinquefasciatus. Permethrin performed poorly in terms of mosquito mortality, however, killing only 15.2% of the An. arabiensis and 9.2% of the Cx. quinquefasciatus exposed to the nets treated with this pyrethroid (after correcting for control mortality). The alpha-cypermethrin and deltamethrin performed marginally better, with respective mortalities of 32.8% and 33.0% for An. arabiensis and 19.4% and 18.9% for Cx quinquefasciatus. The poor killing effect of permethrin was confirmed in a second trial where a commercial, long-lasting insecticidal net based on this pyrethroid (Olyset) produced low mortalities in both An. arabiensis (11.8%) and Cx. quinquefasciatus (3.6%). Anopheles arabiensis survivors collected from the verandahs of the experimental huts and tested on 0.75%-permethrin and 0.05%-deltamethrin papers, in World Health Organization susceptibility kits, showed mortalities of 96% and 100%, respectively. The continued use of permethrin-treated nets is recommended for personal protection against An. arabiensis. In control programmes that aim to interrupt transmission of pathogens by mosquitoes and/or manage pyrethroid resistance in such vectors, a combination of a pyrethroid and another insecticide with greater killing effect should be considered.

Experimental hut evaluation of the pyrrole insecticide chlorfenapyr on bed nets for the control of Anopheles arabiensis and Culex quinquefasciatus

OBJECTIVE

To determine the efficacy of chlorfenapyr against Anopheles arabiensis and Culex quinquefasciatus in East Africa and to identify effective dosages for net treatment in comparison with the commonly used pyrethroid deltamethrin.

METHODS

Chlorfenapyr was evaluated on bed nets in experimental huts against A. arabiensis and C. quinquefasciatus in Northern Tanzania, at application rates of 100-500 mg/m(2).

RESULTS

In experimental huts, mortality rates in A. arabiensis were high (46.0-63.9%) for all dosages of chlorfenapyr and were similar to that of deltamethrin-treated nets. Mortality rates in C. quinquefasciatus were higher for chlorfenapyr than for deltamethrin. Despite a reputation for being slow acting, >90% of insecticide-induced mortality in laboratory tunnel tests and experimental huts occurred within 24 h, and the speed of killing was no slower than for deltamethrin-treated nets.

CONCLUSIONS

Chlorfenapyr induced low irritability and knockdown, which explains the relatively small reduction in blood-feeding rate. Combining chlorfenapyr with a more excito-repellent pyrethroid on bed nets for improved personal protection, control of pyrethroid-resistant mosquitoes and pyrethroid resistance management would be advantageous.

An experimental hut evaluation of Olyset nets against anopheline mosquitoes after seven years use in Tanzanian villages

Background

Long-lasting insecticidal nets (LLINs) are advocated by WHO for protection against malaria. Of the three brands of LLINs currently approved by WHO, Olyset^® is the only one currently granted full recommendation. With this type of LLIN, the insecticide (permethrin) is incorporated into the polyethylene fibre during manufacture and diffuses from the core to the surface, thereby maintaining surface concentrations. It has not been determined for how long Olyset nets remain protective against mosquitoes in household use.

Methods

Examples of Olyset nets, which had been in use in Tanzanian villages for seven years, were tested in experimental huts against naturally entering Anopheles gambiae and Anopheles funestus mosquitoes. Performance was compared with new Olyset nets, conventionally treated ITNs (either newly treated with alphacypermethrin or taken from local villages after 1.5 years of use) and untreated nets. All nets were artificially holed except for the seven-year Olyset nets, which had developed holes during prolonged domestic use.

Results

Anopheles funestus and An. gambiae in NE Tanzania are susceptible to pyrethroids. The new Olyset nets caused high mortality against An. funestus (73.9%) and An. gambiae (62.7%) in experimental huts. The seven-year Olyset nets caused 58.9% mortality against An. funestus and 40.0% mortality against An. gambiae. The freshly treated alphacypermethrin nets also caused high mortality against An. funestus (70.6%) and An. gambiae (72.0%); this decreased to 58.4% and 69.6% respectively after 1.5 years of use. The new Olyset nets inhibited blood-feeding by 40–50%. The 7 year Olyset nets showed no feeding inhibition over that shown by the untreated nets. The alphacypermethrin treated nets failed to inhibit blood-feeding after 1.5 years of use. However iHhhdn laboratory tunnel tests samples of all types of treated net including the 7 year Olyset inhibited blood-feeding by more than 95%.

Conclusion

After seven years of use Olyset nets were still strongly insecticidal. Mosquito mortality decreased by only 20–35% over this period. However, Olyset would not provide personal protection after seven years unless it was in good condition and all holes fully repaired.

Efficacy of pyrethroid-treated nets against malaria vectors and nuisance-biting mosquitoes in Tanzania in areas with long-term insecticide-treated net use

OBJECTIVE

To measure pyrethroid susceptibility in populations of malaria vectors and nuisance-biting mosquitoes in Tanzania and to test the biological efficacy of current insecticide formulations used for net treatment.

METHODS

Anopheles gambiae Giles s.l., An. funestus Giles s.l. and Culex quinquefasciatus Say were collected during three national surveys and two insecticide-treated net (ITN) studies in Tanzania. Knockdown effect and mortality were measured in standard WHO susceptibility tests and ball-frame bio-efficacy tests. Test results from 1999 to 2004 were compared to determine trends in resistance development.

RESULTS

Anopheles gambiae s.l. and An. funestus s.l. were highly susceptible to permethrin (range 87-100%) and deltamethrin (consistently 100%) in WHO tests in 1999 and 2004, while Culex quinquefasciatus susceptibility to these pyrethroids was much lower (range 7-100% and 0-84% respectively). Efficacy of pyrethroid-treated nets was similarly high against An. gambiae s.l. and An. funestus s.l. (range 82-100%) while efficacy against Cx. quinquefasciatus was considerably lower (range 2-100%). There was no indication of development of resistance in populations of An. gambiae s.l. or An. funestus s.l. where ITNs have been extensively used; however, susceptibility of nuisance-biting Cx. quinquefasciatus mosquitoes declined in some areas between 1999 and 2004.

CONCLUSION

The sustained pyrethroid susceptibility of malaria vectors in Tanzania is encouraging for successful malaria control with ITNs. Continued monitoring is essential to ensure early resistance detection, particularly in areas with heavy agricultural or public health use of insecticides where resistance is likely to develop. Widespread low susceptibility of nuisance-biting Culex mosquitoes to ITNs raises concern for user acceptance of nets.

Submicroscopic Plasmodium falciparum gametocyte carriage is common in an area of low and seasonal transmission in Tanzania

OBJECTIVE

Recently developed molecular gametocyte detection techniques have shown that submicroscopic Plasmodium falciparum gametocytes are common in symptomatic patients and can infect mosquitoes. The relevance for the infectious reservoir of malaria in the general population remains unknown. In this study, we investigated submicroscopic asexual parasitaemia and gametocytaemia in inhabitants of an area of hypoendemic and seasonal malaria in Tanzania.

METHODS

Two cross-sectional malariometric surveys were conducted in the dry and wet seasons of 2005 in villages in lower Moshi, Tanzania. Finger prick blood samples were taken to determine the prevalence of P. falciparum parasites by microscopy, rapid diagnostic test and real-time nucleic acid sequence-based amplification (QT-NASBA).

RESULTS

2752 individuals participated in the surveys, of whom 1.9% (51/2721) had microscopically confirmed asexual parasites and 0.4% (10/2721) had gametocytes. In contrast, QT-NASBA revealed that 32.5% (147/453) of the individuals harboured asexual parasites and 15.0% (68/453) had gametocytes. No age dependency or seasonality was observed in submicroscopic parasite carriage.

DISCUSSION

Molecular detection techniques reveal that carriage of submicroscopic asexual parasite and gametocyte densities is relatively common in this low transmission area. Submicroscopic gametocytaemia is likely to be responsible for maintaining malarial transmission in the study area.

Spatial and temporal variation in malaria transmission in a low endemicity area in northern Tanzania

Background

Spatial and longitudinal monitoring of transmission intensity will allow better targeting of malaria interventions. In this study, data on meteorological, demographic, entomological and parasitological data over the course of a year was collected to describe malaria epidemiology in a single village of low transmission intensity.

Methods

Entomological monitoring of malaria vectors was performed by weekly light trap catches in 10 houses. Each house in the village of Msitu wa Tembo, Lower Moshi, was mapped and censused. Malaria cases identified through passive case detection at the local health centre were mapped by residence using GIS software and the incidence of cases by season and distance to the main breeding site was calculated.

Results

The principle vector was Anopheles arabiensis and peak mosquito numbers followed peaks in recent rainfall. The entomological inoculation rate estimated was 3.4 (95% CI 0.7–9.9) infectious bites per person per year. The majority of malaria cases (85/130) occurred during the rainy season (χ² = 62,3, p < 0.001). Living further away from the river (OR 0.96, CI 0.92–0.998, p = 0.04 every 50 m) and use of anti-insect window screens (OR 0.65, CI 0.44–0.94, p = 0.023) were independent protective factors for the risk of malaria infection. Children aged 1–5 years and 5–15 years were at greater risk of clinical episodes (OR 2.36, CI 1.41–3.97, p = 0.001 and OR 3.68, CI 2.42–5.61, p < 0.001 respectively).

Conclusion

These data show that local malaria transmission is restricted to the rainy season and strongly associated with proximity to the river. Transmission reducing interventions should, therefore, be timed before the rain-associated increase in mosquito numbers and target households located near the river.

Occurrence of the leucine-to-phenylalanine knockdown resistance (kdr) mutation in Anopheles arabiensis populations in Tanzania, detected by a simplified high-throughput SSOP-ELISA method

Background

Molecular markers of insecticide resistance can provide sensitive indicators of resistance development in malaria vector populations. Monitoring of insecticide resistance in vector populations is an important component of current malaria control programmes. Knockdown resistance (kdr) confers resistance to the pyrethroid class of insecticides with cross-resistance to DDT through single nucleotide polymorphisms (SNPs) in the voltage-gated sodium channel gene.

Methods

To enable detection of kdr mutations at low frequency a method was developed that uses polymerase chain reaction (PCR) and enzyme-linked immunosorbent assay (ELISA)-based technology, allowing rapid, reliable and cost-effective testing of large numbers of individual mosquitoes. This was used to assay mosquitoes from sites in lower Moshi, Tanzania.

Results

Sequence-specific oligonucleotide probes (SSOP) were used for simultaneous detection of both East and West African kdr mutations with high specificity and sensitivity. Application of the SSOP-ELISA method to 1,620 field-collected Anopheles arabiensis from Tanzania identified the West African leucine-phenylalanine kdr mutation in two heterozygous individuals, indicating the potential for resistance development that requires close monitoring.

Conclusion

The presence of the West African kdr mutation at low frequency in this East African population of An. arabiensis has implications for the spread of the kdr gene across the African continent.

Entomological evaluation of malaria vectors at different altitudes in Hai district, northeastern Tanzania

Entomological monitoring in four villages situated along an altitude transect in the Hai District of Northeastern Tanzania identified Anopheles arabiensis Patton as the principal vector of malaria and detected seasonal changes in vector behavior. Over a 13-mo sampling period, 10,557 mosquitoes were collected with CDC light traps, pyrethrum spray catches, and pit traps of which 5,969 (56.5%) wereAn. arabiensis, 762 (7.2%) wereAnopheles funestus Giles s.l., 3,578 (33.9%) were culicines, and 248 (2.3%) were nonvector anophelines. Vector densities declined rapidly with increasing altitude, demonstrating a 50% decrease in annual human biting rate for every 86-m rise in altitude. Light traps were found to be more efficient than spray catches for the collection of An. arabiensis. This observation was attributed to increased exophily of this species, most notably in the wet season, and is supported by seasonal changes in the human blood index and fed/gravid ratio. These results indicate that spray catches may underestimate the abundance of exophilic vectors such as An. arabiensis and that entomological monitoring may require more than one collection method, especially at low vector densities. The annual entomological inoculation rate (EIR) decreased sharply with increasing altitude, with large variation around the estimate at low vector densities. Increased transmission because of unpredictable short rains at low altitudes and spatial clustering of infective mosquitoes may contribute to elevated EIR estimates.

Irrigated crop production is associated with less malaria than traditional agricultural practices in Tanzania

There is concern that crop irrigation that results in increased numbers of vector mosquitoes will lead to a rise in malaria in local communities. We evaluated the level of malaria experienced in 3 communities in northern Tanzania with different agricultural practices: rice irrigation, sugar-cane irrigation and traditional maize cultivation. Five cross-sectional surveys were used to measure the prevalence of infection with falciparum malaria in 1-4 years old children in each community over a period of 12 months. Active case detection was also carried out to record clinical episodes of malaria during the study period. Information on antimalarial measures was also recorded. Results from the cross-sectional surveys showed that the overall prevalence of malaria parasites was less near the rice irrigation (12.5%) and sugar-cane (16.9%) schemes than the savannah village (29.4%). There were also significantly fewer clinical episodes of malaria in the rice village (15 cases/1000 child-weeks at risk [cwar]) than either the sugar-cane (36 cases/1000 cwar) or savannah (40 cases/1000 cwar) villages. Overall, rice irrigation was associated with less malaria than alternative agricultural practices, despite the considerable numbers of vectors produced in the paddies. This finding supports other studies that indicate that irrigation in much of sub-Saharan Africa will not lead to increased malaria. Nonetheless, African governments planning irrigation projects need effective policies to encourage local communities to use personal protection measures, such as insecticide-treated bednets, and to ensure that these communities have access to effective antimalarial drugs and efficient health services.

Malaria transmission risk variations derived from different agricultural practices in an irrigated area of northern Tanzania

Malaria vector Anopheles and other mosquitoes (Diptera: Culicidae) were monitored for 12 months during 1994-95 in villages of Lower Moshi irrigation area (37 degrees 20' E, 3 degrees 21' S; approximately 700 m a.s.l.) south of Mount Kilimanjaro in northern Tanzania. Adult mosquito populations were sampled fortnightly by five methods: human bait collection indoors (18.00-06.00 hours) and outdoors (18.00-24.00 hours); from daytime resting-sites indoors and outdoors; by CDC light-traps over sleepers. Anopheles densities and rates of survival, anthropophily and malaria infection were compared between three villages representing different agro-ecosystems: irrigated sugarcane plantation; smallholder rice irrigation scheme, and savannah with subsistence crops. Respective study villages were Mvuleni (population 2200), Chekereni (population 3200) and Kisangasangeni (population approximately/= 1000), at least 7 km apart. Anopheles arabiensis Patton was found to be the principal malaria vector throughout the study area, with An. funestus Giles sensu lato of secondary importance in the sugarcane and savannah villages. Irrigated sugarcane cultivation resulted in water pooling, but this did not produce more vectors. Anopheles arabiensis densities averaged four-fold higher in the ricefield village, although their human blood-index was significantly less (48%) than in the sugarcane (68%) or savannah (66%) villages, despite similar proportions of humans and cows (ratio 1:1.1-1.4) as the main hosts at all sites. Parous rates, duration of the gonotrophic cycle and survival rates of An. arabiensis were similar in villages of all three agro-ecosystems. The potential risk of malaria, based on measurements of vectorial capacity of An. arabiensis and An.funestus combined, was four-fold higher in the ricefield village than in the sugarcane or savannah villages nearby. However, the more realistic estimate of malaria risk, based on entomological inoculation rates, indicated that exposure to infective vectors was 61-68% less for people in the ricefield village, due to the much lower sporozoite rate in An. arabiensis (ricefield 0.01%, sugarcane 0.1%, savannah 0.12%). This contrast was attributed to better socio-economic conditions of rice farmers, facilitating relatively more use of antimalarials and bednets for their families. Our findings show that, for a combination of reasons, the malaria challenge is lower for villagers associated with an irrigated rice-growing scheme (despite greater malaria vector potential), than for adjacent communities with other agro-ecosystems bringing less socio-economic benefits to health. This encourages the development of agro-irrigation schemes in African savannahs, provided that residents have ready access to antimalaria materials (i.e. effective antimalaria drugs and insecticidal bednets) that they may better afford for protection against the greater vectorial capacity of An. arabiensis from the ricefield agro-ecosystem.

Evidence for memorized site-fidelity in Anopheles arabiensis

A mark-recapture experiment was carried out in northern Tanzania to determine whether Anopheles arabiensis exhibits memory, by investigating if bloodfed individuals would return to either the location or the host where or on which they had obtained a previous bloodmeal, behaviours termed site-fidelity and host-fidelity respectively. Over 4300 mosquitoes were collected from 2 houses, marked with different fluorescent colours according to whether they were caught in cattle sheds, 'cattle-fed', or within human bednets, 'human-fed', at either location, then released from a third location. Over 17,000 mosquitoes were collected and examined over the next 8 days. In total, 1% of released mosquitoes were recaptured. Of these, 68% had returned to the house where they were first caught, demonstrating site-fidelity (P = 0.007). However, 86% of recaptured mosquitoes were caught on cattle regardless of where they were initially caught (P = 0.185). Bloodmeal identification showed that a high proportion of mosquitoes classed as human-fed contained bovine blood, thereby confounding the investigation into host-fidelity. Notably, the proportion of mosquitoes with mixed bloodmeals depended on the proximity of cattle and humans, with significantly higher proportions of mixed bloodmeals occurring when cattle and humans slept in close proximity. The effects of the observed behaviours on malaria epidemiology are discussed.

The distribution and chromosome polymorphism of Simulium dieguerense (Diptera: Simuliidae)

Simulium dieguerense has been identified from Senegal, Mali and Guinea, in the Western Zone of the World Health Organization/Onchocerciasis Control Programme thus considerably extending its known distribution and indicating the need to reassess its possible importance as a vector of human onchocerciasis. A distribution map based on these identifications has been drawn. Examination of the larval polytene chromosomes revealed the presence of a hitherto unreported sex chromosome system. About 90% of males and 17% of females were found to be heterozygous for an altered segment of the centromere region on chromosome I.

The effect of domestic detergents on the population dynamics of the immature stages of two competitor mosquitoes, Culex cinereus Theobald and Culex quinquefasciatus Say (Diptera, Culicidae) in Kenya

In some rural areas of Kenya pit latrines are the most important breeding places of Culex quinquefasciatus, but this vector is often rapidly displaced by a competitor, Culex cinereus, which then breeds prolifically in the latrines. In urban settlements, however, cesspools are the main breeding sites of C. quinquefasciatus and no such species replacement occurs. These latter habitats contain water contaminated with domestic detergents. When detergents were introduced into a pit latrine colonized only by C. cinereus this mosquito was eliminated after about 3 weeks. When both species were reared in water containing detergents C. cinereus had lower pupal yields than C. quinquefasciatus. In two pit latrines where C. cinereus normally displaced C. quinquefasciatus, the addition of detergent prevented this, and after their coexistence for a few weeks, C. cinereus eventually disappeared. These observations suggest that during the last few decades domestic detergents, together with other pollutants such as insecticides, may have contributed to the elimination of competitors, such as C. cinereus, from C. quinquefasciatus breeding sites.

Biting activity and resting behaviour of Anopheles merus Donitz (Diptera: Culicidae) on the Kenya Coast

Behavioural studies on An. merus were conducted during a period of 11 months in Jimbo, a village on the Kenya Coast. The biting activity of An. merus both outdoors and indoors attained a peak between 24.00 and 01.00 hours. The mosquito showed a stronger tendency to bite outdoors than indoors and was markedly exophilic. Various outdoor resting sites were also identified.

Separation of Anopheles merus from freshwater Anopheles gambiae by salinity tolerance test and morphological characters

The separation methods for Anopheles merus from freshwater A. gambiae s.l. involving the use of salinity tolerance test, sensilla coeloconica, palpal ratio and palpal bands were evaluated for a period of one year on a total of about 340 mosquitoes. The salinity tolerance test method was found to be quite simple and reliable but unsuitable in disease transmission studies due to an interval of 2-3 days between the collection and dissection periods and also due to the fact that only a fraction of the mosquito sample is generally identified by this method. Although significantly higher proportions of sensilla coeloconica and palpal ratio were observed in A. merus as compared to freshwater A. gambiae s.l. these characters were found quite unreliable due to their overlapping between two mosquito groups. Sensilla coeloconica and palpal ratio used separately could separate respective percentages of 11.4 and 11.8 A. merus from freshwater A. gambiae s.l., while in combination they separated up to 40.9%. Percentages 4-banded palp mosquitoes accounted for about 32% in A. merus and 19% in freshwater A. gambiae s.l. All these characters also displayed some seasonal variations in the two mosquito groups.

Laboratory infection of Anopheles pharoensis with Wuchereria bancrofti

A. pharoensis was infected from a donor with a high count of W. bancrofti microfilariae. Of the freshly dissected mosquitos, 81.6% were found to have ingested microfilariae, with an average of 12 +/- 2.2 microfilariae per mosquito. An infectivity rate of 41.9% was observed in mosquitos dissected between the eleventh and fifteenth days after feeding. A mean of 7.6 +/- 1.2 third-stage larvae was found in infective mosquitos. Although A. pharoensis has not yet been found naturally infected with third-stage larvae of W. bancrofti, these studies suggest that it is a potential vector of Bancroftian filariasis.