Insecticide-treated materials, mosquito adaptation and mass effect: entomological observations after five years of vector control in Burkina Faso

Review of Issues on Residual Malaria Transmission

Residual malaria transmission is the actual maintained inoculation of Plasmodium, in spite of a well-designed and implemented vector control programs, and is of great concern for malaria elimination. Residual malaria transmission occurs under several possible circumstances, among which the presence of exophilic vector species, such as Anopheles dirus, or indoor- and outdoor-biting vectors, such as Anopheles nili, or specific behavior, such as feeding on humans indoors, then resting or leaving the house the same night (such as Anopheles moucheti) or also changes in behavior induced by insecticides applied inside houses, such as the well-known deterrent effect of permethrin-treated nets or the irritant effect of DDT. The use of insecticides may change the composition of local Anopheles populations, such as A. arabiensis taking up the place of A. gambiae in Senegal, A. aquasalis replacing A. darlingi in Guyana, or A. harrisoni superseding A. minimus in Vietnam. The change in behavior, such as biting activity earlier than usually reported—for example, Anopheles funestus after a large-scale distribution of long-lasting insecticidal nets—or insecticide resistance, in particular the current spread of pyrethroid resistance, could hamper the efficacy of classic pyrethroid-treated long-lasting insecticidal nets and maintained transmission. These issues must be well documented in every situation to elaborate, implement, monitor, and evaluate tailored vector control programs, keeping in mind that they must be conceived as integrated programs with several well and appropriately coordinated approaches, combining entomological but also parasitological, clinical, and social methods and analyses. A successful integrated vector control program must then be designed to reduce transmission and incidence rates of malaria morbidity and overall mortality.

Emergence of behavioural avoidance strategies of malaria vectors in areas of high LLIN coverage in Tanzania

Despite significant reductions in malaria transmission across Africa since 2000, progress is stalling. This has been attributed to the development of insecticide resistance and behavioural adaptations in malaria vectors. Whilst insecticide resistance has been widely investigated, there is poorer understanding of the emergence, dynamics and impact of mosquito behavioural adaptations. We conducted a longitudinal investigation of malaria vector host choice over 3 years and resting behaviour over 4 years following a mass long-lasting insecticidal nets (LLINs) distribution in Tanzania. By pairing observations of mosquito ecology with environmental monitoring, we quantified longitudinal shifts in host-choice and resting behaviour that are consistent with adaptation to evade LLINs. The density of An. funestuss.l., declined significantly through time. In tandem, An. arabiensis and An. funestuss.l. exhibited an increased rate of outdoor relative to indoor resting; with An. arabiensis reducing the proportion of blood meals taken from humans in favour of cattle. By accounting for environmental variation, this study detected clear evidence of intra-specific shifts in mosquito behaviour that could be obscured in shorter-term or temporally-coarse surveys. This highlights the importance of mosquito behavioural adaptations to vector control, and the value of longer-term behavioural studies.

Evaluation of mosquito electrocuting traps as a safe alternative to the human landing catch for measuring human exposure to malaria vectors in Burkina Faso

BACKGROUND

Measuring human exposure to mosquito bites is a crucial component of vector-borne disease surveillance. For malaria vectors, the human landing catch (HLC) remains the gold standard for direct estimation of exposure. This method, however, is controversial since participants risk exposure to potentially infected mosquito bites. Recently an exposure-free mosquito electrocuting trap (MET) was developed to provide a safer alternative to the HLC. Early prototypes of the MET performed well in Tanzania but have yet to be tested in West Africa, where malaria vector species composition, ecology and behaviour are different. The performance of the MET relative to HLC for characterizing mosquito vector population dynamics and biting behaviour in Burkina Faso was evaluated.

METHODS

A longitudinal study was initiated within 12 villages in Burkina Faso in October 2016. Host-seeking mosquitoes were sampled monthly using HLC and MET collections over 14 months. Collections were made at 4 households on each night, with METs deployed inside and outside at 2 houses, and HLC inside and outside at another two. Malaria vector abundance, species composition, sporozoite rate and location of biting (indoor versus outdoor) were recorded.

RESULTS

In total, 41,800 mosquitoes were collected over 324 sampling nights, with the major malaria vector being Anopheles gambiae sensu lato (s.l.) complex. Overall the MET caught fewer An. gambiae s.l. than the HLC (mean predicted number of 0.78 versus 1.82 indoors, and 1.05 versus 2.04 outdoors). However, MET collections gave a consistent representation of seasonal dynamics in vector populations, species composition, biting behaviour (location and time) and malaria infection rates relative to HLC. As the relative performance of the MET was somewhat higher in outdoor versus indoor settings, this trapping method slightly underestimated the proportion of bites preventable by LLINs compared to the HLC (MET = 82.08%; HLC = 87.19%).

CONCLUSIONS

The MET collected proportionately fewer mosquitoes than the HLC. However, estimates of An. gambiae s.l. density in METs were highly correlated with HLC. Thus, although less sensitive, the MET is a safer alternative than the HLC. Its use is recommended particularly for sampling vectors in outdoor environments where it is most sensitive.

Insecticidal Activity of Local Plants Essential Oils Against Laboratory and Field Strains of Anopheles gambiae s. l. (Diptera: Culicidae) From Burkina Faso

The emergence and intensification of resistance to insecticides in malaria vector populations is the main obstacle to insecticide-based control efforts. The main objective of this study was to evaluate the larvicidal and adulticidal properties of the essential oils (EOs) of Ocimum canum, Hyptis suaveolens, Hyptis spicigera, and Lantana camara on field-collected, pyrethroids-resistant mosquitoes, local laboratory strains, and susceptible 'Kisumu' strain of Anopheles gambiae (Meigen) (Diptera: Culicidae) populations. Larvae and adults of these mosquitoes were challenged against four EOs. The mortality rates of larvae and adults were assessed 24 h after exposure to the EOs. Species identifications and detection of the L1014F and L1014S kdr mutations and the 1575Y super-kdr mutation were carried out using polymerase chain reaction on the pyrethroid-resistant mosquitoes from the field. EO compositions were analyzed by gas chromatography and mass spectrometry. Monoterpene hydrocarbons were the major components of H. suaveolens and H. spicigera EOs (49.8%) and (69.6%), respectively, whereas oxygenated monoterpenes (68.7%) were predominant in the O. canum EO. For L. camara, the component yields were variable, but it was the most effective EO against all strains. The LC50 values for the larvae were 7.73 and 25.63 ppm for the susceptible 'Kisumu' and resistant field strains, respectively. The LC50 for adults was 0.24% for the susceptible strain and 1.98% for the resistant strain. Molecular analysis confirmed the presence of L1014F and N1575Y mutations in resistant Anopheles arabiensis and Anopheles coluzzii mosquitoes from the field. Our results highlighted the potential of the EOs of local plants as insecticides against resistant and susceptible strains of An. gambiae populations.

Malaria Vector Control Still Matters despite Insecticide Resistance

Mosquito vectors' resistance to insecticides is usually considered a major threat to the recent progresses in malaria control. However, studies measuring the impact of interventions and insecticide resistance reveal inconsistencies when using entomological versus epidemiological indices. First, evaluation tests that do not reflect the susceptibility of mosquitoes when they are infectious may underestimate insecticide efficacy. Moreover, interactions between insecticide resistance and vectorial capacity reveal nonintuitive outcomes of interventions. Therefore, considering ecological interactions between vector, parasite, and environment highlights that the impact of insecticide resistance on the malaria burden is not straightforward and we suggest that vector control still matters despite insecticide resistance.

Knowledge and awareness of malaria and mosquito biting behaviour in selected sites within Morogoro and Dodoma regions Tanzania

Background

In Tanzania there has been a downward trend in malaria prevalence partly due to use of insecticide-treated bed nets for protection against Anopheles mosquitoes. However, residual malaria transmission attributed to early biting behaviour of malaria vectors is being reported. Knowledge of mosquito feeding behaviour is key to improvements in control approaches. The present study aimed to assess knowledge and awareness on malaria and malaria vectors in—Morogoro and Dodoma regions of Tanzania.

Methods

A cross sectional study was undertaken in selected sites in Morogoro and Dodoma Tanzania. A structured questionnaire was administered to 218 randomly selected households from each of which the head or second in/charge and the most senior primary school child were interviewed.

Results

A total of 400 participants of whom 56 % were females, were recruited into the study. Their ages ranged between nine and 58 years. Among the participants, 70.7 % had primary school education and the rest attained secondary school (16.8 %), university/college (4.0 %) and not attended school at all (8.5 %). Fifteen per cent of the participants were employed, while 45.5 % were self-employed and 39.5 % were studying. Overall, 58.5 % of respondents were knowledgeable of malaria and its vector. However, 78.8 % were not aware that early mosquito bites can transmit malaria and 86.5 % said that only midnight-biting mosquito bite was responsible for malaria transmission. The majority (66 %) of respondents visited a health facility on observing malaria symptoms while 15.8 % took anti-malaria drugs without medical consultation.

Conclusion

This study has shown that Anopheles is well known as the night-biting vector of malaria. The majority of participants were not aware of changed biting behaviour of malaria-transmitting mosquitoes and that early outdoor mosquito bite is a risk of malaria transmission. School children have shown a better understanding of malaria and its vector. Therefore, more awareness of Anopheles feeding behaviour is needed.

Electronic supplementary material

The online version of this article (doi:10.1186/s12936-016-1332-4) contains supplementary material, which is available to authorized users.

Applications and limitations of Centers for Disease Control and Prevention miniature light traps for measuring biting densities of African malaria vector populations: a pooled-analysis of 13 comparisons with human landing catches

Background

Measurement of densities of host-seeking malaria vectors is important for estimating levels of disease transmission, for appropriately allocating interventions, and for quantifying their impact. The gold standard for estimating mosquito—human contact rates is the human landing catch (HLC), where human volunteers catch mosquitoes that land on their exposed body parts. This approach necessitates exposure to potentially infectious mosquitoes, and is very labour intensive. There are several safer and less labour-intensive methods, with Centers for Disease Control light traps (LT) placed indoors near occupied bed nets being the most widely used.

Methods

This paper presents analyses of 13 studies with paired mosquito collections of LT and HLC to evaluate these methods for their consistency in sampling indoor-feeding mosquitoes belonging to the two major taxa of malaria vectors across Africa, the Anopheles gambiae sensu lato complex and the Anopheles funestus s.l. group. Both overall and study-specific sampling efficiencies of LT compared with HLC were computed, and regression methods that allow for the substantial variations in mosquito counts made by either method were used to test whether the sampling efficacy varies with mosquito density.

Results

Generally, LT were able to collect similar numbers of mosquitoes to the HLC indoors, although the relative sampling efficacy, measured by the ratio of LT:HLC varied considerably between studies. The overall best estimate for An. gambiae s.l. was 1.06 (95% credible interval: 0.68–1.64) and for An. funestus s.l. was 1.37 (0.70–2.68). Local calibration exercises are not reproducible, since only in a few studies did LT sample proportionally to HLC, and there was no geographical pattern or consistent trend with average density in the tendency for LT to either under- or over-sample.

Conclusions

LT are a crude tool at best, but are relatively easy to deploy on a large scale. Spatial and temporal variation in mosquito densities and human malaria transmission exposure span several orders of magnitude, compared to which the inconsistencies of LT are relatively small. LT, therefore, remain an invaluable and safe alternative to HLC for measuring indoor malaria transmission exposure in Africa.

Electronic supplementary material

The online version of this article (doi:10.1186/s12936-015-0761-9) contains supplementary material, which is available to authorized users.

Introgression in Lake Malaŵi: increasing the threat of human urogenital schistosomiasis?

For the last 15 years, we have studied the relationships among cichlid snail-eating fishes, intermediate snail-host density, and the prevalence of human infection of Schistosoma haematobium in Lake Malaŵi and concluded that the increase of human infection is correlated with the decrease in snail-eating fishes in the shallow waters of the lake. We postulated that a strain of S. haematobium from other parts of Africa, which was introduced into the Cape Maclear region of Lake Malaŵi by tourists, was compatible with Bulinus nyassanus-which is a close relative of B. truncatus, and interbred with the indigenous strain of S. haematobium, which ultimately produced via introgression a strain that can use both B. globosus and B. nyassanus as intermediate hosts. This actively evolving situation involving intermediate snail-host switching and decline of Trematocranus placodon, a natural cichlid snail predator, will impact on transmission of urogenital schistosomiasis within the local communities and on tourists who visit Lake Malaŵi.

Dynamics of malaria transmission and susceptibility to clinical malaria episodes following treatment of Plasmodium falciparum asymptomatic carriers: results of a cluster-randomized study of community-wide screening and treatment, and a parallel entomology study

Background

In malaria-endemic countries, large proportions of individuals infected with Plasmodium falciparum are asymptomatic and constitute a reservoir of parasites for infection of newly hatched mosquitoes.

Methods

Two studies were run in parallel in Burkina Faso to evaluate the impact of systematic identification and treatment of asymptomatic carriers of P. falciparum, detected by rapid diagnostic test, on disease transmission and susceptibility to clinical malaria episodes. A clinical study assessed the incidence of symptomatic malaria episodes with a parasite density >5,000/μL after three screening and treatment campaigns ~1 month apart before the rainy season; and an entomological study determined the effect of these campaigns on malaria transmission as measured by entomological inoculation rate.

Results

The intervention arm had lower prevalence of asymptomatic carriers of asexual parasites and lower prevalence of gametocyte carriers during campaigns 2 and 3 as compared to the control arm. During the entire follow-up period, out of 13,767 at-risk subjects, 2,516 subjects (intervention arm 1,332; control arm 1,184) had symptomatic malaria. Kaplan-Meier analysis of the incidence of first symptomatic malaria episode with a parasite density >5,000/μL showed that, in the total population, the two treatment arms were similar until Week 11–12 after campaign 3, corresponding with the beginning of the malaria transmission season, after which the probability of being free of symptomatic malaria was lower in the intervention arm (logrank p < 0.0001). Similar trends were observed in infants and children <5 years and in individuals ≥5 years of age. In infants and children <5 years old who experienced symptomatic malaria episodes, the geometric mean P. falciparum density was lower in the intervention arm than the control arm. This trend was not seen in those individuals aged ≥5 years. Over the year, monthly variation in mosquito density and entomological inoculation rate was comparable in both arms, with September peaks in both indices.

Conclusion

Community screening and targeted treatment of asymptomatic carriers of P. falciparum had no effect on the dynamics of malaria transmission, but seemed to be associated with an increase in the treated community’s susceptibility to symptomatic malaria episodes after the screening campaigns had finished. These results highlight the importance of further exploratory studies to better understand the dynamics of disease transmission in the context of malaria elimination.

Consistently high estimates for the proportion of human exposure to malaria vector populations occurring indoors in rural Africa

Background Insecticide-treated nets (ITNs) and indoor residual spraying (IRS) are highly effective tools for controlling malaria transmission in Africa because the most important vectors, from the Anopheles gambiae complex and the A. funestus group, usually prefer biting humans indoors at night.

Methods Matched surveys of mosquito and human behaviour from six rural sites in Burkina Faso, Tanzania, Zambia, and Kenya, with ITN use ranging from 0.2% to 82.5%, were used to calculate the proportion of human exposure to An. gambiae sensu lato and An. funestus s.l. that occurs indoors (π_i), as an indicator of the upper limit of personal protection that indoor vector control measures can provide. This quantity was also estimated through use of a simplified binary analysis (π_i^B) so that the proportions of mosquitoes caught indoors (P_i), and between the first and last hours at which most people are indoors (P_fl) could also be calculated as underlying indicators of feeding by mosquitoes indoors or at night, respectively.

Results The vast majority of human exposure to Anopheles bites occurred indoors (πiB = 0.79–1.00). Neither An. gambiae s.l. nor An. funestus s.l. strongly preferred feeding indoors (P_i = 0.40–0.63 and 0.22–0.69, respectively), but they overwhelmingly preferred feeding at times when most humans were indoors (P_fl = 0.78–1.00 and 0.86–1.00, respectively).

Conclusions These quantitative summaries of behavioural interactions between humans and mosquitoes constitute a remarkably consistent benchmark with which future observations of vector behaviour can be compared. Longitudinal monitoring of these quantities is vital to evaluate the effectiveness of ITNs and IRS and the need for complementary measures that target vectors outdoors.

Collapse of Anopheles darlingi populations in Suriname after introduction of insecticide-treated nets (ITNs); malaria down to near elimination level

A longitudinal study of malaria vectors was carried out in three villages in Suriname between 2006 and 2010. During 13,392 man hours of collections, 3,180 mosquitoes were collected, of which 33.7% were anophelines. Of these, Anopheles darlingi accounted for 88.1%, and An. nuneztovari accounted for 11.1%. The highest mean An. darlingi human biting rate (HBR) observed per survey was 1.43 bites/man per hour outdoor and 1.09 bites/man per hour indoor; 2 An. darlingi of the 683 tested were infected with Plasmodium falciparum. The anopheline HBR decreased to zero after the onset of malaria intervention activities, including insecticide-treated net (ITN) distribution, in 2006. Malaria transmission decreased to pre-elimination levels. It is concluded that the combination of ITN and climatic events has led to the collapse of malaria vector populations in the study sites in the interior of the country. The results are discussed in relation to the stability of malaria transmission in areas with low-density human populations.

Long-term effects of malaria prevention with insecticide-treated mosquito nets on morbidity and mortality in African children: randomised controlled trial

OBJECTIVE

The objective is to investigate the effect of malaria control with insecticide-treated mosquito nets (ITNs) regarding possible higher mortality in children protected during early infancy, due to interference with immunity development, and to assess long-term effects on malaria prevalence and morbidity.

METHODS

Between 2000 and 2002, a birth cohort was enrolled in 41 villages of a malaria holoendemic area in north-western Burkina Faso. All neonates (n = 3387) were individually randomised to ITN protection from birth (group A) vs. ITN protection from age 6 months (group B). Primary outcome was all-cause mortality. In 2009, a survey took place in six sentinel villages, and in 2010, a census was conducted in all study villages.

RESULTS

After a median follow-up time of 8.3 years, 443/3387 (13.1%) children had migrated out of the area and 484/2944 (16.4%) had died, mostly at home. Long-term compliance with ITN protection was good. There were no differences in mortality between study groups (248 deaths in group A, 236 deaths in group B; rate ratio 1.05, 95% CI: 0.889-1.237, P = 0.574). The survey conducted briefly after the rainy season in 2009 showed that more than 80% of study children carried asexual malaria parasites and up to 20% had clinical malaria.

CONCLUSION

Insecticide-treated mosquito net protection in early infancy is not a risk factor for mortality. Individual ITN protection does not sufficiently reduce malaria prevalence in high-transmission areas. Achieving universal ITN coverage remains a major challenge for malaria prevention in Africa.

Anopheles gambiae resistance to pyrethroid-treated nets in cotton versus rice areas in Mali

The rise and spread of Anopheles gambiae s.l. (the major malaria vector sub-Saharan Africa) resistance to pyrethroids is of great concern owing to the predominant role of pyrethroid-treated nets in the WHO global strategy for malaria control. Use of pyrethroids for agricultural purposes may exert a strong selection pressure, favouring the emergence of insecticide resistance. The objective of this study was to evaluate the efficacy of alpha-cypermethrin treated nets in settings where insecticides are used against pests. This was assessed in two ways, i.e. under laboratory conditions using the WHO standard cones test technique and in experimental huts, on Anopheles gambiae s.l. collected in two Malian rural sites, Koumantou characterised by cotton crops and high insecticide use and Sélingué, a rice field area with low insecticide use. According to the WHO standard cones test technique, there was no difference between mosquitoes collected in the two sites; KD50 time was less than 3 min and the KD95 time below 30 min. Nevertheless, in the experimental huts with alpha-cypermethrin treated bed nets, the mosquito mortality rate was significantly lower in Koumantou (102/361, 28.2%) than in Sélingué (122/233, 52.3%) (RR: 0.65, 95%CI: 0.56-0.76) (p<0.001). In addition, in Koumantou the percentage of unfed mosquitoes found in the veranda was much lower in the huts with untreated (26.0%, 33/127) than in those with treated nets (92.2%, 118/128) (p<0.01) while in Sélingué there was no difference between huts with treated and untreated bed nets. Alpha-cypermethrin treated bed nets had a significant effect on mortality and repelling behaviour of Anopheles gambiae s.l. though in Koumantou treated bed nets were less efficacious, possibly due to the intense use of pesticide for agriculture.

Early biting rhythm in the Afro-tropical vector of malaria, Anopheles arabiensis, and challenges for its control in Ethiopia

The biting cycle of the malaria vector Anopheles arabiensis Patton (Diptera: Culicidae) was assessed by hourly light trap collections in three villages in Tigray, northern Ethiopia. Hourly catches were conducted in two houses in each village, for four consecutive nights. Light traps were set from 18.00 hours to 07.00 hours in houses in which people slept under untreated bednets. Anopheles arabiensis showed early biting activities, which peaked between 19.00 hours and 20.00 hours in the three villages; over 70% of biting activity occurred before 22.00 hours, when people typically retire to bed. This early biting activity may have a negative impact on the efficiency of bednets to control malaria.

Haematological parameters, natural regulatory CD4 + CD25 + FOXP3+ T cells and γδ T cells among two sympatric ethnic groups having different susceptibility to malaria in Burkina Faso

BACKGROUND

Fulani ethnic group individuals are less susceptible than sympatric Mossi ethnic group, in term of malaria infection severity, and differ in antibody production against malaria antigens. The differences in susceptibility to malaria between Fulani and Mossi ethnic groups are thought to be regulated by different genetic backgrounds and offer the opportunity to compare haematological parameters, Tregs and γδT cell profiles in seasonal and stable malaria transmission settings in Burkina Faso. The study was conducted at two different time points i.e. during the high and low malaria transmission period.

RESULTS

Two cross-sectional surveys were undertaken in adults above 20 years belonging either to the Fulani or the Mossi ethnic groups 1) at the peak of the malaria transmission season and 2) during the middle of the low malaria transmission season. Full blood counts, proportions of Tregs and γδ T cells were measured at both time-points.As previously shown the Fulani and Mossi ethnic groups showed a consistent difference in P. falciparum infection rates and parasite load. Differential white blood cell counts showed that the absolute lymphocyte counts were higher in the Mossi than in the Fulani ethnic group at both time points. While the proportion of CD4+CD25high was higher in the Fulani ethnic group at the peak of malaria transmission season (p = 0.03), no clear pattern emerged for T regulatory cells expressing FoxP3+ and CD127low. However CD3+γδ+ subpopulations were found to be higher in the Fulani compared to the Mossi ethnic group, and this difference was statistically significant at both time-points (p = 0.004 at low transmission season and p = 0.04 at peak of transmission).

CONCLUSION

Our findings on regulatory T cell phenotypes suggest an interesting role for immune regulatory mechanisms in response to malaria. The study also suggests that TCRγδ + cells might contribute to the protection against malaria in the Fulani ethnic group involving their reported parasite inhibitory activities.

Schistosomiasis in Lake Malaŵi villages

Historically, open shorelines of Lake Malaŵi were free from schistosome, Schistosoma haematobium, transmission, but this changed in the mid-1980s, possibly as a result of over-fishing reducing density of molluscivore fishes. Very little information is available on schistosome infections among people in lake-shore communities and therefore we decided to summarise data collected from 1998 to 2007. Detailed knowledge of the transmission patterns is essential to design a holistic approach to schistosomiasis control involving the public health, fisheries and tourism sectors. On Nankumba Peninsula, in the southern part of the lake, inhabitants of villages located along the shores of Lake Malaŵi have higher prevalence of S. haematobium infection than those living in inland villages. Overall prevalence (all age classes combined) of urinary schistosomiasis in 1998/1999 ranged from 10.2% to 26.4% in inland villages and from 21.0% to 72.7% in lakeshore villages; for school children prevalence of infection ranged from 15.3% to 57.1% in inland schools and from 56.2% to 94.0% in lakeshore schools. Inhabitants on the islands, Chizumulu and Likoma, also have lower prevalence of infection than those living in lakeshore villages on Nankumba Peninsula. This increased prevalence in lakeshore villages is not necessarily linked to transmission taking place in the lake itself, but could also be due to the presence of more numerous typical inland transmission sites (e.g., streams, ponds) being close to the lake. Temporal data witness of intense transmission in some lakeshore villages with 30-40% of children cleared from infection becoming reinfected 12 months later (also lakeshore village). The level of S. mansoni infection is low in the lakeshore communities. Findings are discussed in relation to fishing in the lake.

Density of Trematocranus placodon (Pisces: Cichlidae): a predictor of density of the schistosome intermediate host, Bulinus nyassanus (Gastropoda: Planorbidae), in Lake Malaŵi

From the mid-1980s, we recorded a significant increase in urinary schistosomiasis infection rate and transmission among inhabitants of lakeshore communities in the southern part of Lake Malaŵi, particularly on Nankumba peninsula in Mangochi District. We suggested that the increase was due to over-fishing, which reduced the density of snail-eating fishes, thereby allowing schistosome intermediate host snails to increase to higher densities. In this article, we collected data to test this hypothesis. The density of both Bulinus nyassanus, the intermediate host of Schistosoma haematobium, and Melanoides spp. was negatively related to density of Trematocranus placodon, the most common of the snail-eating fishes in the shallow water of Lake Malaŵi. Both these snails are consumed by T. placodon. Transmission of S. haematobium through B. nyassanus only occurs in the southern part of the lake and only at villages where high density of the intermediate host is found relatively close to the shore. Thus, we believe that implementation of an effective fish ban up to 100-m offshore along these specific shorelines in front of villages would allow populations of T. placodon to increase in density and this would lead to reduced density of B. nyassanus and possibly schistosome transmission. To reduce dependence on natural fish populations in the lake and still maintain a source of high quality food, culture of indigenous fishes may be a viable alternative.

Distribution systems of insecticide-treated bed nets for malaria control in rural Burkina Faso: cluster-randomized controlled trial

BACKGROUND

Insecticide-impregnated bed nets (ITNs) have been shown to be a highly effective tool against malaria in the endemic regions of sub-Saharan Africa (SSA). There are however different opinions about the role of ITN social marketing and ITN free distribution in the roll-out of ITN programmes. The objective of this study was to evaluate the effects of free ITN distribution through antenatal care services in addition to an ITN social marketing programme in an area typical for rural SSA.

METHODS

A cluster-randomised controlled ITN trial took place in the whole Kossi Province in north-western Burkina Faso, an area highly endemic for malaria. Twelve clusters were assigned to long-term ITN (Serena brand) social marketing plus free ITN (Serena brand) distribution to all pregnant women attending governmental antenatal care services (group A), and 13 clusters to ITN social marketing only (group B). The intervention took place during the rainy season of 2006 and thereafter. The trial was evaluated through a representative household survey at baseline and after one year. Serena ITN household ownership was the primary outcome measure.

FINDINGS

A total of 1052 households were visited at baseline in February 2006 and 1050 at follow-up in February 2007. Overall Serena ITN household ownership increased from 16% to 28% over the study period, with a significantly higher increase in group A (13% to 35%) than in group B (18% to 23%) (p<0.001).

INTERPRETATION

The free distribution of ITNs to pregnant women through governmental antenatal care services in addition to ITN social marketing substantially improved ITN household ownership in rural Burkina Faso.

TRIAL REGISTRATION

Controlled-Trials.com ISRCTN07985309.

Malaria transmission pattern resilience to climatic variability is mediated by insecticide-treated nets

Background

Malaria is an important public-health problem in the archipelago of Vanuatu and climate has been hypothesized as important influence on transmission risk. Beginning in 1988, a major intervention using insecticide-treated bed nets (ITNs) was implemented in the country in an attempt to reduce Plasmodium transmission. To date, no study has addressed the impact of ITN intervention in Vanuatu, how it may have modified the burden of disease, and whether there were any changes in malaria incidence that might be related to climatic drivers.

Methods and findings

Monthly time series (January 1983 through December 1999) of confirmed Plasmodium falciparum and Plasmodium vivax infections in the archipelago were analysed. During this 17 year period, malaria dynamics underwent a major regime shift around May 1991, following the introduction of bed nets as a control strategy in the country. By February of 1994 disease incidence from both parasites was reduced by at least 50%, when at most 20% of the population at risk was covered by ITNs. Seasonal cycles, as expected, were strongly correlated with temperature patterns, while inter-annual cycles were associated with changes in precipitation. Following the bed net intervention, the influence of environmental drivers of malaria dynamics was reduced by 30–80% for climatic forces, and 33–54% for other factors. A time lag of about five months was observed for the qualitative change ("regime shift") between the two parasites, the change occurring first for P. falciparum. The latter might be explained by interspecific interactions between the two parasites within the human hosts and their distinct biology, since P. vivax can relapse after a primary infection.

Conclusion

The Vanuatu ITN programme represents an excellent example of implementing an infectious disease control programme. The distribution was undertaken to cover a large, local proportion (~80%) of people in villages where malaria was present. The successful coverage was possible because of the strategy for distribution of ITNs by prioritizing the free distribution to groups with restricted means for their acquisition, making the access to this resource equitable across the population. These results emphasize the need to implement infectious disease control programmes focusing on the most vulnerable populations.

How is childhood development of immunity to Plasmodium falciparum enhanced by certain antimalarial interventions?

The development of acquired protective immunity to Plasmodium falciparum infection in young African children is considered in the context of three current strategies for malaria prevention: insecticide-impregnated bed nets or curtains, anti-sporozoite vaccines and intermittent preventive therapy. Evidence is presented that each of these measures may permit attenuated P. falciparum blood-stage infections, which do not cause clinical malaria but can act as an effective blood-stage "vaccine". It is proposed that the extended serum half-life, and rarely considered liver-stage prophylaxis provided by the anti-folate combination sulphadoxine-pyrimethamine frequently lead to such attenuated infections in high transmission areas, and thus contribute to the sustained protection from malaria observed among children receiving the combination as intermittent preventative therapy or for parasite clearance in vaccine trials.

Interdependence of domestic malaria prevention measures and mosquito-human interactions in urban Dar es Salaam, Tanzania

BACKGROUND

Successful malaria vector control depends on understanding behavioural interactions between mosquitoes and humans, which are highly setting-specific and may have characteristic features in urban environments. Here mosquito biting patterns in Dar es Salaam, Tanzania are examined and the protection against exposure to malaria transmission that is afforded to residents by using an insecticide-treated net (ITN) is estimated.

METHODS

Mosquito biting activity over the course of the night was estimated by human landing catch in 216 houses and 1,064 residents were interviewed to determine usage of protection measures and the proportion of each hour of the night spent sleeping indoors, awake indoors, and outdoors.

RESULTS

Hourly variations in biting activity by members of the Anopheles gambiae complex were consistent with classical reports but the proportion of these vectors caught outdoors in Dar es Salaam was almost double that of rural Tanzania. Overall, ITNs confer less protection against exophagic vectors in Dar es Salaam than in rural southern Tanzania (59% versus 70%). More alarmingly, a biting activity maximum that precedes 10 pm and much lower levels of ITN protection against exposure (38%) were observed for Anopheles arabiensis, a vector of modest importance locally, but which predominates transmission in large parts of Africa.

CONCLUSION

In a situation of changing mosquito and human behaviour, ITNs may confer lower, but still useful, levels of personal protection which can be complemented by communal transmission suppression at high coverage. Mosquito-proofing houses appeared to be the intervention of choice amongst residents and further options for preventing outdoor transmission include larviciding and environmental management.

An epidemiologic model of severe morbidity and mortality caused by Plasmodium falciparum

The intensity of Plasmodium falciparum transmission has multifarious and sometimes counter-intuitive effects on age-specific rates of severe morbidity and mortality in endemic areas. This has led to conflicting speculations about the likely impact of malaria control interventions. We propose a quantitative framework to reconcile the various apparently contradictory observations relating morbidity and mortality rates to malaria transmission. Our model considers two sub-categories of severe malaria episodes. These comprise episodes with extremely high parasite densities in hosts with little previous exposure, and acute malaria episodes accompanied by co-morbidity or other risk factors enhancing susceptibility. In addition to direct malaria mortality from severe malaria episodes, the model also considers the enhanced risk of indirect mortality following acute episodes accompanied by co-morbidity after the parasites have been cleared. We fit this model to summaries of field data from endemic areas of Africa, and show that it can account for the observed age- and exposure-specific patterns of pediatric severe malaria and malaria-associated mortality in children. This model will allow us to make predictions of the long-term impact of potential malaria interventions. Predictions for children will be more reliable than those for older people because there is a paucity of epidemiologic studies of adults and adolescents.

Costing the distribution of insecticide-treated nets: a review of cost and cost-effectiveness studies to provide guidance on standardization of costing methodology

BACKGROUND

Insecticide-treated nets (ITNs) are an effective and cost-effective means of malaria control. Scaling-up coverage of ITNs is challenging. It requires substantial resources and there are a number of strategies to choose from. Information on the cost of different strategies is still scarce. To guide the choice of a delivery strategy (or combination of strategies), reliable and standardized cost information for the different options is required.

METHODS

The electronic online database PubMed was used for a systematic search of the published English literature on costing and economic evaluations of ITN distribution programmes. The keywords used were: net, bednet, insecticide, treated, ITN, cost, effectiveness, economic and evaluation. Identified papers were analysed to determine and evaluate the costing methods used. Methods were judged against existing standards of cost analysis to arrive at proposed standards for undertaking and presenting cost analyses.

RESULTS

Cost estimates were often not readily comparable or could not be adjusted to a different context. This resulted from the wide range of methods applied and measures of output chosen. Most common shortcomings were the omission of certain costs and failure to adjust financial costs to generate economic costs. Generalisability was hampered by authors not reporting quantities and prices of resources separately and not examining the sensitivity of their results to variations in underlying assumptions.

CONCLUSION

The observed shortcomings have arisen despite the abundance of literature and guidelines on costing of health care interventions. This paper provides ITN specific recommendations in the hope that these will help to standardize future cost estimates.

Comparative evaluation of carbosulfan- and permethrin-impregnated curtains for preventing house-entry by the malaria vector Anopheles gambiae in Burkina Faso

Pyrethroid-impregnated bednets and curtains are widely employed to reduce the risk of malaria transmission, but pyrethroid-resistance is becoming more prevalent among malaria vector Anopheles mosquitoes (Diptera: Culicidae). As an alternative treatment for curtains, we assessed carbosulfan (a carbamate insecticide) in comparison with permethrin as the standard pyrethroid, against endophilic female mosquitoes of the Anopheles gambiae Giles complex in a village near Ouagadougou, Burkina Faso. The main criterion evaluated was the impact of curtains (hung inside windows, eaves and doorways) on the number of An. gambiae s.l. females active indoors at night. Light-traps were operated overnight (21.00-06.00 hours beside occupied untreated bednets) to sample mosquitoes in houses fitted with net curtains treated with carbosulfan 0.2 g ai/m2 or permethrin 1 g ai/m2 or untreated, compared with houses without curtains. The treated and untreated curtains significantly reduced the numbers of mosquitoes collected indoors, compared with houses without curtains. Carbosulfan-treated curtains had a highly significantly greater effect than permethrin-treated or untreated curtains, the scale of the difference being estimated as three-fold. However, there was no significant difference between the impact of untreated and permethrin-treated curtains on densities of An. gambiae s.l. trapped indoors. Samples of the An. gambiae complex comprised An. arabiensis Patton and both the S- and M-forms of An. gambiae Giles s.s. Susceptibility tests revealed some resistance to DDT and low frequencies of permethrin-resistance, insufficient to explain the poor performance of permethrin on curtains. Among survivors from the diagnostic dosage of permethrin were some specimens of all three members of the An. gambiae complex, but the kdr resistance mechanism was detected only in the S-form of An. gambiae s.s. Questions arising for further investigation include clarification of resistance mechanisms in, and foraging behaviour of, each member of the An. gambiae complex in this situation and the need to decide whether carbosulfan-treated curtains are acceptably safe for use to reduce risks of malaria transmission.

Do insecticide-treated bednets have an effect on malaria vectors?

The use of insecticide-treated bednets (ITNs) has been widely adopted as an important method for malaria control. Few data exist on effects of ITNs on mosquito biology and ecology, other than the development of insecticide resistance against the insecticides used. There is no hard evidence that the insecticide resistance recorded is the result of insecticidal use on bednets or from agricultural use. Resistance against pyrethroids, the preferred class of insecticides for ITN use, has been recorded from countries in Asia, Africa and South America. Resistance is expressed as reduced excito-repellency and mortality of mosquitoes exposed to insecticide-treated materials. In the absence of resistance, however, most studies on ITN effects report a reduced survival of adult mosquitoes as well as mass killing. Other effects are highly variable, and shifts in time of biting, feeding site and blood hosts have occasionally been reported, but not in proportion to the scale of ITN use. In general, a reduced sporozoite rate is recorded in ITN programmes. Because many of the anticipated behavioural effects caused by insecticidal use will be avoided by the use of untreated nets, studies on the efficacy of untreated nets are required. Examples are presented in which untreated nets provided a reasonable degree of protection against malaria.