In-vitro susceptibility and ex-vivo evaluation of macrocyclic lactone endectocides sub-lethal concentrations against Plasmodium vivax oocyst development in Anopheles arabiensis

BACKGROUND

Asymptomatic malaria transmission has become a public health concern across malaria-endemic Africa including Ethiopia. Specifically, Plasmodium vivax is more efficient at transmitting earlier in the infection and at lower densities than Plasmodium falciparum. Consequently, a greater proportion of individuals infected with P. vivax can transmit without detectable gametocytaemia. Mass treatment of livestock with macrocyclic lactones (MLs), e.g., ivermectin and doramectin, was suggested as a complementary malaria vector tool because of their insecticidal effects. However, the effects of MLs on P. vivax in Anopheles arabiensis has not yet been fully explored. Hence, comparative in-vitro susceptibility and ex-vivo studies were conducted to evaluate the effects of ivermectin, doramectin and moxidectin sub-lethal concentrations on P. vivax oocyst development in An. arabiensis.

METHODS

The 7-day sub-lethal concentrations of 25% (LC25) and 5% (LC5) were determined from in-vitro susceptibility tests on female An. arabiensis in Hemotek® membrane feeding assay. Next, an ex-vivo study was conducted using P. vivax gametocytes infected patient's blood spiked with the LC25 and LC5 of the MLs. At 7-days post-feeding, each mosquito was dissected under a dissection stereo microscope, stained with 0.5% (w/v) mercurochrome solution, and examined for the presence of P. vivax oocysts. Statistical analysis was based on a generalized mixed model with binomially distributed error terms.

RESULTS

A 7-day lethal concentration of 25% (LC25, in ng/mL) of 7.1 (95% CI: [6.3;8.0]), 20.0 (95%CI:[17.8;22.5]) and 794.3 (95%CI:[716.4;1516.3]) were obtained for ivermectin, doramectin and moxidectin, respectively. Similarly, a lethal concentration of 5% (LC5, in ng/mL) of 0.6 (95% CI: [0.5;0.7]), 1.8 (95% CI:[1.6;2.0]) and 53.7 (95% CI:[ 48.4;102.5]) were obtained respectively for ivermectin, doramectin and moxidectin. The oocyst prevalence in treatment and control groups did not differ significantly (p > 0.05) from each other. Therefore, no direct effect of ML endectocides on P. vivax infection in An. arabiensis mosquitoes was observed at the sub-lethal concentration (LC25 and LC5).

CONCLUSIONS

The effects of ivermectin and doramectin on malaria parasite is more likely via indirect effects, particularly by reducing the vectors lifespan and causing mortality before completing the parasite's sporogony cycle or reducing their vector capacity as it affects the locomotor activity of the mosquito.

On the haem auxotrophy of the soft tick Ornithodoros moubata

Genomes of ticks display reductions, to various extents, in genetic coding for enzymes of the haem biosynthetic pathway. Here, we mined available transcriptomes of soft tick species and identified transcripts encoding only half of the enzymes involved in haem biosynthesis. Transcripts identified across most species examined were those coding for porphobilinogen synthase, coproporphyrinogen oxidase, protoporphyrinogen oxidase, and ferrochelatase. Genomic retention of porphobilinogen synthase seems to be soft tick-restricted as no such homologue has been identified in any hard tick species. Bioinformatic mining is thus strongly indicative of the lack of biochemical capacity for de novo haem biosynthesis, suggesting a requirement for dietary haem. In the hard tick Ixodes ricinus, depletion of dietary haem, i.e. serum feeding, leads to oviposition of haem-free eggs, with no apparent embryogenesis and larvae formation. In this work, we show that serum-fed Ornithodoros moubata females, unlike those of I. ricinus, laid haem-containing eggs similarly to blood-fed controls, but only by a small proportion of the serum-fed females. To enhance the effect of dietary haem depletion, O. moubata ticks were serum-fed consecutively as last nymphal instars and females. These females laid eggs with profoundly reduced haem deposits, confirming the host origin of the haem. These data confirm the ability of soft ticks to take up and allocate host haem to their eggs in order to drive reproduction of the ticks.

A standardised low-cost membrane blood-feeder for Aedes aegypti made using common laboratory materials

Blood feeding is a necessary part of laboratory studies involving mosquitoes and other hematophagous arthropods of interest in medical and ecological research. However, methods involving hosts may present serious risks, require ethics approvals and can be expensive. Here we describe an insect blood feeder made using common laboratory materials, which is low cost (<US$100) and can be constructed and operated with little technical expertise. We compared the blood feeder containing an artificial blood diet, Skitosnack, to direct human arm feeding for Aedes aegypti (Diptera: Culicidae), in terms of engorgement rate, fecundity and hatch rate. No significant difference in fecundity between the two approaches was found, (mean ± SD); direct human arm: 56 ± 26 eggs/female, artificial method: 47 ± 25 eggs/female, P = 0.569. Engorgement rates (direct human arm: 97.8 ± 4%, artificial: 64.1 ± 23%, P < 0.05) and hatch rates (direct human arm: 75 ± 12%, artificial: 59 ± 14%, P < 0.05) were lower in the artificially fed mosquitoes. Despite these differences, we maintained a healthy mosquito colony for 10 generations using the artificial feeding approach. Results from this comparison are within the range of other studies which compared direct host feeding with an artificial feeding method. We anticipate that the blood feeder presented here could substantially reduce costs usually required to establish a standardised and effective blood feeding method for maintaining mosquito colonies or conducting experiments, extending the capability of laboratories especially where research resources are limited, but vector-borne diseases common.

Plasmodium vivax gametocytes and transmission

Malaria elimination means cessation of parasite transmission. At present, the declining malaria incidence in many countries has made elimination a feasible goal. Transmission control has thus been placed at the center of the national malaria control programs. The efficient transmission of Plasmodium vivax from humans to mosquitoes is a key factor that helps perpetuate malaria in endemic areas. A better understanding of transmission is crucial to the success of elimination efforts. Biological delineation of the parasite transmission process is important for identifying and prioritizing new targets of intervention. Identification of the infectious parasite reservoir in the community is key to devising an effective elimination strategy. Here we describe the fundamental characteristics of P. vivax gametocytes - the dynamics of their production, longevity, and the relationship with the total parasitemia - as well as recent advances in the molecular understanding of parasite sexual development. In relation to malaria elimination, factors influencing the human infectivity and the current evidence for a role of asymptomatic carriers in transmission are presented.

A simple and affordable method for estimating the fluid volume a mosquito sucks using food dyes

BACKGROUND

Blood-sucking by mosquitoes is an inevitable behavior when pathogens are transmitted among humans. Adenine nucleotides such as ATP are known as phagostimulants for mosquitoes and are widely used to induce and enhance the blood-sucking activity in an artificial manner. Although using ATP solution is convenient to introduce a variety of substances (for example chemicals and pathogens) into the mosquito body via sucking, establishing an easy and cost-effective method to quantify the amount of solution ingested has yet to be reported.

RESULTS

A set of commercial food dyes (green, blue, yellow, and red) was employed in this study. Each dye was added to ATP solution used to colorize the abdomen of Ae. aegypti female mosquitoes after ingestion. The intake of food dyes did not show any toxicity to the mosquitoes, affecting neither ATP-sucking behavior nor survival of the mosquitoes. We observed that quantifying the color intensity of green dye in the mosquito abdomen by spectral analysis, as well as distinguishing the size of the colored abdomen using the naked eye, allowed the estimation of the amount of ingested solution. Using this method, capsaicin, a pungent component of chili peppers, was identified as an aversive tastant that can discourage mosquitoes from sucking the ATP solution.

CONCLUSIONS

Employing commercially available, non-toxic food dyes converted ATP-driven membrane feeding into an easy-to-use method to estimate the amount of solution ingested by mosquitoes. This method can be further applied for a variety of experiments such as introducing a certain quantity of chemical compounds or microbes into the mosquito body.

Evaluation of a simple polytetrafluoroethylene (PTFE)-based membrane for blood-feeding of malaria and dengue fever vectors in the laboratory

Background

Controlled blood-feeding is essential for maintaining laboratory colonies of disease-transmitting mosquitoes and investigating pathogen transmission. We evaluated a low-cost artificial feeding (AF) method, as an alternative to direct human feeding (DHF), commonly used in mosquito laboratories.

Methods

We applied thinly-stretched pieces of polytetrafluoroethylene (PTFE) membranes cut from locally available seal tape (i.e. plumbers tape, commonly used for sealing pipe threads in gasworks or waterworks). Approximately 4 ml of bovine blood was placed on the bottom surfaces of inverted Styrofoam cups and then the PTFE membranes were thinly stretched over the surfaces. The cups were filled with boiled water to keep the blood warm (~37 °C), and held over netting cages containing 3–4 day-old inseminated adults of female Aedes aegypti, Anopheles gambiae (s.s.) or Anopheles arabiensis. Blood-feeding success, fecundity and survival of mosquitoes maintained by this system were compared against DHF.

Results

Aedes aegypti achieved 100% feeding success on both AF and DHF, and also similar fecundity rates (13.1 ± 1.7 and 12.8 ± 1.0 eggs/mosquito respectively; P > 0.05). An. arabiensis had slightly lower feeding success on AF (85.83 ± 16.28%) than DHF (98.83 ± 2.29%) though these were not statistically different (P > 0.05), and also comparable fecundity between AF (8.82 ± 7.02) and DHF (8.02 ± 5.81). Similarly, for An. gambiae (s.s.), we observed a marginal difference in feeding success between AF (86.00 ± 10.86%) and DHF (98.92 ± 2.65%), but similar fecundity by either method. Compared to DHF, mosquitoes fed using AF survived a similar number of days [Hazard Ratios (HR) for Ae. aegypti = 0.99 (0.75–1.34), P > 0.05; An. arabiensis = 0.96 (0.75–1.22), P > 0.05; and An. gambiae (s.s.) = 1.03 (0.79–1.35), P > 0.05].

Conclusions

Mosquitoes fed via this simple AF method had similar feeding success, fecundity and longevity. The method could potentially be used for laboratory colonization of mosquitoes, where DHF is unfeasible. If improved (e.g. minimizing temperature fluctuations), the approach could possibly also support studies where vectors are artificially infected with blood-borne pathogens.

Do the venous blood samples replicate malaria parasite densities found in capillary blood? A field study performed in naturally-infected asymptomatic children in Cameroon

Background

The measure of new drug- or vaccine-based approaches for malaria control is based on direct membrane feeding assays (DMFAs) where gametocyte-infected blood samples are offered to mosquitoes through an artificial feeder system. Gametocyte donors are identified by the microscopic detection and quantification of malaria blood stages on blood films prepared using either capillary or venous blood. However, parasites are known to sequester in the microvasculature and this phenomenon may alter accurate detection of parasites in blood films. The blood source may then impact the success of mosquito feeding experiments and investigations are needed for the implementation of DMFAs under natural conditions.

Methods

Thick blood smears were prepared from blood obtained from asymptomatic children attending primary schools in the vicinity of Mfou (Cameroon) over four transmission seasons. Parasite densities were determined microscopically from capillary and venous blood for 137 naturally-infected gametocyte carriers. The effect of the blood source on gametocyte and asexual stage densities was then assessed by fitting cumulative link mixed models (CLMM). DMFAs were performed to compare the infectiousness of gametocytes from the different blood sources to mosquitoes.

Results

Prevalence of Plasmodium falciparum asexual stages among asymptomatic children aged from 4 to 15 years was 51.8% (2116/4087). The overall prevalence of P. falciparum gametocyte carriage was 8.9% and varied from one school to another. No difference in the density of gametocyte and asexual stages was found between capillary and venous blood. Attempts to perform DMFAs with capillary blood failed.

Conclusions

Plasmodium falciparum malaria parasite densities do not differ between capillary and venous blood in asymptomatic subjects for both gametocyte and trophozoite stages. This finding suggests that the blood source should not interfere with transmission efficiency in DMFAs.

Electronic supplementary material

The online version of this article (doi:10.1186/s12936-017-1978-6) contains supplementary material, which is available to authorized users.

High susceptibility of wild Anopheles funestus to infection with natural Plasmodium falciparum gametocytes using membrane feeding assays

Background

Anopheles funestus is a major vector of malaria in sub-Saharan Africa. However, because it is difficult to colonize, research on this mosquito species has lagged behind other vectors, particularly the understanding of its susceptibility and interactions with the Plasmodium parasite. The present study reports one of the first experimental infections of progeny from wild-caught An. funestus with the P. falciparum parasite providing a realistic avenue for the characterisation of immune responses associated with this infection.

Methods

Wild-fed resting An. funestus females were collected using electric aspirators and kept in cages for four days until they were fully gravid and ready to oviposit. The resulting eggs were reared to adults F1 mosquitoes under insectary conditions. Three to five day-old An. funestus F1 females were fed with infected blood taken from gametocyte carriers using an artificial glass-parafilm feeding system. Feeding rate was recorded and fed mosquitoes were dissected at day 7 to count oocysts in midguts. Parallel experiments were performed with the known Plasmodium-susceptible An. coluzzii Ngousso laboratory strain, to monitor our blood handling procedures and infectivity of gametocytes.

Results

The results revealed that An. funestus displays high and similar level of susceptibility to Plasmodium infection compared to An. coluzzii, and suggest that our methodology produces robust feeding and infection rates in wild An. funestus progeny. The prevalence of infection in An. funestus mosquitoes was 38.52 % (range 6.25–100 %) and the median oocyst number was 12.5 (range 1–139). In parallel, the prevalence in An. coluzzii was 39.92 % (range 6.85–97.5 %), while the median oocyst number was 32.1 (range 1–351).

Conclusions

Overall, our observations are in line with the fact that both species are readily infected with P. falciparum, the most common and dangerous malaria parasite in sub-Saharan Africa, and since An. funestus is widespread throughout Africa, malaria vector control research and implementation needs to seriously address this vector species too. Additionally, the present work indicates that it is feasible to generate large number of wild F1 infected An. funestus mosquitoes using membrane feeding assays, which can be used for comprehensive study of interactions with the Plasmodium parasite.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-016-1626-y) contains supplementary material, which is available to authorized users.

A simple and affordable membrane-feeding method for Aedes aegpyti and Anopheles minimus (Diptera: Culicidae)

This study developed an artificial feeding (AF) method to replace direct host feeding (DHF) for the maintenance of Aedes aegypti and Anopheles minimus mosquito colonies. The procedure can be adopted by all laboratories due to its simple and affordable materials and design. The apparatus consists of heparinized cow blood contained in a 5cm diameter glass petri dish with 5cm(2) Parafilm M (Bemis(®)) stretched thinly over the top, with a pre-heated bag of vegetable oil placed underneath to keep the blood warm. Both parts are contained within an insulated Styrofoam™ box with a hole in the lid for mosquitoes to access the membrane. Mosquitoes are fed by AF for 15min at a time. Feeding rate and fecundity of Ae. aegypti mosquitoes feeding on the AF device were compared to those feeding on a live rat (DHF(r)), and of Anopheles minimus mosquitoes feeding on the AF device compared to those feeding on a human arm (DHF(h)). Aedes aegypti mosquitoes fed by AF or DHF(r) had similar feeding rates (38.2±21.5% and 35.7±18.2%, respectively) and overall egg production (1.5% difference). Anopheles minimus mosquitoes fed by the AF method had a lower feeding rate (52.0±1.0% for AF compared to 70.7±20.2% for DHF(h)) and overall egg production (40% reduction compared to DHF(h)). However, the number of eggs produced by AF-fed mosquitoes (1808 eggs per 100 mosquitoes) was still sufficient for colony maintenance, and with increased feeding time both parameters are expected to increase. Reduced feeding rate and overall egg production was observed when Ae. aegypti mosquitoes were fed on blood refrigerated for over two weeks. In conclusion, an AF device has been developed which can replace DHF for Ae. aegypti and An. minimus colony maintenance when using blood refrigerated for a maximum of two weeks.

Comparison of field-based xenodiagnosis and direct membrane feeding assays for evaluating host infectiousness to malaria vector Anopheles gambiae

Several techniques are currently being used to study host infectiousness to mosquitoes, including the experimental possibility of laboratory reared mosquitoes acquiring infections through membrane feeders or directly on host skin. Here, the relative performance of the laboratory-based membrane feeding method (DMFA) and the field-based xenodiagnosis (XD) of malaria infectious hosts using wild Anopheles mosquitoes were compared. A cross-sectional survey involving a sample of 70 children (aged 3-12 years) living in a malaria endemic area in Western Burkina Faso, was carried out to measure their infectiousness to Anopheles mosquitoes using two approaches. The first approach used the xenodiagnostic procedure in which children were exposed to mosquito bites overnight, being sleeping individually in different sentinel huts from 6 pm to 6 am (4 nights per child). Anopheles sp that had acquired blood-meal on each child were subsequently collected early in the morning, and examined for Plasmodium falciparum oocyst infection on day 7 post-feeding. In the second approach, the infectiousness of the same children was estimated by whole-blood membrane feeding procedure using F0 An. gambiae s.l. that emerged from field-collected larvae cohorts. In the DMFA, 41.4% of the children successfully infected at least one mosquito with the mean oocyst prevalence of only 4.6±1.1% in the 2171 mosquitoes that were examined (mean oocyst intensity: 2.0±(std error of mean) 0.3 oocysts per infected midgut). Comparatively 78.6% of children yielded oocysts infection in mosquitoes during the XD approach (Chi square=20.11, df=1; p<0.001), with a mean rate of 19.6±2.0 in the 3752 wild caught mosquitoes (mean intensity: 3.93±0.2 oocysts per infected mosquito). The DMFA failed to reveal a portion (n=26) of infectious individuals that were sharply evidenced by the XD, particularly at low gametocyte densities or at levels that could not be detected by the classical microscopic examination of blood smears. As opposed to the resource consuming DMFA, which is often mined by technical constraints, using the XD method could be an advantage in experimental investigations of host infectiousness in areas where anopheline species cannot be conveniently reared for the experimental studies. Ethical aspects of this approach, mainly related to exposure of the human subjects to potentially infectious mosquito bites are discussed.