Fatty acids in anopheline mosquito larvae and their habitats

Larvae of the three important Central American malaria vectors, Anopheles albimanus, An. vestitipennis, and An. darlingi, are found in distinctly different habitats broadly defined by hydrology and aquatic vegetation, but little is known about the actual food quality and quantity of these habitats. Polyunsaturated fatty acids (PUFA) are of special interest, because mosquitoes require 20:5ω3 (EPA), 20:4ω6 (ARA), and 22:6ω3 (DHA) and without an adequate supply of these PUFAs they are not able to complete their life cycle. We collected samples of larvae and their corresponding habitats and analyzed their fatty acid (FA) composition to reveal if there are any species‐specific and habitat‐specific differences in FA composition, and if habitat FA differences can be linked to differences in the mosquito FA pattern and, ultimately, mosquito performance. We also assessed how FA of wild larvae compare to the laboratory‐reared larvae. Habitats were generally low in essential PUFAs and there were no significant differences among the FA composition of habitat samples. There were significant differences in FA composition of larvae. An. darlingi contained significantly higher amounts of FA, specifically a higher content of ω‐6 PUFA, represented mainly by the linoleic acid (18:2ω‐6). Large differences were found between field‐collected and laboratory‐reared An. vestitipennis larvae, especially in the content of PUFAs. The laboratory‐reared larvae contained significantly more of the total FA, ω3 PUFA, and MUFA. The laboratory‐reared larvae contained three to five times more essential PUFAs, EPA, and DHA. However, there were no differences in the total dry weight of the 4^th instar larvae between the wild vs laboratory‐reared larvae. Total FA in both larvae and habitats of An. albimanus and An. darlingi were positively correlated with the concentration of particulate organic carbon and nitrogen (POC, PON) in their respective habitats, but no such correlation was found for An. vestitipennis. PUFA are a good indicator of nutritional quality, although factors controlling the success of anopheline development from larval habitats are likely to be more complex and would include the presence of predators, pathogens, and toxins as interacting factors.     

Insensitivity to the Spatial Repellent Action of Transfluthrin in Aedes aegypti: A Heritable Trait Associated with Decreased Insecticide Susceptibility

Background

New vector control paradigms expanding the use of spatial repellents are promising, but there are many gaps in our knowledge about how repellents work and how their long-term use might affect vector populations over time. Reported here are findings from a series of in vitro studies that investigated the plasticity and heritability of spatial repellent (SR) behaviors in Aedes aegypti exposed to airborne transfluthrin, including results that indicate a possible link between repellent insensitivity and insecticide resistance.

Methodology/principal findings

A dual-choice chamber system was used to observe directional flight behaviors in Aedes aegypti mosquitoes exposed to passively emanating transfluthrin vapors (1.35 mg/m³). Individual SR responder and SR non-responder mosquitoes were identified, collected and maintained separately according to their observed phenotype. Subsequent testing included re-evaluation of behavioral responses in some mosquito cohorts as well as testing the progeny of selectively bred responder and non-responder mosquito strains through nine generations. At baseline (F₀ generation), transfluthrin actively repelled mosquitoes in the assay system. F₀ mosquitoes repelled upon initial exposure to transfluthrin vapors were no more likely to be repelled again by subsequent exposure 24h later, but repelled mosquitoes allowed to rest for 48h were subsequently repelled at a higher proportion than was observed at baseline. Selective breeding of SR responders for nine generations did not change the proportion of mosquitoes repelled in any generation. However, selective breeding of SR non-responders did produce, after four generations, a strain of mosquitoes that was insensitive to the SR activity of transfluthrin. Compared to the SR responder strain, the SR insensitive strain also demonstrated decreased susceptibility to transfluthrin toxicity in CDC bottle bioassays and a higher frequency of the V1016I^kdr mutation.

Conclusions/significance

SR responses to volatile transfluthrin are complex behaviors with multiple determinants in Ae. aegypti. Results indicate a role for neurotoxic irritation of mosquitoes by sub-lethal doses of airborne chemical as a mechanism by which transfluthrin can produce SR behaviors in mosquitoes. Accordingly, how prolonged exposure to sub-lethal doses of volatile pyrethroids might impact insecticide resistance in natural vector populations, and how already resistant populations might respond to a given repellent in the field, are important considerations that warrant further monitoring and study. Results also highlight the critical need to develop new repellent active ingredients with novel mechanisms of action.     

Feldspar Tunneling by Fungi along Natural Productivity Gradients

Recently, it was hypothesized that ectomycorrhizas hyphae are involved in mineral tunneling. We evaluated the role of ectomycorrhizas in mineral weathering and the ecosystem influx of basic cations by correlating mineral tunnel density to ectomycorrhizas density in two forest productivity gradients. The gradients, two gentle slopes in northern Sweden, are the result of groundwater movement and are characterized by reduced productivity upslope due to lower nitrogen availability. As expected, ectomycorrhizas density in the O horizon was higher upslope, where nitrogen availability was lower and where the vegetation was dominated by ectomycorrhizas plant species. We consistently found that tunnel formation in mineral grains was more intense in nutrient-poor sites, indicating a higher contribution of fungi to ecosystem influx of potassium and calcium. ectomycorrhizas density was positively correlated with feldspar tunnel density in the upper 2 cm of the E horizon. This suggests that ectomycorrhizas are involved in mineral tunneling. We discuss the possible involvement of ericoid mycorrhizas and saprotrophic fungi in feldspar tunneling and the role of the weathering status of mineral grains as additional factors controlling mineral funneling.     

The effect of host type on movement patterns of Aedes aegypti (Diptera: Culicidae) into and out of experimental huts in Thailand.

Flight behavior studies were carried out from December 2004 through February 2005 at two sites in Thailand to compare the movement patterns of Aedes aegypti into and out of experimental huts baited with a human host, dog host, or without a host using a mark-release-recapture study design. Studies were conducted in isolated villages of Kanchanaburi and Chiang Mai Provinces, Thailand. In the presence of a human host only 4.9% (39/800) of the Ae. aegypti females departed the hut as compared to 46.5% (372/800) when a dog was present. There was no significant difference in the numbers of Ae. aegypti exiting when comparing dog to no host. A peak in exiting behavior in the absence of any host (human or dog) was observed between 1400-1700 h. Ingress behavior was much stronger when a human host was present in the hut with the peak of entering occurring in the morning (0830-1130 h) compared to 1000-1200 h without a host. Overall, significant differences between the two host types were observed with Ae. aegypti females being more attracted to humans (p < 0.05) than dogs. There was no significant difference between numbers of Ae. aegypti entering the hut baited with a dog and the hut containing no host source. The experimental hut design used in the present study can serve as a protocol for testing the exiting and entering behavior of Ae. aegypti in response to chemical compounds.     

Short-term and long-term alterations in neuronal excitability during injury-induced axonal regeneration in ganglia and cell culture

Helisoma neurons B5, regenerating axonal arbors following crush-axotomyin vivo, displayed a transient (<24 h) reduction in excitability followed by a sustained period of hyperexcitability (>13 d). Neurons isolated into outgrowth-permissive cell culture conditions expressed a similar pattern of hypo- and hyperexcitability; however, excitability of neurons B5 in culture was elevated for only 5 d and then declined to a lower, stable level. The expression of these alterations in excitability was neurite outgrowth-independent and required the presence of ganglia-derived conditioning factors in the culture medium. Excitability of neurons in medium lacking conditioning factors fell by day 3 to minimal levels. Conditioned medium was effective in rescuing excitability of neurons deprived of conditioning factors during their first 3 days of cell culture, but not following longer periods of deprivation. Exposure to the protein synthesis inhibitor, anisomycin, blocked the ability of conditioning factors to rescue B5 neuronal excitability. Together, results from cell culture suggest that mechanisms underlying neuronal excitability following nerve injury are independent of process outgrowth state, but require exposure to conditioning factors derived from injured neural tissue within several days of axonal insult. Although changes in B5 neurite outgrowth and neuronal excitability were expressed simultaneously following axonal injury, their independence suggests the existence of an underlying regenerative state that regulates both cellular modifications.     

Maps of the Sri Lanka malaria situation preceding the tsunami and key aspects to be considered in the emergency phase and beyond

In Vietnam, a large proportion of all malaria cases and deaths occurs in the central mountainous and forested part of the country. Indeed, forest malaria, despite intensive control activities, is still a major problem which raises several questions about its dynamics. A large-scale malaria morbidity survey to measure malaria endemicity and identify important risk factors was carried out in 43 villages situated in a forested area of Ninh Thuan province, south central Vietnam. Four thousand three hundred and six randomly selected individuals, aged 10–60 years, participated in the survey. Rag Lays (86%), traditionally living in the forest and practising "slash and burn" cultivation represented the most common ethnic group. The overall parasite rate was 13.3% (range [0–42.3] while Plasmodium falciparum seroprevalence was 25.5% (range [2.1–75.6]). Mapping of these two variables showed a patchy distribution, suggesting that risk factors other than remoteness and forest proximity modulated the human-vector interactions. This was confirmed by the results of the multivariate-adjusted analysis, showing that forest work was a significant risk factor for malaria infection, further increased by staying in the forest overnight (OR= 2.86; 95%CI [1.62; 5.07]). Rag Lays had a higher risk of malaria infection, which inversely related to education level and socio-economic status. Women were less at risk than men (OR = 0.71; 95%CI [0.59; 0.86]), a possible consequence of different behaviour. This study confirms that malaria endemicity is still relatively high in this area and that the dynamics of transmission is constantly modulated by the behaviour of both humans and vectors. A well-targeted intervention reducing the "vector/forest worker" interaction, based on long-lasting insecticidal material, could be appropriate in this environment.     

Contact Irritant Responses of Aedes aegypti Using Sublethal Concentration and Focal Application of Pyrethroid Chemicals

Background

Previous studies have demonstrated contact irritant and spatial repellent behaviors in Aedes aegypti following exposure to sublethal concentrations of chemicals. These sublethal actions are currently being evaluated in the development of a push-pull strategy for Ae. aegypti control. This study reports on mosquito escape responses after exposure to candidate chemicals for a contact irritant focused push-pull strategy using varying concentrations and focal application.

Methods

Contact irritancy (escape) behavior, knockdown and 24 hour mortality rates were quantified in populations of female Ae. aegypti under laboratory conditions and validated in the field (Thailand and Peru) using experimental huts. Evaluations were conducted using varying concentrations and treatment surface area coverage (SAC) of three pyrethroid insecticides: alphacypermethrin, lambacyhalothrin and deltamethrin.

Results

Under laboratory conditions, exposure of Ae. aegypti to alphacypermethrin using the standard field application rate (FAR) resulted in escape responses at 25% and 50% SAC that were comparable with escape responses at 100% SAC. Significant escape responses were also observed at <100% SAC using ½FAR of all test compounds. In most trials, KD and 24 hour mortality rates were higher in mosquitoes that did not escape than in those that escaped. In Thailand, field validation studies indicated an early time of exit (by four hours) and 40% increase in escape using ½FAR of alphacypermethrin at 75% SAC compared to a matched chemical-free control. In Peru, however, the maximum increase in Ae. aegypti escape from alphacypermethrin-treated huts was 11%.

Conclusions/Significance

Results presented here suggest a potential role for sublethal and focal application of contact irritant chemicals in an Ae. aegypti push-pull strategy to reduce human–vector contact inside treated homes. However, the impact of an increase in escape response on dengue virus transmission is currently unknown and will depend on rate of biting on human hosts prior to house exiting.