Selection of biting sites by mosquitoes

The behaviour of adult Anopheles gambiae, sub-Saharan Africa's principal malaria vector, and its relevance to malaria control: a review

BACKGROUND

Mosquitoes of the Anopheles gambiae complex are one of the major vectors of malaria in sub-Saharan Africa. Their ability to transmit this disease of major public health importance is dependent on their abundance, biting behaviour, susceptibility and their ability to survive long enough to transmit malaria parasites. A deeper understanding of this behaviour can be exploited for improving vector surveillance and malaria control.

FINDINGS

Adult mosquitoes emerge from aquatic habitats at dusk. After a 24 h teneral period, in which the cuticle hardens and the adult matures, they may disperse at random and search upwind for a mate or to feed. Mating generally takes place at dusk in swarms that form over species-specific 'markers'. Well-nourished females may mate before blood-feeding, but the reverse is true for poorly-nourished insects. Females are monogamous and only mate once whilst males, that only feed on nectar, swarm nightly and can potentially mate up to four times. Females are able to locate hosts by following their carbon dioxide and odour gradients. When in close proximity to the host, visual cues, temperature and relative humidity are also used. Most blood-feeding occurs at night, indoors, with mosquitoes entering houses mainly through gaps between the roof and the walls. With the exception of the first feed, females are gonotrophically concordant and a blood meal gives rise to a complete egg batch. Egg development takes two or three days depending on temperature. Gravid females leave their resting sites at dusk. They are attracted by water gradients and volatile chemicals that provide a suitable aquatic habitat in which to lay their eggs.

CONCLUSION

Whilst traditional interventions, using insecticides, target mosquitoes indoors, additional protection can be achieved using spatial repellents outdoors, attractant traps or house modifications to prevent mosquito entry. Future research on the variability of species-specific behaviour, movement of mosquitoes across the landscape, the importance of light and vision, reproductive barriers to gene flow, male mosquito behaviour and evolutionary changes in mosquito behaviour could lead to an improvement in malaria surveillance and better methods of control reducing the current over-reliance on the indoor application of insecticides.

Landing mosquitoes bounce when engaging a substrate

In this experimental study we film the landings of Aedes aegypti mosquitoes to characterize landing behaviors and kinetics, limitations, and the passive physiological mechanics they employ to land on a vertical surface. A typical landing involves 1-2 bounces, reducing inbound momentum by more than half before the mosquito firmly attaches to a surface. Mosquitoes initially approach landing surfaces at 0.1-0.6 m/s, decelerating to zero velocity in approximately 5 ms at accelerations as high as 5.5 gravities. Unlike Dipteran relatives, mosquitoes do not visibly prepare for landing with leg adjustments or body pitching. Instead mosquitoes rely on damping by deforming two forelimbs and buckling of the proboscis, which also serves to distribute the impact force, lessening the potential of detection by a mammalian host. The rebound response of a landing mosquito is well-characterized by a passive mass-spring-damper model which permits the calculation of force across impact velocity. The landing force of the average mosquito in our study is approximately 40 [Formula: see text]N corresponding to an impact velocity of 0.24 m/s. The substrate contact velocity which produces a force perceptible to humans, 0.42 m/s, is above 85% of experimentally observed landing speeds.

Flight behaviour of malaria mosquitoes around odour-baited traps: capture and escape dynamics

Host-seeking mosquitoes rely on a range of sensory cues to find and approach blood hosts, as well as to avoid host detection. By using odour blends and visual cues that attract anthropophilic mosquitoes, odour-baited traps have been developed to monitor and control human pathogen-transmitting vectors. Although long-range attraction of such traps has already been studied thoroughly, close-range response of mosquitoes to these traps has been largely ignored. Here, we studied the flight behaviour of female malaria mosquitoes (Anopheles coluzzii) in the immediate vicinity of a commercially available odour-baited trap, positioned in a hanging and standing orientation. By analysing more than 2500 three-dimensional flight tracks, we elucidated how mosquitoes reacted to the trap, and how this led to capture. The measured flight dynamics revealed two distinct stereotypical behaviours: (i) mosquitoes that approached a trap tended to simultaneously fly downward towards the ground; (ii) mosquitoes that came close to a trap changed their flight direction by rapidly accelerating upward. The combination of these behaviours led to strikingly different flight patterns and capture dynamics, resulting in contrasting short-range attractiveness and capture mechanism of the oppositely oriented traps. These new insights may help in improving odour-baited traps, and consequently their contribution in global vector control strategies.

The location of Australian Buruli ulcer lesions-Implications for unravelling disease transmission

Background

Buruli ulcer (BU), caused by Mycobacterium ulcerans, is increasing in incidence in Victoria, Australia. To improve understanding of disease transmission, we aimed to map the location of BU lesions on the human body.

Methods

Using notification data and clinical records review, we conducted a retrospective observational study of patients diagnosed with BU in Victoria from 1998–2015. We created electronic density maps of lesion locations using spatial analysis software and compared lesion distribution by age, gender, presence of multiple lesions and month of infection.

Findings

We examined 579 patients with 649 lesions; 32 (5.5%) patients had multiple lesions. Lesions were predominantly located on lower (70.0%) and upper (27.1%) limbs, and showed a non-random distribution with strong predilection for the ankles, elbows and calves. When stratified by gender, upper limb lesions were more common (OR 1·97, 95% CI 1·38–2·82, p<0·001) while lower limb lesions were less common in men than in women (OR 0·48, 95% CI 0·34–0·68, p<0·001). Patients aged ≥ 65 years (OR 3·13, 95% CI 1·52–6·43, p = 0·001) and those with a lesion on the ankle (OR 2·49, 95% CI 1·14–5·43, p = 0·02) were more likely to have multiple lesions. Most infections (71.3%) were likely acquired in the warmer 6 months of the year.

Interpretation

Comparison with published work in Cameroon, Africa, showed similar lesion distribution and suggests the mode of M. ulcerans transmission may be the same across the globe. Our findings also aid clinical diagnosis and provide quantitative background information for further research investigating disease transmission.

Buruli ulcer is an emerging tropical disease that is also increasingly common in the temperate Australian state of Victoria. The mode of transmission of this geographically restricted infection remains elusive. We have accurately mapped the location of 649 PCR-confirmed Buruli lesions affecting 579 patients and displayed their position on front and back human body diagrams. Lesion distribution density was assessed with computer-generated heat-maps. Buruli lesion distribution was most common on exposed parts of the body (distal limbs). However, even on exposed areas, lesion distribution was highly unevenly distributed and focused towards ankles, backs of calves and elbows. The palmar and plantar surfaces of hands and feet were rarely affected. We propose that targeting behavior by biting insects rather than direct contact with a contaminated environment best explains the lesion distribution we observed.

Geographic distribution, age pattern and sites of lesions in a cohort of Buruli ulcer patients from the Mapé Basin of Cameroon

Buruli ulcer (BU), a neglected tropical disease of the skin, caused by Mycobacterium ulcerans, occurs most frequently in children in West Africa. Risk factors for BU include proximity to slow flowing water, poor wound care and not wearing protective clothing. Man-made alterations of the environment have been suggested to lead to increased BU incidence. M. ulcerans DNA has been detected in the environment, water bugs and recently also in mosquitoes. Despite these findings, the mode of transmission of BU remains poorly understood and both transmission by insects or direct inoculation from contaminated environment have been suggested. Here, we investigated the BU epidemiology in the Mapé basin of Cameroon where the damming of the Mapé River since 1988 is believed to have increased the incidence of BU. Through a house-by-house survey in spring 2010, which also examined the local population for leprosy and yaws, and continued surveillance thereafter, we identified, till June 2012, altogether 88 RT-PCR positive cases of BU. We found that the age adjusted cumulative incidence of BU was highest in young teenagers and in individuals above the age of 50 and that very young children (<5) were underrepresented among cases. BU lesions clustered around the ankles and at the back of the elbows. This pattern neither matches any of the published mosquito biting site patterns, nor the published distribution of small skin injuries in children, where lesions on the knees are much more frequent. The option of multiple modes of transmission should thus be considered. Analyzing the geographic distribution of cases in the Mapé Dam area revealed a closer association with the Mbam River than with the artificial lake.

Buruli ulcer (BU) is an infectious disease caused by Mycobacterium ulcerans that is affecting mostly children in endemic areas of West Africa. Proximity to slow flowing water is a risk factor, but the exact mode of transmission of BU remains unclear. Man-made environmental changes, such as sand mining, damming of rivers and irrigation have been implicated with increases in disease incidence. Here, we report findings from a survey for BU and continued case detection thereafter in the Bankim Health District of Cameroon. In this area, the local population believed that the damming of the Mapé River has led to the emergence of BU. In 28 months we identified 88 laboratory confirmed cases of BU. Studying these cases, we found that the age adjusted cumulative incidence of BU in the elderly is similar to that in children and that the distribution pattern of BU lesions neither matches mosquito biting patterns nor the distribution of small skin injuries. Multiple modes of transmission should therefore be considered. Our data further showed that the patients appear to have closer contact to the local Mbam River than to the artificial Mapé dam reservoir.

New protective battle-dress impregnated against mosquito vector bites

BACKGROUND

Mixing repellent and organophosphate (OP) insecticides to better control pyrethroid resistant mosquito vectors is a promising strategy developed for bed net impregnation. Here, we investigated the opportunity to adapt this strategy to personal protection in the form of impregnated clothes.

METHODS

We compared standard permethrin impregnated uniforms with uniforms manually impregnated with the repellent KBR3023 alone and in combination with an organophosphate, Pirimiphos-Methyl (PM). Tests were carried out with Aedes aegypti, the dengue fever vector, at dusk in experimental huts.

RESULTS

Results showed that the personal protection provided by repellent KBR3023-impregnated uniforms is equal to permethrin treated uniforms and that KBR3023/PM-impregnated uniforms are more protective.

CONCLUSION

The use of repellents alone or combined with OP on clothes could be promising for personal protection of military troops and travellers if residual activity of the repellents is extended and safety is verified.

Visual and olfactory responses of haematophagous Diptera to host stimuli

Key biotic and environmental constraints on the host-orientated behaviour of haematophagous Diptera are summarized. For each major group of biting Diptera, responses to host stimuli are reviewed, including activation and ranging behaviour, long-range and short-range olfactory responses and visual responses. Limitations to the comparison of results between groups of species, and the practical problems of experimental method and equipment are discussed.