Mosquito communities and disease risk influenced by land use change and seasonality in the Australian tropics

Background: Anthropogenic land use changes have contributed considerably to the rise of emerging and re-emerging mosquito-borne diseases. These diseases appear to be increasing as a result of the novel juxtapositions of habitats and species that can result in new interchanges of vectors, diseases and hosts. We studied whether the mosquito community structure varied between habitats and seasons and whether known disease vectors displayed habitat preferences in tropical Australia. Methods: Using CDC model 512 traps, adult mosquitoes were sampled across an anthropogenic disturbance gradient of grassland, rainforest edge and rainforest interior habitats, in both the wet and dry seasons. Nonmetric multidimensional scaling (NMS) ordinations were applied to examine major gradients in the composition of mosquito and vector communities. Results: We captured ~13,000 mosquitoes from 288 trap nights across four study sites. A community analysis identified 29 species from 7 genera. Even though mosquito abundance and richness were similar between the three habitats, the community composition varied significantly in response to habitat type. The mosquito community in rainforest interiors was distinctly different to the community in grasslands, whereas forest edges acted as an ecotone with shared communities from both forest interiors and grasslands. We found two community patterns that will influence disease risk at out study sites, first, that disease vectoring mosquito species occurred all year round. Secondly, that anthropogenic grasslands adjacent to rainforests may increase the probability of novel disease transmission through changes to the vector community on rainforest edges, as most disease transmitting species predominantly occurred in grasslands. Conclusion: Our results indicate that the strong influence of anthropogenic land use change on mosquito communities could have potential implications for pathogen transmission to humans and wildlife

Mosquito (Diptera: Culicidae) communities across land uses in tropical Australia

Mosquito-borne diseases cause mortality and morbidity worldwide. Diseases, such as malaria, dengue and yellow fever, have emerged or re-emerged in recent decades and are often induced by anthropogenic land use changes. Deforestation, road construction and conversions of land use (e.g. from natural environment to urbanisation or agriculture) are examples of human modified environments. These modifications alter original habitats and species compositions and, in relation to mosquito-borne diseases result in novel juxtapositions of vectors, hosts and pathogens. Hot spots for the emergence or re-emergence of mosquito-borne diseases are tropical regions due to high biodiversity, vast land clearances and human encroachment into these areas. Furthermore, remote, tropical regions are especially vulnerable to the emergence of mosquito-borne diseases as surveillance can be logistically demanding and expensive and thus prevent early disease detection. My thesis examined if mosquito communities respond to land use changes in tropical Australia and additionally, if a newly developed technique to capture adult mosquitoes can be applied in remote localities. I explored mosquito communities across anthropogenic disturbance gradients in several ways: (1) capturing adult mosquito from three habitats (man-made grassland, forest edge and rainforest interior) during the wet and dry season in a peri-urban environment near Cairns; (2) sampling immature mosquitoes from small, artificial containers from the same habitats; and (3) capturing adult mosquitoes from urban and sylvan habitats from four remote islands (Saibai, Boigu, Badu and Moa) in the Torres Strait where I used a novel trap design. To evaluate if mosquito communities are influenced by anthropogenic land use, I carried out adult mosquito sampling in three habitats (man-made grassland, rainforest edge and rainforest interior) in the outskirts of Cairns and in two habitats (sylvan and urban) in the Torres Strait. I found that adult mosquito communities varied in response to anthropogenic modified habitats in both locations. The Cairns sampling revealed that the mosquito community from rainforest interior was distinctly different to the grassland community and that forest edge acted as an ecotone with shared communities from both forest interior and grasslands. I also found that important vector species (Aedes vigilax, Culex annulirostris) were able to persist all year round and occurred mainly in grasslands (Chapter 2). A similar pattern was evident from the Torres Strait sampling where urban and sylvan habitats supported distinctly different mosquito communities with disease-competent species, such as Aedes albopictus, Aedes aegypti and Culex quinquefasciatus occurring more in urban areas than in sylvan habitats (Chapter 5). I sampled immature mosquitoes from small, artificial ovitraps across the three habitats in Cairns and from two trap locations (traps were either placed on the ground or above ground) to evaluate female oviposition preferences. I found that most species chose to lay their eggs in grassland traps and that none of the species preferred to oviposit in forests traps. I also found that traps located on the ground had four times more emergents than traps located above ground. Aedes notoscriptus, an important disease vector, was mostly reared from grassland traps. Additionally, I observed that water temperature (ranging between 13.7°C and 43.5°C) had no influence on the number of emergents and that mosquito eggs were able to hatch in instalments (Chapter 3). Mosquito sampling in remote areas poses unique challenges for disease surveillance and detection. Sampling female mosquitoes is heavily dependent on attractants to lure them into traps. Carbon-dioxide (CO₂), in the form of dry ice or from gas cylinders, is commonly used. However, dry ice is unavailable in remote areas and gas cylinders are difficult or even prohibited to transport. I therefore aimed to assess the usefulness of CO₂ derived from sugar and yeast as an attractant and trialled different CO₂ concentrations in temperature controlled experiments. The concentrations which produced the most CO₂ were then compared to dry ice in field situations (in three tropical forest habitats). I found that traps baited with dry ice captured more mosquitoes than yeast-baited traps; but more importantly that there were no differences in the mosquito community composition (Chapter 4). An additional challenge is that most mosquito traps require a source of electricity which is rarely obtainable in remote field locations. I developed a novel sampling technique to capture mosquitoes by coupling a non-powered trap with CO₂ derived from sugar/yeast fermentation (Chapter 5). Conventional disease detection methods involve sentinel animals, such as chickens or pigs, or large pools of dead mosquitoes which can be very expensive and labourintense in remote areas. A suitable alternative are honey-soaked Flinders Technology Associates cards (FTA® cards) which preserve viruses but at the same time deactivate them. Mosquitoes taking a honey-meal from the FTA® cards expel saliva which can be used for disease detection by eluting viral RNA. I used four FTA® cards in each of the non-powered traps in the Torres Strait and even though weak infections were initially detected, they were not significant (Chapter 5). In summary, my thesis demonstrates that mosquito communities in peri-urban environments and on remote islands in tropical Australia are strongly influenced by land use. This could have potential impacts for disease transmission to humans, domestic animals and wildlife, especially where immense anthropogenic pressures continue to change natural environments irrevocably. Strong projected growth in human population and the subsequent demand for space will further impact already fragile environments. My thesis may contribute to achieve a more cost-effective and logistically less demanding method to monitor mosquitoes in remote localities and thus allowing permanent and continuous disease surveillance

Land use influences mosquito communities and disease risk on remote tropical islands: a case study using a novel sampling technique

Land use changes, such as deforestation and urbanization, can influence interactions between vectors, hosts, and pathogens. The consequences may result in the appearance and rise of mosquito-borne diseases, especially in remote tropical regions. Tropical regions can be the hotspots for the emergence of diseases due to high biological diversity and complex species interactions. Furthermore, frontier areas are often haphazardly surveyed as a result of inadequate or expensive sampling techniques, which limit early detection and medical intervention. We trialed a novel sampling technique of nonpowered traps and a carbon dioxide attractant derived from yeast and sugar to explore how land use influences mosquito communities on four remote, tropical islands in the Australian Torres Strait. Using this technique, we collected > 11,000 mosquitoes from urban and sylvan habitats. We found that human land use significantly affected mosquito communities. Mosquito abundances and diversity were higher in sylvan habitats compared with urban areas, resulting in significantly different community compositions between the two habitats. An important outcome of our study was determining that there were greater numbers of disease-vectoring species associated with human habitations. On the basis of these findings, we believe that our novel sampling technique is a realistic tool for assessing mosquito communities in remote regions

Development and field evaluation of a system to collect mosquito excreta for the detection of arboviruses

Mosquito-borne diseases are a major public health concern globally and early detection of pathogens is critical to implement vector management and control strategies. Existing methods for pathogen detection include screening sentinel animals for antibodies and analyzing mosquitoes for pathogen presence. While these methods are effective, they are also expensive, labor-intense, and logistically challenging. To address these limitations, a new method was developed whereby mosquito saliva is collected on honey-coated nucleic acid preservation cards which are analyzed by molecular assays for detection of pathogens. However, mosquitoes only expel small amounts of saliva when feeding on these cards, potentially leading to false negatives. Another bodily fluid that is expelled by mosquitoes in larger volumes than saliva is excreta, and recent laboratory experiments have demonstrated that a range of mosquito-borne pathogens can be detected in mosquito excreta. In the current study, we have modified light and passive mosquito traps to collect their excreta and assessed their efficacy in field evaluations. From these field-collections, we detected West Nile, Ross River, and Murray Valley encephalitis viruses. Our findings suggest that mosquito traps are easily modified to collect excreta and, that this system has the potential to enhance detection of pathogens

Searching for the proverbial needle in a haystack: advances in mosquito-borne arbovirus surveillance

Abstract Surveillance is critical for the prevention and control of mosquito-borne arboviruses. Detection of elevated or emergent virus activity serves as a warning system to implement appropriate actions to reduce outbreaks. Traditionally, surveillance of arboviruses has relied on the detection of specific antibodies in sentinel animals and/or detection of viruses in pools of mosquitoes collected using a variety of sampling methods. These methods, although immensely useful, have limitations, including the need for a cold chain for sample transport, cross-reactivity between related viruses in serological assays, the requirement for specialized equipment or infrastructure, and overall expense. Advances have recently been made on developing new strategies for arbovirus surveillance. These strategies include sugar-based surveillance, whereby mosquitoes are collected in purpose-built traps and allowed to expectorate on nucleic acid preservation cards which are submitted for virus detection. New diagnostic approaches, such as next-generation sequencing, have the potential to expand the genetic information obtained from samples and aid in virus discovery. Here, we review the advancement of arbovirus surveillance systems over the past decade. Some of the novel approaches presented here have already been validated and are currently being integrated into surveillance programs. Other strategies are still at the experimental stage, and their feasibility in the field is yet to be evaluated