Why would a Californian drought trigger an outbreak of mosquito-borne disease?

CalifornianBushfireSunset_DawnEllnerMosquitoes need water almost as much as they need blood so why is it a drought could cause an outbreak of mosquito-borne disease? Why does the drought in California mean less water but more mosquito-borne disease?

More than just water

All mosquitoes need water. It could be a teaspoon of water in a pot plant base or an expanse of wetlands inundated by tides. Following flooding, health authorities are typically quick to issue public health warnings about increased risk of mosquito-borne disease. However, more mosquitoes doesn’t always mean more mosquito-borne disease.

Mosquitoes need blood. As well as biting people, they also bite animals. Outbreaks of mosquito-borne disease typically requires the presence of wildlife, animals that act as reservoirs for the disease-causing viruses.

CaliforniaDrought_EPA

Drought is hitting California hard (Source EPA via Huffington Post)

Mosquitoes, drought and West Nile virus

West Nile virus is a mosquito-borne pathogen generally spread between birds and people by mosquitoes. Culex mosquitoes they appear to play the most important role in West Nile virus transmission in urban environments, particularly Culex pipiens.

These mosquitoes are generally not breeding in wetlands. They’re found in artificial structures ranging from backyard containers and neglected swimming pools to stormwater pipes and drains. These mosquitoes have moved out of the swamps and into the suburbs! They’ve also moved into the constructed wetlands popping up throughout the suburbs too.

Rather than water birds associated with wetland environments, the birds playing a key role in West Nile virus transmission are small songbirds common in urban areas. These birds roost in large numbers and are the target the the Culex mosquitoes that preferentially feed on birds. It is important to keep in mind that there is still a lot of learn about how the roosting behaviour of birds influences their exposure to West Nile virus.

During “dry” conditions, bird populations are concentrated in urban areas (where humans provide water and food) and mosquito populations associated with urban water-holding structures increase. During “wet” summers, bird populations may be more widely dispersed through the environment with many birds roosting and foraging well away from residential areas and reducing the contact between birds, mosquitoes and people. When the “dry” summers arrive, birds move back close to the people. People who provide water.

Culex_molestus_Photo_StephenDoggett

The Culex pipiens group of mosquitoes play an important role in the transmission of West Nile virus and are closely associated with urban environments. They like biting birds. (Photo: Stephen Doggett, NSW Health Pathology)

In the absence of rain, water stagnates in stormwater pipes and drains providing favourable conditions for mosquitoes. During “wet” summers, the mosquitoes are flushed out by increased water flows and, even if they don’t, permanent habitats are more likely to support populations of mosquito predators such as fish.

During “dry” summers, people also start storing water around the home. Once water restrictions kick in, the desire to keep the garden looking healthy can potentially pose an indirect health risk to the homeowner as they hoard water around the home that provides habitat for mosquitoes.

In short, dry conditions help concentrate mosquitoes and birds in close proximity to people and increase the risk of mosquito-borne disease outbreaks.

WestNileSign

Mosquito control in Texas in response to an outbreak of West Nile virus raised much concern within the community. It can sometimes be difficult to balance the need for mosquito control with community engagement to allay fears of insecticide -based human health risks (Source: CDC)

An outbreak in Texas in 2012

West Nile virus was first introduced into North America in 1999. Despite rapidly spreading across the continent in the subsequent years, the numbers of outbreaks steadily declined and, to some extent, it fell of the radar as a serious public health concern. There was a resurgence of the disease in 2012 with an outbreak primarily focused in Texas.

There was a substantial increase in the number of cases compared to previous years with an unusually warm spring thought to have played an important role in driving the outbreak. Health authorities were warned that outbreaks of this nature may continue.

USDroughtMonitors_7April2015West Nile virus and the Californian drought

For the past couple of years, California has been hit with one of its worst droughts in decades. It is having widespread impacts and may also be increasing mosquito-borne disease. Californian authorities have been battling potential public health risks associated with mosquitoes on many fronts. There were record numbers of deaths due to West Nile virus disease in 2014 and exotic mosquitoes were detected. This included an Australian mosquito that was found in Los Angeles.

It is relatively early in California’s mosquito season but West Nile virus has already been detected. Health authorities are warning that another bad year for West Nile virus activity could be ahead despite the ongoing drought. There is already a suggestion that the severity of the current drought may be exacerbated by climate change and that climate change may be playing a role in future West Nile virus risk internationally.

There is little doubt that prolonged drought will impact Californian residents in many ways and an increased risk of mosquito-borne disease is just one of them. Fortunately, mosquito and vector control agencies in California work closely with local health authorities to monitoring mosquito and pathogen activity to provide warnings of increased risk. However, there is responsibility for everyone to ensure that the ways in which water is conserved around the home doesn’t increase the risks associated with mosquitoes.

potplantsaucer

If you’re worried about keeping your pot plants well watered but don’t want to provide a home for mosquitoes, fill the saucer with sand. It will keep the moisture in place but there is no “free water” for mozzies to use!

If you’re not able to “dump and drain” water holding containers, make sure that they’re covered to stop mosquitoes getting in or out. If you’ve got a swimming pool that’s neglected, start chlorinating it or release fish to eat through any mosquitoes. There are also a few mosquito control products that could be used, the most appropriate would probably be the insect growth regulator methoprene, it will stop mosquitoes emerging from the water holding container.

Why not share your tips on saving water around the home while not increasing opportunities for mosquitoes on Twitter?

The photo at the top of this post is taken by Dawn Ellner (see original photo here)

6th International Congress of the Society for Vector Ecology

Culex_molestus_Photo_StephenDoggettThis month, medical entomologists from across the globe will come together in California for the 6th International Congress of the Society for Vector Ecology. With thanks to a travel grant provided by the Bill and Melinda Gates Foundation, two of my (recently completed) PhD students will be attending and presenting work on the role of mosquitoes in urban environments and mosquito-borne disease risk.

The Society for Vector Ecology was established in 1968 to bring together individuals interested in the management of vector-borne disease. This includes professionals mostly involved in mosquito research, mosquito control and surveillance operations and communications. Every four years, the society holds a congress either in North America or Europe. The couple of congresses that I’ve attended have been fantastic and I’m greatly disappointed not to be able to attend this year’s meeting.

The 6th International Congress of the Society for Vector Ecology is being held 22-27 September in La Quinta, California, USA. You can have a look at a PDF of the program here.

Although I won’t be able to make it, two of my PhD students will be attending after being awarded travel grants by Bill and Melinda Gates Foundation. They will be presenting some of the work they completed as part of their PhD candidature.

The titles and abstracts of their presentations are below.

Understanding the ecological importance of mosquitoes to insectivorous bats and the implications for mosquito-borne disease management in coastal Australia

Leroy Gonsalves, Bradley Law, Cameron Webb, Vaughan Monamy and Brian Bicknell

Manangement of mosquito-borne disease risk in coastal Australia faces many challenges. Urbanisation is increasing the size and proximity of the community to productive mosquito habitats. Coastal wetlands are also the focus of conservation and rehabilitation efforts. Mosquitoes associated with these wetlands, in particular the saltmarsh mosquito, Aedes vigilax, are abundant, widely dispersing and key vectors of Ross River and Barmah Forest viruses. These mosquitoes may also represent an abundant prey resource for threatened and endangered insectivorous bat species and local authorities are reluctant to approve broadscale mosquito control programs due to concerns regarding indirect impacts on local bat populations. A combination of diet analysis, radio-tracking and prey abundance studies were undertaken. Analysing prey DNA within guano collected from 52 individuals representing five local bat species demonstrated that bats consumed a diverse range of prey dominated by lepidopterans. Consumption of Ae. vigilax was restricted to two species, Vespadelus pumilus and V. vulturnus. Radiotracking of 13 V. vulturnus individuals during periods of relatively large and small population abundances of Ae. vigilax, together with monitoring of prey abundance, revealed that foraging ranges of bats shifted in response to mosquito abundance (and no other prey). These findings suggest that there are species-specific relationships between bats and mosquitoes and that there may be site-specific strategies required to balance mosquito management and bat conservation.

The biology, distribution and genetics of Culex molestus in Australia?

Nur Faeza A Kassim, Cameron E Webb & Richard C Russell

The Culex pipiens subgroup of mosquitoes includes some of the most important vector species involved in mosquito-borne disease transmission internationally and four species within this subgroup are found in Australia. One of these species, Culex molestus, is thought to have been introduced into Australia in the 1940s. Closely associated with subterranean urban habitats, this mosquito has the potential to cause serious nuisance biting impacts but also may cause significant public health risks through the transmission of endemic arboviruses. Exotic pathogens, such as West Nile virus, may also pose a potential threat to biosecurity of Australia. Our review of the literature has confirmed that the current Australian distribution of Cx. molestus is limited to areas south of latitude -28.17ºS. However, given that the mosquito is established in habitats south of the corresponding zone in the northern hemisphere, there is potential for Cx. molestus to spread north into QLD and NT. Molecular analysis of the mosquito indicated that Australian Cx. molestus shared stronger genetic similarity with specimens from Asia than specimens from Europe or North America. Laboratory and field studies have shown that the mosquito is uniquely adapted to urban environments through the expression of autogeny (ability to lay their first batch of eggs without a blood meal) and stenogamy (ability to mate in confined spaces). Culex molestus is active throughout the year and the current trend towards increased water storage in urban areas of Australia has raised concerns of increased nuisance-biting and public health risks in the future. However, the results of our studies indicate that there may be biological and ecological barriers that may lessen the importance of this mosquito in urban mosquito-borne disease cycles. A delay in blood feeding resulting from their obligatory autogeny, combined with limited access to potential reservoir hosts, may reduce the likelihood of them playing a significant role in pathogen transmission.