Do outbreaks of mosquito-borne disease always follow floods?

Water, water everywhere…and mosquitoes soon to follow. It makes sense that with more water you’ll get more mosquitoes and with more mosquitoes you’ll get more mosquito-borne disease. Right? Well, not always.

With floods hitting parts of inland NSW, health authorities have issued warnings about mosquitoes and mosquito-borne disease.

Western NSW, has been substantially impacted by flooding this month and the region has been declared a natural disaster zone. The Lachlan River at Forbes has reached a level not seen for 25 years. There is a lot of water about. 35,000 mega litres of water has also been released from Wyangala dam resulting in further flooding. There could be more to come as “Superstorm 2016” continues to bring rain to south-east Australia. Evacuations continue.

The flooding has come at a time when the weather in warming up and there are already reports of mosquito numbers increasing. The biggest concern is that once the flood water recede, how long will pools of water remain, have mosquitoes got a “jump start”on the season?

On the other side of the world, Hurricane Matthew is threatening Florida. The Bahamas and Haiti have already been hit and more than 2 million people in the US have been told to evacuate their homes. Flooding is expected.

Mosquitoes need water

There is no doubt that mosquito populations can increase rapidly following flood. There is even a group of mosquitoes commonly called “floodwater mosquitoes“. The desiccation resistant eggs of these mosquitoes are laying dormant in the cracks and crevices of flood plains, just waiting for the water to arrive. When it floods, the eggs hatch and in about a week or so, swarms of mosquitoes emerge.

For the most part, it isn’t immediately following the flooding, but in the weeks and even months following that can provide the most ideal conditions for mosquitoes. If temperatures aren’t high enough to drive rapid evaporation of ponding (or if additional rainfall keeps them topped up), mosquitoes can start building impressing population abundances. With more mosquitoes, the risk of mosquito-borne disease outbreak can increase.

Rainfall records provided by the Bureau of Meteorology indicate that over the three months to September 2016, some regions of NT, QLD, NSW and Victoria received some of their highest rainfall on record for the period. (Bureau of Meterology)

A look back to floods and mosquito surveillance

In 2011-2012, QLD, NSW and Victoria saw incredible flooding. For those of us working in the field of mosquito-borne disease, we’re well aware of what that flooding can cause. Our attention was sparked when stories starting coming out from locals about this being the biggest flooding since the 1970s. Why was this important? Following flooding in the 1970s, we saw one of the biggest outbreaks of the potentially fatal Murray Valley encephalitis virus Australia has seen. This outbreak, and the response to the actual and potential health impacts, was essentially the genesis of many mosquito-borne disease surveillance programs across the country.

One of those programs was the NSW Arbovirus Surveillance and Mosquito Monitoring Program. Following the flooding in early 2012, there was a huge jump in mosquito populations in western NSW and one of the largest collections of mosquitoes in the history of the program was recorded with over 18,000 mosquitoes collected! Fortunately, we didn’t see any substantial activity of Muray Valley encephalitis virus but elsewhere in Australia, cases were reported.

Mosquito-borne disease outbreaks need more than just mosquitoes

There is little doubt you need mosquitoes about for pathogen transmission. However, for many mosquito-borne diseases, the pathogens that cause the illness in people are naturally found in wildlife. Person to person transmission may occur but for pathogens such as West Nile virus, Ross River virus or Murray Valley encephalitis virus, the mosquitoes that inject their virus-filled saliva into people have bitten birds or mammals previously.

The role of wildlife is important to consider as the flooding may influence mosquito populations but they can also influence wildlife. While kangaroos and wallabies may be adversely impacted by floods, flood waters can provide a major boost for waterbirds.

In some instances, as is the case for Murray Valley encephalitis virus, floods provide ideal conditions for both mosquitoes and birds!

Do floods really cause outbreaks of mosquito-borne disease?

There are few studies that have demonstrated that outbreaks of mosquito-borne disease always occur following floods.

Studies in North America had previously concluded that there wasn’t a direct link between hurricanes and flooding and mosquito-borne disease. But, that doesn’t mean there won’t potentially be a boost in nuisance-biting mosquitoes following flooding.There is often widespread spraying to control these pest mosquito populations.

Interestingly, after Hurricane Katrina hit New Orleans in 2005, there was an increase in mosquito-borne disease with more than a 2-fold increase in West Nile neuroinvasive disease. However, other reports noted no significant increase in cases of either West Nile or St. Louis encephalitis viruses. Surveillance for 6 weeks following the hurricane, authorities found no arboviruses circulating in local mosquito populations. These results highlight that much more than water and mosquitoes are required for outbreaks of disease.

In Australia, a recent review looked at the influence of flooding on cases of Ross River virus disease. They found that the evidence to support a positive association between flooding and RRV outbreaks is largely circumstantial. The trouble in predicting outbreaks of Ross River virus disease is that there can be complex biological, environmental and climatic drivers at work and, irrespective of local flooding, there may be other region-specific issues that either increase or decrease the potential for an outbreak.

What should we expect in Australia as summer approaches?

There is no doubt mosquito repellent will come in handy over the coming months. There are already reports of increased mosquito populations in some parts of the country. While nuisance-biting impacts will be a worry, if mosquito populations further increase following flooding, authorities need to remain mindful of a range of other health risks too.

The good news is that unless higher than normal mosquito populations persist into the warmer months, we may not see major outbreaks of disease. It typically isn’t until November-December that we start to see pathogens circulate more widely among wildlife and mosquitoes. Hopefully, if some hot weather arrives, the flood waters will quickly evaporate and abundant mosquitoes populations won’t continue.

Current outlooks suggest that between now and December 2016, south-eastern regions of Australia are likely to receive above average rainfall. Temperatures, though, are likely to be a little cooler than normal. We’re probably lucky that this cooler weather will keep the really big mosquito population increases that we saw a few years ago at bay.

On balance, we’re expecting plenty of mosquitoes to be about as summer starts, hopefully not “mozziegeddon” but enough to ensure the community should stay aware of the health risks associated with mosquito bites and how best to avoid their bites.

Have you seen mosquitoes about already this season? Join the conversation and tweet some shots of local mosquitoes!

The buzz of bat conservation

Illustration by Golly Bard (Check out some of the other wonderful illustrations of plants and animals by Golly Bard at her Etsy site)

What is the ecological role of mosquitoes?

There is a huge gap in our knowledge of mosquitoes and their place in the world. There are very few studies that have investigated what environmental “good’ they may do. This would actually be a pretty handy thing to find out. A better understanding of their ecological role may assist balancing the objectives of mosquito control and wetland/wildlife conservation that all too often appears to be (or is perceived to be) in conflict. The results of our newly published study in PLoS ONE may help in understanding the ecological role of one mosquito species in coastal wetlands.

There have been plenty of studies that have documented mosquitoes in the diet of a range of animals such as predatory aquatic macroinvertebrates, fish, frogs, lizards, birds and bats. However, the importance of mosquitoes in the diet of these animals either hasn’t been quantified or can often be overstated. One example often cited is that tadpoles eat mosquito larvae. When we tested some common Australian tadpoles, results demonstrated that they rarely consume mosquito larvae. We need to build our understanding of how important mosquitoes are (or perhaps are not) to local ecosystems.

There are almost 80 species of bat in Australia. While fruit bats may be the most commonly encountered, insectivorous bats are a less well known but diverse group of bats. These often tiny bats (often weighing less than 10g) are usually found in small and far less conspicuous groups. Many of them are found in close proximity to humans and many even be present in in major cities like Sydney. Many are listed as threatened or endangered.

The management of these bat species faces many challenges. Notwithstanding the threat of urbanisation and direct impacts on roost and foraging habitats, indirect impacts may also be important. One potential prey in the diet of bats that may be regionally important is the mosquito. Could broadscale mosquito control programs impact local bat populations?

Saltmarshes have already been identified as important habitats for the saltmarsh mosquito, Aedes vigilax. The abundance of these mosquitoes can dramatically increase in response to tidal flooding of coastal wetlands and nuisance-biting impacts can be widespread in the local community as these species disperse widely from local wetlands. This mosquito is also the major vector of Ross River virus in coastal Australia. There are about 5,000 cases of human illness caused by Ross River virus each year across Australia.

An example of saltmarsh habitats in Empire Bay, Central Coast, NSW

Mosquito-borne disease is an ongoing concern in many parts of Australia, particularly coastal regions. There is a range of environmentally sensitive mosquito control strategies available that have been demonstrated to provide effective mosquito control without directly adversely impacting the local environment. However, studies have shown that it is preemptive, not reactive, mosquito control efforts that have a significant impact on reducing mosquito-borne disease risk. Some concerns have been expressed by local authorities regarding the potential indirect non-target impacts on insectivorous bats of reducing local mosquito abundance.

In 2007, a multidisciplinary research project, funded by a research grant from the NSW Environmental Trust, kicked off to investigate the importance of saltmarsh mosquitoes to local bat populations on the Central Coast of NSW. The 4 year project included researchers from the Australian Catholic University, University of Sydney and Forest Science Centre (NSW Department of Primary Industries). The hard work in this project was done by PhD candidate Leroy Gonsalves (Dr Gonsalves now!).

Leroy Gonsalves (Photo ACU Media)

The project was undertaken in a region where mosquitoes had been identified as both a nuisance-biting problem and potential public health risks but local authorities were reluctant to undertake broadscale mosquito control. For full details, see the Living with Mosquitoes document I produced for local councils in the region.

Our previously published studies analysing bat call recordings have shown that the activity of insectivorous bats can be influenced by the abundance of prey as well as the structure of local habitats. Bats use sound for the detection and capture of prey, as well as navigation, and, as calls are species-specific, an analysis of recordings can determine the bat species present in local habitats and what they’re doing there.

In this newly published paper, we radio-tracked Vespadelus vulturnus (little forest bat), a species known to be a predator of saltmarsh mosquitoes. The activity of these radiotracked bats could be monitored across a range of habitats (including saltmarsh and adjacent coastal swamp forest) when mosquito populations were at naturally relatively high and low abundances.

Bats were collected using harp nets set along flyways in the coastal swamp forest close to the estuarine wetlands. The collected bats were then fitted with small radio-transmitters and released. The activity of each bat was tracked over a period of up to 10 days whereby triangulating the signal direction could record where the bats were active, as well as allowing the identification of day roost sites. This data was then analysed together with prey (mosquito and moth) abundance data recorded using light and carbon dioxide baited traps.

An example of a harp net set to collect microbats (Photo Doug Beckers)

The results were interesting.

When mosquito abundance was high, the saltmarsh was selected preferentially for foraging instead of the coastal swamp forest. However, at times of low mosquito abundance, the bats spread their activity across a range of habitats. The results suggest that this species of bat may be moving into habitats of high mosquito abundance to feed.

Little Forest Bat (Vespadelus vulturnus) (Photo: Melbourne Museum)

This is the first radio-tracking study to demonstrate a shift in habitat use by an insectivorous bat species in association with fluctuations in the abundance and distribution of a particular prey population. The shift in habitat use by the little forest bat suggests that, for this species, the saltmarsh mosquito may be an important food item (at least at times when mozzies are abundant).

Does this mean that broadscale mosquito control activities should stop? There is no doubt that the results of our research indicates that the little forest bat moves into areas where the saltmarsh mosquito is abundant. However, whether reducing the abundance of mosquitoes will have ecological implications is yet to be shown. The bats in our study site certainly weren’t eating enough to reduce nuisance-biting impacts of the mozzies!

The Saltmarsh Mosquito (Aedes vigilax) (Photo: Stephen Doggett)

The implications for wetland management are likely to change from location to location. In areas where there is abundant alternative prey such as moths, or mosquitoes not targeted by control activity, are present, reducing populations of key pest species like the saltmarsh mosquito may be achieved without any significant ecological impact. It is important to note that there is no evidence from our studies that the little forest bat is targetting the saltmarsh mosquito specifically. That mosquito was most abundant in this local area but in some regions along the coast, there are equally abundant mosquitoes (e.g. mosquitoes associated with freshwater or brackish water environments), many of which are not the focus of control efforts.

Perhaps an adaptive management approach is required whereby careful monitoring of bat populations is recommended and that, in areas where local bat populations may be shown to be susceptible, avoid control activities during the lactation period of bats, a time when their energetic demands are greatest. As these periods generally fall in late spring and early summer, as opposed to peak periods of  mosquito-borne disease risk that generally fall in late summer and early autumn, an integrated approach to management of both public health and environmental health can be achieved.

One of the key findings, with implications for further research directions and mosquito management, is that species-specific studies are required to understand the ecological role of mosquitoes. This result indicates that the importance of local mosquitoes for insectivorous bats is likely to be specific species (perhaps both the bat species and mosquito species), as well as no doubt varying from region to region. Managing not just the habitats but the insects associated with them may be important. Maintaining suitable habitats for bats under the pressures of expanding urbanisation along much of coastal Australia is critical. It will take more than just retaining these remnant habitats, it will be the strategies used to manage these wetlands and surrounding habitats that will also be important.

You can read the media release regarding this project and publication at the University of Sydney news page.

The full citation for our new paper is: Gonsalves L, Law B, Webb C, Monamy V (2013) Foraging Ranges of Insectivorous Bats Shift Relative to Changes in Mosquito Abundance. PLoS ONE 8(5): e64081. doi:10.1371/journal.pone.0064081