Do outbreaks of mosquito-borne disease always follow floods?

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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.

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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.

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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.

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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!

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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!

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.

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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.

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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.

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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.

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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)