West Aussies versus the local mozzies

This is a special guest post from Dr Abbey Potter, Senior Scientific Officer, Environmental Health Hazards, WA Health. I’m currently mentoring Abbey as part of The Public Health Advocacy Institute of WA (PHAIWA) Mentoring Program. Its been a great experience as we navigate through some of the strategies to raise awareness of mosquito-borne disease and advocate for better approaches to addressing the public health risks associated with mosquitoes.

Living in WA, we’re all too familiar with the pesky mosquito. We know they bite but what we often don’t consider is that they can transmit serious and sometimes deadly diseases. In fact, a recent survey of locals indicated that knowledge of mosquito-borne disease is pretty limited, particularly among younger adults aged 18-34 years and those living in the Perth Metro. It’s pretty important we’re aware of the risks posed by these pint-sized blood suckers and how you can avoid them… and here’s why!

The Facts

On average, more than 1,000 people will be infected with a mosquito-borne disease in WA every year. Our mossies can transmit Ross River virus, Barmah Forest virus, West Nile virus (Kunjin substrain) and Murray Valley encephalitis virus. All four cause diseases that are debilitating at best, causing weeks to months of symptoms. Murray Valley encephalitis is limited to the north of the State but is so serious it can result in seizures, coma, brain damage and even death.

Forget the bush, most people bitten in their own backyard. West Aussies are all very prone to getting eaten alive while socialising outdoors but if you’re up in the north of the State, you’ve also got a much higher likelihood of being bitten while boating, camping or fishing or working outside, compared to the rest of the state.

And don’t think you’re off the hook when you head off on holidays. A further 500 WA residents return from overseas travel with an exotic mosquito-borne disease every year. Heading to Bali? Beware of dengue, especially young adult males who return home with the illness more than others. There is limited mosquito management in many overseas countries where disease-transmitting mozzies can bite aggressively both indoors and throughout the day. This catches West Aussies off guard, as we are accustomed to mozzies biting outdoors, around dusk and dawn. When you’re in holiday mode it’s likely that you’ll be relaxing, having a couple of drinks and not thinking about applying repellent. Oddly enough, mosquitoes may actually be more attracted to people whose body temperature is higher. This happens naturally when you consume alcohol, so best pull out the repellent before you crack your first beer.

Despite our attractiveness to mosquitoes, we aren’t really aware of the most effective ways to avoid bites or how we can do our bit to reduce breeding in our own backyards. If you live by the mantra Cover Up. Repel. Clean Up you’ll have no problems!

Western Australia has some amazingly beautiful wetlands but these saltmarshes around Mandurah can produce large populations of nuisance-biting mosquitoes!

Cover Up

If you know you are going to be outdoors when mosquitoes are active, wear loose, long-fitting clothing that is light in colour. Believe it or not, mosquitoes can bite through tight pants as tough as jeans – I’ve witnessed it!

If you’re staying in accommodation that isn’t mosquito-proof, consider bed netting.

Try to keep children indoors when mosquitoes are most active. If exposure can’t be avoided, dress them appropriately and cover their feet with socks and shoes. Pram netting can also be really useful.

Admittedly, it’s not always practical to wear long sleeves during our warm summer nights, so there are going to be times when you need to use repellent. Choose a product that actually works and apply it appropriately so it does the job. Despite our best intentions, this is where we often go wrong. There are a few basic things to cover here, so stick with it!

Ingredient: Science tells us that the best active ingredient for repelling mosquitoes is diethyltoluamide (DEET for short) or picaridin. You need to look for either one of these names on the repellent label under the ‘active constituents’ section.

Unfortunately, natural repellents and anything wearable (e.g. bands, bracelets or patches) have very limited efficacy. Experts don’t recommend you use them and I consider this very wise advice. It only takes a single mosquito bite to become infected and chances are you will receive at least one if you rely solely on a product of this nature. It just isn’t worth the risk.

Percentage: The next thing to consider is the percentage of the active ingredient. This can range anywhere from 7% to 80% which can make choosing a repellent confusing. Just remember, the higher the percentage, the LONGER the product will remain active for. It doesn’t mean it will repel mosquitoes better.

A repellent containing 16-20% DEET will provide around 4-6 hours of protection, and is a good place to start. Repellents labelled ‘tropical strength’ usually contain greater than 20% DEET – they are useful when you spend longer periods exposed to mosquitoes or if you are heading to a region where dengue, malaria or Zika is problematic. Kids repellents usually contain picaridin or <10% DEET.

Sometimes it can be tricky to work out the percentage of the active ingredient. You can see the Bushmans example below states this clearly, but the other bottles list the ingredient in grams per litre (g/L). No need for complex maths – just divide by 10 and you have the magic number! For example, the RID label below reports the product contains 160g/L of DEET. This would convert to 16% DEET – easy!

You can see a few examples here of effective repellents:

How to Apply: No doubt we would all prefer if repellents didn’t feel quite so gross on our skin or didn’t smell so bad. Even I have to admit that before I moved into this field, I was guilty of putting just a dab here and a dab there. Unfortunately, this is flawed logic that will only result in you being bitten!

Repellents must be applied correctly to be effective. That means reading the label and applying it evenly to all areas of exposed skin. Remember to reapply the product if you are exposed to mosquitoes for longer than the repellent protects you for. You’ll also have to reapply the repellent after sweaty activity or swimming.

For more information on repellent use in adults and children, click here.

Clean Up

Mosquitoes need water to breed, but only a very small amount. Water commonly collects in a range of things you may find in your backyard including pot plant drip trays, toys, old tyres, trailers and clogged up gutters. Mosquitoes also love breeding in pet water bowls, bird baths and pools if the water is not changed weekly or they are not well maintained. Rain water tanks can also be problematic so place some insect proof meshing over any outlets. When you’re holidaying, cover up or remove anything that may collect water.

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If you need more official info from WA Health about mosquito-borne disease or simple ways to prevent being bitten click here. And if you want to read more about how much West Aussies know (or don’t know) about mossies, check out Abbey’s excellent paper here! Joint the conversation too on Twitter by following Abbey and Cameron.

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!