Why is mosquito-borne disease risk greater in autumn?

Repel_illustration_DSMosquito-borne disease risk in Australia is greatest during summer right? Wrong. Imagine you’re heading off to football training for the first time this season. Training kicks off in mid-March ahead of an April start to the season. Training is once a week, typically at dusk. Playing fields are located close to wetlands. There mozzies aren’t as bad as they were back in January so why put on insect repellent? The reality is you are more likely to catch Ross River virus during autumn. Recent health warnings confirm it. Why?

The most common mosquito-borne disease in Australia is caused by Ross River virus. Infection can cause symptoms that can be highly variable in their severity but may include  rash, fever, fatigue as well as arthritic pain. Disease caused by infection with this mosquito-borne virus is reported in over 4,000 people each year from across the country.

Can we predict outbreaks of Ross River virus?

Predicting outbreaks of Ross River virus can be difficult. There are differing drivers of outbreaks between both regions and season. While outbreaks typically occur when mosquito populations are high, abundant mosquito populations don’t guarantee an outbreak of disease. Similarly, outbreaks can still occur when mosquito populations are low. The reasons for this are complex.

Perhaps most importantly, mosquitoes don’t emerge from the wetlands infected with Ross River virus, they must bite an infected animal first. The animals most likely to pass the virus onto mosquitoes are kangaroos and wallabies (waterbirds are most likely to be involved in the transmission cycles of other Australian mosquito-borne viruses such as Murray Valley encephalitis virus). This means that, despite the relative abundance of mosquitoes, without the presence of kangaroos and wallabies, the risks of Ross River virus transmission are low. This is why Ross River virus is generally considered a rural disease. However, there have been small outbreaks in metropolitan Sydney where kangaroos and wallabies are present. Outbreaks will almost always required the presence of macropods.

mosquitoes_undermicroscopeWhat has been happening in 2014?

There has been plenty of activity of Ross River virus in coastal NSW this year, particularly around the Georges River region of Sydney. This is one of my key study sites and provides a good opportunity to study Ross River virus in a region where mosquitoes, wildlife, wetlands and people are all present. The detection of Ross River virus, along with Barmah Forest virus and Stratford virus, in mosquitoes in the suburbs of Lugarno, Alfords Point, Illawong and Bankstown in early 2014 has prompted warnings from local health authorities. Similarly, Ross River virus has been isolated from mosquitoes in the Newcastle/Port Stephens region, also prompting the release of health warnings. So too around Port Macquarie. There were plenty of mosquitoes about in summer but why are the risks comparatively much higher in autumn?


Sharks may patrol the waters of Georges River but mosquitoes rule the wetlands. When it comes to mosquito-borne disease risk, it is the swamp wallabies that really raise the risks.

Why are there more Ross River virus cases in autumn?

Remember when I posted the question back in January 2014… “could the Autumn of 2014 see a surge in cases of disease?” When national notification data for Ross River virus infections are reviewed, there is a distinct peak in cases around March. With regard to overall seasonal averages, there are more cases in the three months of autumn than those of summer. In part, the official statistics may reflect a delay between initial infection of an individual and the collection of official statistics but there are also other factors at play.


Average monthly notifications of Ross River virus across Australia between 1992-2013 (National Notifiable Diseases Surveillance System)

What may be some of the driving factors in the autumn peak of disease notifications?

  1. It takes time for the pathogens to circulate amongst animals and mosquitoes. We don’t exactly know what kick starts outbreaks of Ross River virus. Is the virus present at very low levels all the time, only to explode when conditions are right or is it introduced by an animal or travelling human? Either way, it may take time for the pathogens to spread amongst local wildlife and, subsequently, sufficient numbers of mosquitoes becoming infected to drive increases in human disease notifications.
  2. Diversity of mosquito populations. While the abundance of estuarine mosquitoes (such as Aedes vigilax), and those associated with ephemeral habitats, will tend to go up and down over the summer and autumn, freshwater mosquitoes like Culex annulirostris will tend to build in their abundance from mid-summer through until autumn. These mosquitoes have been identified as key to outbreaks of Ross River virus. Perhaps it is the overall diversity, rather than abundance of a particular mosquito species that is important?
  3. Mosquitoes are living longer. As mentioned earlier, mosquitoes must bite an infected animal to become infective themselves. The longer a mosquito lives, the more likely it is to acquire an infected bloodmeal from a wallaby or kangaroo and then more likely to pass it on to one or more people. Like all insects, the mortality of mosquitoes is influenced by climate. Mosquitoes live longer during the cooler and, generally, more humid conditions in autumn than the hot and dry summer.
  4. We’re getting complacent. It may have been a long hot summer full of mosquitoes. Back in January you may have been chased out of the local wetlands due to the number of mosquitoes. As the intensity of nuisance-biting declines, most people are probably increasingly likely to skip the application of insect repellents or be vigilant about mosquito bites. Similarly, we’re probably less likely to apply a sunscreen in cooler months. I’ve worked with local health authorities to coordinate public health messages ahead of the Easter long weekend. I’ve often thought that this is a time when many head off on a camping weekend and may not be thinking about mozzies. Fortunately, in 2014, the Easter holiday falls towards the end of April, greatly reducing the risks when compared to early April holidays.
  5. More people are being tested in autumn. There is probably not an even spread of awareness of mosquito-borne disease amongst health professionals. Recently, my GP told me I’m the only person he’s ever requested a Barmah Forest virus test for (just routine testing for me). I live in a very low risk region but I’m only a 20min drive from a region currently experiencing Ross River virus and Barmah Forest virus activity amongst mosquito populations. When there are outbreaks, or when there is media coverage of Ross River virus activity, GPs and the community will be more aware of potential disease and perhaps may be more willing to request tests. This awareness is likely to build over the summer and into autumn so that, perhaps, more people are being tested in autumn than summer.

How does this influence public health messages?

While working for the Living with Mosquitoes group in the Hunter region, we discussed many different ways to increase awareness of mosquito-borne disease within the region. As well as developing a range of different approaches to raising awareness amongst the community, it was suggested that perhaps a local sports person could become a spokesperson for a television campaign. What sports person would you choose? A cricketer obviously right? Perhaps it should be a footballer given their considerably higher profile at this time of the year?

Should a NRL player, rather than summer sports star, be the face of a mosquito-borne disease awareness? (Photo: Sharks)

For me, I’m always trying to think of groups in the community that may be at higher risks or activities that may hold greater potential for exposure to mosquito bites. Bushwalking, camping and fishing are obvious cases in point. This was one of the reasons why we developed “mosquito risk tide charts” for a couple of years back, highlighting periods of the summer when mosquitoes were most likely to be in high abundance (as well as providing some general advice on mosquito repellents and other personal protection strategies).

The autumn peak in Ross River virus risk is always a consideration ahead of Easter holidays. This year, we’re fortunate in that the long weekend falls in mid-late April so the risks may be less had Easter fallen at the end of March or early April. Nonetheless, it is always important to spread the word as Australian’s head off for a weekend of camping. Make sure you’ve packed the repellent!

Lastly, I’ve been thinking back to the start of the football season. I’ve seen plenty of people out training hard throughout March in the lead up to the stat of the season. You may be less likely to be bitten by mozzies while actually running about but how about down down and the coaches, support staff and parents standing about. I must stress that there is no evidence from the human notification data that football training is resulting in increased numbers of cases but I wonder if this is a group of people to whom public health messages could be directed. They are typically a well connected group of people with good lines of communications, perhaps a reminder at the start of the football season about mosquito repellent use would be handy?

UPDATE [11 April 2014] NSW and Victorian Health authorities have issued new health warnings to avoid mosquito bites during the school holidays and Easter long weekend following the detection of the potentially fatal Murray Valley encephalitis virus in sentinel chicken flocks in the Riverina region. Other mosquito-borne viruses including Ross River and Barmah Forest viruses continue to be detected in mosquito populations collected along the coast. Current understandings of the more serious Murray Valley encephalitis virus and Kunjin virus are not thought to be active in coastal regions of NSW and SE QLD.

Why not join the conversation on Twitter by following me at @mozziebites?

The illustration at the top of this piece comes from the 1943 booklet “This is Ann – She’s dying to meet you” by the US War Department. Recognise the work of a very famous illustrator?


World Health Day 2014: An Australian perspective

WHO_WHD2014_SmallBiteBigThreatMonday 7 April is World Health Day. Every year, to mark the anniversary of the founding of WHO in 1948, the World Health Organization draws attention to a public health problem of global proportions and what needs to be done to address it. The theme is vector-borne disease with the key message that a small bite can be a big threat.

There are some wonderful resources available at the WHO website including information on the key vector-borne diseases internationally and what can be done to combat the vectors and reduce the burden of disease. One of the key messages is that while more than half of the world’s population is at risk of vector-borne diseases and increased travel, trade and migration make even more people vulnerable, these diseases are preventable.

What about Australia?

Australia may be fortunate in that we’re free of some of the nastiest pathogens spread by mosquitoes, we still have our own home grown viruses. Exotic pathogens are also increasingly knocking at our door. While countries in our region battle with outbreaks of mosquito-borne dengue, chikungunya and zika viruses, and Australian travellers are increasingly returning home either infected with these pathogens, or potentially carrying exotic mosquitoes.

In recent years we’ve had a case of “airport dengue” in Darwin, the first case of locally acquired dengue in Western Australia for 70 years, the first imported human case of zika virus, detection of yellow fever mosquitoes in Melbourne and activity of Ross River virus making its way into residential areas of Sydney.

Notwithstanding our “home grown” pathogens, that circulate amongst endemic mosquitoes and local wildlife,  exotic threats continue to threatened our shores.

It wasn’t until the 1980s that Australia was declared free of malaria. Historically, there were cases throughout the country but now the risks are considered minimal. It is, however, important to note that there are small risks of local outbreaks as highlighted by a small cluster of cases in Far North QLD in 2002. While there remains a risk to travellers, the availability of effective prophylaxis reduces those risks. However, the number and type of imported cases of malaria into Australia in the future may be determined by a range of factors including the resettlement of people from endemic countries, as well as military and civilian activities.

Aedes aegypti (Photo: Stephen Doggett)

The Yellow Fever mosquito, Aedes aegypti (Photo: Stephen Doggett)

Yellow Fever hasn’t impacted Australia to the extent of many other regions of the world. While we’ve seen increasing numbers of imported dengue and chikungunya, there have been very few imported cases of Yellow Fever reported with only two since 1991. However, recent studies have suggested that Australian mosquitoes readily become infected with and can transmit representative African and South American strains of Yellow Fever virus. The question remains as to the potential for local outbreaks to be triggered by returning travellers but perhaps we are fortunate that the availability of an effective vaccine and quarantine reviews of travellers returning from yellow fever endemic regions (Australians visiting yellow fever-endemic countries may be required to show proof of vaccination with a WHO International Certificate of Vaccination ) assists in reducing outbreak risk. Be careful if you’re heading to the World Cup in Brazil!

WHO_WHD2014_SmallBiteBigThreat_boyJapanese encephalitis virus has been knocking on our door for a while. While it is not considered a high risk for short-term travellers to endemic regions, northern Australia may be impacted. Most notable was an outbreak in the Torres Strait in 1995. While the availability of a vaccine can greatly reduce the risks, given the similarities between the mosquito populations of Torres Strait and northern Australia, concerns were raised as to the risks that may exist for transmission to occur more widely on mainland Australia. Studies assessing the risk to mainland Australia revealed some fascinating factors that may drive public health risks, from high altitude wind dispersed mosquitoes to genetic differences between local mosquito species. Despite the virus being isolated from mosquitoes collected on mainland Australia and debate continues as to any likely activity in northern mainland Australia there generally appears to be very low risk that transmission will occur more widely. Perhaps the greatest risk will be if human population grows in northern Australia?

Although imported cases in travellers, as well as refugees, are occasionally reported, until recently, Australia was considered free of Leishmaniasis. However, the discovery of parasites in red kangaroos (as well as a black wallaroo and agile wallabies) and subsequent incrimination of biting midges has changed the way local health authorities look at Australia’s susceptibility to these parasites. One of the most interesting aspects of recent research is the identification of biting midges as playing a potential role in local transmission. Biting midges (Diptera: Ceratopogonidae) are commonly known as sand flies in Australia but true sand flies (Diptera: Phlebotominae) are considered the primary vectors of Leishmania parasites. The absence of human biting species of Australian sand flies has often been cited as a reason the disease poses little risk here. More research is needed to better understand the local risks factors. However, like many of the pathogens discussed here, increased global travel from endemic regions is likely to be the biggest future risk factor.

It is probably dengue that poses the biggest threat to Australia. Historically, dengue posed a risk to communities along the east and west coasts of the country but since the 1950s, activity has been limited to Far North QLD. The current risk areas of dengue are determined by the presence of the only mosquito species in Australia currently capable of transmitting the viruses, Aedes aegypti. Local outbreaks are triggered by travellers introducing the pathogen to local mosquito populations and the number of cases reported can range from less than 100 to around 10000 each year. The future risks posed to Australia by dengue will be driven by the introduction of new vector species, such as the Asian Tiger Mosquito, Aedes albopictus, as well as increasing activity of dengue viruses in our region.

Think global and think local

It is easy for Australians and Australian authorities to become complacent of these vector-borne diseases. When we read news of outbreaks on a massive scale overseas, it is difficult to connect a threat to our local suburb. However, the critical factor across many of these pathogens is that increasing globalisation and fast international travel can expose Australia to both the pathogens and their vectors. We need to appreciate the risks and anticipate the threats. Ensuring that we maintain the capacity for quarantine surveillance and strategic responses to the detection of pathogens and vectors will be critical.

This wonderful info-graphic below from WHO on the simple steps you can take to reduce the risks of mosquito-borne disease to you and your family apply just as much to Australia as any other country on earth. It only takes one bite and over half the world’s population at risk of these pathogens. With cases of mosquito-borne Ross River virus reported from every state and territory in Australia, that includes us!

WHO_VectorborneDiseaseguidelinesFor more information, visit the official World Health Day site.