After the floods come the mosquitoes – but the disease risk is more difficult to predict


Cameron Webb, University of Sydney

We’re often warned to avoid mosquito bites after major flooding events. With more water around, there are likely to be more mosquitoes.

As flood waters recede around Townsville and clean-up efforts continue, the local population will be faced with this prospect over the coming weeks.

But whether a greater number of mosquitoes is likely to lead to an outbreak of mosquito-borne disease is tricky to predict. It depends on a number of factors, including the fate of other wildlife following a disaster of this kind.

Mozzies need water

Mosquitoes lay their eggs in and around water bodies. In the initial stages, baby mosquitoes (or “wrigglers”) need the water to complete their development. During the warmer months, it doesn’t take much longer than a week before they are grown and fly off looking for blood.

So the more water, the more mosquito eggs are laid, and the more mosquitoes end up buzzing about.

But outbreaks of disease carried by mosquitoes are dependent on more than just their presence. Mosquitoes rarely emerge from wetlands infected with pathogens. They typically need to pick them up from biting local wildlife, such as birds or mammals, before they can spread disease to people.




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Mosquitoes and extreme weather events

Historically, major inland flooding events have triggered significant outbreaks of mosquito-borne disease in Australia. These outbreaks have included epidemics of the potentially fatal Murray Valley encephalitis virus. In recent decades, Ross River virus has more commonly been the culprit.

A focal point of the current floods is the Ross River, which runs through Townsville. The Ross River virus was first identified from mosquitoes collected along this waterway. The disease it causes, known as Ross River fever, is diagnosed in around 5,000 Australians every year. The disease isn’t fatal but it can be seriously debilitating.

Following substantial rainfall, mosquito populations can dramatically increase. Carbon dioxide baited light traps are used by local authorities to monitor changes in mosquito populations.
Cameron Webb (NSW Health Pathology)

In recent years, major outbreaks of Ross River virus have occurred throughout the country. Above average rainfall is likely a driving factor as it boosts both the abundance and diversity of local mosquitoes.

Flooding across Victoria over the 2016-2017 summer produced exceptional increases in mosquitoes and resulted in the state’s largest outbreak of Ross River virus. There were almost 1,700 cases of Ross River virus disease reported there in 2017 compared to an average of around 300 cases annually over the previous 20 years.




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Despite plagues of mosquitoes taking advantage of flood waters, outbreaks of disease don’t always follow.

Flooding resulting from hurricanes in North America has been associated with increased mosquito populations. After Hurricane Katrina hit Louisiana and Mississippi in 2005, there was no evidence of increased mosquito-borne disease. The impact of wind and rain is likely to have adversely impacted local mosquitoes and wildlife, subsequently reducing disease outbreak risk.

Applying insect repellent is worthwhile even if the risk of mosquito-borne disease isn’t known.
From shutterstock.com

Australian studies suggest there’s not always an association between flooding and Ross River virus outbreaks. Outbreaks can be triggered by flooding, but this is not always the case. Where and when the flooding occurs probably plays a major role in determining the likelihood of an outbreak.

The difficulty in predicting outbreaks of Ross River virus disease is that there can be complex biological, environmental and climatic drivers at work. Conditions may be conducive for large mosquito populations, but if the extreme weather events have displaced (or decimated) local wildlife populations, there may be a decreased chance of outbreak.

This may be why historically significant outbreaks of mosquito-borne disease have occurred in inland regions. Water can persist in these regions for longer than coastal areas. This provides opportunities not only for multiple mosquito generations, but also for increasing populations of water birds. These birds can be important carriers of pathogens such as the Murray Valley encephalitis virus.




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In coastal regions like Townsville, where the main concern would be Ross River virus, flood waters may displace the wildlife that carry the virus, such as kangaroos and wallabies. For that reason, the flood waters may actually reduce the initial risk of outbreak.

Protect yourself

There is still much to learn about the ecology of wildlife and their role in driving outbreaks of disease. And with a fear of more frequent and severe extreme weather events in the future, it’s an important area of research.

Although it remains difficult to predict the likelihood of a disease outbreak, there are steps that can be taken to avoid mosquito bites. This will be useful even if just to reduce the nuisance of sustaining bites.

Cover up with long-sleeved shirts and long pants for a physical barrier against mosquito bites and use topical insect repellents containing DEET, picaridin, or oil of lemon eucalyptus. Be sure to apply an even coat on all exposed areas of skin for the longest lasting protection.The Conversation

Cameron Webb, Clinical Lecturer and Principal Hospital Scientist, University of Sydney

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Here’s what you need to know about melioidosis, the deadly infection that can spread after floods



File 20190214 1721 1pkznl1.jpg?ixlib=rb 1.1
People typically become sick between one and 21 days after being infected.
Goran Jakus/Shutterstock

Sanjaya Senanayake, Australian National University

The devastating Townsville floods have receded but the clean up is being complicated by the appearance of a serious bacterial infection known as melioidosis. One person has died from melioidosis and nine others have been diagnosed with the disease over the past week.

The bacteria that causes the disease, Burkholderia pseudomallei, is a hardy bug that lives around 30cm deep in clay soil. Events that disturb the soil, such as heavy rains and floods, bring B. pseudomallei to the surface, where it can enter the body through through a small break in the skin (that a person may not even be aware of), or by other means.

Melioidosis may cause an ulcer at that site, and from there, spread to multiple sites in the body via the bloodstream. Alternatively, the bacterium can be inhaled, after which it travels to the lungs, and again may spread via the bloodstream. Less commonly, it’s ingested.




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Melioidosis was first identified in the early 20th century among drug users in Myanmar. These days, cases tend to concentrate in Southeast Asia and the top end of northern Australia.

What are the symptoms?

Melioidosis can cause a variety of symptoms, but often presents as a non-specific flu-like illness with fever, headache, cough, shortness of breath, disorientation, and pain in the stomach, muscles or joints.

People with underlying conditions that impair their immune system – such as diabetes, chronic kidney or lung disease, and alcohol use disorder – are more likely to become sick from the infection.

The majority of healthy people infected by melioidosis won’t have any symptoms, but just because you’re healthy, doesn’t mean you’re immune: around 20% of people who become acutely ill with melioidosis have no identifiable risk factors.

People typically become sick between one and 21 days after being infected. But in a minority of cases, this incubation period can be much longer, with one case occurring after 62 years.

How does it make you sick?

While most people who are sick with melioidosis will have an acute illness, lasting a short time, a small number can have a grumbling infection persisting for months.

One of the most common manifestations of melioidosis is infection of the lungs (pneumonia), which can occur either via infection through the skin, or inhalation of B. pseudomallei.

The challenges in treating this organism, though, arise from its ability to form large pockets of pus (abscesses) in virtually any part of the body. Abscesses can be harder to treat with antibiotics alone and may also require drainage by a surgeon or radiologist.

How is it treated?

Thankfully, a number of antibiotics can kill B. pseudomallei. Those recovering from the infection will need to take antibiotics for at least three months to cure it completely.

If you think you might have melioidosis, seek medical attention immediately. A prompt clinical assessment will determine the level of care you need, and allow antibiotic therapy to be started in a timely manner.

Your blood and any obviously infected body fluids (sputum, pus, and so on) will also be tested for B. pseudomallei or other pathogens that may be causing the illness.

While cleaning up after these floods, make sure you wear gloves and boots to minimise the risk of infection through breaks in the skin. This especially applies to people at highest risk of developing melioidosis, namely those with diabetes, alcohol use disorder, chronic kidney disease, and lung disease.




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The Conversation


Sanjaya Senanayake, Associate Professor of Medicine, Infectious Diseases Physician, Australian National University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Queensland’s floods are so huge the only way to track them is from space


Linlin Ge, UNSW

Many parts of Queensland have been declared disaster zones and thousands of residents evacuated due to a 1-in-100-year flood. Townsville is at the epicentre of the “unprecedented” monsoonal downpour that brought more than a year’s worth of rain in just a few days, and the emergency is far from over with yet more torrential rain expected.

Such monumental disruption calls for emergency work to safeguard crucial infrastructure such as bridges, dams, motorways, railways, power substations, power lines and telecommunications cables. In turn, that requires accurate, timely mapping of flood waters.

For the first time in Australia, our research team has been monitoring the floods closely using a new technique involving European satellites, which allows us to “see” beneath the cloud cover and map developments on the ground.




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Given that the flooding currently covers a 700km stretch of coast from Cairns to Mackay, it would take days to piece together the big picture of the flood using airborne mapping. What’s more, conventional optical imaging satellites are easily “blinded” by cloud cover.

But a radar satellite can fly over the entire state in a matter of
seconds, and an accurate and comprehensive flood map can be produced in less than an hour.

Eyes above the skies

Our new method uses an imaging technology called “synthetic aperture radar” (SAR), which can observe the ground day or night, through cloud cover or smoke. By combining and comparing SAR images, we can determine the progress of an unfolding disaster such as a flood.

In simple terms, if an area is not flooded on the first image but is inundated on the second image, the resulting discrepancy between the two images can help to reveal the flood’s extent and identify the advancing flood front.

To automate this process and make it more accurate, we use two pairs of images: a “pre-event pair” taken before the flood, and a “co-event pair” made up of one image before the flood, and another later image during the flooding.

The European satellites have been operated strategically to collect images globally once every 12 days, making it possible for us to test this new technique in Townsville as soon as flooding occurs.

To monitor the current floods in Townsville, we took the pre-event images on January 6 and January 18, 2019. The co-event pair was collected on January 18 and January 30. These sets of images were then used to generate the accurate and detailed flood map shown below.

The image comparisons can all be done algorithmically, without a human having to scrutinise the images themselves. Then we can just look out for image pairs with significant discrepancies, and then concentrate our attention on those.

Satellite flood mapping along the Queensland coast, compiled using images from the European radar satellite Sentinel-1A.
European Space Agency/Smart Spatial Technology Development Laborator (SSTD), UNSW, Author provided



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Our technique potentially avoids the need to monitor floods from airborne reconnaissance planes – a dangerous or even impossible task amid heavy rains, strong wind, thick cloud and lightning.

This timely flood intelligence from satellites can be used to switch off critical infrastructure such as power substations before flood water reaches them.The Conversation

Linlin Ge, Professor, UNSW

This article is republished from The Conversation under a Creative Commons license. Read the original article.