Field of Science

Flooding and disease

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I’m not sure what the coverage has been like overseas but most of the east coast of Australia has been hit pretty hard. First there were biggest floods Australia has seen for a VERY long time that started in Queensland and continue to affect the east cost of Australia. Then, instead of letting Queensland off the hook for a few weeks nature hit the coast with a cyclone THE SIZE OF THE U.S.A. that might move so far inland that it could dump rain into my state, which is a desert, halfway across our island continent.

A map showing Tropical Cyclone Yasi superimposed over the USA /
While all of this was occurring I was thinking about what kind of diseases we can expect in flood damaged towns. Luckily, nothing has surfaced and this has been attributed to the integrity of the water supply being maintained (against all odds it should be added) but it could have been so much worse.

Floods can potentially increase the transmission of water and vector transmissible diseases. A vector transmissible disease is the fancy way of saying an infection that spreads between humans but requires an intermediate host, a vector. Malaria is a vector transmissible disease due to the requirement of mosquitoes for example.

Rates of diarrhoea (including cholera and dysentery), respiratory infections, hepatitis A and E, typhoid fever, leptospirosis, and diseases borne by insects are often seen to increase in flood affected areas. However it has also been observed that the increased rates are often small unless water supplies are contaminated or significant portions of the population are displaced causing crowding in the remaining safe areas.

Of particular importance during floods is leptospirosis. This disease (known by lots of names with my personal fav being Rat Catcher’s Yellows) is caused by a spirochaete (kind of bacteria) called leptospira that is commonly found in rodent urine. In floods rodent populations explode and of course rodent urine gets mixed into the water. The leptospira can then catch a free ride to your insides though open wounds (plenty in a crisis) or directly onto your airways or gastrointestinal system. Once there the disease goes through two phases, the first of which is annoyingly general flu-like symptoms but the second phase, separated by a brief period without symptoms, sees the development of meningitis, jaundice and renal failure.

Vector transmissible diseases are generally only a problem if flood waters become still. The initial flow of water will generally wash away any established breeding sites for insects like mosquitoes but after the flow water tends to pool and with lots of water comes lots of mosquitoes. Malaria is always the concern with flooding but previous major floods have resulted in outbreaks of dengue and West Nile Fever so it very difficult to predict what will happen. Luckily, it seems that despite all the extra water its actually the change in human behaviour (sleeping outside, temporary break in disease control and overcrowding of disaster centres) that is the major issues. This is lucky because changes in human behaviour can always be modified with experience of disaster situations.

The final concern many have about disease spread during disasters and particularly flooding is the danger posed by corpses. The Queensland floods took many lives and there were concerns that these individual tragedies could result in the spread of disease. Corpses actually pose very little threat as most agents in a body will die very soon after the host itself with the notable exception of HIV which can persist for up to a week. The major problem is if death occurred due to cholera or haemorrhagic fever both of which were not reported in the Queensland floods.

Flood-waters even got into the CBD
The Queensland floods and the continued devastation of Australia’s east coast (there have been reports of a monsoon in Victoria now as well) have been traumatic but it could have been much worse. The water supply staying safe for the majority of the crisis has undoubtedly saved thousands of lives and so while our hearts must go out to all those effected our thanks should go to the volunteers who helped in any way they could, including those charged with protecting the precious water supply and the medical staff who protected a state during a phenomenal crisis.

WHO: Flooding and communicable diseases fact sheet
Ohl, C. (2000). Flooding and human health BMJ, 321 (7270), 1167-1168 DOI: 10.1136/bmj.321.7270.1167
Howard MJ, Brillman JC, & Burkle FM Jr (1996). Infectious disease emergencies in disasters. Emergency medicine clinics of North America, 14 (2), 413-28 PMID: 8635416


  1. Hey, nice post!

    Coupla things I hope you don't mind my adding...
    There's been a history of vector-borne diseases in Australia. A few thousand Australians are affected every year. But the government and science agencies have made efforts in the last few decades to monitor and curb them. (Google Richard Russell, USyd). This has had some success, i.e. locally acquired Malaria hasn't actually occurred in decades. And this is because of what you suggest ~ changes in human behaviour and clean up immediately after a flood. With any luck, hopefully there won't be a rise in disease incidence!

  2. Of course I don't mind people adding stuff! Makes it easy when others write my posts for me :)
    I was really surprised to find that unless there is already disease in the community floods don't 'generally' increase disease incidence. I thought dysentery would develop into a major problem but I haven't heard anything so far and now I don't expect to.

  3. Thanks, James, for putting this together. I've been watching the news from Queensland and wondering about the sorts of diseases that could strike, but hadn't gotten so worked up that I started to look for information yet.

    Really glad to hear the information about malaria, especially. Had worried a bit about what was likely to happen.

  4. No problem Greg, glad you liked it :)


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