Note Number: AG1440
Published: March 2011
Dr Cameron Bell
Mosquitoes and related insects can carry a range of diseases that affect horses. Fortunately, Australia is free of many of the serious mosquito-borne diseases of horses. Nevertheless horse owners need to be vigilant and take steps to minimise exposure of their horses.
There are over 300 species of mosquito in Australia but only a small proportion represent a significant risk to animal and human health, either as nuisance-biting pests or vectors of disease.
Mosquito larvae develop into adults over warmer months of the year in stagnant water that has been standing for at least 7-10 days. Some mosquito species will also develop in brackish/saline habitats, so high tides together with flooding in coastal regions may also increase mosquito numbers. Flood conditions and high rainfall in other areas may do the same.
Female mosquitoes suck blood from animals, including humans, as part of their eating and breeding habits. When a mosquito bites, she also injects saliva and anti-coagulants into the blood which may contain disease-causing organisms.
Mosquitoes are primarily attracted to carbon dioxide and the "smell" of skin. Each species varies in its propensity to bite animals. Conversely, different animals vary in both their attractiveness to mosquitoes and their sensitivity to a bite.
Adult mosquitoes typically live for up to 3 weeks.
Mosquitoes and related insects can transmit a range of diseases to animals, including humans. Not every mosquito however spreads disease: only those that have been infected after feeding on an infected animal.
Viruses carried by mosquitoes and related insects are called arboviruses. Although a number of well-studied arboviruses of animals occur in Australia, there are some about which very little is known.
Unexplained cases of disease in horses have been recorded in Victoria in the past and they usually coincide with periods of very high mosquito activity after a high rainfall period. Associating these cases with nearby water bodies suggests mosquito-borne infection is the primary cause.
Affected horses typically show either nervous (neurological) signs, including being wobbly on their feet, or a reluctance to move and swollen joints. Affected horses usually recover uneventfully and deaths are rare.
Viruses causing two mosquito-borne human diseases, Ross River fever and Murray Valley encephalitis, have been implicated in these unexplained cases. However no definitive evidence has been found at this stage.
It is highly likely a majority of horses infected with arboviruses will not show any signs of illness.
There is no known risk to humans from direct contact with horses infected with arboviruses in Australia.
As for all diseases, prevention is better than cure. All horse owners should make efforts to reduce the risk of exposure of their horse(s) to mosquitoes. It may be impossible to completely prevent a horse being bitten but the following suggestions may minimise the risk of horses contracting mosquito-borne diseases.
- Eliminate breeding sites on the premises by removing stagnant/standing water. This includes disposing of tins, tyres and rubbish containers that may hold water, and ensuring that mosquitoes cannot access rainwater tanks.
- House horses during peak periods of mosquito activity (ie. between dusk and dawn).
- Turn off lights inside stables at night.
- Use fluorescent lights in stables which do not attract mosquitoes.
- Place incandescent bulbs around the stable perimeter to attract mosquitoes away from horses.
- Use commercial mosquito traps.
- Screen stable windows.
- Use fogging, fans and automatic overhead misting systems to eliminate mosquitoes within stables.
- Apply topical insect repellents.
- Use physical barriers such as rugging and hooding of horses in lightweight material. Other barriers such as mesh fly veils with ear covers and fly boots are commercially available.
Horse owners are encouraged to contact their local veterinarian for further information about mosquito-borne diseases of horses and reducing the risk of exposure to mosquitoes. Any horse showing unusual or unexplained signs of disease should be promptly reported to a veterinarian.