Analysis of the recent Australian equine influenza outbreak has improved understanding of how the disease spreads.
Researchers from Australia and New Zealand looked back at the outbreak, which took place between August and December 2007. It was estimated to have cost the Australian equine industry over $350 million. Almost 70,000 horses on more than 9000 premises in Queensland and New South Wales were affected. The country had previously been free of the disease.
Simon Firestone, lead researcher and PhD candidate at the University of Sydney's Faculty of Veterinary Science. and colleagues matched meteorological data with information from infected premises to assess the effect of the weather on the spread of infection.
They discovered an association between the occurrence of new cases and the weather conditions three days previously. This coincides with the incubation period for equine flu – the first signs are usually seen from 3 days after the animal becomes infected.
The researchers found that horses were more likely to get infected on drier days when relative humidity was low. There was minimal risk once relative humidity exceeded 80%.
Infection was also less likely on days when the maximum daily air temperature was between 20 and 25°C. Extreme temperatures, either high ( >28°C) or low (<16°C) were associated with the highest risk of infection. Lower minimum daily temperatures were associated with higher risk of infection.
The researchers also found that wind speeds in excess of 30km/hour from the direction of infected premises were associated with increased risk of infection.
In conclusion, the authors write: “by combining influenza outbreak and concurrent meteorological data, we have shown how relative humidity, air temperature and wind velocity combined to influence the spread of an actual influenza outbreak.
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