Scientists have identified Hendra hotspots across eastern Australia, and fingered two species of fruit bat that appear to pose the greatest risk of passing the deadly virus to horses.
Hendra is a dangerous virus, with fruit bats of the genus Pteropus, commonly known as flying-foxes, known to be the natural reservoir.
Hendra is able to be transmitted to horses, and seven people are known to have contracted the virus from close contact with infected horses. Of the seven, four died.
Effective treatment is lacking in both horses and humans and, even though a vaccine is now available for horses, minimising the risk of exposure remains an important infection control strategy.
Scientists Craig Smith, Chris Skelly Nina Kung, Billie Roberts and Hume Field set about investigating area-dependent factors that may have an influence on nearby Hendra virus cases.
They identified a significant cluster of equine cases within 40km of flying fox roosts, which they said was entirely consistent with the foraging “footprint” of bats.
Their analysis identified multiple equine infection hot spots along the eastern Australia coast, from far north Queensland to central New South Wales. The largest hotspot extended for nearly 300km from southern Queensland to northern New South Wales.
Their analysis showed the density of Pteropus alecto and Pteropus conspicillatus to have the strongest correlation with equine case locations, suggesting these species were more likely a source of infection of Hendra virus for horses than Pteropus poliocephalus or Pteropus scapulatus.
The density of horses, climate variables and vegetation variables were not found to be significant risk factors, but their geographically weighted regression (GWR) analysis suggested that extra unidentified risk factors also existed at the property level.
These factors might include property attributes and husbandry and management practices that reduced the likelihood of bat-horse contact, such as excluding horses from paddocks where flying-foxes were active. Further work was needed to confirm these local risk factors, they said.
“Further investigations and comparisons between case and control properties are needed to identify these local risk factors,” said the researchers, whose findings have been published in the open-access journal, PLoS ONE.
Hendra virus was first identified in September 1994 in Australia, when it caused an outbreak in horses and two people who had close contact with the infected equines.
Sporadic cases continue to occur in horses and humans, with some 80 confirmed or possible equine cases and seven confirmed human cases identified to the end of 2012.
There has been an increase in reported cases since 2006.
Reported equine Hendra virus cases to date have been restricted to the adjoining eastern Australia states of Queensland and New South Wales.
The researchers used for their analysis the 80 recorded cases of Hendra infection in the two states up until the end of 2012, involving 40 properties that were epidemiologically unrelated. Thirty-one of the properties were in Queensland and nine in New South Wales.
For comparison, they randomly selected 1189 control properties from 118,900 registered horse properties in Queensland, New South Wales and Victoria that had not had a confirmed or suspected case of Hendra virus.
Their work also identified what appeared to be “cold spots” – a clustering of control properties on the Southern Downs of south-east Queensland and along the central coast of New South Wales.
The researchers assessed a total of 15 variables, with the density of P. alecto and P. conspicillatus having the strongest positive correlation with reported Hendra virus equine cases. The annual minimum temperature had a weak positive correlation and the density of P. scapulatus had a weak negative correlation.
Negligible correlations were identified for P. poliocephalus, relative humidity and horse population.
Analysis found the combined density of P. alecto and P. conspicillatus to be the strongest positive correlation with reported equine case locations.
They said their research suggested P. scapulatus and P. poliocephalus played a limited role in the infection of horses.
“We found that the density of horses had negligible correlation with equine case locations, and is evidently not a significant risk factor.”
Smith and his colleagues concluded that it was important that horse owners continued to implement risk management strategies that minimise horse exposure to Hendra virus.
They said their research had confirmed the presence of equine infection hot spots along the eastern Australia coast, and that the density of P. alecto and P. conspicillatus strongly correlated with case locations. “This finding suggests that these species play a primary role in Hendra virus infection of horses.”
Smith C, Skelly C, Kung N, Roberts B, Field H (2014) Flying-Fox Species Density – A Spatial Risk Factor for Hendra Virus Infection in Horses in Eastern Australia. PLoS ONE 9(6): e99965. doi:10.1371/journal.pone.0099965
The fully study can be read here.