Periodic spillover of Hendra virus (HeV) from its natural host (fruit bats, flying-foxes) results in catastrophic disease in horses and occasionally the subsequent infection of humans. Prior to 2011, fourteen equine incidents involving seven human cases (four fatal) were recorded. In 2011, in contrast to the sporadic incidents and limited case numbers of previous 17 years, a super-cluster of 18 incidents (23 equine cases) occurred in a 12-week period in eastern Australia. What was predominantly different in 2011 was the number of spillover events, the geographic clustering in south-east QLD and northern NSW, and the unprecedented number and proportion of incidents (8/18) in NSW. Furthermore, a dog with neutralising antibodies to HeV was associated with one of the infected horse property in south-east QLD has added another factor in the complexity of HeV transmission dynamic.
Epidemiologic evidence suggests that additional host and/or environmental factors were at play in 2011. Considering that all but one of the 18 incidents occurred within a 350km radius of the first reported 2011 HeV infected property (IP) at Kerry, 75km south of Brisbane, it is suggested that there could be some local/regional factors involved. In addition, preliminary phylogenetic study of HeV identified from samples collected from infected horses consistent with that seen in previous years, suggesting the 2011 cluster of spillovers was not due to a new more virulent or easily transmissible strain of HeV, but to factors external to the virus. Furthermore, preliminary analysis of the urine samples collected from flying-fox colonies proximate to the 2011 IPs suggests that excretion levels at some areas were higher than in the previous years. During the epidemiological investigation of the 2011 HeV IPs, it was observed that additional factors such as horse management practice and horse behaviour may also play a role in the probability of flying-fox - horse interaction thus virus transmission. The cause and relevance of these factors to the magnitude of the 2011 cluster is yet to be determined, and are the focus of an accelerated research program funded by the state governments of Queensland and New South Wales and the Australian federal government.