In 1994, an outbreak of respiratory and neurological disease was reported in a racing stable in the Brisbane suburb of Hendra. Of the 20 horses affected, 14 died or were euthanized, seven of these within 12 hours of the onset of clinical signs.
Sick horse coughing and showing signs of respiratory distress.
Hendra virus is a significant zoonotic risk to Australian horses, horse owners and handlers and especially to equine veterinarians practicing in Australia.
© 2017 by S. Hanusch
African horse sickness was quickly ruled out as the cause, which left veterinary authorities with another problem. What was causing these deaths? To make the situation more serious, two of the people looking after the horses were also hospitalized, and one of these people died.
After a significant collaborative effort between veterinary virologists, a novel Paramyxovirus, subsequently named Hendra virus (HeV), was identified as the causal organism. Since the first recognized occurrence in 1994, HeV infections have been identified sporadically in horses showing clinical signs of respiratory disease, neurological disease or sudden death.
More than 100 horses have been infected in this time, mostly occurring as single cases, but all of these horses have died or were euthanized. Significantly, seven people have been reported to have been infected with HeV, and four of these people have died because of the infection. To date, all of the people infected with HeV have contracted the infection through exposure to an infected horse.
Extensive investigations led to the identification of Pteropid fruit bats, also called flying foxes, as the natural host of HeV. There are four species of Pteropid bats that inhabit the Australian mainland, and antibodies to HeV have been detected in all species.
Extensive epidemiological investigations have identified a likelihood of equine cases of HeV infection associated with Black flying foxes (Pteropus alecto) and Spectacled flying foxes (P. conspicillatus), although a recent report has identified a new HeV variant in Greyheaded flying foxes (P. poliocephalus).
No evidence of HeV infection has been reported in other wildlife species, although dogs have been found to be infected in both outbreaks and in experimental studies. There is no evidence of direct, bat-to-human HeV infection.
There is experimental evidence that HeV can infect horses, ferrets, pigs, cats, , mice, guinea pigs and hamsters, however, only bats, horses, humans and dogs have been identified as hosts in an outbreak situation. To date, Hendra virus infections have only been reported on the Australian mainland, although a closely related virus, Nipah virus, has been reported as a significant cause of human mortality in South-East and South Asia.
Hendra virus infection has been identified in many different organs in bats, however, it is present in highest concentrations in kidney tissue and urine, and it is thought that contamination of the environment with Hendra virus infection has been identified in many different organs in bats, however, it is present in highest concentrations in kidney tissue and urine, and it is thought that contamination of the environment with HeV-laden bat urine is the mechanism by which horses are infected with HeV.
Subsequent horse-to-horse transmission has been reported. Risk factors associated with an increased likelihood of equine HeV infection are the presence of fruiting or flowering plants and trees in the yards or paddocks where horses are housed, lack of stables and feeding of horses outside where feeding stations are exposed to bat urine.
There is considerable season-to-season variation in the occurrence of equine HeV infections and it is likely that this is associated with seasonal conditions that affect the availability of flying fox food sources and roosting sites.
Additionally, equine HeV cases have only been identified in Queensland and coastal areas of NSW north of Sydney, despite the virus being detected in flying foxes in a much broader geographical range, around the coast from Adelaide to Cairns.
Experimental challenge studies in horses have identified a wide range of clinical signs associated HeV infection, from the severe respiratory and neurological conditions identified in early outbreaks, to less obvious signs such as discomfort, increased heart rate and fever, all of which could be confused with other diseases, such as colic.
This poses a significant workplace safety problem for veterinarians who see sick horses routinely. Two veterinarians and a veterinary assistant have died and a vet nurse was seriously affected as a result of occupational exposure to HeV. There are no HeV vaccines registered for use in humans, however, since all human cases are associated with equine infections, the risk of human infection has been moderated by vaccination of horses with the available equine HeV vaccine.
Hendra virus vaccine has been shown to protect horses from challenge doses of HeV that were previously demonstrated to be lethal in unvaccinated horses. To date, HeV vaccination has been extremely effective in preventing equine HeV disease.
Since the vaccine was commercially released in 2012, there have been no HeV cases reported in vaccinated horses. All cases since 2012 have been in unvaccinated horses.
Hendra virus is a significant zoonotic risk to Australian horses, horse owners and handlers and especially to equine veterinarians practicing in Australia. Vaccination is the most effective means of protecting horses from HeV infection and consequently protecting the people who look after them from being exposed to a potentially life-threatening disease.
Press release by Equine Disease Quarterly - Article by James R. Gilkerson Professor of Veterinary Microbiology, Centre for Equine Infectious Disease, Melbourne Veterinary School