Hendra Virus: An Unprecedented Australian Outbreak

Hendra Virus: An Unprecedented Australian Outbreak

Serological evidence of hendra infection has been demonstrated in all four species of fruit-eating bats found on the Australian mainland; this evidence of seroconversion is not restricted to the areas where equine cases have been reported.

Photo: Justin Welbergen

Hendra virus is a potentially fatal zoonotic virus that is transmitted from horses to humans. Together with the closely related Nipah virus, a cause of respiratory disease and encephalitis in pigs and humans, these two viruses form the genus Henipavirus in the family paramyxyoviridae.

First described following an outbreak of acute, fulminant (sudden, severe, and intense) respiratory disease in horses in 1994 in a north Brisbane, Australia, suburb, hendra virus has since been found to naturally infect a variety of Pteropid bat species native to Australia. Since 1994 there have been 26 reported incidents of hendra infection affecting 63 horses, one dog, and seven people. Of these 26 incidents, 12 have occurred in 2011. No reports of bat-to-human transmission of this virus have been made. All seven cases of hendra in humans have, to date, been associated with equine infection. The case-fatality rate of human hendra virus infection is 57%. Horses testing positive for hendra virus are euthanized, according to national guidelines.

Although the host bat species are native to large regions of coastal Australia, equine and human cases of hendra virus infection have been reported only in the north of New South Wales and in Queensland. Serological evidence of hendra infection has been demonstrated in all four species of fructivorous (fruit-eating) bats found on the Australian mainland; this evidence of seroconversion is not geographically restricted to the areas where equine cases have been reported. Bats with antibodies to hendra have been identified from Melbourne in the south to the far north of Queensland. Seroprevalence increases with age.

Hendra virus infection of horses is a very problematic disease for equine clinicians. Cases present with a variety of clinical signs, which poses serious human safety concerns. The first large outbreak of hendra virus infection in 1994 was associated with severe respiratory distress and sudden death that occurred in the majority of affected horses within 36 hours of the onset of clinical signs. hendra virus has a definite tropism (a biological phenomenon, indicating growth or turning movement of a biological organism) for the vascular endothelium (organ lining), particularly the arterial endothelium, which explains many of the post-mortem findings, such as foam-filled airways, dilated pulmonary lymphatics, severe pulmonary edema, and congestion.

However, in 2008 a second large outbreak of hendra virus infection occurred in a different suburb of Brisbane, involving five horses in residence at a veterinary clinic. The presenting signs in this outbreak were predominantly neurological and included depression, anorexia, and ataxia (incoordination) with rapid deterioration resulting in euthanasia. These clinical signs were not consistent with previoushendra cases. Both of these outbreaks were associated with human infections and deaths.

Currently no vaccine is commercially available for humans or horses. Recently, vaccine trials at the Australian animal health laboratories in Geelong (Victoria) have demonstrated that vaccination with a soluble Hendra virus g protein protected horses against virus challenge. It is hoped that this vaccine will become available for wider use in 2012.

Quantitative reverse transcriptase polymerase chain reaction (RT-PCR) assays have facilitated the provision of rapid diagnostic results, minimizing potential exposure of the laboratory worker to infectious material. Serological assays to detect hendra antibodies, including rapid enzyme-linked immunosorbant assay (ELISA)-based serological screening tests, are available, but specificity is not high. Any positive or suspect positive serum samples are then sent to a high security, BSL-4 level laboratory for testing by virus neutralization assay.

Currently, public health authorities in Australia are dealing with an unprecedented number and distribution of Hendra cases. In 2011 there have been 15 equine cases of hendra on 12 premises (Editor's Note: As of Oct. 15, the case total has since risen to 22 equine cases confirmed in Australia in 2011). These cases have occurred farther south and west than had been reported previously. Also, the first canine case of hendra infection has been reported, which is the first report of the infection in a domestic species other than horses. The dog is seropositive, but has not, to date, shown any clinical signs of disease and no virus has been detected (Editor's Note: The affected dog has since been euthanized). The reasons for the dramatic increase in incidence are not known.

Ongoing virological monitoring of bat populations in eastern Australia has shown an increase in the amount of virus shed by the bats. This outbreak has focused attention on the immensity of what is unknown about hendra and has stimulated a significant increase in research into this serious disease of horses and humans.

CONTACT: James Gilkerson, BVSc, BSc(Vet) Hons, PhD--61 3 8344 9969--jrgilk@unimelb.edu.au--Director, Equine Infectious Disease Laboratory, The University of Melbourne, Victoria, Australia

This is an excerpt from Equine Disease Quarterly, funded by underwriters at Lloyd's, London, brokers, and their Kentucky agents.

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