Experimental Infection of Horses with Hendra Virus/Australia/Horse/2008/Redlands

Glenn A. Marsh

, Jessica Haining, Timothy J. Hancock, Rachel Robinson, Adam Foord, Jennifer A. Barr, Shane Riddell, Hans G. Heine, John R. White, Gary Crameri, Hume E. Field, Lin-Fa Wang, and Deborah Middleton

Author affiliations: Commonwealth Scientific and Industrial Research Organization Livestock Industries, Geelong, Victoria, Australia (G.A. Marsh, J. Haining, T.J. Hancock, R. Robinson, A.J. Foord, J.A. Barr, S. Riddell, H.G. Heine, J.R. White, G. Crameri, L.-F. Wang, D. Middleton); Queensland Centre for Emerging Infectious Disease, Coopers Plains, Queensland, Australia (H.E. Field)

Abstract

Hendra virus (HeV) is a highly pathogenic zoonotic paramyxovirus harbored by Australian flying foxes with sporadic spillovers directly to horses. Although the mode and critical control points of HeV spillover to horses from flying foxes, and the risk for transmission from infected horses to other horses and humans, are poorly understood, we successfully established systemic HeV disease in 3 horses exposed to Hendra virus/Australia/Horse/2008/Redlands by the oronasal route, a plausible route for natural infection. In 2 of the 3 animals, HeV RNA was detected continually in nasal swabs from as early as 2 days postexposure, indicating that systemic spread of the virus may be preceded by local viral replication in the nasal cavity or nasopharynx. Our data suggest that a critical factor for reducing HeV exposure risk to humans includes early consideration of HeV in the differential diagnosis and institution of appropriate infection control procedures.

Hendra virus (HeV) is a zoonotic paramyxovirus harbored by Australian mainland flying foxes, from which it is believed to be transmitted directly to horses. In horses, HeV causes a severe, often fatal, febrile illness associated with respiratory and neurologic signs (1). Since its emergence in Queensland, Australia, in 1994, HeV infection of horses has regularly recurred. Of the 32 equine outbreaks, 5 have extended to involve infection of humans; of the 7 known human case-patients, 4 have died. Human infection has typically occurred after close contact with infected horses, usually horses in the terminal stages of disease or at postmortem examination, except for 1 person for whom epidemiologic findings suggested the most likely exposure to an infected horse occurred during incubation (2). Currently, HeV is an unmanaged emerging infectious disease.

Since the serious zoonotic potential of HeV was confirmed, clinical and laboratory evaluation of disease horses from outbreaks has been limited. In particular, the relationship between the onset of clinical signs and duration of viral shedding has not been determined, and the understandably few equine experimental infection studies conducted in the mid-1990s (3) yielded limited data that could guide effective management of the exposure risk to humans.

Further concern arose after an HeV outbreak in the Brisbane suburb of Thornlands (Redlands Shire), Queensland, in 2008, in which the major clinical signs in horses were attributable to disease of the central nervous system (4). Although nervous system signs have been associated with previous outbreaks, HeV is more commonly considered to induce a respiratory syndrome in horses. In the Redlands 2008 outbreak, credible alternate provisional diagnoses and thus delay in definitive diagnosis likely contributed to an increased HeV exposure risk to attending staff and to in-contact horses; 2 staff members became infected, 1 fatally (4).

The objectives of this study were to monitor potential routes of shedding for evidence of HeV replication in horses experimentally exposed to Hendra virus/Australia/Horse/2008/Redlands and to compare the associated clinical syndrome with that observed after infection with the HeV isolate from the first outbreak in 1994. These data would provide a framework for assessing the relative transmission risk posed by horses at various times during acute HeV infection and permit incorporation of recommendations for reducing the transmission risk to humans and other horses into advisory and outbreak management strategies. Following the observation in the Redlands outbreak of a predominantly neurologic disease, an experimental challenge was carried out under BioSafety Level 4 conditions at the Australian Animal Health Laboratory, Geelong, Victoria, Australia, in late 2008.

Suggested citation for this article: Marsh GA, Haining J, Hancock TJ, Robinson R, Foord AJ, Barr JA, et al. Experimental infection of horses with Hendra virus/Australia/Horse/2008/Redlands. Emerg Infect Dis [serial on the Internet]. 2011 Dec [date cited]. http://dx.doi.org/10.3201/eid1712.111162