Hendra Virus Infection in Horses

Hendra virus (HeV), a zoonotic virus first identified in Australia in 1994, is a henipavirus within the subfamily Paramyxoviridae. Close contact with infected horses plays a primary role in disease transmission to other horses and to humans. The viral agent is endemic in specific species of fruit bats (also called flying foxes). Contact with these bats is suspected to have facilitated transfer of the HeV to horses. Horses are infected via ingestion of contaminated feed or water or via contact with infected horses. Infected horses can excrete HeV in urine, saliva, and respiratory secretions and can be infectious for several days before the onset of clinical signs. 

There have been multiple (> 60), sporadic incidents of equine disease in Australia between 1994 and 2020, with < 10 reported human cases. Hendra virus has not been identified outside Australia. The case fatality rate in humans is high (~57%) and even higher in horses (~75%). Infection in humans can occur via exposure to respiratory secretions or blood in live horses or at postmortem examination.

Equine veterinarians are considered at occupational risk of contracting HeV, particularly when treating or performing postmortem examinations on infected horses. Horses and humans infected with HeV develop severe and often rapidly fatal respiratory disease as well as vasculitis and encephalitis characterized by dyspnea, vascular endothelial damage, and pulmonary edema. Common clinical signs include:

• depression and anorexia

• fever

• respiratory distress

• ataxia

• tachycardia

• frothy nasal discharge

A commercial HeV vaccine for use in horses became available in 2012 under minor permit. The vaccine has become fully registered for use by veterinarians. Vaccinated horses are identified by microchip and the information entered into the HeV Vaccine National Online Registry of Australia. The vaccine consists of soluble forms of G glycoprotein of HeV; it does not contain modified or inactivated virus. Recent confirmed cases of Hendra virus have been in unvaccinated horses. A human vaccine for Hendra virus is in clinical trials as of 2021.