Hendra virus (HeV) infection is a rare but severe zoonotic disease that poses a significant public health concern, primarily within Australia. The virus is a member of the Henipavirus genus, which belongs to the family Paramyxoviridae. First identified in 1994 during an outbreak in Brisbane, Queensland, HeV is characterized by its high case fatality rate. The mortality rate for confirmed human cases is approximately 57-60%, highlighting the severity of infection.
The Natural Host
The natural reservoir for Hendra virus is the Pteropid fruit bat, commonly known as the flying fox, found across coastal and forested regions of Australia. These bats rarely show any signs of illness. They continuously shed the virus into the environment, with the highest concentration of viral RNA found in their urine. Viral excretion also occurs via feces, saliva, and birthing products, which can contaminate areas below their roosts. Viral shedding increases during reproductive periods or when colonies are under nutritional or environmental stress.
Transmission Pathways
The transmission of Hendra virus involves a distinct two-step process, with horses acting as the intermediate or spillover host. The initial spillover occurs when horses ingest feed or water sources that have been contaminated by flying fox excretions, particularly urine. The second step involves horse-to-human transmission, which is the only known route for human infection. People contract HeV through close occupational contact with an infected, symptomatic horse or its tissues and body fluids, typically involving high-level exposure to respiratory secretions, blood, or other excretions from a sick or deceased horse. There is no evidence of direct bat-to-human or human-to-human spread of Hendra virus.
Clinical Presentation in Horses and Humans
Hendra virus infection in horses is typically acute, severe, and rapidly progressive, with a case fatality rate that can approach 75-90%. Infected equines often present with a rapid onset of illness, high fever, and an increased heart rate. Clinical signs include severe respiratory distress and neurological impairment, leading to incoordination, weakness, and collapse. The disease presentation in humans is variable but serious, typically developing between 5 and 21 days after exposure. The illness may manifest initially as an acute influenza-like syndrome, but the most severe form involves the central nervous system, leading to neurological signs such as disorientation and vertigo, which can quickly advance to meningitis or encephalitis.
Prevention and Management Strategies
The primary preventative measure against Hendra virus is the vaccination of horses, which breaks the transmission chain to humans. The equine vaccine, Equivac HeV, is effective at reducing the risk of horses becoming infected and subsequently passing the virus to people, and works against the recently identified variant, HeV-g2. Horse owners should implement biosecurity protocols to mitigate risk, such as covering water troughs and removing feed bins from under trees where bats roost. For human exposure, no licensed vaccine exists, but a human monoclonal antibody, m102.4, is used as a post-exposure prophylactic treatment under emergency authorization. This therapeutic is designed to neutralize the virus following confirmed high-risk exposure.