Bit & Bridle

Equine Hendra Virus

Other Names

  • Equine Morbillivirus (historical)
  • Acute Equine Respiratory Syndrome (associated)
  • HeV

What is Equine Hendra Virus?

Equine Hendra Virus (HeV) is a highly lethal zoonotic paramyxovirus that causes acute, rapidly progressive respiratory and/or neurological disease in horses. It is one of a small number of equine diseases with serious implications for human health — the virus can be transmitted from horses to humans, where it also causes fatal encephalitis. HeV is classified as a Biosafety Level 4 (BSL-4) pathogen and is subject to strict international reporting requirements.

The virus is a member of the Henipavirus genus (family Paramyxoviridae), closely related to Nipah virus. It was first identified in 1994 following an outbreak in Hendra, a suburb of Brisbane, Queensland, Australia, which killed 13 horses and one person. Since that initial outbreak, all confirmed cases have occurred in Australia, where the virus is endemic in fruit bats (flying foxes) of the genus Pteropus, which serve as the reservoir host.

While Hendra virus is currently geographically restricted to eastern Australia, it is included in global equine disease awareness resources due to its exceptionally high case fatality rates in both horses and humans, its biosafety implications, and the importance of awareness for anyone working with horses in or travelling to affected regions. A licensed equine vaccine is available in Australia.

Causes

  • Reservoir host: Australian fruit bats (Pteropus spp. — grey-headed flying foxes, black flying foxes, little red flying foxes, and spectacled flying foxes) are the natural reservoir of HeV and do not develop disease themselves. The virus is present in their urine, saliva, faeces, and birthing products.
  • Horse-to-human transmission: Horses are considered “spillover” hosts — they become infected through contact with bat secretions (typically via contaminated pasture, water, or food sources). Infected horses then shed large amounts of virus in their body secretions, including respiratory secretions, urine, and blood, creating the risk of transmission to humans through close contact.
  • Human risk factors: Veterinarians, horse owners, and handlers performing procedures on sick horses without adequate PPE are at greatest risk. Transmission requires close physical contact — typically through mucosal exposure to infected horse secretions. No sustained human-to-human transmission has been documented.
  • Seasonality: Cases tend to cluster in winter and spring in Australia, coinciding with periods of bat food stress when flying foxes are more likely to shed virus in areas used by horses.

Symptoms and Signs

Clinical signs in horses develop rapidly and can progress to death within days:

  • Fever: Often the earliest and most consistent sign, usually high (above 40°C)
  • Depression and lethargy
  • Anorexia
  • Respiratory signs: Laboured breathing, increased respiratory rate, nasal discharge (which may be frothy and blood-tinged in severe cases), pulmonary oedema
  • Neurological signs: Ataxia (incoordination), head tilting, apparent blindness, aimless circling, muscle tremors, facial twitching, and recumbency
  • Facial swelling
  • Rapid deterioration: The disease course is extremely fast — horses may die within 1–3 days of first clinical signs

Some cases present primarily as respiratory disease, others primarily as neurological disease, and many as a combination of both. The case fatality rate in horses is approximately 75–80%.

Diagnosis

Suspected Hendra virus infection in Australia is a notifiable disease and triggers an immediate biosecurity response. Any horse presenting with acute severe respiratory and/or neurological signs of unknown cause in Queensland or New South Wales should be treated as a potential HeV case until proven otherwise:

  • Isolation immediately: The horse should be isolated and no unnecessary personnel should approach without full PPE (full protective suit, gloves, eye protection, and N95 respirator minimum)
  • Notify authorities: State animal health authorities must be notified immediately. In Australia, this is a mandatory reportable disease.
  • Laboratory testing: Nasal swabs, blood (EDTA and serum), urine, and cerebrospinal fluid are submitted to the Australian Centre for Disease Preparedness (ACDP) or designated BSL-4 laboratories. PCR testing detects viral RNA. Serology detects HeV-specific antibodies.
  • Post-mortem examination: Conducted under strict containment conditions by qualified personnel with full PPE

Important: No routine equine practice vet should attempt detailed examination of a horse suspected to have Hendra virus without following the recommended biosafety protocols. Contact state authorities before intervening.

Treatment

There is no licensed specific antiviral treatment for Hendra virus in horses. Treatment is supportive, but given the rapid progression and extremely high case fatality rate, and the biosafety constraints that limit close examination and intervention, most affected horses do not survive:

  • Supportive care is limited by biosafety requirements — only properly protected, trained personnel should provide care
  • Euthanasia is frequently necessary on welfare and biosafety grounds in confirmed or highly suspected cases
  • The monoclonal antibody m102.4, developed for human post-exposure treatment, has been used experimentally in horses under compassionate use protocols in Australia, but it is not a licensed equine treatment

Prevention

Prevention is centred on vaccination in Australia and biosafety management:

  • Hendra Virus Vaccine (Equivac HeV): A licensed vaccine is available in Australia and provides effective protection against HeV infection in horses. Vaccination is strongly recommended by the Australian Veterinary Association (AVA) and equine health authorities for all horses in Queensland and New South Wales. Vaccinated horses significantly reduce the human public health risk.
  • Reduce bat contact: Remove horses from paddocks where large numbers of bats are roosting. Cover water sources and hay/grain storage. Avoid feeding horses under or near bat roosts.
  • PPE when examining sick horses: Any veterinary professional or handler examining a horse with unexplained acute respiratory and neurological disease in HeV-endemic areas should use appropriate PPE until HeV has been excluded
  • Biosecurity protocols: Isolate any horse with unexplained severe illness until HeV can be excluded
  • Awareness: Horse owners in and travelling to affected regions of Australia should understand the signs of HeV and know to contact state animal health authorities immediately

Prognosis

The prognosis for horses with confirmed Hendra virus infection is grave. Case fatality rates of approximately 75–80% are reported across all documented outbreaks. Horses that survive are considered a significant ongoing biosafety risk and must remain isolated and managed under official veterinary oversight. Euthanasia of confirmed cases is often the recommended course of action on both welfare and public health grounds. For humans exposed to an infected horse, post-exposure prophylaxis with m102.4 has been used in Australia.

When to Call the Vet

In Australia (Queensland, New South Wales, and northern regions):

  • Any horse with unexplained acute fever, respiratory distress, and/or neurological signs — treat as a HeV suspect until proven otherwise
  • Do not approach the horse without PPE and do not allow unnecessary personnel near it
  • Call the state animal health authority emergency line immediately — do not wait for a standard vet appointment
  • If you have had unprotected close contact with a sick horse with these signs, seek urgent medical attention and inform the treating physician of the potential HeV exposure

Outside Australia: No confirmed cases have ever been reported outside Australia. However, any horse with acute severe respiratory and neurological disease should always receive prompt veterinary assessment.

Frequently Asked Questions

Can Hendra virus spread outside Australia?

All confirmed HeV cases in horses and humans have occurred in Australia. The reservoir hosts — Australian fruit bats of the genus Pteropus — do not exist outside Australia, which currently limits the geographical distribution of the virus. There is theoretical concern about the virus spreading via infected horses in transit, which is why it remains a notifiable disease at international level, but there has been no evidence of establishment elsewhere to date.

Is the Hendra vaccine safe for horses?

Yes. Equivac HeV has been through rigorous regulatory evaluation by the Australian Pesticides and Veterinary Medicines Authority (APVMA) and is considered safe and effective. The Australian Veterinary Association strongly recommends vaccination of all horses in endemic areas. Some horses may experience mild local reactions at the injection site, which is normal for many vaccines.

What should I do if I think my horse in Australia has Hendra virus?

Immediately isolate the horse. Do not allow others to approach without full PPE. Phone your state agricultural department emergency line or the Emergency Animal Disease Watch Hotline (1800 675 888 in Australia) immediately. Do not attempt to examine the horse without appropriate biosafety equipment. If you have already had unprotected contact with the animal, seek urgent medical advice and inform the doctor of your exposure.

How does Hendra virus differ from other equine respiratory diseases?

The key distinguishing features are the combination of acute severe respiratory AND neurological signs together, the extremely rapid progression to death, and the very high case fatality rate — unusual for most equine respiratory diseases, which rarely kill healthy adult horses so quickly. Equine influenza, equine herpesvirus, and strangles can all cause respiratory illness, but none carries the zoonotic risk or the mortality rate of Hendra virus. Any horse with this combination of signs in a HeV-endemic area must be treated as a potential Hendra case until laboratory results confirm otherwise.