Assessing Hendra virus risk factors

Before making direct contact with a patient, an attending veterinarian should assess factors that may contribute to Hendra virus (HeV) spill-over and put steps in place to manage work health and safety obligations and human health risks.

Consider the possible combination of risk factors below when you decide on the risk management measures and actions you will take while dealing with equine patients, including providing sound advice to clients.

Always take the following into account:

  • Consider standard work health and safety responsibilities and appropriate risk management strategies when dealing with sick horses. More information on this can be found in the Australian Veterinary Association's Guidelines for veterinary personal biosecurity and Workplace Health and Safety Queensland's information on work health and safety and diseases from animals.
  • Horses have shown variable, and sometimes vague, clinical signs, particularly in the early phase of HeV infection. You should therefore conduct an assessment before you have direct contact with equine patients. Your risk assessment may evolve while attending the case as more information comes to hand.
  • Vaccination of horses is the most effective way to help manage HeV disease. Vaccination of horses provides a public health and work health and safety benefit by reducing the risk of HeV transmission to humans and other susceptible animals. However, whenever HeV infection is suspected, even in vaccinated horses, appropriate biosecurity precautions including personal protective equipment (PPE) should be used as no vaccine can provide 100% guaranteed protection.

Factors to consider before making direct contact with the patient

Patient location

Proximity of flying fox roosts

You should consider HeV wherever horses and flying foxes are in proximity to each other. Known flying fox roosts in the locality have been demonstrated as an additive factor for HeV spill-over risk.

Keep in mind that flying fox roosts vary every year, both in the population and in the prevalence of HeV infection. Give greater consideration to roosts with black and/or spectacled flying foxes as they've been shown to be more likely to excrete HeV.

Find maps of flying-fox roost locations or an interactive flying fox web viewer.

The absence of noted flying fox activity does not remove the risk of HeV infection in horses. Smith CS, (unpublished data) found a statistically significant increase in the risk of equine cases within 7km of a known roost, however a 40km foraging 'footprint' was identified in another risk assessment study (Smith et al. 2014). However during the Bowen incident (2008-09) no roosts were mapped within 50km of the spill-over site (McFarlane et al. 2011).

History of the area

You should also take previous incidents in the area into consideration. Analysis of past events has identified areas with a higher prevalence of incidents. However, the interplay between the climate, location and the reservoir species should be taken into account, (i.e. these areas are most strongly correlated to species densities of black and spectacled flying-foxes) (Smith et al. 2014).

Find a summary of HeV incidents, including general locations.

Time of year

Don't use season as a risk criteria on its own. Seasonality has been identified in South East Queensland and northern New South Wales (NSW), but this winter peak is much less evident in other regions.

All regions sampled in Queensland (Field et al. 2015), show a similar background of 'endemic' infection. In the southern Queensland/northern NSW and central NSW regions, there has historically been a strong winter epidemic pattern.

One study did demonstrate an association with the dry season, May to October, with lower temperatures and rainfall (Mcfarlane et al. 2011). A more recent study (Martin et al. 2018) analysed risk based on latitude. It concluded there are 2 distinct regions:

  • above latitude -22° (approximately in-line with Claireview, 220km north of Rockhampton), spill-over is not affected by seasons
  • below latitude -22°, spill-over risk increases from April to October.

Patient factors

Vaccination history

Vaccination of horses is the most effective way to help manage HeV disease. Whenever HeV infection is suspected, even in vaccinated horses, appropriate biosecurity precautions including PPE should be used as no vaccine can provide 100% guaranteed protection.

Acute versus chronic presentation

HeV infection in horses typically causes an acute illness that is rapidly fatal.

Clinical signs

There are no pathognomonic signs that define HeV infection in horses. Horses infected with HeV have shown variable and sometimes vague clinical signs. There is a range of clinical signs recorded from confirmed cases.


There are a number of measures horse owners can take to reduce the risk of horses becoming infected. Attending veterinarians should also take these measures into consideration when assessing a case.


Field, H, Jordon, D, Edson, D, Morris, S, Melville, D and Parry-Jones K 2015, 'Spatiotemporal aspects of Hendra virus infection in pteroptid bats (flying foxes) in Eastern Australia', PLOS ONE 10(12): e0144055.

Smith, C, Skelly, C, Kung, N, Roberts, B, Field, H 2014, 'Flying-foxes species density – a spatial risk factor for Hendra virus infection in horses in Eastern Australia', PLOS ONE 9(6): e999965. https://doi:10.1371/journal.pone.0099965

McFarlane, R, Becker, N, Field, H 2011, 'Investigation of the climatic and environmental context of Hendra Virus spillover events 1994-2010'. PLOS ONE 6(12): e28374.

Martin, G, Yanez-Arenas, C, Plowright, R.K, Chen, C, Roberts, B, Skerratt, L.F 2018, 'Hendra Virus spillover is a bimodal system driven by climatic factors', EcoHealth.