Hendra Virus Update
The recent case of Hendra virus in Mackay is a timely reminder that this dangerous disease presents a real risk to horses and their handlers. First diagnosed in Australia in 1994, there have been over 100 confirmed horse deaths as a result of Hendra virus in Australia. So, what do we know about Hendra virus?
Hendra virus is a zoonotic disease, meaning it is transmissible between animals and humans. It is a notifiable/emergency disease that has been recorded in QLD, NSW and VIC. At this stage WA, SA, NT, TAS and ACT are free from it.
Hendra virus is an endemic disease in fruit bat (flying fox) populations in Australia. Bats become infected, and re-infected, through normal socialising interactions, similar to the common cold in humans. The disease does not cause illness in bats, with infection lasting a couple of weeks. Other than in bats, clinical disease has been limited to horses, humans and dogs.
On occasion, the virus can leave the bat population and enter the horse population. Horses, in particular, are highly susceptible to the disease, and the fatality rate is very high (70%). Whilst the occurrence and number of cases have been relatively limited, historically, the severity of the disease, and the rapid deterioration of infected horses, make it a significantly dangerous one.
The transmission of Hendra virus is interesting. Bat-bat transmission is highly common, however, bat-human transmission does not occur. Bat-horse transmission occurs and, subsequently, horse-human transmission can occur. Horse-dog transmission has been reported, however, dog-human transmission has not. Dogs do not seem to be clinically affected but can be carriers. In terms of mode of transmission, bat urine is the highest risk bat fluid, with horses often becoming infected by urine contamination of their feed or water sources. Horse-horse and horse-dog transmission can occur via direct or indirect contact with body fluids/discharges, whilst horse-human transmission can occur via oral and respiratory discharges and aerosols, blood, and contact with contaminated tissue, including post-mortem.
One of the concerns with Hendra virus transmission from horses, is the pre-clinical shedding that occurs. This means that horses are potentially infectious to other horses and humans for up to 3 days prior to showing clinical symptoms of the virus. The virus also has a 5-16 day incubation period in horses and early signs can be vague, and sometimes similar to colic (lethargy, depression, inappetence). Deterioration is rapid, with respiratory distress or neurological impairment, and horses are often found severely ill or deceased.
Hendra symptoms in humans occur approximately 5-21 days after exposure and include flu-like symptoms such as cough, fever, sore throat and headache. The disease progresses to inflammation of the brain (encephalitis) with severe headaches, high fever, and drowsiness. Convulsions and coma can occur.
Being aware of this disease, and understanding the importance of incoming horse quarantine, are vital tools to reduce the risk of Hendra virus. A vaccination for horses is available, which offers effective protection against infection. For new horse arrivals quarantine questions such as: has the horse been in a known Hendra area in the past 16 days; what is the horse’s vaccination status; where has the horse been residing; and are there known bat roosts in that area; can be useful for risk assessment. For horse owners concerned about bat roosts on their property, removing horses from paddock areas where bats roost, fencing off roosting areas (to remove horse access), covering outdoor water troughs and feed bins, and stabling horses at night can also reduce the risk of exposure.
With no reported cases in Western Australia, is there a risk? Some flying fox species (black and little red) are present in northern WA, the Gascoyne region and occasionally in small numbers in the mid-west and northern Wheatbelt areas. See the map of main flying fox species.
Horses arriving from the eastern states, particularly QLD and northern NSW, should be assessed for Hendra risk and potential exposure. Isolation for 20 days upon arrival should be considered. Due to the small fruit bat populations in WA, the risk is considered low, however, horse owners should remain vigilant. Always assess the risk of Hendra virus and seek veterinary advice for any suspected cases.
Sources:
https://www.business.qld.gov.au/industries/service-industries-professionals/service-industries/veterinary-surgeons/guidelines-hendra/incident-summary
https://www.agric.wa.gov.au/livestock-biosecurity/hendra-virus-managing-risk-western-australia#:~:text=Hendra%20virus%20is%20an%20emergency,is%20some%20risk%20of%20infection
https://www.researchgate.net/publication/349739999_Threatened_but_not_conserved_flying-fox_roosting_and_foraging_habitat_in_Australia/figures?lo=1
Smith CS, McLAUGHLIN A, Field HE, Edson D, Mayer D, Ossedryver S, Barrett J, Waltisbuhl D. Twenty years of Hendra virus: laboratory submission trends and risk factors for infection in horses. Epidemiol Infect. 2016 Nov;144(15):3176-3183. doi: 10.1017/S0950268816001400. Epub 2016 Jun 30. PMID: 27357144; PMCID: PMC9150281.
https://pubmed.ncbi.nlm.nih.gov/27357144/