Arboviral diseases and Horses – What you need to know

Arboviral diseases are diseases spread by biting insects and may affect many different species, including horses, water birds and on rare occasions, humans. Arboviral diseases are caused by viruses belonging to the genus Flavivirus, including Murray Valley encephalitis (MVE) virus, West Nile virus (Kunjin strain) and Ross River virus, which are all known to occur in Victoria.

As transmission relies on the bite of infected mosquitoes, humans are not at risk of infection from direct contact with infected animals.

Clinical signs in horses

In horses, infection with arboviral disease viruses will not always cause clinical disease, however in some cases animals may present with neurological or musculoskeletal signs such as:

• Staggering
• Incoordination
• Weakness
• Depression.

The clinical signs of arboviral disease also mimic a number of other illnesses, including notifiable diseases such as Hendra. It is therefore important to get an accurate diagnosis from your veterinarian.

History of outbreaks

Ross River virus (RRV), Murray Valley encephalitis virus (MVEV), and West Nile (Kunjin)virus (WNVKUN) are all known to exist in Australia.

RRV is found across Australia, and the flaviviruses, MVEV and KUNV are found in Northern Australia in the Northern Territory, Queensland and northern Western Australia and most recently in NSW (Jan 2022).

Sporadic outbreaks of MVEV have been reported in southeast Australia following periods of high rainfall and flooding, such as occurred in 1974, 2011 and 2021.

Most recently, West Nile virus – Kunjin strain (WNVKUN) has been diagnosed in four foals in two distant parts of NSW. Two foals showing severe neurological signs have recovered with appropriate supportive care. The other two are still undergoing treatment.

RRV is the most common mosquito-borne human pathogen in Australia, and each year thousands of cases are reported through state health departments. Until the 2011 outbreaks, arboviruses were rarely diagnosed as the cause of disease in horses. Consequently, the prevalence of these viruses in horses was considered low.

Over the spring and summer of 2010 to 2011, and again in 2021-2022, the southeast of Australia experienced unusually wet weather and flooding resulting in increased mosquito and wild bird populations. These conditions resulted in increased occurrence of arboviral disease in horses.

Preventative measures

Disease prevention involves reducing contact with mosquitoes, though this can be very challenging in a stable or paddock environment. The following measures can be used:

• Remove sources of stagnant water such as old buckets, tubs and tyres to reduce breeding sites for mosquitoes.
• Use suitable repellents or protective coverings such as summer rugs or fly veils.
• Stable animals during peak periods of mosquito activity (during dusk and dawn).
• Use commercial mosquito traps.