Ross River virus (RRV) infection, a mosquito-borne disease, affects humans and horses. Named after the Ross River, which flows through Townsville on the northeastern coast of Queensland, the infection is also referred to as epidemic polyarthritis or Ross River fever.

Transmitted to humans by several species of mosquitoes, humans infected with RRV typically develop fever, arthritis in multiple joints, and rashes. Infection is seldom fatal, however. Joint difficulties may be profound and linger for years after other signs have diminished.

In horses, RRV infection was first identified in the 1970s and has been recognized throughout Australia with especially high rates of infection in Victoria, southeastern New South Wales, and southeastern and northern Queensland.

The virus is transmitted enzootically between mosquitoes and kangaroos, wallabies, flying foxes, horses, and other vertebrates. Human–mosquito–human transmission also occurs.

Clinical signs associated with RRV infection in horses include fever, musculoskeletal abnormalities (swollen joints, swelling of the lower limbs, reluctance to move, lameness), abnormal clinical pathology (anemia, lymphopenia), and behavioral changes (lethargy, inappetence, increased lying down). Other documented clinical signs include pinpoint hemorrhages of oral mucosa and submandibular lymphadenopathy.

Persistence of certain signs, especially those that affect athletic performance such as musculoskeletal deficiencies and exercise intolerance, can last for months. In one case-series report, most horses did not return to work for 7 to 12 months after initial clinical signs.

“Despite a long convalescence, many horses return to their previous level of performance, so that is encouraging for owners of affected horses,” explained Peter Huntington, B.V.Sc., M.A.C.V.Sc., director of nutrition at Kentucky Equine Research Australasia.

Diagnosis of RRV is typically made based on serologic testing for specific antibodies. Paired serum samples taken 2-4 weeks apart assist in pinpointing time of infection. An IgM response is generally detectable 7-10 days after infection and peaks within 2-3 weeks before declining. At that point, antibody class switches, and IgG becomes the predominant antibody detected. IgG antibodies to RRV are believed to be lifelong, so detection of IgG in horses demonstrates only prior exposure to RRV.

Supportive treatment of RRV includes the use of nonsteroidal anti-inflammatory drugs (NSAID) to control fever and to alleviate musculoskeletal pain. Some horses are sensitive to the effects of NSAID, so gastrointestinal support should be offered simultaneously. Systemic corticosteroids are not generally used.

Nutritional antioxidants, including vitamins E and C, have been prescribed by some veterinarians.

“Antioxidants combat oxidative stress and counter the buildup of free radicals in joints and muscles of affected horses. In choosing a vitamin E supplement, natural sources are preferred over synthetic sources because of their superior bioavailability. Nano-E, a natural-source vitamin E developed by Kentucky Equine Research, features a nanodispersion delivery system that ensures its absorption, leading to rapid increases in serum levels and availability for body-wide use,” explained Huntington.

Preserve, available in Australia, contains vitamin C and selenium in addition to natural vitamin E. Other high-quality antioxidants are also appropriate for horses during their recovery, including coenzyme Q10.

Moreover, chondroprotective agents, such as sodium hyaluronan or polysulfated glycosaminoglycans, support joint health and are therefore appropriate for horses exhibiting musculoskeletal problems.

“The anti-inflammatory benefits of long-chain omega-3 fatty acids are also recommended. Research-proven EO-3 increases plasma and tissue levels of DHA and EPA, two important omega-3s,” Huntington said.

No vaccine is available for RRV, so prevention strategies involve reducing mosquito exposure, including:

  • Eradication of stagnant water wherever possible, including old tires, cisterns, or water troughs that collect rainwater. Low-lying areas in paddocks or pastures that create slow-draining puddles should be addressed.
  • Rugs, fly sheets, and fly masks provide a physical barrier against mosquitoes, and use of the topical insecticide permethrin is encouraged.
  • Turnout scenarios that leave horses stabled during dawn and dusk, when vectors are most active.
  • Modifications to stables, such as window and door screens; installation of fogging machines, overhead fans, or misting systems; and use of incandescent bulbs.

Ross River virus is considered an arbovirus, defined as a virus transmitted by biting arthropods, which include mosquitoes, biting midges, sandflies, and ticks. As mentioned previously, the primary vectors for RRV are mosquitoes. Arboviruses replicate in the bodies of vectors, which differentiates them from viruses that have a mechanical mode of transportation, e.g., carried in the mouth of an insect as it travels between animals. Other well-known arboviruses include Eastern equine encephalomyelitis, Western equine encephalomyelitis, Venezuelan equine encephalomyelitis, and West Nile virus.

Azuolas, J.K., E. Wishart, S. Bibby, and C. Ainsworth. 2003. Isolation of Ross River virus from mosquitoes and from horses with signs of musculoskeletal disease. Australian Veterinary Journal 81:344-347.

Barton, A.J., and H. Bielefeldt-Ohmann. 2017. Clinical presentation, progression, and management of five cases of Ross River virus infection in performance horses located in southeast Queensland: A longitudinal case series. Journal of Equine Veterinary Science 51:34-40.

Chapman, G.E., M. Baylis, D. Archer, and J.M. Daly. 2018. The challenges posed by equine arborviruses. Equine Veterinary Journal 50:436-445.

El-Hage, C.M., N.J. Bamford, J.R. Gilkerson, and S.E. Lynch. 2020. Ross River virus infection of horses: Appraisal of ecological and clinical consequences. Journal of Equine Veterinary Science 93:103143.

El-Hage, C.M., M.J. McCluskey, and J.K. Azuolas. 2008. Disease suspected to be caused by Ross River virus infection in horses. Australian Veterinary Journal 86:367-370.

Huntington, P.J. 1984. Arbovuirus infection in horses. Australian Equine Veterinarian 2:9-10.