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ROSS RIVER VIRUS T. S. Mair* and P. J. Timoney† Bell Equine Veterinary Clinic, Mereworth, Maidstone, Kent ME18 5GS; and †Gluck Equine Research Center, University of Kentucky, Lexington, Kentucky 40546-0099, USA. Keywords: horse; Ross River virus; ; mosquito-borne; horse; zoonosis

Summary locomotor difficulties can persist for several months Ross River virus is an arthropod-borne virus even years in a percentage of affected individuals () and the cause of the most common (Boughton 1996). Whereas infection with Ross River mosquito-borne human disease in Australia, being virus is commonly encountered in horses in many frequently associated with a debilitating polyarthritis. areas of Australia, especially in the northern tropical Serological evidence would indicate that subclinical regions where there is year-round virus activity infections with the virus are widespread in horses in (Russell 2002), the overall clinical attack rate would many areas of the country. Clinical disease can occur appear to be low (Azuolas 1998). in horses, with affected animals displaying any or all of the following signs: pyrexia, inappetence, lameness, Aetiology stiffness, swollen joints, reluctance to move, ataxia, Ross River virus is a single-stranded, positive sense mild colic and poor performance. Persistence of certain RNA virus with quasi-species structure belonging to clinical signs such as limb soreness and impaired the genus Alphavirus, family Togaviridae. It is performance for months or even years has also been classified in the Semliki Forest complex along with reported in a small percentage of cases. The horse is Semliki Forest, Bebaru, Getah, Chikungunya, considered a possible amplifying host for the virus, O’nyong-nyong, Una and Mayaro and belongs with the ability to develop a high-titred viraemia and to the Getah virus serogroup of agents (van infect various mosquito species. Regenmortel et al. 2000). Considerable sequence homology exists between the genomes of Ross River Introduction and Getah viruses (Strauss and Strauss 1994), with Ross River virus is closely related to Getah virus in evidence of geographic variability among isolates of terms of its taxonomic classification, ecology and the Ross River virus (Lindsay et al. 1993). Ross River nature of the clinical disease it can cause in horses. virus is primarily a mosquito-borne infection and an In contrast to Getah virus, however, Ross River virus important human pathogen. is also an important human pathogen (Russell 2002). It is an arthropod-borne virus or arbovirus that can Epidemiology be transmitted by a wide range of mosquito species As previously indicated, Ross River virus is and which is endemic in Australia, in Papua-New arthropod-borne, and infection principally results Guinea and certain islands in the South Pacific from the bite of an infected mosquito (Russell 1994, (Spradbrow 1972; Russell 1994, 1996, 2002; Azuolas 1996 and 2002). Different mosquito species are 1998). Disease caused by Ross River virus is the most involved as vectors in various regions of Australia or common arboviral infection of man in Australia. It is wherever the virus is found. The species vary a nonfatal disease that is characterised principally by depending on different climatic and environmental a debilitating polyarthritis syndrome (Fraser 1986; conditions, with mosquitoes belonging to the genera Russell 1994). Joint pain, limb soreness and , (Ochlerotatus), Anopheles, Coquillettidia and Mansonia implicated in transmission under field *Author to whom correspondence should be addressed. or experimental circumstances (Aaskov 1997; Russell EVE Man 08-046 Mair v2:Layout 1 14/08/2009 13:10 Page 160

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2002). Notwithstanding the wide range of mosquito source of infection for mosquitoes and may well play species that can serve as potential vectors of Ross a role in disseminating and perhaps maintaining the River virus, only a small number are of primary virus in certain areas/regions of the country. There is importance in transmission in coastal or inland, some evidence to suggest that horses may not urban or rural settings. Principal vectors in costal necessarily be efficient amplifying hosts (Kay et al. situations are the northern saltmarsh mosquito, Ae. 1987). The prevalence of Ross River virus infection in vigilax and the southern saltmarsh mosquito, Ae. horses is high in endemic areas of Australia. In camptorhynchus, whereas in inland areas, Cx. Queensland (an area believed to have year-round annulirostris is the major vector. Similar to Getah mosquito activity), the seropositivity rate was virus, and many other , Ross River virus approximately 80%, compared to 50% in the region of is maintained in a mosquito-vertebrate-mosquito host the Gippsland Lakes in southern Australia (an area cycle. Vertical transmission of the virus has been with seasonal mosquito activity) (Azuolas 1998). In a confirmed in species of culicine mosquitoes serological survey of vertebrate sera (1706 samples) (Dhileepan et al. 1996). obtained from 5 animal species in the area around The natural vertebrate hosts of Ross River virus Brisbane, sera from dogs and horses were most are considered to be nonmigratory native macropodids commonly found positive for antibodies to Ross River e.g. kangaroos and wallabies. Other marsupials may virus, with antibodies present in 22.5% and 25.5% of also serve as possible reservoir hosts (Russell 2002). samples, respectively. Kangaroos and wallabies are assumed to be the most important amplifying hosts. In the area around Clinical signs Brisbane, marsupials (brushtail possums and Ross River virus causes a nonfatal infection in horses. Macropods), flying foxes and horses were identified as Although seroconversion to Ross River virus is the most important host species (Kay et al. 2007). In common, the majority of horses infected with the the latter study, mosquitoes collected in the Brisbane area were analysed for host blood meals. The most virus develop minimal, if any, signs of disease (Kay commonly identified blood meal was dog blood et al. 1987; Azuolas 1998; Azuolas et al. 2003; El-Hage (average of 37.4% of all identified blood meals), et al. 2008). Experimental infection of horses with followed by bird (18.4%), horse (16.8%), brushtail Ross River virus has failed to result in the possum (13.3%), human (11.6%), cat (1.7%), flying fox development of disease. No clinical signs were (0.7%) and macropod (0.2%). Since horses infected reported in either of 2 experimental studies with Ross River virus may develop high-level comprising a total of 14 horses (Gard et al. 1977). viraemias of up to 106.3 suckling mouse intracerebral Accepting that variation in pathogenicity probably exists among Ross River virus genotypes (Fraser LD50/ml (Kay et al. 1987), they serve as an important 1986; Russell 1994), it may be that many virus strains cause only subclinical or inapparent infection. Nonetheless, infection with this virus is considered responsible for musculoskeletal disease in performance horses that has occurred for many years in many riverland and northern regions of Australia (El-Hage et al. 2008), notwithstanding the lack of laboratory confirmation of the disease. Clinical signs associated with Ross River virus infection in horses include pyrexia, inappetence, serous nasal discharge (Fig 1), lethargy (Fig 2), submaxillary lymphadenopathy, distal limb swelling, lameness, stiffness, swollen joints (Fig 3), ataxia, reluctance to move, petechial haemorrhages on the gingival FIGURE 1: Serous nasal discharge associated with Ross River mucous membranes and mild colic (Pascoe et al. 1978; virus infection. Azuolas 1998; Azuolas et al. 2003; El Hage et al. EVE Man 08-046 Mair v2:Layout 1 14/08/2009 13:10 Page 161

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2008). Persistence of certain clinical signs, especially Diagnosis limb soreness and poor performance are believed to No matter how suggestive the clinical signs exhibited occur in a small percentage of affected horses (El- by an affected horse(s) are of infection with Ross Hage et al. 2008). While seroconversion to Ross River River virus, laboratory confirmation of a provisional virus is quite common in horses, to date there have clinical diagnosis of the disease is essential. Diagnosis been few reports of clinical disease associated with is based on detection of the virus in serum or seroconversion or virus isolation (Gard et al. 1977; heparinised blood by virus isolation in cell culture or Pascoe et al. 1978; Azuolas 1998; Azuolas et al. 2003; intracerebral inoculation of suckling mice, or by Studdert et al. 2003). In a recent report, Ross River reverse transcription-polymerase chain reaction virus was believed to be the cause of acute illness in assay of blood or synovial fluid (Azuolas et al. 2003; 4 horses around the Bellarine peninsula in south- Studdert et al. 2003). To optimise the chances of virus west Victoria, Australia, that were naturally exposed isolation, specimens should be collected as early as to the virus (El-Hage et al. 2008). The clinical signs possible after the onset of clinical signs. Recent exhibited by the affected individuals included exposure to and infection with Ross River virus can petechial haemorrhages, lymphadenopathy, distal also be established by demonstration of a detectable limb swelling and reluctance to move. Fibrinogen IgM antibody response in a single serum sample or levels were also elevated in 3 of the 4 horses. Whilst in the acute sample, where paired blood samples no virus was isolated, serological testing revealed taken 2–3 weeks apart are collected from suspect increased IgM titres to the virus in all the horses, cases of the disease (Azuolas et al. 2003). An IgM confirming recent infection. The outbreak occurred at antibody response is generally detectable by Day a time when a known Ross River virus vector, the 7–10 after exposure, peaks within 2–3 weeks before mosquito Aedes camptorhynchus was recorded in very declining rapidly and disappearing as IgG antibody high numbers in the region. Based on the serological levels increase and eventually replace IgM antibodies finding of IgM antibodies, there is every reason to in the circulation (Azuolas et al. 2003). relate the clinical signs observed in these 4 horses to recent infection with Ross River virus. The signs displayed were consistent with those identified Treatment with this virus infection in humans in whom there is Treatment of horses with suspected Ross River virus targeting of synovial and musculoskeletal tissues. infection is supportive and symptomatic, with an ‘Poor performance’ is a frequently reported outcome emphasis on the use of analgesics and nonsteroidal in horses suspected of being infected with Ross River anti-inflammatory drugs to mitigate the muscle virus (Azuolas 1998). soreness, stiffness in gait and swollen joints.

FIGURE 2: Depression, lethargy and inappetence are common clinical signs associated with Ross River virus FIGURE 3: Limb oedema and swollen joints associated with infection. Ross River virus infection. EVE Man 08-046 Mair v2:Layout 1 14/08/2009 13:10 Page 162

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Prevention Kay, B.H., Pollit, C.C., Fanning, I.D. and Hall, R.A. (1987) The experimental infection of horses with Murray Valley encephalitis There is no vaccine currently available against Ross and Ross River viruses. Aust. vet. J. 64, 52-55. River virus infection in horses. Preventing exposure Kay, B.H., Boyd, A.M., Ryan, P.A. and Hall, R.A. (2007) Mosquito of horses to the virus is dependent primarily on feeding patterns and natural infection of vertebrates with Ross controlling the vector (mosquito) population through River and Barmaf Forest viruses in Brisbane, Australia. Am. J. reduction of breeding sites and use of larvicides, as Trop. Med. Hyg. 76, 417-423. well as the natural vertebrate hosts of the virus. The Lindsay, M.D., Coelen, R.J. and MacKenzie, J.S. (1993) Genetic heterogeneity among isolates of Ross River virus from different use of topical insect repellents is recommended on geographical regions. J. Virol. 67, 3576-3585. horses at risk of infection. Pascoe, R.R., St George, T.D. and Cybinski, D.H. (1978) The isolation of a Ross River virus from a horse. Aust. vet. J. 54, 600. References Russell, R.C. (1996) A Colour Photo Atlas of Mosquitoes of Aaskov, J. (1997) Towards prevention of epidemic polyarthritis. Southeastern Australia, Department of Medical Entomology, Arbovirus Res. Aust. 7, 1-4. University of Sydney and Westmead Hospital. pp 43-153. Azuolas, J.K. (1998) Ross River virus disease of horses. Aust. Equine Russell, R.C. (1994) Ross River virus: disease trends and vector Vet. 16, 56-58. ecology in Australia. Bull. Soc. Vector Ecol. 19, 73-81. Azuolas, J.K., Wishart, E., Bibby, S. and Ainsworth, C. (2003) Russell, R.C. (2002) Ross River virus: ecology and distribution. Annu. Isolation of Ross River virus from mosquitoes and from horses Rev. Entomol. 47, 1-31. with signs of musculo-skeletal disease. Aust. vet. J. 81, 344–347. Spradbrow, P.B. (1972) Arbovirus infections of domestic animals of Boughton, C.R. (1996) Australian Arboviruses of Medical Importance, Australia. Aust. vet. J. 48, 181-185. Royal Australian College of General Practitioners, Sydney. Strauss, J.H. and Strauss, E.G. (1994) The : gene Dhileepan, K., Azuolas, J.K. and Gibson, C.A. (1996) Evidence of expression, replication and evolution. Microbiol. Rev. 58, vertical transmission of Ross River and Sindbis viruses 491-562. (Togaviridae: alphaviruses) by mosquitoes (Dyptera: Culicidae) in Studdert, M.J., Azuolas, J.K., Vasey, J.R., Hall, R.A., Ficorilli, N. and southeastern Australia. J. Med. Entomol. 33, 180-182. Huang, J.-A. (2003) Polymerase chain reaction tests for the El-Hage, C.M., McCluskey, M.J. and Azuolas, J.K. (2008) Disease identification of Ross River, Kunjin and Murray Valley suspected to be caused by Ross River virus infection of horses. encephalitis infections in horses. Aust. vet. J. 81, 76-80. Aust. vet. J. 86, 367–370. Van Regenmortel, M.H.V., Fauquet, C.M., Bishop, D.H.L., Carstens, Fraser, J.R. (1986) Epidemic polyarthritis and Ross River virus E.B., Estes, M.K., Lemon, S.M., Maniloff, J., Mayo, M.A., disease. Clin. Rheum. Dis. 12, 369-388. McGeoch, D.J., Pringle, C.R. and Wickner, R.B. (2000) Virus Gard, G.P., Marshal, I.D., Walker, K.H., Ackland, H. and De Sarem, taxonomy: Classification and nomenclature of viruses. In: 7th W.G. (1977) Association of Australian arboviruses with nervous Report of the International Committee on Taxonomy of Viruses, disease in horses. Aust. vet. J. 53, 61-66. Academic Press, San Diego. pp 884-887.