Ross River virus (RRV) is a small encapsulated single-strand RNA Alphavirus endemic to Australia, Papua New Guinea and other islands in the South Pacific. It is responsible for a type of mosquito-borne, non-lethal but extremely debilitating tropical disease known as Ross River fever, previously termed "epidemic polyarthritis". The virus is suspected to be enzootic in populations of various native Australian mammals, and has been found on occasion in horses.
Classification and morphology
Taxonomically, Ross River virus belongs to the virus genus Alphavirus, which is part of the family Togaviridae. The alphaviruses are a group of small enveloped single-strand positive-sense RNA viruses. RRV belongs to a subgroup of "Old World" (Eurasian-African-Australasian) alphaviruses, and belongs to the SF antigenic complex of the genus Alphavirus. The virions (virus particles) themselves contain their genome in a protein capsid 700 Å in diameter. They are characterised by the presence of two glycoproteins (E1 and E2) embedded as trimeric dimers in a host-derived lipid envelope. Because RRV is transmitted by mosquitos, it is considered an arbovirus, a non-taxonomic term for viruses borne by arthropod vectors.
History In 1928, an outbreak of acute febrile arthritis was recorded in Narrandera and Hay in New South Wales, Australia. In 1943, several outbreaks of arthralgia and arthritis were described in the Northern Territory, Queensland and the Schouten Islands, off the northern coast of Papua New Guinea. The name epidemic polyarthritis was coined for the disease. In 1956, an epidemic occurred in the Murray Valley which was compared to acute viral polyarthritis caused by the Chikungunya virus. The Australian disease seemed to progress in a milder fashion. In 1956, serological testing suggested an unknown new species of alphavirus (group A arbovirus) was the likely culprit. In July and August 1956 and 1957, a virus recovered from mosquitoes collected near Tokyo, Japan, and was dubbed Sagiyama virus. For a time, it was thought to be a separate species, but is now considered conspecific with Ross River virus. In 1959, a new alphavirus was identified in samples from a mosquito (Aedes vigilax) trapped in the Ross River, located in Townsville, Queensland, Australia. Further serological testing showed that patients who had suffered "epidemic polyarthritis" in Queensland had antibodies to the virus. The new virus was named Ross River virus, and the disease Ross River fever. The virus itself was first isolated in 1972 using suckling mice. It was found that RRV isolated from human serum could kill mice. However, the serum containing the virus that was used had come from an Aboriginal boy from Edward River, North Queensland. The child had a fever and a rash but no arthritis, making the link between RRV and Ross River fever less than concrete. The largest-ever outbreak of the virus was in 1979–1980 and occurred in the western Pacific. The outbreak involved the islands of Fiji, Samoa, the Cook Islands, and New Caledonia. However, RRV was later isolated in humans following a series of epidemic polyarthritis outbreaks in Fiji, Samoa and the Cook Islands during 1979. RRV was isolated in an Australian patient suffering from Ross River fever in 1985.
Ecology In rural and regional areas of Australia, the continued prevalence of Ross River virus is thought to be supported by natural reservoirs such as large marsupial mammals. Antibodies to Ross River virus have been found in a wide variety of placental and marsupial mammals, and also in a few bird species. It is not presently known what reservoir hosts support Ross River virus in metropolitan areas such as Brisbane. The southern saltmarsh mosquito (Aedes camptorhynchus), which is known to carry the Ross River virus, was discovered in Napier, New Zealand, in 1998. Due to an 11-year program by the New Zealand Ministry of Health, and later the Ministry of Agriculture & Fisheries, the species was declared completely eradicated from New Zealand in July 2010. As of September 2006, there has never been a report of a case of Ross River virus acquired within New Zealand. Separate mosquito species may act as vector, widespread across areas and seasonal/geographical locations. In southern and northern regions, the Aedes group (A. camptorhynchus and A. vigilax) are the main RRV carriers. However, inland the Culex annulirostris is the main carrier with Aedes mosquitoes becoming active during wet seasons.
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