see also:

• zoonoses
• Barmah Forest virus (BFV)
• Murray Valley encephalitis (MVE)
• Japanese encephalitis virus (JEV)
• viruses

• mosquito-borne viral infection causing epidemic polyarthritis
• no evidence of transmission from person to person as transmission requires a female mosquito vector
• infection with the RRV confers lifelong immunity
• occurs in all states of Australia and is the most common mosquito transmitted disease with over 6,500 cases per year with occasional local epidemics with much higher numbers
  • in 2020:
    • 3,500 cases in Qld
    • 2,000 cases in NSW - a record high for NSW - mainly Hunter-New England region and northern NSW
    • ~450 cases in Vic - mainly Bellarine Peninsula, Surf Coast, Loch Sport region and East Gippsland (usually there are 200 cases pa, but late 2016 floods resulted in 2000 cases in 2017 )
• the most common mosquito-borne disease in Australia with around 5000 notifications annually
• infections in Australia are 10-100x more common than Barmah Forest virus (BFV)

• can be spread by more that 40 mosquito species, and hosts can include kangaroos, wallabies, bats, horses, and dogs.
• warmer temperatures significantly increase the risk of Ross River Virus outbreaks, particularly in riverland and coastal regions. The study found that infections are most likely when temperatures range between 17°C and 31°C, with cases peaking around 26°C. ¹⁾
• inland areas:
  • Culex annulirostris
    • Common Banded Mosquito, has a distinctive median pale band on its proboscis and has banded legs
    • native to Australia, Fiji, Micronesia, the Philippines and Indonesia
    • one of the most important nuisance-biting pests and vectors of mosquito-borne pathogens associated with freshwater habitats in Australia.
    • breeding takes place anywhere there is standing water, from swamps and ponds to man-made puddles—irrigation channels, bamboo stumps, cacao shells, the bottoms of canoes. Water can be clean or polluted, in sun or shade, and fresh or brackish
    • active between spring and late autumn when they appear most commonly at dusk, though can also be active during the day and indoors.
    • can travel 5–10 km from their place of birth and feed on mammals and birds.
    • an important vector for a number of arboviruses, including Murray Valley encephalitis virus, Ross River virus, Barmah Forest virus, Kunjin virus and Japanese encephalitis, Jogalong virus, as well as dog heartworm and the roundworm Wuchereria bancrofti in New Guinea
• coastal regions:
  • Ochlerotatus vigilax in New South Wales
    • can disperse large distances
  • Ochlerotatus camptorhynchus (the southern saltmarsh mosquito, aka Aedes camptorhynchus) in southern parts of WA, Victoria and Tasmania
    • it also spread to NZ in 1998 but was declared eradicated in 2010
    • it is active year round and feeds readily on humans and other animals during the day, particularly around dusk and dawn
    • most disperse 1-3km but can disperse large distances 3-6km

female Ochlerotatus camptorhynchus image courtesy of Tasmanian insect field guide

Culex annulirostris image courtesy https://wrbu.si.edu/vectorspecies/mosquitoes/annulirostris

• primary mosquito–mammal cycle involving macropods (particularly the western grey kangaroo), and possibly other marsupials and wild rodents
• human–mosquito cycle may occur in explosive outbreaks
• horses can act as amplifier hosts and appear to develop joint and nervous system disease after infection
• fruit bats might act as vertebrate hosts in some areas
• vertical transmission in desiccation-resistant eggs of Ochlerotatus spp. mosquitoes may be a mechanism to enable the virus to persist in the environment for long periods and then rapidly appear after heavy rains

• 60% are subclinical, and clinical disease is rare in pre-pubertal children
• incubation period post mosquito bite: 7-10 days
• variable severity flu-like illness
  • fever, chills, headache, myalgias and arthralgias
• 2/3rds with symptoms have a rash which can occur up to 2 weeks before or after other symptoms, and in some may be the only symptom
  • may be associated with buccal and palatal enanthems
• cervical lymphadenopathy occurs frequently
• rheumatic symptoms are present in most patients and consist of arthritis or arthralgia, primarily affecting the wrist, knee, ankle and small joints of the extremities
• paraesthesiae and tenderness of the palms and soles are present in a small percentage of cases
• some have long lasting joint damage and pain
  • can cause incapacity and inability to work for 2–3 months
  • 25% have have rheumatic symptoms that persist for a year or more
  • some cases, there may be remissions and exacerbations of decreasing intensity for years

• serology:
  • IgG seroconversion, a significant increase in antibody level, or a fourfold or greater rise in titre to RRV
  • detection of RRV-specific IgM in the presence of RRV IgG
• isolation of RRV
• detection of RRV by nucleic acid testing

• supportive care, rest, analgesia / anti-inflammatories