With Summer Comes Mosquitoes: Australia Health Dept. Warns of Ross River Virus

On December 9, Australia’s Northern Territory health department warned residents that this season’s mosquito numbers and high tide do not bode well for risk of mosquito-borne illnesses. Health officials were particularly concerned about Ross River virus, a virus we don’t see too often on HealthMap.

Ross River virus is a mosquito-borne infectious virus, from the genus alphavirus and the family Togaviridae, which is endemic to Australia, Papua New Guinea and a number of islands in the South Pacific.

The first recorded incidence of the virus is believed to be from 1928, when two epidemics of rash and joint pain were reported from Narrandera and Hay in New South Wales. In 1946, after several similar outbreaks had occurred in Australia and Papua New Guinea, the syndrome was referred to as “epidemic polyarthritis.” Scientists isolated the Ross River virus in 1972.

In Australia, it is the most common and widespread arbovirus. In recent years, the country has reported an average of 5,000 cases annually.

The alert on Monday was issued by the Northern Territory health department and voiced concern over the rising number of mosquitos this time of year. Stagnant water becomes a breeding ground for vectors for the virus. The alert encourages inhabitants of Top End to remove stagnant water sources, such as filled buckets, plant drip trays, and unused swimming pools, near their homes. December and January are usual high-risk months for humans to contract the virus.

Ross River fever is characterized by influenza-like symptoms and polyarthritis, or arthritis in 5 or more joints simultaneously. Though not fatal, the disease is nevertheless debilitating, with some cases reporting difficulty getting out of bed. Severity of the disease varies widely. Swollen joints, headache, fever, lethargy, muscle aches and skin rash are common. Meanwhile approximately one-third of infections are asymptomatic, especially among children.

The incubation period lasts seven to ten days. Once the symptoms appear, they can last a few weeks up to a year, with the majority of case feeling progressively better within three months. Typically the flu-like symptoms resolve in a month, and it is the joint/muscle stiffness and pain that lingers.

Ross River virus is not contagious and thus cannot pass person-to-person. It spreads via mosquitos that are infected after biting infected animals. The infected mosquito then bites a human, passing along the disease through its salvia. Several different species of mosquitoes can carry Ross River virus, including Culex annulirostris, Aedes vigilax, and Aedes notoscriptus. The main natural reservoirs are kangaroos and wallabies, though in urban areas possums, horses, birds and flying foxes may also act as hosts.  

A blood test is the only way to confirm a case. Once confirmed, treatment includes antiviral drugs as well as drugs to reduce inflammation, pain and fever. Patients are instructed to rest, hydrate, and allow the virus to run its course.

Currently there is no vaccine against Ross River virus. Preventative measures include the use of bug spray and insecticide, wearing long clothing, outfitting your house with screens, using nets when camping and avoiding mosquito-infested areas. 

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