The MARC works closely with local and state governments and industry to develop better ways of predicting and preventing the transmission of endemic mosquito-borne diseases such as those caused by Ross River virus and Barmah Forest virus. These are examples of zoonotic diseases, spread between animals and humans by a mosquito vector. The MARC also works to understand and prevent the transmission of non-endemic viruses such as dengue, chikungunya and Zika, transmitted from human to human by the highly invasive vectors Aedes aegypti and Aedes albopictus. The incursion and establishment of either of those mosquito species in Australia’s major population centres would create a major disease risk because of the high rates of importation of viruses acquired overseas. People who have been infected overseas import over 1000 cases of dengue to Australia annually. Currently, most of those travellers return to areas where there is no competent vector. Today, Aedes albopictus is confined to the Torres Strait islands while Aedes aegypti is only found in Queensland, largely in the tropical north.
Of the viral mosquito borne diseases detailed below, only Japanese Encephalitis has an effective vaccine. None of the viral diseases listed can be cured or prevented by drugs: all that clinicians can do for their patients is manage their symptoms. The only way to manage these diseases is to reduce the risks of being bitten through manipulation of the habitat, the use of insecticides or the application of personal protective measures such as repellents and bed nets. There is a partially effective vaccine for malaria, and a variety of antibiotics can be used for its treatment or prevention, but even for this disease the greatest impacts on transmission in endemic areas have occurred as a result of mosquito control measures.
Ross River virus (RRV) is an endemic, mosquito-borne pathogen that occurs all over Australia. The virus was first isolated from Aedes vigilax mosquitoes collected on the banks of the Ross River in Townsville in 1959 but it was not associated with clinical symptoms until some years later. Approximately 5,000 cases are notified every year with sporadic outbreaks also recorded in the Pacific Islands. The largest Australian epidemic (9,550 cases) occurred in 2015, with the majority of cases (6,193) recorded in Queensland. Acute symptoms usually develop within 7 to 9 days of contracting the virus and include fever, rash, joint and muscle pain, swollen lymph glands, sore throat and fatigue. Debilitating chronic joint pain may last for several month or even years. The transmission of RRV is complex and multiple mosquito species may be responsible. Across Australia, more than 40 mosquito species have been associated with the virus in the field. These include the common pest species Culex annulirostris, Aedes vigilax, Aedes procax, and Aedes notoscriptus. RRV infects mosquitoes when they bite viraemic vertebrates that include humans, horses, possums and birds. We have a poor understanding of the key virus reservoirs and transmission pathways in different habitats.
Barmah Forest virus (BFV) is closely related to Ross River virus. It was first isolated from Culex annulirostris mosquitoes collected in 1974 in the Barmah Forest on the Murray River near Echuca but it was not associated with human disease until 1988. Little is known about the vertebrate hosts of BFV although marsupials and domestic animals such as cattle and horses have been implicated. In 2014/2015, 650 cases of Barmah Forest virus were reported from across Australia, the majority from Queensland. The disease causes similar symptoms to Ross River virus but usually lasts for a shorter duration. Many infected people, especially children, show no symptoms.
Dengue symptoms generally appear 7 to 21 days after infection with dengue, and typically include headache and retro-orbital pain, fever, nausea, joint, bone or muscle pain, rash and swollen glands. On rare occasions, when a patient has been infected previously with a different dengue serotype (there are 4 distinct serotypes in circulation) they may experience dengue haemorrhagic fever (DHF), characterised by internal bleeding and requiring urgent medical attention.
The only mosquito on mainland Australia that can transmit dengue is Aedes aegypti. It is found throughout northern Queensland and as far south as Wondai and Goomeri. Historically, most locally acquired dengue cases have occurred in northern Queensland where Aedes aegypti is most common. In the Torres Strait islands, Aedes albopictus is also a vector with a small dengue outbreak reported from Erub and Badu Islands in 2016.
Outbreaks of dengue in Queensland increased in frequency and intensity from the early 1990s to 2000s. In 2003 and 2004, there were six outbreaks of dengue in north Queensland. Nearly 900 cases were reported from Cairns, Townsville and the Torres Strait. Two Torres Strait Island residents were hospitalised with severe and life-threatening symptoms of DHF and were the first recorded fatalities in Australia in more than a century. Again in 2008 and 2009 over 1,000 cases were reported in north Queensland - the worst dengue outbreak in Queensland for 50 years.
The deployment of the wMel strain of Wolbachia into local Aedes aegypti populations across many parts of north Queensland has resulted in a dramatic reduction of local dengue transmission.
Murray Valley encephalitis (MVEV) is the most serious of the endemic arboviruses in Australia with a 15-30% fatality rate. Large outbreaks in the South East of Australia are uncommon (58 cases in 1974 and 17 cases in 2011) but individual cases occur sporadically in the north of Western Australia and the Northern Territory. The transmission cycle is maintained via viraemic water birds and the freshwater mosquito Culex annulirostris.
Kunjin virus (KUNV) is related to West Nile virus and is often referred to as West Nile virus Kunjin subtype. Although only a small number of cases of KUNV are reported annually, the virus is known to occur in many parts of Australia. Human disease is characterized by febrile illness with rash or mild encephalitis. It also affects horses, sometimes with severe consequences. KUNV shares a similar epidemiology and ecology with Murray Valley encephalitis. In northern regions of Australia, KUNV is also maintained in a cycle that involves water birds and Culex annulirostris.
Japanese encephalitis virus (JEV) can cause serious infections of the brain. JEV is restricted to far North Queensland and the Torres Strait, but is widely dispersed in areas of eastern Asia, including Papua New Guinea and Indonesia. Human cases of JEV were first notified from the Torres Strait in 1995 (with 2 deaths) and the Australian mainland (Cape York Peninsula) in 1998 (a single non-fatal case). Most human cases of JEV are asymptomatic but severe neurological signs associated with encephalitis occur in up to 5% of cases. Of symptomatic cases, 20–30% are fatal, and among the survivors, approximately 30–50% will have ongoing neurological complications. Queensland Health now vaccinates all long-term residents of the Torres Strait and recommends vaccination for all non-residents who spend more than 30 days in the Torres Strait during the wet season. The main mosquito vector in Australia is Culex annulirostris with waterbirds (herons and egrets) as the main reservoirs and pigs serving as important amplifying hosts.
Chikungunya virus (CHIKV) is emerging as a significant vector borne disease and since 2004 has caused large epidemics in many regions of the world. These outbreaks have had a severe impact on individuals, communities and public health resources. Most people infected with CHIKV will develop some symptoms 3–7 days after being bitten by an infected mosquito. The most common symptoms are fever and joint pain, headache, muscle pain, joint swelling and a rash. Chikungunya disease does not often result in death, but the symptoms can be severe and disabling. People at risk for more severe disease include newborns infected around the time of birth, older adults (≥65 years), and people with medical conditions such as high blood pressure, diabetes, or heart disease. The principle vectors are Aedes aegypti and Aedes albopictus.
Zika virus (ZIKV) infection during pregnancy can cause infants to be born with microcephaly and other congenital malformations. Infection is also associated with other complications of pregnancy including preterm birth and miscarriage and an increased risk of neurologic complications including Guillain-Barré syndrome, neuropathy and myelitis. Zika virus is primarily transmitted through the bite of Aedes aegypti mosquitoes but may also be transmitted sexually, from human to human. Imported cases are detected in Australia each year, but there has not yet been any local transmission. The vast majority of Zika cases in adults are asymptomatic but where infection is reported, its symptoms appear similar to mild dengue.
Malaria is caused by single-celled parasites of the Plasmodium genus. It is spread from human to human by infected female Anopheles mosquitoes. The disease is widespread in the tropical and subtropical regions of the world and is most devastating in Sub-Saharan Africa. In 2017, an estimated 219 million cases of malaria occurred worldwide with 435 000 deaths. Children aged under 5 years are the most vulnerable group accounting for 61% of all malaria deaths.
Australia was declared 'malaria free' by the World Health Organization in 1981. However, mosquito species that can transmit the malaria parasite remain distributed throughout Australia and the number of imported cases has increased dramatically with the increase in global travel. Since the 1980s there have been several outbreaks associated with the introduction of Plasmodium vivax or Plasmodium falciparum in an infected human or mosquito from overseas. In 2002, 7 locally acquired cases of Plasmodium vivax were reported from a campsite in north Queensland. In 2011, 8 cases of locally-acquired Plasmodium falciparum malaria occurred on the Torres Strait islands of Saibai and Dauan.
Dog heartworm (caused by Dirofilaria immitis) has been an increasing veterinary problem in recent decades and human infections are occasionally reported with eye, lung, and skin involvement. Local mosquitoes, including species in southern Queensland (Aedes notoscriptus and Culex annulirostris) have been shown to be highly susceptible to infection and have close contact with dogs in domestic and peridomestic urban and rural situations.