In Australia, as in many other countries, mosquitoes are not just an irritation but can carry and spread bacterial, viral and parasitic diseases to humans - known as ‘vector-borne diseases’. The infections transmitted by mosquitoes can range from mild to severe, occasionally resulting in death.
The major Australian mosquito-borne disease risks are:
- Ross River Fever
- Barmah Forest infection
- Murray Valley encephalitis and Kunjin virus disease
- Dengue fever
- Japanese encephalitis
ROSS RIVER VIRUS (RRV)
Infection with Ross River Virus, an arthritogenic Alpha virus, can result in the illness Ross River fever, the most commonly reported and widespread mosquito-borne disease in Australia, averaging around 5,000 infections each year, although this is likely to be an underestimate of the true number.
In Australia, more than 40 types of mosquitoes from a diverse range of habitats are known to play a role in the transmission of RRV between animals and humans. And many of our species are suitable virus reservoirs: wild and domestic animals, particularly kangaroos and wallabies, but also humans who have sparked epidemics in urban areas and in the islands of the South Pacific. As our population grows and development moves out into rural areas near to wildlife and abundant mosquito breeding grounds, the risk of infection increases.
While RRV infection is not fatal it can be quite debilitating as a result of peripheral arthritic joint pain which affects over 80% of patients - along with Barmah Forest infection (see below), Ross River fever is referred to as epidemic polyarthritis. The severity of symptoms, which appear 3-11 days following the bite of an infected mosquito, range from asymptomatic - more common in children - to mild (can include fatigue, fever, swollen, aching joints, muscle pain, and a week later, a rash) to severe, lasting from a couple of weeks to several months, with some patients experiencing persistent chronic joint pain and tiredness for many years.
There is no specific treatment for Ross River Virus disease, only supportive care to treat the symptoms – generally analgesics and anti-inflammatory medication. An effective vaccine has been developed but is not licensed (and therefore is not available).
Where is it found?
Ross River virus is widespread in Australia but more commonly found in Queensland, the Northern Territory and the Kimberley region of Western Australia. During the wet season from January to April, the incidence is generally higher. It is prevalent during late spring, summer and early autumn in the southern regions of Auistralia.
BARMAH FOREST INFECTION (BFV)
As with RRV, Barmah Forest virus (BFV) is categorised as epidemic polyarthritis due to its effect on the joints. Most infections are asymptomatic, but in the event of a clinical illness, symptoms that develop 7-10 days after exposure include fever, chills, headache, rash, muscle pain, joint pain, stiffness and swelling, fatigue and weakness. Most people recover in a few weeks however some experience ongoing fatigue and joint pain for many months.
Treatment entails the use of analgesia and anti-inflammatory medications to manage symptoms.
Where is it found?
Barmah Forest virus is also widespread throughout most regions of Australia, including Tasmania, but the incidence is greatest along inland waterways and coastal areas. Higher rates of infections are related to warm, humid conditions, particularly following floods, heavy rains and high temperatures.
Australia-wide, roughly 250 to 750 BFV infections were recorded each year since 2014 - QLD and NSW (particularly northern districts) have the highest case numbers (also RRV).
MURRAY VALLEY ENCEPHALITIS (MVE) & KUNJIN VIRUS (KUN)
Both viruses cycle between water birds via infected mosquitoes which can then also bite and infect humans (and wild/domestic animals), however the low levels of virus reproduction in man don’t allow for the onward transfer of the infection. The main vector, a Culex spp. mosquito, generally feeds for a couple of hours around dusk and dawn.
While most people with MVE virus are asymptomatic, apparent clinical infection is often severe due to neurological involvement, causing death in approximately 15-30% of cases and residual mental or functional disability in up to half of those who survive. The incubation period is generally 1–4 weeks and early symptoms can include sudden onset of fever, headache, nausea, vomiting, diarrhoea, rash and cough. Other symptoms that can occur in young children are irritability, floppiness and drowsiness.
Progression of the infection is evidenced by neurological symptoms such as lethargy, irritability, headache, neck stiffness, confusion and seizures.
There is no vaccine or specific treatment for Murray Valley encephalitis so the recommended course is early diagnosis and supportive care (generally as an in-patient in hospital). Infection with MVE virus confers lifelong immunity against future infections.
The Kunjin virus (a subtype of the West Nile virus) is related to MVE but is more likely to cause a milder infection. Incubation times are shorter (2-14 days) and symptoms, if they occur, can include fatigue, rash, muscle weakness, enlarged lymph nodes and swollen, painful joints. Of the infections that do produce symptoms, some can result in encephalitis which requires treatment in hospital. Fatalities are rare.
As with MVE, infection with KUN provides immunity from another bout (but there is no cross protection against MVE and vice versa).
Where are they found?
MVE virus is enzootic (endemic in animals) in freshwater habitats of the Kimberley region of Western Australia and the northern area of the Northern Territory. Occasional outbreaks occur as far south as NSW following the migration of infected birds and with local mosquito populations boosted by rainfall/flooding.
Kunjin virus is more widespread than MVE, ranging across the tropical north and down through Qld into SE Australia and Victoria.
DENGUE FEVER (DENV)
The dengue virus is no longer endemic in Australia but is introduced each year by infected travellers arriving from overseas. Most tropical and sub-tropical regions across the globe report locally-acquired cases of dengue fever – the WHO estimates that around half the world’s population is at risk of infection.
Infection with one of the four dengue viruses causes disease ranging from asymptomatic right through to fatal. In symptomatic cases, the incubation period of 4 –10 days is followed by a high fever and at least 2 of these symptoms: severe headache, pain behind the eyes, muscle and joint pains, nausea and vomiting, swollen glands and a rash. Infection with one dengue serotype does not confer lasting immunity to the other three and a second infection is more likely to result in haemorrhagic complications. More information about dengue fever can be found under the Resources tab.
Where is it found?
The primary mosquito vector capable of transmitting the dengue virus - Aedes aegypti - is more commonly found in far north QLD, in and around urban areas, although in the past it was also present in the NT, WA and southern NSW. Another Aedes species with a wider range of habitats, Aedes albopictus, is a secondary vector of DENV in Australia. Both species are most active just after dawn and before dusk but will feed during the day as well as indoors; they are most likely to use man-made containers with clean water to lay their eggs in.
JAPANESE ENCEPHALITIS (JEV)
Japanese encephalitis is a mosquito-borne viral disease that mainly occurs in rural regions of Asia and the Pacific region, although through increased urbanisation cases are being reported more often in and around urban areas. It is more prevalent where rice is cultivated using flooding irrigation or in swampy areas, however seasonal rainfall and high temperatures play a part in the variable risk patterns seen in some countries.
The JE virus exists in a transmission cycle between the Culex species of mosquito (dusk to dawn feeder) and pigs and wading birds, and occasionally to humans. Humans are considered dead-end virus hosts as mosquitoes are unlikely to become infected through feeding on the low levels of virus circulating during an infection (unlike pigs which are known as virus amplifiers).
While most JEV infections are mild or even without obvious symptoms, roughly 1 in 250 infections will result in a severe illness. Symptoms can vary, and following the 5-15 day incubation period, children are more likely to report abdominal pain and vomiting; in adults fever and headache are more common. The rapid onset of a high fever, neck stiffness, disorientation, coma and seizures signals severe disease which is fatal in around one third of cases. Of the remainder, another third will suffer some sort of neurological after-effects such as paralysis, epilepsy and problems with speech.
There is no cure for Japanese encephalitis and so treatment is focused on relieving severe clinical signs and supporting the patient to overcome the infection. There are however preventive vaccines – two formulations are available. Within Australia vaccination is recommended for the following groups:
- residents of the outer islands in Torres Strait
- non-residents who will be living or working on the outer islands of Torres Strait for a cumulative total of 30 days or more during the wet season from Dec-May (JE virus transmission in Torres Strait usually peaks in February and March)
Where is it found?
Japanese encephalitis is found in many parts of Asia, the Indian subcontinent, Southeast Asia and China, however the virus has become more widespread, with cases also occurring in Indonesia, islands of the Torres Strait, Papua New Guinea and in the past, one case in North QLD. Active surveillance was discontinued in the Torres Strait in 2005 so the JEV status in the north is unknown. More detailed information on JEV, vaccines and the indications for travellers can be found under the Resources tab.
RISKS AND PREVENTION FOR TRAVELLERS
The prevalence of mosquito-borne diseases depends on many factors - season, temperature, rainfall and breeding sites among them and the disease vectors are active and feeding at different times of the day i.e. daytime biting for Aedes species but dusk to dawn for the species that transmit Japanese encephalitis and Ross River Virus. For this reason, using precautions to avoid mosquito bites at all times is the safest option.
- Avoid travelling into outbreak areas
- Be aware of peak mosquito exposure times and places
- Use effective insect repellents & contact insecticides
- Wear appropriate clothing
- Avoid scents & perfumes
- Mosquito nets, insect coils and sprays
One other infection of note is Buruli ulcer (also known as Bairnsdale or Daintree ulcer) that is predominantly found in coastal parts of Victoria as well as in the Cairns and Capricorn Coast regions of Queensland during the warmer months. The causative organism, Mycobacterium ulcerans, precipitates a slow growing bacterial infection of the skin that leads to the development of a painless necrotising ulcer, subsequent deformity and possibly functional disability of the affected area. Exactly how Mycobacterium ulcerans infection is transmitted has not as yet been determined, however studies are underway to find why it is increasing in incidence and the role that mosquitoes may play. As a result, when in endemic regions, it is strongly advisable to take measures to avoid mosquito bites.
The information we have provided in this factsheet is a guide and overview only. If you have any questions or need advice regarding your travel plans in Australia, speak to your doctor or a travel health provider before you leave.
References:
Australian Government Department of Health: https://www.healthdirect.gov.au
Better Health: https://www.betterhealth.vic.gov.au
PLOS: https://journals.plos.org/plospathogens/article?id=10.1371/journal.ppat.1005070#ppat.1005070.ref006
Medical Journal of Australia: https://www.mja.com.au/journal/2012/196/5/murray-valley-encephalitis-review-clinical-features-diagnosis-and-treatment
Arboviruses in the Australian region, 1990 to 1998:
NSW Government Department of Health https://www.health.nsw.gov.au/Infectious/factsheets/Pages/barmah_forest_virus_infection.aspx
The Conversation Media https://theconversation.com/la-nina-will-give-us-a-wet-summer-thats-great-weather-for-mozzies-147180
Tropical Medicine and Infectious Diseases:
Japanese Encephalitis Virus in Australia: From Known Known to Known Unknown https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6473502/
Australian Immunisation Handbook: Japanese encephalitis
https://immunisationhandbook.health.gov.au/vaccine-preventable-diseases/japanese-encephalitis
Flesh-eating bacteria cases on the rise and we need an urgent response: https://theconversation.com/flesh-eating-bacteria-cases-on-the-rise-and-we-need-an-urgent-response-94975