Australia has had some major historical success in the battle against mosquito-borne diseases.

The country was declared malaria free in the 1980s and the threat of dengue transmission in tropical north Queensland has been greatly reduced by the introduction of a bacteria into the mosquito population.

However, the recent discovery of Japanese Encephalitis (JE) in the south of Australia reminds us that we need to continually survey the horizon for new threats.

The spread of JE from its previously known distribution (far north Queensland), would appear unpredictable, but the virus bears many similarities to another potential deadly virus, Murray Valley Encephalitis (MVE).

MVE’s biggest outbreaks occurred in the south of Australia in 1974 and 2011 – also “La Nina” years.

These diseases are here to stay, hidden in the wildlife reservoir. Only occasionally will they “spill over” into the human population – the result of a “perfect storm” of conditions.

In the case of JE, this “perfect storm” involved “La Nina”, her impacts on bird migration (new wetlands), her impact on mosquito abundance (a proliferation of the main vector), and the appearance of both infected birds and new wetlands in close proximity to piggeries.

Regardless of “La Nina”, if our climate is to become warmer, and challenged by flood events, we may find that JEV will circulate widely and continuously. JEV is expanding its range globally.

In the last two decades, we’ve also seen the introduction of new, exotic mosquito vectors of JE, of unknown significance in Australia. We are doing nothing to track these.

Mosquito-borne disease has always been a threat to public health with millions of people infected every year worldwide. A number of mosquito transmitted viruses have spread globally due to the geographical expansion of the virus mosquito vectors and increased international travel, such as Chikungunya virus, Zika virus and Japanese encephalitis virus (JEV).

JEV infection in humans could lead to acute encephalitis, the symptoms of which may include headache, vomiting, fever and seizures. JEV is transmitted primarily through Culex species mosquitoes which are widely distributed in temperate countries. The largest outbreak of JEV was the pandemic in 1924 in Japan, involving more than ~6000 clinical cases with up to 60 per cent fatality.

According to the World Health Organization (WHO), JEV outbreaks occur every 2-15 years and could be intensified during the rainy season. A few cases of JEV were reported in the Torres Strait Islands and northern Australia in 1998.  However, due to the increased spreading of Culex species mosquitoes in Australia and the recent “rain bomb” along the east coast, the current spread of JEV is a serious concern.

To keep ourselves safe, vaccination (under a doctor’s direction) is highly recommended. Apart from JEV, cases of other mosquito transmitted viruses, such as Ross River virus, may also rise during and after the rainy season. It is therefore essential to avoid mosquito bites at all times. For livestock keepers, it is also very important to take measures to reduce the chance of mosquito bites on cattle, as pigs, horses and birds are natural hosts of JEV.

The reports of multiple cases of Japanese encephalitis (JE) acquired in Australia occurring at the same time as severe flooding serves as a warning of the significant potential for new human health threats associated with climate change, including the emergence of new pathogens and the appearance of known infections in new localities.

The cases now occurring represent the first time a cluster of locally-acquired JE cases has been reported on the Australian mainland.

JE infections have been reported among Australians previously, but apart from some cases in the Torres Strait Islands, reports are very rare and have occurred almost exclusively in travellers to Papua New Guinea or to areas in Asia with known JE transmission.

There are two effective JE vaccines available in Australia, one of which requires two doses 7 - 28 days apart, and the other requiring a single dose. While the single dose approach may seem preferable, there are some people – such as those with compromised immune systems and pregnant or breastfeeding women – for whom this particular vaccine is not recommended.

JE vaccines administered in Australia have mainly been given to laboratory workers who may have contact with the virus, or to travellers visiting areas with known JE transmission and who intend to stay there for greater than one month or are planning repeated trips.

As further data on the number and extent of local JE cases emerges, public health authorities may need to consider expanding vaccine recommendations for those likely to be exposed. In the meantime, people can reduce their risk of infection through measures that minimize exposure to mosquito bites including protective clothing, insecticides and repellants.

Japanese Encephalitis virus (JEV) belongs to the same family of viruses (called Flaviviruses) as Dengue virus, Yellow Fever virus and other viruses found in Australia, like Murray Valley Encephalitis virus (MVE). 

JEV is not a new virus in Australia, last outbreak was in 1995 in the top end. JEV has - until now - been confined to the tropical north (Torres Strait / Cape York Peninsula). A more recent appearance in 2005 suggests that birds (especially migratory birds) might be carrying the virus around, and that pigs are good amplifying hosts.

In most cases, human infection with JEV is asymptomatic, but in about one per cent of people this can progress to disease that includes encephalitis (inflammation of the brain). In approximately 20 per cent of people who do become ill, JEV can be lethal.

The extent of JEV distribution needs to be assessed and confirmed through mosquito surveillance and mapping studies. Currently, reports indicate that rural areas (where commercial piggeries and cattle stations are located) are most at risk, but these mosquitoes can be found in peri-urban areas, which is why better and broader surveillance are needed.

JEV is one of the few mosquito-borne viruses for which there is an approved vaccine (two types of vaccines). It is normally recommended for people who travel to places where JEV is more present (Indonesia, PNG and Torres Strait Islands) and for people who work/live around livestock in areas where mosquitoes are prevalent. 

Other mosquito-borne viruses like Ross River virus (which causes acute arthritis and fever) are also likely to be on the rise - there is no vaccine for Ross River virus. Using mosquito repellents with DEET or Picaridin, avoiding being outdoors at dusk and dawn and eliminating pooled stagnant water (pots, saucers, puddles) around the house are the best ways to limit these mosquitoes breeding.

JEV (Japanese encephalitis virus) has likely been introduced to mainland Australia from Papua New Guinea via the Torres Straits (as we have seen previously) or possibly from Indonesia through the Kimberly region or the Northern Territory.

Its spread to south east Australia follows heavy rainfalls in central Australia and in the Murray Darling basin where the virus is carried to the south east by infected waterbirds flying south, cycling between the birds and mosquitoes at flooded bodies of water where they rest and so on.

Symptoms include headache, nausea, mental confusion, stiff neck, seizures and possibly muscle weakness or partial paralysis.

Protect yourself from mosquito bites by covering up, wearing repellents etc.

Ross River virus is also prevalent in these areas currently.