Asian mosquito capable of spreading Japanese encephalitis virus detected in northern Australia

Distribution of Culex (Culex) tritaeniorhynchus specimens sequenced in the study. Image: Lessard et al.
Distribution of Culex (Culex) tritaeniorhynchus specimens sequenced in the study. Image: Lessard et al.

A species of mosquito capable of spreading Japanese encephalitis virus (JEV), which can infect humans and horses, has been detected in northern Australia.

Culex (Culex) tritaeniorhynchus is an important vector of JEV, which can cause disease in feral pigs, native mammals, humans and horses. The mosquito species is widely distributed throughout Southeast Asia, Africa and Europe, but was thought to be absent in Australia.

Bryan Lessard and his fellow researchers, in a paper published in the journal Parasites & Vectors, reported the detection of Cx. tritaeniorhynchus in the Darwin and Katherine regions of the Northern Territory.

“The vector is likely to be already established in northern Australia, given the wide geographical spread throughout the Top End of the Northern Territory,” they said.

The establishment of Cx. tritaeniorhynchus in the country is a concern to health officials, they said, as the species is an important vector of JEV.

It is now the sixth species from the subgenus Culex capable of transmitting JEV in Australia.

“We suggest that the species must now be continuously monitored during routine mosquito surveillance programmes to determine its current geographical spread and prevent the potential transmission of exotic JEV throughout Australia.”

The 19 female specimens were collected in February and May 2020 by the Medical Entomology unit of the Northern Territory Top End Health Service from the Darwin and Katherine regions.

Molecular-based identification revealed that specimens shared 99.7% nucleotide identity to Cx. tritaeniorhynchus from Dili, Timor-Leste, and clustered with other Cx. tritaeniorhynchus from Southeast Asia and the Middle East.

JEV is the leading cause of viral brain inflammation in humans, with 68,000 cases reported globally each year, resulting in 20,400 deaths (a 25% mortality rate) and 14,000 to 24,000 neurological impairments, many of which occur in children under 12.

JEV also affects animals that act as reservoir hosts, including birds, cows, pigs, horses and other domestic animals, and can cause reproduction disorders and abortions in pigs.

Discussing their findings, the research team said Cx. tritaeniorhynchus appears to be established in the Northern Territory, with confirmed collection records from the Darwin region, extending 270km further southeast to Katherine.

“While the introduction pathways are unconfirmed, it is plausible that Cx. tritaeniorhynchus may have travelled to Australia from Timor-Leste via windblown adult mosquitoes, given the relatively short distance of 465km between Timor-Leste and Melville Island near Darwin, and that Cx. tritaeniorhynchus has been previously recorded as flying 200–500km over sea waters in the Northwest Pacific.

“Alternatively, the vector may have arrived in Australia with adults being transported on board aircraft, or most likely as larvae and/or pupae inadvertently stowed on cargo ships.

“The vector may have been first introduced into Australia several decades ago,” they said, “since Cx. tritaeniorhynchus larvae were reportedly collected during larval surveys from Darwin and the Kimberley Research Station in the state of Western Australia in the 1950s. However, the larvae were not illustrated and the whereabouts of the original specimens are unknown.”

Elimination of the mosquito is most likely unfeasible, they said.

The authors noted there were an estimated 2.3 to 6.3 million feral pigs in Australia, with 6.1 pigs per square kilometre estimated from the Mary River region in the Northern Territory alone.

“As pigs are known amplifier hosts for JEV, the establishment of Cx. tritaeniorhynchus in Australia may be considered a public health concern due to the abundance of feral pigs occurring across northern Australia, which may increase infection rates and potentially lead to emerging JEV outbreaks.”

Recent vector competence testing has shown that possums and the black flying fox bat, Pteropus alecto, are potential amplifying hosts for JEV in Australia, compared to those considered to be poor hosts, such as the eastern grey kangaroos, agile wallabies and tammar wallabies.

Other researchers have suggested that flying foxes could play a prominent role in the transmission of JEV into northern Australia, since thousands of individuals migrate to Australia from Torres Strait and New Guinea, where the virus is more prevalent.

“Although JEV is relatively rare and yet to be established in Australia, northern Australia is a JEV risk area,” the study team warned.

“The first outbreak of the arbovirus was last detected in 1995 in humans and pigs from the Torres Strait and Cape York Peninsula, northern Queensland, most likely introduced by migratory birds or windblown mosquitos from New Guinea and amplified by the native JEV vector mosquito Cx. annulirostris.

“Therefore, the additional JEV vector Cx. tritaeniorhynchus must now be continuously monitored to prevent the potential health risk of transmitting this exotic disease in Australia.”

They said accurate species identification of exotic species with known affinities for disease transmission is essential for improving the monitoring of high-risk mosquito species to better predict and manage emerging mosquito-borne diseases in Australia.

The study team comprised Lessard, Juanita Rodriguez and Christopher Hardy, with the science agency CSIRO; and Nina Kurucz and Jane Carter, with Medical Entomology, NT Health–Royal Darwin Hospital, Top End Health Service.

Lessard, B.D., Kurucz, N., Rodriguez, J. et al. Detection of the Japanese encephalitis vector mosquito Culex tritaeniorhynchus in Australia using molecular diagnostics and morphology. Parasites Vectors 14, 411 (2021).

The study, published under a Creative Commons License, can be read here

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