Japanese encephalitis could emerge in North America

January 2008

Japanese encephalitis has the potential for introduction and establishment in North America and could, in theory, produce a threat greater than West Nile virus, according to recent findings.

Increased commerce with endemic areas and a lack of vector surveillance could increase the viability and establishment of the viral encephalitis into new regions, experts warn.

“The likelihood of introduction and establishment is greater than that of West Nile virus,” said Frank L. Mannix, MPH, a researcher in tropical medicine at the Tulane University School of Public Health. “We need to be aware of issues around the world, not only because they may affect us, but because what is happening elsewhere could be happening here.”

Mannix presented data regarding Japanese encephalitis at the 56th Annual Meeting of the American Society of Tropical Medicine and Hygiene, held recently in Philadelphia. Increased commerce, vector characteristics and Pacific flyways were examined to determine the viability and transmissibility of Japanese encephalitis.

Of particular concern is the lack of Japanese encephalitis surveillance. “Now that West Nile virus is not in the news as much, many regions are losing their vector control programs,” Mannix said. “We shouldn’t wait for a crisis to occur so we can react, we should have good public health structures and agricultural surveillance in place now.”

Between 30,000 and 50,000 cases of Japanese encephalitis are reported each year in Southeast Asia. The case-fatality ratio is 30%.

Commerce and travel

Japanese encephalitis is associated with pig and rice agro-ecosystems and is historically endemic in Asia. In recent years, the United States has increased trade with countries where Japanese encephalitis is endemic. With increased trade, increased opportunities for vector transport also increase. Several types of mosquitoes can transmit transovarially and eggs are desiccation-resistant. Any item that holds water, including the sides of ships, can be a possible laying ground for mosquito eggs. The eggs can also survive colder temperatures to reach busy ports such as Vancouver and Seattle.

Travel to endemic areas also increases chance of virus transmissibility to Northern America, where competent viral reservoirs have been long established.

Researchers mapped the successful geographical spread of the virus, noting that it emerged in Western Papua in the 1990s, an outbreak followed in the Torres Straits in 1995 and the virus reached the Australian mainland in 1998.

Smuggled animals and legal exotic pets, including birds and snakes, may provide an additional route for introduction. Wild birds travelling Pacific flyways could also bring Japanese encephalitis to North America, where several types of mosquitoes are established capable vectors.

For more information:

• Mannix FL, Wesson DW. Potential for introduction and establishment of Japanese encephalitis virus in North America. Presented at: The 56th Annual Meeting of the American Society of Tropical Medicine and Hygiene; Nov. 4-8, 2007; Philadelphia.