Japanese Encephalitis

Abstract & Commentary

Synopsis: Guangdong province in China, having brought SARS under control, is now facing its seasonal epidemic of Japanese encephalitis.

Source: Japanese encephalitis—China (Guangdong). Pro-MED. http://www.promedmail.org.

While just now recovering from SARS, the Guangdong Provincial Bureau of Public Health on June 27 reported that 77 patients with Japanese encephalitis (JE) had been hospitalized in the previous 5 days. That brought the total to 297 since the middle of May. Approximately 90% have been children younger than 11 years of age. One adult acquired his infection in Shenzhen in the "Special Economic Zone" just north of Hong Kong. Ten of the patients with JE virus infection were from the provincial capital, Guangdong, where 4 cases of dengue fever had also been identified

Comment by Stan Deresinski, MD, FACP

JE virus is a flavivirus closely related to, among others, West Nile virus, Murray Valley virus, and St. Louis encephalitis virus.1 It is transmitted in an enzootic cycle involving mosquitoes and vertebrate hosts, especially domestic pigs and wading birds. The larvae of the Culex mosquito vectors breed in pools of ground water, with flooded rice fields providing a favored site. The intrusion of settings conducive to JE virus amplification adjacent to or even within urban areas has led to occasional acquisition of infection in and near cities.

Transmission is seasonal, with peaks in summer and early fall in temperate areas. (Cases in Guangdong province usually peak between May and July.) Seasonality in tropical and subtropical areas depends on a number of factors, such as rainfall, although widespread irrigation may be associated with year-round transmission.

JE virus infection is the leading cause of viral encephalitis in Asia, with approximately 50,000 cases reported annually from China, Korea, Japan, Southeast Asia, the Indian subcontinent, and Oceania. Its recent introduction into the Cape York Peninsula of Australia may have resulted from the transit-infected wind-blown mosquitoes from New Guinea during some unusual atmospheric conditions.2

Most JE virus infections are asymptomatic, and central nervous system involvement occurs in only a small proportion of those infected. When, however, encephalitis occurs, it is usually severe, with a 25% case fatality rate and with 30% of survivors left with significant persisting neuropsychiatric sequelae.1

In addition to the usual clinical features of encephalitis, patients with JE may commonly exhibit findings such as movement disorders and flaccid paralysis, both of which have recently been associated with West Nile virus encephalitis in the United States.3 Consistent with the frequency of movement disorders is MRI evidence of predominant involvement of the thalami and basal ganglia.

Management is supportive. A recent randomized, double-blind, placebo-controlled trial failed to demonstrate benefit from treatment with interferon a2-a.4

A total of 100,000 children have been vaccinated in Guangdong province since the beginning of the outbreak, and an attack on the mosquito vector has been launched. The risk to most travelers is low but depends upon season, as well as intensity and duration of exposure to infected mosquitoes. This, together with the potential adverse effects associated with the Biken vaccine in Westerners, has led to recommendations by the CDC that vaccination be limited. In the absence of other factors such as epidemic transmission and high-risk activities, only individuals spending 30 or more days in endemic areas during the local transmission season, especially if travel includes rural areas, should be vaccinated. Fortunately, new vaccines are under investigation, including a live attenuated chimeric product that uses yellow fever virus 17D, the virus used in the United States against yellow fever, as a living vector for envelope genes of a strain of JE virus.5

References

1. CDC. Japanese encephalitis. http://www.cdc.gov/ncidod/dvbid/jencephalitis/index.htm.

2. Ritchie SA, Rochester W. Wind-blown mosquitoes and introduction of Japanese encephalitis into Australia. Emerg Infect Dis. 2001;7:900-903.

3 Deresinski S. It’s time for West Nile Virus Again! Infectious Disease Alert. 2003;22:129-130.

4. Solomon T, et al. Interferon alfa-2a in Japanese encephalitis: A randomized double-blind placebo-controlled trial. Lancet. 2003;361:821-826.

5. Monath TP. Japanese encephalitis vaccines: Current vaccines and future prospects. Curr Top Microbiol Immunol. 2002;267:105-138.

Dr. Derenski is Clinical Professor of Medicine, Stanford; Associate Chief of Infectious Diseases, Santa Clara Valley Medical Center.