Potential for the Emergence of Japanese Encephalitis Virus in California

The potential risk for the introduction and establishment of Japanese encephalitis virus (JEV) within California is described based on the literature. JEV is a mosquito-borne arbovirus endemic to Asia that when transmitted to humans can lead to Japanese encephalitis (JE), a disease affecting mostly children with a fatality rate up to 30%. The geographical expansion of JEV in Asia along with the recent introduction and rapid spread of West Nile virus (WNV) across the United States, demonstrates the ability of arboviruses to rapidly extend their distributions. California is at particular risk for the introduction of JEV because it is a large state functioning as a hub for international travel and commerce with Asia, potentially allowing the introduction of mosquitoes infected with JEV. If JEV is introduced into California, the virus might become established due to the significant number of susceptible mosquito vectors and vertebrate hosts. Once introduced, the lack of active surveillance for JEV, the ambiguous clinical presentation of JE, the cross reactivity of serological testing between JEV and other flaviviruses, and the probability that clinicians and laboratories would not consider JE as a possible diagnosis would likely delay recognition. A significant delay in detection of JEV in California would make control and eradication of the virus very difficult and costly. Public health authorities should consider the need for future control efforts if JEV emerges in the United States.