Arboviral Encephalitis outside North America

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Chapter 260 Arboviral Encephalitis outside North America

The principal causes of arboviral encephalitis outside North America are Venezuelan equine encephalitis (VEE) virus, Japanese encephalitis (JE) virus, tick-borne encephalitis (TBE), and West Nile (WN) virus (Table 260-1).

260.1 Venezuelan Equine Encephalitis

The VEE virus was isolated from an epizootic in Venezuelan horses in 1938. Human cases were first identified in 1943. Hundreds of thousands of equine and human cases have occurred over the past 70 yr. During 1971, epizootics moved through Central America and Mexico to southern Texas. After 2 decades of quiescence, epizootic disease emerged again in Venezuela and Colombia in 1995.

260.2 Japanese Encephalitis

Epidemics of encephalitis were reported in Japan from the late 1800s.

Epidemiology

JE is a mosquito-borne viral disease of humans as well as horses, swine, and other domestic animals that causes human infections and acute disease in a vast area of Asia, northern Japan, Korea, China, Taiwan, Philippines, and the Indonesian archipelago and from Indochina through the Indian subcontinent. Culex tritaeniorhynchus summarosus, a night-biting mosquito that feeds preferentially on large domestic animals and birds but only infrequently on humans, is the principal vector of zoonotic and human JE in northern Asia. A more complex ecology prevails in southern Asia. From Taiwan to India, C. tritaeniorhynchus and members of the closely related Culex vishnui group are vectors. Before the introduction of JE vaccine, summer outbreaks of JE occurred regularly in Japan, Korea, China, Okinawa, and Taiwan. Over the past decade, there has been a pattern of steadily enlarging recurrent seasonal outbreaks in Vietnam, Thailand, Nepal, and India, with small outbreaks in the Philippines, Indonesia, and the northern tip of Queensland, Australia. Seasonal rains are accompanied by increases in mosquito populations and JE transmission. Pigs serve as amplifying host.

The annual incidence in endemic areas ranges from 1-10/10,000 population. Children <15 yr of age are principally affected, with nearly universal exposure by adulthood. The case : infection ratio for JE virus has been variously estimated at 1 : 25 to 1 : 1,000. Higher ratios have been estimated for populations indigenous to enzootic areas. JE occurs in travelers visiting Asia; therefore, a travel history in the diagnosis of encephalitis is critical.

Prevention

Travelers to endemic countries who plan to be in rural areas of the endemic region during the expected period of seasonal transmission and travelers in rural areas experiencing endemic transmission should receive JE vaccine. An inactivated vaccine manufactured in Japan by intracerebral injection of young mice and available throughout the world has been taken off the market owing to a high incidence of adverse events. In 2008-2009, tissue culture–based JE vaccine (IXIARO) was licensed in Europe, Australia, and the USA. In the USA, this vaccine (also called IC51) is licensed for use in older children and adults and is distributed by Novartis (Basel). For this vaccine, JE virus is grown in Vero cells, then formalin inactivated and administered intramuscularly as two 0.5-mL doses, 28 days apart. The final dose should be completed at least 1 wk prior to the patient’s expected arrival in a JE endemic area. This vaccine contains alum and protamine sulfate and has exhibited only mild adverse events. A highly efficacious live-attenuated single-dose JE vaccine developed in China for children is licensed and marketed in some Asian countries. This vaccine can be co-administered with live-attenuated measles vaccine without altering the immune responses to either vaccine. In humans, prior dengue virus infection provides partial protection from clinical JE.

Personal measures should be taken to reduce exposure to mosquito bites, especially for short-term residents in endemic areas. They consist of avoiding evening outdoor exposure, using insect repellents, covering the body with clothing, and using bed nets or house screening.

Commercial pesticides, widely used by rice farmers in Asia, are effective in reducing populations of C. tritaeniorhynchus. Fenthion, fenitrothion, and phenthoate are effectively adulticidal and larvicidal. Insecticides may be applied from portable sprayers or from helicopters or light aircraft.

260.3 Tick-Borne Encephalitis

TBE was identified by Russian scientists in 1937 and was subsequently shown to be widespread in Europe, where it was identified as the cause of milk-borne encephalitis.