Japanese encephalitis (JE) is a mosquito-born infection endemic to parts of Asia. The ﬂaviviral neurologic infection is closely related to St. Louis encephalitis and West Nile virus. This infection causes an average of 35,000 reported cases and 10,000 deaths each year (209), although the majority of infec- tions are subclinical. Viremia develops after a bite from an infected mosquito and 1 out of 250 infections leads to symp- tomatic disease (211). Most infections clear before the virus enters the central nervous system. However, once neurologic invasion occurs, large areas of the brain may be involved. The resulting encephalitis is typically severe, with a 25–40% fatal- ity rate (212,213). Residual neurologic sequelae are evident in 10–30% of cases (212). Japanese encephalitis is seasonal with most cases occurring after infection during the rainy season; in temperate areas, this is from June through September. In the more tropical areas, the season begins in March and extends until October.
Several ﬁndings related to JE infection are:
1. Poorer performance on standardized tests (compared with uninfected subjects).
2. Those who had dengue fever infection earlier may have decreased morbidity and mortality rates, possibly due to the presence of other antiﬂavivirus antibodies.
3. Risk factors for death include documented virus in CSF, low levels of IgG or IgM, and decreased sensorium.
Control of vectors and reservoirs of infection aid in decreasing cases of JE. These measures are: 1) control of mos- quitoes and avoidance of areas where mosquitoes are likely to occur; 2) draining or spraying of swamps and other areas with standing water; 3) humans and other mammals may be dead- end hosts requiring no containment; and 4) agricultural ani- mals (pigs) and endemic birds (egrets and herons) may be amplifying hosts with high-grade viremia.
Three vaccines are available worldwide. The one used com- mercially for travelers is derived from mouse brain and is a form- aldehyde inactivated vaccine. The vaccine contains a Beijing-1
strain, thimersol, gelatin, and other components. The vaccine is administered as 3 doses on days 0, 7, and 30. More frequent inoculations may be given (5–7 days apart) when there is a need for a quick immunization schedule, although antibody response is lower and may not last as long. The vaccine is licensed for persons >1 year of age in the United States (214,215). The vaccine is recommended for travelers to Asia who will be spending a month or longer in endemic areas dur- ing the transmission season of the virus (which varies accord- ing to geographic region) (213). Two other JE vaccines are licensed in China: an inactivated JE vaccine derived from hamster kidney and a live attenuated vaccine from the same source combined with the SA14-14-2 viral strain. The latter is less costly and is replacing the inactivated virus vaccine. The efﬁcacy record of this vaccine is reported to be greater (Table 4.6).
Systemic side effects. Fever, headache, malaise, chills, dizziness, rash, myalgia, abdominal pain, and nausea and vomiting are reported.
Adverse neurologic events. Encephalitis, peripheral neuropathy, or other adverse neurologic events occur in
1.0 to 2.3 cases per 1 million vaccinations (216).
Allergic mucocutaneous reactions. The mouse brain vaccine has been associated with 73 allergic mucocuta- neous reactions (217).
Adverse allergic reaction. May occur within minutes or as late as 17 days after vaccination; most occur with- in 48 hours. Those with a history of allergic rhinitis or urticaria development (insect stings or bites) have a great risk (218,219).
General. 1 of 260 vaccinees complains of a general rash, itching, or swelling, especially in the areas of the face, lips, and throat, and/or the extremities.
Travelers. A 10-day period following vaccination is rec- ommended before traveling due to possibility of ad- verse events.
Live virus vaccines. Should live virus vaccines (such as MMR) be necessary, it is better to administer two doses of JE vaccines before the live virus vaccines for maxi- mum efﬁcacy.
Malaria. The efﬁcacy of JE vaccine is lessened if chloro- quine is being taken for prophylaxis against malaria.