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66 Alfuy-Qld 91 Alfuy-WA 51 Murray Valley-Vic

72 Murray Valley-WA

73 Murray Valley-WA 66 Murray Valley-PNG 56 Murray Valley-PNG

JE Ja0ArS982 JE Sa V

95 JE-Torres Strait 95 JE-Torres Strait

? West Nile - Nigeria ? West Nile - Sarafend ? 66 Kunjin-Borneo 66 Kunjin-Qld 60 Kunjin-Qld 68 Koutango-Senegal

Subgroup I

Subgroup II

Subgroup III

Subgroup IV

Subgroup V

FIGURE 3.27 Phylogenetic tree of JE subgroup flaviviruses, based on nucleotide identity in the NS5 gene and the 3'NTR. Virus names in green are Australasia isolates; blue-green are PNG viruses; blue are from Japan, orange are from Africa. Underlined names are isolates from humans; the remainder are mosquito isolates. Isolates are named with year of isolation, name, and location of isolation where known. Yellow fever virus (YF) from Africa is shown as an outgroup. JE, Japanese encephalitis virus; Qld, Queensland; WA, Western Australia; NSW, New South Wales; VIC, Victoria; PNG, Papua New Guinea. [Adapted from Poidinger et al. (1996).]

encephalitis is 2-40% in different outbreaks, but 45-70% of survivors have neurological sequelae. In endemic areas, virtually all people have been infected by the time they reach adulthood. The vertebrate reservoirs for the virus are birds and pigs. Domestic pigs are particularly important amplifying hosts for human disease because they are found in proximity to their human owners. Various species of Culex mosquitoes transmit the virus. During peak transmission seasons, up to 1% of Culex mosquitoes around human habitations may be virus infected. Travelers to endemic regions have a probability of about 10-4/week of contracting JE, and 24 cases of JE encephalitis in travelers were reported between 1978 and 1992. Inactivated virus vaccines are in use in different regions of Asia. The Japanese have long used such a vaccine to eliminate JE encephalitis from their population, and the Chinese have recently developed a vaccine that is being used in China and Thailand. The Japanese vaccine is also available in the United States for travelers to endemic regions. Of considerable interest is the finding that JE virus infection may reactivate in mice after the immune system first damps it out. Reactivation in other animals may also occur and could be important for persistence of the virus in nature.

MVE virus and Kunjin virus are Australian viruses that are closely related to JE virus. They cause encephalitis in man, but the number of cases is small. Birds are the primary vertebrate reservoir, and epidemics of MVE have been associated with wet years when the mosquito population expands and nomadic waterfowl invade regions that are normally too dry to support them. Culex annulirostris is the primary vector for MVE.

SLE virus is a North American virus that belongs to the JE complex and that causes regular epidemics of encephalitis in the United States. The virus is widely distributed and cases of SLE encephalitis have been recorded in every state, with the majority of cases occurring in the Mississippi River valley, Texas, California, and Florida. Data for the years 1964-1993 are shown in Fig. 3.29. In the epidemic year 1975 there were 1815 cases of SLE encephalitis officially reported in the United States, but in nonepidemic years there may be fewer that 50 cases. The most recent epidemic occurred in 1990 in Florida with 223 cases and 11 deaths. The case fatal-

Reported Cases 1986-1990

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