Murray Valley Encephalitis

Outbreaks of an acute, severe, encephalitic illness, clinically similar to Japanese and St. Louis encephalitis, occurred in rural areas of southeastern Australia in 1917, 1918, 1922, 1925, 1951, and 1974[1,9,14-16] and in north and northwestern Australia in 1981, 1993, and 2000.[8,12,41] Approximately 420 cases were reported in these nine outbreaks.[41] They are thought to represent a single entity for which various names (Australian X disease, Murray Valley encephalitis, Australian encephalitis) have been used. Twenty-two cases were diagnosed in the 5 years between 2007 and 2011; three were fatal, and one of the fatalities occurred in a Canadian tourist on return from a holiday in northern Australia. Case-fatality rates, as high as 70 percent in the early years,[9,11] declined to 20 percent in the 1974 outbreak and have remained at about this level since then.[5,10,12] However, significant residual neurologic disability occurs in as many as 50 percent of survivors.[10,12] The presence of this disease in Papua New Guinea was confirmed in 1956.[20] The causative virus was transmitted to experimental animals as early as 1918,[6,11] although those strains could not be maintained. The definitive isolation and characterization of Murray Valley encephalitis virus in 1951[19] led to epidemiologic studies that suggested its survival in bird-mosquito cycles in northern Australia but not in the area of epidemic occurrence in southern Australia.[1] Murray Valley encephalitis is caused by Murray Valley encephalitis virus. In an effort to dissociate a disease from a specific locality, the term Australian encephalitis was proposed by residents of Murray Valley for the disease caused by Murray Valley encephalitis virus. Some researchers subsequently have attempted to expand the term Australian encephalitis to include encephalitis caused by any Australian arbovirus. Because the term Australian encephalitis has no scientific validity and is ambiguous, it should not be used.