Arboviral Encephalitis, 2002
La Crosse encephalitis and Western equine encephalitis (WEE) historically have been the primary arboviral encephalitides found in Minnesota. During July 2002, however, West Nile Virus (WNV) was identified in Minnesota for the first time.
Confirmed cases of arboviral encephalitis are defined as those that are clinically and epidemiologically compatible with arboviral encephalitis and meet one or more of the following laboratory criteria: a four-fold or greater rise in antibody titer to the virus, isolation of virus from or detection of viral antigen in tissues or body fluids, or detection of specific IgM antibody in cerebrospinal fluid. Probable cases are defined as clinically compatible cases occurring during a period when arboviral transmission is likely, with an elevated and stable (i.e., two-fold change or less) antibody titer to the virus. Medical providers' interest in WNV has led to the diagnosis and reporting of substantial numbers of less severe WNV infections (i.e., not classified as arboviral encephalitis cases). Consequently, a case definition for West Nile fever was developed, which includes a febrile illness with headache and the same laboratory confirmation criteria as for arboviral encephalitis cases.
WNV is maintained in a mosquito-to-bird transmission cycle. Several mosquito and bird species may be involved in this cycle, and regional variation in vector and reservoir species is likely. Nationally during 2002, WNV was found in 44 states and the District of Columbia. During 2002, 4,156 human cases of WNV disease were reported in the U.S., including 284 fatalities. Forty-eight cases of WNV disease (no fatalities) were reported in Minnesota residents. Thirty-one (65%) Minnesota case patients had West Nile fever; an additional 16 (33%) case-patients had either encephalitis or meningitis. Three case-patients (including two with encephalitis) were diagnosed with acute flaccid paralysis, an uncommon manifestation with a presentation similar to Guillain Barre syndrome.
A large epizootic among horses also was reported nationally; 14,539 equine cases were reported nationwide, including 992 cases in Minnesota. In addition to the detection of human and equine WNV cases, MDH focused surveillance efforts on dead birds (especially American Crows and Blue Jays). In much of the country, dead bird surveillance has been the most sensitive indicator of the presence of WNV in an area. Evidence of WNV was detected in humans, horses, or birds in 85 of 87 Minnesota counties in 2002.
A more complete review of WNV can be found in the June 2003 issue of the Disease Control Newsletter (vol. 31, no. 4).
La Crosse encephalitis was reported in several southeastern Minnesota counties in 2002. The disease, which primarily affects children, is transmitted through the bite of infected Ochlerotatus triseriatus (Eastern Tree Hole) mosquitoes. Persons are exposed to infected mosquitoes in wooded or shaded areas inhabited by this mosquito species, especially in areas where water-holding containers (e.g., waste tires, buckets, or cans) are abundant; these containers may be utilized as mosquito breeding habitats. During 2002, 13 cases of La Crosse encephalitis were reported to MDH. From 1985 through 2002, 114 cases were reported from 19 southeastern Minnesota counties (Figure 1), with a median of five cases (range, three to 13 cases) reported yearly. Disease onsets have been reported from June through September; most onsets have occurred from mid-July through mid- September.