Epidemiologic aspects of a St. Louis encephalitis outbreak in Mesa County, Colorado.

An epidemic of St. Louis encephalitis in 1985 in Mesa County, Colorado, led to 17 cases, including one fatality. Risk was associated with advanced age and residence in Grand Junction, the county's principal city. A trend was observed toward higher risk in females. However, increased risk in females was not associated with higher infection rates (increased exposure). Capture enzyme immunoassays detected specific immunoglobulin M (IgM) and immunoglobulin A after infection. A serosurvey of Grand Junction residents disclosed an infection rate of 4.0%, indicating that 1,123 epidemic St. Louis encephalitis infections may have occurred in the city. Evidence of previous St. Louis encephalitis virus infection was found in 11.2% of survey respondents who had neutralizing antibody to the virus without specific IgM. The prevalence of St. Louis encephalitis virus antibody was similar to rates observed in serosurveys undertaken 30 years earlier, indicating that the level of endemic St. Louis encephalitis transmission in the city had not changed appreciably in that interval.

Epizootic vesicular stomatitis in Colorado, 1982: infection in occupational risk groups.

In 1982-1983, an epizootic of vesicular stomatitis occurred in the western United States. Veterinarians, research workers, and regulatory personnel who were exposed to vesicular stomatitis virus were examined for patterns of human infection and prevalence of vesicular stomatitis New Jersey serotype neutralizing antibody. Insight into the mechanism of transmission was sought by comparing activities of antibody-positive and antibody-negative persons. A statistically significant risk factor was a history of infected animals sneezing in the face of serosurvey participants. Elevated odds ratios were also calculated for those who usually examined the oral cavity of affected animals, had open wounds on hands or arms, and had exposure to saliva through the eye or skin. Relatively intimate direct contact was required; a higher risk was associated with examining horses than cattle. Neutralizing antibody prevalence was significantly higher among exposed persons with illness (23%) than in exposed persons without a history of clinical illness (7%). Overall, however, infectivity of VSNJ for humans during the epizootic was low.

Rapid and specific serodiagnosis of western equine encephalitis virus infection in horses.

Paired sera from 28 nonvaccinated horses with serologically confirmed western equine encephalitis (WEE) virus infections were evaluated for immunoglobulin (Ig)M and IgG directed against WEE virus, by use of enzyme immunoassay. Twenty-one of the horses developed greater than or equal to 4-fold increases or decreases in serum IgM titers in paired serum samples, confirming the diagnosis of WEE in these horses. Of the remaining 7 horses, 1 had stable IgM titers, 1 had a 2-fold increase in IgM titer between paired sera, 2 had 2-fold decreases in IgM titer, and for 3 horses adequate volumes were not available for both sera of the pair. Twenty-nine of 56 blood samples collected from these 28 horses had been collected within the first 3 days after clinical disease was recognized; all 28 horses and 48 of 53 available serum samples had IgM antibody to WEE virus. Immunoglobulin M also was detected in sera of 27 of 45 other nonvaccinated horses that had illnesses clinically compatible with WEE. Sera with IgM did not have cross-reacting IgM against eastern equine encephalitis virus. Therefore, the sensitivity, specificity, and lack of persistence of IgM was useful in the rapid diagnosis of WEE virus infections in horses.

Serodiagnosis of western equine encephalitis virus infections: relationships of antibody titer and test to observed onset of clinical illness.

Sera from horses and human beings with clinically diagnosed western equine encephalitis (WEE) virus infections were tested for hemagglutination-inhibition (HI), complement-fixation (CF), and neutralizing (N) antibody to WEE virus. These tests confirmed infection in 43.8% (HI), 56.3% (CF), and 80.4% (N) of horses and 54.5% (HI), 59.1% (CF), and 77.3% (N) of human beings. Use of the N test as an adjunct to the HI and CF tests increased the likelihood of serologic confirmation to 91.7%. In both horses and human beings, N antibody increased steeply at the end of the 1st week after onset. The results suggested that the presence of a high HI, CF, and/or N antibody titer in a single serum obtained from horses during the acute phase of illness caused by WEE virus can be used as presumptive evidence for infection with this virus.

Tick-borne relapsing fever in Colorado. Historical review and report of cases.

Since 1915 the front range of the Colorado Rocky Mountains has been postulated as a focus of endemic tick-borne relapsing fever. However, the disease has rarely been identified: only two cases have been reported in Colorado since 1944. Three sporadic cases in 1977--tightly grouped geographically and temporally--prompted an epidemiologic review. Tick-borne relapsing fever should be considered in the differential diagnosis of recurrent paroxysmal fever--with or without known presence of ticks--whenever exposure in an endemic area is part of a patient's history.