Surveillance of Rocky Mountain spotted fever, United States, 1981-1983.

The increase in the incidence of RMSF observed in the 1970s appears to have plateaued at a median of 0.49 cases/100,000 for the 6-year period 1978-1983. In 1981-1983, for the first time, Oklahoma had the highest incidence of RMSF in the United States (4.10 cases/100,000/year); increased numbers of cases were also reported from two of the other three West South Central states-Texas and Arkansas. Texas, which reported 217 cases in the 3-year period, had an average incidence of 0.47 cases/100,000/year. This apparent regional increase in RMSF activity may reflect multiple factors, including increased recognition and reporting of cases and environmental factors that may be associated with a true increase in RMSF. Simultaneously, a decrease in the number of cases reported and in the incidence of RMSF have been observed in the South Atlantic states, although these states still account for most (51%) of the cases in the United States. The slight decrease in the case-fatality ratio observed over the last 9 years is unexplained. It may reflect increased awareness of RMSF and result in appropriate and timely treatment. The increasing number and percentage of cases that are laboratory confirmed suggest that the more sensitive and specific laboratory tests now required for serologic confirmation are being more widely used and are contributing to an increased accuracy of reporting of RMSF in this country.(ABSTRACT TRUNCATED AT 250 WORDS)