Decreasing prevalence of Q fever in Illinois.

There were 858 (37.7 percent) Q fever-infected dairy herds among the 2,277 tested in Illinois in 1963. The percentage decreased to 19.2 percent (380 of 1,975) in 1967. Reaction rates (complement-fixation test titer of 1:8 or greater) in serum samples from veterinarians decreased from 13.3 percent in 1956 to 3.9 percent in 1964 and from 3.6 percent in 1966 to 0 percent in 1968, 1970, 1972. There were 14 (2.7 percent) reactive serum samples among 526 abattoir workers tested in 1966; reaction rates were higher among workers having contact with swine (8.2 percent) than among workers having contact with cattle (1.8 percent). Two (0.1 percent) of 1,432 serum samples collected from 1967 to 1971 during preemployment examinations at another abattoir were reactive. Only two clinical cases of Q fever were reported to the Illinois Department of Public Health in the period 1963-80. All evidence evidence points to a decreasing prevalence of Q fever in Illinois.