Seroepidemiology of Q fever in Nova Scotia: evidence for age dependent cohorts and geographical distribution.

To determine the incidence of Q fever in Nova Scotia, a randomly selected sample of 492 volunteers aged 18-70 years was recruited by mail from all 18 urban and rural counties in this province. Volunteers were followed from 1988 to 1991 for antibody titres to Coxiella burnetii antigens. Analysis of seroprevalence by age revealed two statistically different cohorts. Those younger than 35 years had a low, stable seroprevalence while those 35 years and older had a seroprevalence statistically correlated with increasing age. The finding of age related cohorts suggests a sporadic or cyclical infection rate. An age-independent geographical clustering was also noted, with higher seroprevalence in a major agricultural area of the province. No seroconversions were observed, but four subjects had seroreversions. At baseline, 72 (14.6%) of the 492 subjects were considered seropositive (> or = 1:8 C. burnetti phase II antibody titre by microimmunofluorescence). Of 47 seropositive subjects who provided yearly serum samples, two (4.3%) showed > or = 4-fold increase in antibody titre and did not react to antigens of the 11 other respiratory pathogens tested, suggesting a reactivation of Q fever. All 22 subjects positive to phase I antigen were positive to phase II antigen. This study suggests that the rate of infection in Nova Scotia has been low since the 1960's and that risk of infection is associated more with a geographically clustered reservoir of infection than with occupation or gender.