Vaccination of dairy cattle against Q fever (Coxiella burneti): results of field trials.

A phase I formalin-inactivated Q fever vaccine, using the Nine Mile strain of the organism, was tested for its ability to prevent dairy cows from shedding Coxiella burnetti in their milk. More than 1,400 Holstein-Friesian dairy calves and heifers from 5 dairies were used in field trials lasting over a 3-year period. Vaccination of 476 calves resulted in a geometric mean antibody titer of 1:123.3 compared with 1:2.4 for 486 nonvaccinated calves. The milk samples from 163 vaccinated calves were tested by mouse inoculation after the cows commenced lactation and were placed in their respective milking herds. Of these vaccinated animals, only 2 cows (1%) from 1 herd were suspected shedders, but on subsequent testing gave negative results. Among 164 nonvaccinated (control) cows, 39 (24%) were shedding C burnetii in their milk; this figure corresponded to the prevalence (23%) of shedders in the general population of dairy cows in California. The results of the current field trials indicated that vaccination greatly reduced the shedding of the Q fever organism in the milk of dairy cows.