Interference in diagnostic tests for brucellosis in cattle recently vaccinated against leptospirosis.

The aim of the current study was to verify if cattle vaccinated against leptospirosis may react in diagnostic tests for brucellosis. Sixty cows were divided into 5 groups, each comprising 12 animals. Four groups were given different vaccines against leptospirosis, while the control group received only saline. Two doses of vaccine were given, as recommended by the manufacturers. Serum samples were collected on the first day of immunization (day 0) and on postvaccination days 7, 14, 21, 28, 35, 42, 49, 56, 96, and 126. All the serum samples were tested for brucellosis and leptospirosis. Twenty animals were reactive at least once to the Rose Bengal test, but by day 96, no further reactions were elicited by this test. Twenty-six samples were reactive to the Rose Bengal test, but only 7 remained positive in confirmatory tests: 1 to the 2-mercaptoethanol test, 2 to the fluorescence polarization assay, and 6 to indirect enzyme-linked immunosorbent assays. None of the samples was reactive in the complement fixation test. None of the animals in the control group was reactive. A significant difference was found between the control group and the groups vaccinated against leptospirosis, according to Fisher exact test. However, the groups were found to respond independently of the vaccine brand. The results indicate that cattle vaccinated against leptospirosis may show reactivity on screening tests for brucellosis.