Seroconversion and semen shedding in rams experimentally infected with Brucella ovis.

AIM

To determine the time taken for rams to develop antibodies to Brucella ovis in serum, shed B. ovis in semen and develop lesions of epididymitis following infection with B. ovis.

METHODS

Fifteen 19-month-old rams were artificially infected with B. ovis by inoculation of infected semen onto the nasal and rectal mucus membranes (Day 0). Serum was collected from each ram at 2 to 8-day intervals and tested at commercial laboratories using a complement fixation test (CFT) and an ELISA. Cut-off values for the CFT were 0-4/4 negative; 1/8-3/8 suspicious and 4/8-4/128 positive, and for the ELISA were <10% negative; ≥10 to <50% suspicious and ≥50% positive. Selected serum samples were also tested using a gel diffusion test (GDT). At 7 to 8-day intervals semen was collected for bacterial culture and the scrotal contents were palpated to identify lesions of epididymitis. The study was terminated after 56 days.

RESULTS

On Day 28 B. ovis was isolated from the semen of one ram and by Day 49 it was isolated from the semen of 10 rams. All 10 rams had suspicious or positive ELISA or CFT titres by Day 36 and 56, respectively. The GDT results were all negative on Day 36 and in general did not become positive in individual rams until 7-28 days after semen shedding commenced. Epididymitis was detected in one ram on Day 36; by Day 56 eight rams had epididymitis detectable by scrotal palpation.

CONCLUSIONS

The B. ovis ELISA test identified infected rams at an earlier stage than the CFT; this was at 19-36 days after exposure. Rams can begin shedding B. ovis in semen as early as 28 days after exposure and lesions of epididymitis develop as early as 36 days after exposure.

CLINICAL RELEVANCE

During a test and slaughter campaign for the control of B. ovis, the most appropriate serological re-testing interval is likely to be around 28 days (4 weeks) using the ELISA with or without the CFT, although caution is required in interpretation of "suspicious" ELISA results. Following a B. ovis breakdown, two negative CFT or ELISA tests 60 days apart are recommended to confirm freedom from infection, supporting current guidelines.