Brucellosis Testing

Brucellosis Testing

Ovine brucellosis (OB) is a bacterial disease of sheep found throughout Australia and characterised by infertility in rams. In countries without control procedures as many as 75% of ram flocks and 80% of rams within infected flocks can be infected. The prevalence of OB in Australia has been reduced by the operation of OB-free accreditation programs for registered flocks and other regulatory and voluntary control measures. OB exists in between 15% and 60% of untested ram flocks, depending on region, farmer awareness of the problem and their ability to keep rams within property boundaries.

Nature of infection

The causative bacterium, Brucella ovis, is confined to sheep. Infection can be spread from ram to ram directly or via the ewe. Ewes rarely become infected but can infect clean rams by exposing them to infected semen from a recent previous mating. By contrast, rams usually become permanently infected and excrete bacteria periodically in the semen. Sodomy is the most common way that rams become infected. Obvious lesions can be palpated in the testes and epididymides of about 40% of infected rams. The percentage tends to be lower in recently infected flocks. Rams in chronically infected flocks tend to have more gross lesions and the lesions are more distorted in shape.

Most OB lesions are in the tail of the epididymis although lesions in the testes are also common. Both sides can be affected, but unilateral lesions are more common. Infection causes inflammation and swelling. Swelling in the epididymis, which functions as a tube for the transport of semen, can cause a partial or complete blockage and a build-up of semen that increases the size of the swelling. Affected testes can be swollen, or shrunken, hard and irregular in shape.

Most infected rams without obvious scrotal lesions have microscopic lesions in the epididymides or testes, or lesions in non-scrotal tissues such as the accessory sex glands. Most scrotal lesions, but not all, are due to OB. Abscesses or infected wounds can be distinguished from OB because they usually discharge pus. Swelling due to an injury or hernia will usually affect only one ram. However some causes of lesions (eg. Actinbacillus seminis) can affect several rams and cannot be palpably distinguished from OB.

Effect on flock fertility

Infected rams usually produce semen of lower quality, making them either subfertile or sterile, depending on the site and severity of the lesions. The effect on flock fertility will also depend on the percentage of infected rams and flock breeding management.

In flocks joined for a restricted period, say six to eight weeks, a reduction of up to 30% in lamb marking percentage is likely if more than 10% of the rams have OB. In flocks joined for extended periods, ewes that fail to conceive because of low semen quality will return to service at 17 day intervals and many will eventually conceive. This will result in an extended lambing period, with attendant problems of inefficient feed utilisation, the simultaneous underfeeding and overfeeding of ewes in the flock at any given time (due to ewes being at different stages in the pregnancy/lactation cycle), higher ewe and lamb death rates (due to metabolic disorders and mismothering), decreased wool value (more variable in length and quality) and less efficient marketing of prime lambs (variable weight and condition score). In these flocks lamb marking percentage can be a very inadequate indicator of breeding efficiency.

The management of infected ram flocks is very important. If the presence of infected rams is ignored, the patterns of low lamb marking percentages or extended lambing periods can be repeated year after year, and accepted as normal. If rams with palpable lesions or that react positively to irregular ram-flock blood tests are culled, without completing an eradication program, the turnover and cost of rams will be high, and the problem of poor fertility will remain. Eradication is the only real solution.

The effects of OB are often insidious and unrecognised, especially in areas where marked fluctuations in fertility occur due to variation in ewe nutrition, clover disease, predation by foxes, or other circumstances.

Breed differences in susceptibility to OB are seen in some regions. For example, in the Mallee the prevalence of OB infection is less in Merino than in non-Merino ram flocks, and the proportion of infected rams is less in infected Merino flocks. However all breeds are susceptible.

Diagnosis

Systematic scrotal palpation

This is a vital step in examining rams for soundness and monitoring them for OB. This should be done by the farmer as a normal part of the preparation of a ram team for mating and when rams to be purchased are being examined. The detection of any lesions, irregularities or differences between the contents of the two sides of the scrotum that cannot be otherwise explained should result in either confirmatory blood testing for OB or, in the case of rams being considered for purchase, in the rejection of all rams from that source. OB-free accreditation is a valuable insurance, but not a guarantee, that the rams being offered for sale are OB-free.

Blood testing

Blood testing detects antibodies that indicates exposure to OB infection. Culling the positive reactors and the small percentage of infected rams that develop scrotal lesions without a positive blood test reaction is the usual basis for eradicating OB from infected flocks.

Semen examination

This can reveal sperm abnormalities or Brucella ovis organisms. It can be useful to detect the very small proportion of infected rams that do not develop gross lesions or a positive blood test.

 

Our vets are all capable of bleeding, palpating and semen testing your rams, as well the ability to help map out OB-free accreditation programs.