Ovine brucellosis is an infection of sheep caused by a bacterium, Brucella ovis (B. ovis). The disease occurs in most sheep-producing areas of the world.
Ovine brucellosis occurs naturally only in sheep. It is a disease primarily of rams. Ewes usually only carry the infection for one or two oestrus cycles.
In rams the disease causes inflammation of the epididymis (the vessel which transports semen from the testes). One or both ram testes may be affected. In pregnant ewes, B. ovis causes inflammation of the placenta.
Signs in rams
Initial infection by B. ovis causes an elevated temperature and depressed appetite, which usually pass unnoticed by owners.
The infection first affects the epididymis causing inflammation and swelling in the surrounding tissues. The epididymides may become completely blocked, causing sperm to build up. Obvious swelling, hardening or differences in size can often be seen and palpated through the scrotum. Rams with chronic infection may have a grossly enlarged tail of the epididymis at the lower end of the testis and a shrunken testis.
In some rams, the blockage is permanent and the tail of the epididymis continues to swell. In other rams the blockage may break down, the swelling reduce and the testes feel normal, while the ram may still be infected.
Other causes of epididymitis and scrotal lesions include infections with other bacteria, such as Actinobacillus seminis and Corynebacterium pyogenes, and physical injury.
Palpate scrotal contents of all rams to help determine soundness before mating.
Signs in ewes
Clinical signs are not common in ewes.
Most ewes will not remain infected for more than two oestrus cycles after being served by an infected ram. Infection causes inflammation of placenta, early embryonic death usually without outward signs or uncommonly the foetus may be aborted.
The brucella organism may be present in vaginal discharges and milk, after abortion or parturition.
Infection in ewes is self-limiting, so ewes usually breed normally in subsequent years.
Effect on flock fertility
Reduced lamb marking percentages
When clinically affected rams are used the lambing percentages may be reduced. If brucellosis continues in the flock lambing percentages will progressively reduce.
Extended lambing period
When infected rams service ewes, the chance of conception is low and many ewes will return to service 17 days later. If rams are left with ewes, the number of pregnant ewes will increase, but this will result in an unusually long lambing period.
High ram culling rate
If rams are culled when abnormalities are detected on palpation, then infected rams are culled one or two years after infection. If brucellosis is not eliminated, this high culling rate will continue.
Spread of infection
Rams are infected by exposure to infected semen or vaginal discharges from infected or aborting ewes.
B. ovis can be spread within a flock through:
- rams joining ewes that have been served by infected rams in the same heat period
- homosexual activity, common among young rams
- rams exposed to infected ewes that have aborted and are excreting B. ovis infection through the nasal passage due to sniffing and nosing behaviour of rams in ewe flocks, rams running with killer mobs or culls. Several rams may serve any ewe on heat.
Introduction of B. ovis into a previously clean flock is through buying, borrowing or straying infected rams.
There are three methods of diagnosing ovine brucellosis: palpation of the scrotum, examination of semen and blood testing.
Palpation of the scrotum contents (testes and epididymes)
Examine ram in a standing or sitting (shearing) position. Use both hands and start above the testes, moving down to the base of the testes and the epididymal tails. Check for symmetry, free movement in the scrotum and the absence of lumps within the testes and epididymides. Testes should feel firm and springy.
Abnormalities will usually be felt as a swelling below either or both testes, due to inflammation of the tail of the epididymis. There may be a swelling at the head of the epididymis, above the testes. Not all brucella-infected rams will have palpable abnormalities, and conversely, rams with detectable abnormalities may not have brucellosis.
Examination of semen may reveal a reduced volume of semen, poor sperm motility and/or a high proportion of abnormal sperm. B. ovis may also be cultured from semen.
Blood is tested to detect the presence of B. ovis antibodies, which indicates previous exposure to infection.
Eradicating ovine brucellosis from breeding flocks
When ovine brucellosis is detected in a flock, the owner should discuss the management options with their veterinarian.
Eradication can be achieved by a combination of manual examination, blood testing, and removal of infected rams as soon as they are detected. There is no effective vaccine.
During an eradication program it is important to prevent contact between infected and uninfected young rams.
Eradication is not compulsory. Eradication can be a slow and expensive process, and it may fail if testing, segregation and culling are not practised carefully.
Once eradication is achieved, precautions must be taken to prevent infected rams re-entering the flock, whether as strays or introductions.
What to do with infected rams?
Separate infected rams immediately from other tested-negative and young rams. Sell infected rams for slaughter.
Treatment is rarely undertaken and likely to be economical only in valuable rams. Prolonged antibiotic treatment may be effective in early cases before irreparable epididymis damage occurs. Unilateral castration is unlikely to be successful as the bacteria may also be present in the other epididymis and the internal accessory sex glands.
Owners must ensure that introduced rams are from a flock with a low risk of having ovine brucellosis. A single negative blood test prior to purchase can be misleading because antibodies are not detected in the blood for up to seven weeks after a ram is infected.
Ovine Brucellosis Accreditation Scheme (OBAS)
The Department of Primary Industries and Regional Development, Western Australia (DPIRD) administers a voluntary Ovine Brucellosis Accreditation Scheme for ram breeders. Registered veterinarians inspect and advise on property biosecurity and undertake inspection and blood test rams on studs participating in the scheme.
A private veterinarian, chosen by the owner, inspects and assesses the suitability of the fences, yards, paddock layout, handling facilities and ram identification. The initial accreditation requires palpation of the testes and two consecutive blood tests of all rams and teasers in the flock 60–120 days apart.
Accreditation is granted when two successive blood test results are negative for all rams and teasers on the property.
To remain accredited this on-property assessment and ram testing is conducted annually. The frequency of the on-property inspection and blood testing may be reduced after several years of negative testing results.
The continuing disease-free status of rams in accredited flocks depends upon the stock being maintained with the appropriate level of biosecurity, secure fencing and only purchasing rams or teasers from accredited flocks.
The Western Australian Voluntary Ovine Brucellosis Accreditation Scheme guidelines and application to join are available for download from the document links on this webpage.
Lists of accredited flocks – Ovine Brucellosis Accreditation Scheme
The inclusion of a ram breeder on the list of accredited sheep studs (see document links at right) indicates that breeder has met the requirements of the voluntary Ovine Brucellosis Accreditation scheme and been recommended for accreditation by an approved veterinarian on the basis that the rams available from the stud property are at low risk of ovine brucellosis. The Department of Primary Industries and Regional Development, Western Australia (DPIRD) maintains the list of accredited studs but is not able to, and does not, make any representation or warranty that the rams available from a listed stud are in fact free from ovine brucellosis either when the stud is added to the list or at any time subsequently. DPIRD will not be liable for any loss or damage, however caused, arising from reliance on or use by any person of the information contained in the list.
To contact the OBAS administrator, email email@example.com or telephone +61 (0)8 9780 6235.