Ovine brucellosis (OB) remains an important disease in the Australian sheep industry. Previous studies have described the economic impact of OB in flocks from reduced lambing percentages, prolonged lambing periods and loss of rams (especially young stud rams). An increased rate of turnover of the ram team is another effect and when the proportion of rams with palpable lesions exceeds 10-20% there is an impact on fertility4. However, there may not be palpable lesions in the testes of all infected rams, particularly early in the course of infection.
Prevalence in South Australia
During May and June 2010, 45 commercial sheep flocks were selected at random to assess the prevalence of OB in the sheep population of south-eastern South Australia (SA). The work was undertaken by Biosecurity SA Animal Health staff. None of the selected flocks had previously been tested for OB. The number of rams in each flock ranged from six to 84 with the mean number of rams being 28. Overall, there were 1,271 rams tested. The primary test used was the Complement Fixation Test (CFT). For a few animals where the CFT test was indeterminate, the enzyme-linked immunosorbent assay (ELISA) test was used.
Previous surveys in other regions of SA found that 5% of flocks in the Eyre Peninsular, 10% of flocks in the pastoral area and 13% of the Mallee flocks contained rams that tested positive for OB. Initial results of this 2010 survey showed that about 22% of flocks had test positive rams present. About 7.4% of rams tested positive.
Region | % flocks infected |
---|---|
Southeast | 22 |
Eyre Peninsular | 5 |
Mallee | 13 |
Pastoral area | 10 |
Some breed societies in SA require member flocks to be OB accredited and in 2023 there were only 130 flocks enrolled in the SA accreditation program compared to 496 in 2000, as in Table 2 below. (Data from edis.animalhealthaustralia.com.au)
Year | SA | NSW | Vic | WA | Tas | Qld | NT | Total |
---|---|---|---|---|---|---|---|---|
2000 | 496 | 1250 | 747 | 86 | 130 | 67 | 0 | 2776 |
2023 | 130 | 996 | 267 | 257 | 116 | NA | 0 | 1766 |
Voluntary accreditation schemes for ram breeders exist in most Australian states, with oversight from state authorities. This accreditation includes documenting and adherence to a biosecurity plan, initial palpation and testing of all rams on a property over 10 months of age and ongoing testing at regular intervals in the flock to monitor the flock. In SA, and in large flocks, sample sizes are described to give a 95% confidence of detecting disease if present in a flock at a prevalence of 10% for example, 25 tested from a flock of 100. Similar provisions also exist in other states1.
In SA, the CFT is used as the initial flock test followed by ELISA tests in reactors. The CFT is chosen as the initial test as it offers the best compromise with a very high specificity (i.e. few false positives to follow up), but it has a slightly lower sensitivity compared to the ELISA (i.e. some infected rams will test negative 3). The Gel Diffusion Test (GD) is not used in SA.
Sensitivity | Specificity | |
---|---|---|
CFT | 96.3% | 99.3% |
ELISA | 97.2% | 98.6% |
GD | 91.7% | 100% |
Ref: Worthington RW, Weddell W & Penrose ME (1984) A comparison of three serological tests for the diagnosis of B. ovis infection in rams New Zealand Veterinary Journal 32:58-60