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Caseous lymphadenitis in sheep

Can vaccinating sheep reduce the occurrence of the infectious disease?

06 March 2020, at 9:00am
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Caseous lymphadenitis (CLA) is an important clinical disease, found mainly in sheep. It can have a high economic impact on sheep production, as it can decrease the value and quality of the carcass or wool of the affected animal. The disease causes abscesses in the lymph nodes, which can rupture and release infectious pus. CLA is caused by the bacterium Corynebacterium pseudotuberculosis, which is highly contagious, and the presence of these lesions can contribute to the spread of the disease.

Imagine this scenario: a sheep breeder asks you how they could avoid this disease affecting their flock. They know that vaccines are available on the market, but wonder if they are effective. What evidence exists to indicate the efficacy of vaccines for reducing the occurrence of CLA?

The evidence

Ten studies were identified that assessed whether there is a decrease in the incidence of CLA in vaccinated sheep compared to unvaccinated sheep. In all of the critically appraised studies, animals were randomly allocated to experimental treatment groups, which were compared against a control group (where the animals were either given a placebo or no vaccination), providing a high degree of confidence.

One study compared a vaccinated group and a control group, where the bacterium C. pseudotuberculosis was introduced periodically. Vaccinated lambs had a 97 percent lower infection rate compared to unvaccinated lambs. In addition, vaccinated lambs that were infected with CLA had 96 percent fewer lung abscesses compared to unvaccinated infected lambs, and were, therefore, less likely to spread the disease. A different study compared vaccinated and unvaccinated animals, with one of the vaccinated groups receiving two doses of the vaccine. Vaccinated animals had fewer internal lesions and abscesses, especially those who received two doses. These findings were supported by two further studies, both of which found that vaccinated animals were at a lower risk of contracting the clinical disease compared to unvaccinated animals.

Some of the assessed studies also compared the efficacy of several different vaccines, in addition to comparing against a control group. One such study verified the effectiveness of several vaccines, including a combined vaccine formula, concluding that the toxoid phospholipase D vaccine alone was more efficient and provided the highest specific immune response against CLA in animals. One study tested the efficacy of commercial vaccines and a previously untested experimental vaccine, through three experiments in different periods. All vaccines studied were found to be efficient in provoking an immune response that was significantly higher than those of the control lambs.

Two studies found similar efficacy among all vaccines studied, indicating the overall effectiveness of vaccination in the control of CLA. Of the vaccinated sheep, 91 percent exhibited no lesions, compared to 51.5 percent of affected sheep in the control group. Affected vaccinated sheep also had a lower number of average lesions compared to control sheep. A further study found a significant difference in the proportion of vaccinated sheep that developed CLA compared to unvaccinated sheep at each time interval except less than six months post initial vaccination. A final study demonstrated that both the prepared CLA vaccine and a CLA vaccine injected simultaneously with a Bacillus Calmette-Guérin vaccine induced cellular and humoral immunity, therefore making both treatments suitable for prevention and control of CLA.

Conclusion

In all assessed studies, it was found that animals vaccinated against CLA were less likely to acquire the disease when compared to unvaccinated animals. The evidence also demonstrates that vaccination for CLA is an effective measure for prophylaxis of the disease, since vaccinated animals were significantly less affected by the disease when compared to unvaccinated animals. All the studies were randomised, controlled trials.

Limitations of the findings include that not all the studies explained their methods of randomisation and that some studies did not clarify whether the control animals received a placebo vaccination or no vaccination. In addition, two of the studies did not describe whether the vaccine was administered intramuscularly or subcutaneously.

Despite these limitations, there is strong supporting evidence for the use of vaccination as an efficient prophylactic measure against caseous lymphadenitis for sheep flocks.

The full Knowledge Summary can be found online.