The TB risk letters issued by the Department of Agriculture in recent weeks included several lists of animals considered to be high risk, with farmers advised to cull these animals if possible.
They included animals that passed clear after testing inconclusive, animals returned from marts unsold and animals that were alive during previous TB breakdowns.
The reasons why certain animals were considered high risk were detailed by Minister for Agriculture Charlie McConalogue in response to a question from Fianna Fáil TD Niall Collins.
Animals testing inconclusive
Animals that previously tested inconclusive to a TB skin test but passed all tests thereafter are classed as higher risk. This is because, on average, the skin test will only identify 80% of infected animals.
The test fails to identify all animals because, in some cases, the immune system fails to mount a strong enough reaction to leave visible lumps.
Therefore, an inconclusive test followed by a clear test does not guarantee an animal is TB-free. These animals are restricted to the herd for life and removed in the event of a TB breakdown.
Presence at a previous breakdown
The reason why the Department classifies animals present during a previous TB breakdown as higher risk is similar to the reasons above.
The animals were exposed to TB during the breakdown and may have become infected, but for immunological reasons may fail to react to the skin test.
These animals are therefore ‘false negatives’ - ie testing clear when in reality they are infected.
When the test does identify an animal, it is 99.98% accurate, meaning only one in 5,000 animals are false positives, compared with one in five being false negatives.
This is known as residual TB and is a significant reason why the Department’s risk categories extend out over 10 years.
It can take up to 10 years for a herd to return to normal levels of risk after experiencing a breakdown, based on previous research.
Greater number of movements
Animals that have had multiple movements are at higher risk of TB than animals which have not, according to the Department.
This is due to the increased potential for exposure to sources of infection during movement between different herds and lands.
Older animals are also considered higher risk, as studies have reported increased prevalence of TB with age.
This is similarly associated with the increased chance that an animal was exposed to an infection source.
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