Better testing in pipeline for inconclusive TB reactors in Wales

The Welsh government is reviewing how inconclusive reactors are tested for bovine TB following concerns raised by a Carmarthenshire vet.

Currently, all inconclusives within herds on enhanced measures – that have been shut down with TB for more than 18 months – are removed, along with cattle that have failed the test.

See also: New test available to aid early detection of bovine TB

But Cath Tudor of Prostock Vets voiced concerns the skin test isn’t good enough at accurately detecting disease in inconclusive reactors. It is estimated to be about 85% sensitive at picking up reactors, but studies have shown that when inconclusive reactors are retested and read on severe interpretation, the sensitivity of the skin test falls to 38%. 

This means 62% of animals could be culled without just cause.

Terminology explained

What is an inconclusive reactor?
Inconclusive reactors are animals that have a skin test result that shows a greater reaction to the bovine than avian tuberculin, but not enough to be classified as a reactor.

What is severe interpretation?
Using severe interpretation involves lowering the cut-off point for an animal to be classified as a reactor. What this means in practice is that some animals initially considered inconclusive reactors (IR) at standard interpretation will be reclassified as reactors under severe interpretation.

In Wales, severe interpretation is used in TB breakdown herds with lesion and/or culture-positive animals and in those that meet certain criteria that indicate they are at an increased risk of infection – for example, any holding where more than one reactor have been found. Severe interpretation is applied to the first short-interval test and also to the second if any IRs are found at the first test.

Source: TBAS

Ms Tudor believes it is resulting in too many animals being removed from herds with long-term breakdowns.

Furthermore, repeated skin testing of inconclusive reactors can suppress the immune response, with animals becoming desensitised to the test, which means some positive animals may get missed too.

Instead, Ms Tudor has called on the Welsh government to use other testing methods, such as gamma interferon, to improve the sensitivity and specificity of testing to help correctly identify infected animals and those free from infection.

“As vets, we have to try to clear the disease, but we can also see how important these cattle are for the farmer and we want an accurate test to work with for that reason.”

Alternatives

Speaking to Farmers Weekly at the recent Aber TB conference in Aberystwyth, Wales’ chief veterinary officer, Christianne Glossop, said she accepted Ms Tudor’s concerns and was currently working with the government and the TB Hub to find alternative solutions.

“The reason we adopted the approach was to pick up the infected ones. Have we gone that step too far and can we pull back a bit? Is there a better approach? We are actively looking at it.”

Prof Glossop said gamma testing is currently used in herds with long-term TB breakdowns at least once a year, but it isn’t routinely used because there is currently no evidence to suggest it is the right approach to take.

She confirmed she had commissioned a piece of work to see if the government could fine-tune its approach to testing inconclusive reactors within herds on enhanced measures.

She hopes this will be available within months and said as new evidence comes to light, it will be fed into the programme.

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