Hazell Mullins: The TB test - the anxious wait

This TB test is not our annual test, meaning it is at a different time of year than usual, so the normal routine of animals being inside for the test is a different set-up on the day of the test with cattle still out grazing. This meant there were extra, more unfamiliar logistics for getting all the cattle near the yard to make sure the morning went smoothly for everyone.

In the two weeks leading up to the test, my Dad likes to make sure everything is organised and safe for everyone involved on the day. Checking all the animals are present and correct with their two tags and that all the blue cards are in order. 

The crush is my Dad’s main priority, it’s cleaned out and ready to go the evening before and I even spotted him oiling the head gate, that’s service. The final job on the morning of the test was to refill the foot bath, it was then all systems go once the vet arrived.

Bovine TB statistics in Ireland at the moment are far from where we would like to be with an increase in herd incidence of nearly 1% on 2023 figures, a rise from 4.73% to 5.62% as of the end of September. The number of reactors is up over 10,000 animals in the last year, which is an extremely worrying place to be when it comes to control of TB.

There doesn't appear to be one particular reason either; the situation is far from simple. At Vet24 last weekend in Killarney, the Department of Agriculture held a lecture session in the afternoon that was completely dedicated to TB updates and the current data analysis. As with all lectures over the weekend, I learned something new from all of them, which is a sign of a good conference.

The reproductive ratio (RO) number was mentioned in the session, a term that we were all very aware of during the pandemic on the news each evening. The ideal figure we were waiting for each day was below a figure of 1. 

As a refresher, the RO is the number of new cases that were predicted from one single case. A UK TB study recently showed that the RO of cattle-to-cattle spread from positive TB animals in a herd of 30 animals was 1.5; however, in a herd of over 400 animals, it was up to 4.9.

This makes sense in an Irish context also as most of the high numbers of positive TB animals are in areas with denser populations of cattle. The RO of Badger to cattle spread was also investigated in the UK study, it was interesting that an unvaccinated badger has an RO of 1.2 but in a highly dense population of badgers, it rises to 1.5. Again, the population of a species has been a correlating factor in the spread of the disease.

Factory lesions are always a topic of conversation when it comes to TB; the interesting finding from the conference also was that lesions take up to four months to form, which means the infection will be well established in the animal and, unfortunately, perhaps the herd. 

This hit home for me as I have often seen lesions as validation that I was interpreting the test correctly. Whereas, for the herd’s benefit, having no lesions in the factory could mean a return to de-restriction in a shorter time frame. 

The skin test has a high specificity, meaning a positive test is usually positive; the issue is the sensitivity is lower, which means it may not pick up all positive animals on the day, leaving behind possible reservoirs of infection. This is why, in some selected herds, the Gamma interferon blood tests are used, as this test has a higher sensitivity to pick up false negative animals from the skin test. 

The TB TASAH from Animal Health Ireland is a valuable resource for implementing TB biosecurity measures on farms., It is a funded consultation with your testing vet to see if there is any particular cause for the TB breakdown on the farm. It is not only herds experiencing a breakdown but also herds classed as negative high-risk herds that qualify for the TASAH, which aids in the prevention of TB on the farm.

This spring I utilised the TB-resistant bulls available through the use of AI. In ICBF’s bull selection tool, I requested that I wanted high TB resistance as a requirement when selecting bulls to match with my cows. I feel it’s something that we can add to our control programme that may aid reduce the future risk of infection. There is no silver bullet but the evidence is there that it can help reduce the incidence of TB.

At the time of writing this article, the result of the test is unknown, and it is difficult to express the worry we currently have in words.

If it is a clear test, that a sigh of relief won’t last more than six months when we must face this all over again, the sad reality of farming in Cork at the moment. Bovine TB is never an easy topic to discuss, there are so many factors involved and opinions to navigate. 

One common dominator between the vets and farmers is that gut-wrenching feeling we have on the reading day. If farmers are struggling with TB issues at the moment please reach out for help, you are not alone.

• Hazell Mullins BVM BVS, is a large animal vet based in Carrignavar, Co Cork.