Bovine TB: the skin test

As you all know, bovine TB is a very real threat in our local area and unfortunately dealing with a breakdown becomes less about "if" and more about "when".

The cornerstone of the UK Government's 25 year TB eradication strategy is the accurate identification and rapid removal of animals infected with TB before they can spread the disease to other cattle. The primary screening test for TB in cattle in the UK is the tuberculin skin test which you will all be familiar with; but here's a review of the test, how we interpret the results and what those results mean about TB on your farm.

TB in cattle is a chronic infection; the bacteria (Mycobacterium bovis or M. bovis) hide away in white blood cells and the body tries to contain infection in lymph nodes by building thick walled abscesses; the "lesions" found at post-mortem. Signs of TB are very rare nowadays, but infected cattle are infectious well before they develop lesions in their internal organs.

The skin test measures the immune response of the animal to injections of tuberculin - a mix of proteins made from cultures of TB bacteria grown in the lab and then killed by heat. Two separate cultures of TB bacteria are used: M. bovis (cattle TB) to make the bovine tuberculin and M. avium (bird TB) to make the avian tuberculin.

The skin test compares the animal’s immune response to injections of bovine tuberculin and avian tuberculin by injecting the two types into the skin of the neck. If an animal’s immune system has been exposed to infection with cattle TB an inflammatory response will be triggered at the bovine injection site at the bottom of the neck; if the animal has been exposed to bird type mycobacteria then an inflammatory response will be triggered at the avian site at the top of the neck. There can be a certain amount of cross-over but cattle infected with bovine TB will have a bigger reaction to the bovine tuberculin injection.

When you are clear of TB the test is read at standard interpretation; an increase of >4mm at the bovine site compared to the reaction at the avian site would be classed as a reactor. If you have had TB confirmed then the test will be read at severe interpretation and an increase of >2mm at the bovine site compared to the reaction at the avian site would be classed as a reactor.

How accurate is the skin test?

The skin TB test has a terrible reputation for being inaccurate; no diagnostic test is perfect but we can measure how accurate a test is by looking at the balance of SPECIFICITY and SENSITIVITY.

Specificity: is a reactor really infected?

Skin test specificity: 99.98%

This means there is 1 wrong reactor in every 5000 cattle tested at standard interpretation

Sensitivity of the skin test: how many reactors do we miss?

Skin test sensitivity: 80%

This means we can miss 1 in 5 reactors at standard interpretation

So, just like Johne's disease testing, the skin TB test is limited by its sensitivity. This means we can trust the positives; the vast majority of skin test reactors are truly infected, but because the test misses some positives it is difficult to trust a single negative skin test.

What affects the sensitivity of the test?

1. Stage of disease:

In very early infections cattle may not yet be able to react to the skin test, and in late stages of disease the animals immune system gives up and so chronic, infectious reactors with lesions can pass a skin test without any lumps.

2. Interpretation:

If you have a TB breakdown your TB test will be read on "severe interpretation" which means a smaller bovine increase counts as a reactor. Severe interpretation improves sensitivity to 94% so we only miss 1 in 20 reactors, but that comes at a cost to specificity; 1 in 800 will be false positives rather than 1 in 5000.

3. Testing procedure:

How well we perform the test has a very obvious impact on the results - at HFV we pride ourselves on testing every animal properly. A missed reactor can do a lot of damage in a group - and is one reason why increasing testing frequency to 6 monthly can be protective.

"There were no lesions at post-mortem - did she really have TB?"

Remember the skin test at standard interpretation wrongly identifies only 1 reactor out of every 5000 skin tests

The aim of an accurate skin TB test is to pick up an infected cow before she has time to develop lesions as lesions mean she's increasingly infectious to her herd mates.

"Lesions" are TB abscesses found in the lymph nodes in the throat or lungs. These lymph nodes are inspected as part of the meat inspection process in every abattoir, in every cow, in every post-mortem, regardless if it is a reactor, a barren or a fat animal.

If we are testing every animal accurately then we should pick up reactors before they develop lesions and so a post-mortem result of "no visible lesions" is a good thing - we've picked it up early and there will be much less chance of spread between cattle.

" So why do we still find visible lesions (VL)?"

We find VL in around 30% of skin test reactors. Here are a few reasons why:

1. The infection was missed by the skin test in the past and has been progressing slowly for a long time (months to years)
2. The infection is recent but there was a high infectious dose to start with so the disease has progressed quickly (months)
3. The cattle have poor immunity so even a low infectious dose results in disease. Active BVD infection and circulating virus is one of the most common causes of poor immunity in cattle.

TB testing at HFV:

TB testing has often been thought of as a thankless task by farmers and vets alike - but at HFV we take a bit of a different view. TB may be under Government control but it is still an infectious disease, just like every other infectious disease that we control, manage, monitor and prevent. And although many practices now outsource their TB testing or use non-vet TB testers, we see the value in our vets doing your TB work.

TB control is about more than just TB testing; it's about knowing your farm, us working with you and your APHA case vet, liaising to get reactors moved off quickly, working out patterns of disease and identifying risk factors specific to your farm.

TB shouldn't just be an inevitable consequence of farming in a High Risk Area.

Turn out and protecting the bulk tank SCC

Historically, turn out and "Dr Green" meant an improvement in udder health with fewer cases of clinical mastitis and lower cell counts, but as mastitis pathogens and patterns change on farm, and winter housing environments improve, we can see a seasonal spike in cell counts during the summer months.

Strep. uberis is the usual suspect; an environmental bug that can also pass cow to cow and there are typical hot spots at pasture especially around favourite sheltered lying spots, high traffic areas and gateways.

Strep. uberis can survive for up to 21 days on pasture before UV light destroys it so make sure your rotation is set to reduce this risk.

Pre-dipping what look like already clean teats can seem a bit OTT, and obviously adds time to the milking routine, but it dramatically reduces the environmental load of bacteria sat on the teat skin. It is these bacteria that get into the udder during the milking process and cause the sub-clinical infections which show up as an increased cell count.

Remember the pre-dip is a rapid kill product but still needs at least 30 seconds contact time in order to be effective.