In this issue
- Bovine TB - the skin test, results and lesions
- Ketosis & LDA risk - a spike in cases
- FAM30 is back - but the approved dilution rate has changed
- Animal Health & Welfare FETF grants - don't forget to include vet advice for 20% extra points
- VAN numbers - needed to sell fat stock from 13th April 2024
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 can feel 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 (and possibly unfair...) 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 found 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 from 80% 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 TB test has a very obvious impact on the results - the cows immune response can only be interpreted if we inject correctly and make a bleb at the avian and bovine site - 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 head, 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:
- The infection was missed by the skin test in the past and has been progressing slowly for a long time (months to years). These older lesions are more often "calcified" on the post mortem results
- The infection is recent but there was a high infectious dose to start with so the disease has progressed quickly (months)
- 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 try and take a bit of a different view. TB may be under Government control, and TB policy can become politicised, but it is still an infectious disease and we can do some things to control, manage, monitor and prevent it.
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, quickly accessing any visible lesion information, working out any possible patterns of disease and identifying risk factors specific to your farm.
Ketosis & LDA risk - a spike in cases
We've had a spike in ketosis and LDA in the last six weeks. Both tend to be seen in early lactation and highlight a bit of a challenge in transition.
An LDA happens when the fourth stomach - the abomasum, moves from its normal position on the bottom of the abdomen, and drifts to the left-hand side, ending up on the wrong side of the rumen. It can then fill with gas and rise like a balloon, trapping it in position.
Why does it affect freshly calved cows?
During late pregnancy the growing calf and uterus take up an enormous amount of room in the abdomen of the cow and her feed intakes drop off. Once the cow calves there is suddenly a lot more space in the abdomen, and there's a lag in getting intakes back up so a delay in getting the rumen back up to capacity plus there's a diet change from dries to highs which can lead to gas accumulation in the abomasum. It is a combination of all these risk factors, that mean the abomasum is at risk of moving into an abnormal position on the left side.
On top of these "normal" risk factors, there are abnormal risk factors which increase the risk of LDAs further:
FAT COWS: have really poor intakes before and after calving, and are a high risk for milk fever and ketosis
TWINS: increases the risk of retained cleansing, early calvings and poor intakes before calving
FRESH COW DISEASE: if the fresh cow is lame, gets mastitis, has a retained cleansing or gets milk fever - all reduce intakes which can then lead to LDA
GROUP CHANGES: every group change hit intakes hard as the cows resettle hierarchies and feed intakes take a hit
DIET CHANGE: the rumen bugs have a lot of adapting to do changing from the dry ration to highs, or if forage stocks are running low.
What to look out for:
- Older cow (3rd plus lactation)
- Freshly calved (7-21 days)
- Decreased milk yield
- Off feed - sometimes will eat forage but not concentrates
- Some may scour, others may have stiff muck
- Appear ‘empty’ on the left hand side
- Some may appear dehydrated, with sunken eyes
- Arched back due to abdominal pain
- ‘Pear-drop’ smell to their breath
For those of you with stethoscopes on farm; you'll know the characteristic high pitched ping when you listen to the left hand side of a cow with an LDA. Remember to listen along a line between elbow and hip (red line below), stethoscope on the rib and using the other hand flick in a circle around the stethoscope as you move it along that imaginary line.
Treatment of LDA:
The first job is to find and treat any underlying disease (ketosis, metritis, mastitis) - stabilising her medically before surgery helps with recovery post-op. The vast majority of LDA will need surgery to replace the abomasum back to its normal position.
There are a few different surgical methods that you'll see us do - Tom opens up on the right, Paula opens up on the left and Katie does a bit of both - but both methods end up with the abomasum permanently stitched into position and a rapid recovery for the cow - often they're back to normal intakes in a few hours after the op.
How many are too many LDA?
Some farms aim to be LDA free; transition management of every individual is key on these farms to protect rumen fill for every cow. Most farms would run at around 1% LDA; any more than that and we would want to have a chat about common risk factors and prevention strategies for your herd. Sometimes risk factors can affect multiple dry / fresh cows and you can get runs of LDAs - tricky grazing conditions in late Spring can be a common risk factor.
FAM30 is back - but the dilution has changed
There had been a temporary change to the approved list of TB disinfectants on farm and FAM30 had been suspended - it has now been restored but the approved dilution rate is now 1 in 14 not 1 in 20 as it used to be. This is particularly important if you are down with TB and need to fill in your BT05 Cleaning & Disinfection forms for APHA.
FAM30 Dilution Rate: 1:14
= 71ml FAM30 in 1 litre water
= 357ml FAM30 in 5 litres water
= 710ml FAM30 in 10 litres water
Don't forget not all disinfectants cover all diseases - if you struggle with Cocci or Crypto in your calves, it is vital you use a disinfectant that can crack the eggs open - we stock "Cyclex" as our Cocci/Crypto specific disinfectant.
Animal Health & Welfare grants - don't forget to include vet advice for 20% extra points
For those of you that have been waiting - the next round of FETF grant funding is now open for applications.
This time there will be three separate "themes": Productivity, Slurry management and Animal health & welfare.
The grants for animal health & welfare are now open and will run until midday 1st May 2024.
Each item will have a published number of "points" which will then be multiplied by how many of each item you apply for. This total will then be divided by the overall grant funding for these items to give you your "overall points".
As your vets we can increase your "overall points" by 20% by giving evidence that we've discussed the items with you and that they are needed and would be beneficial so make sure you get in touch if you're thinking of applying.
Have a look at the links below to see what items are available on each theme:
VAN numbers - needed to sell fat stock from 13th April 2024
Remember to get in touch if you aren't farm assured and need a Vet Attestation - you'll need to give your "VAN" number when selling fat stock from the 13th April 2024.
If you are Farm Assured Dairy - make sure you have cover for the beef that you sell fat; this will then cover barren cows you sell as well as any finished beef. If you're just covered for Dairy with Farm Assurance, you will need a Vet Attestation and VAN number for your beef sales too
Get in touch
Paula: 07764 747855 paula@haywoodfarmvets.com
Tom: 07837 291097 tom@haywoodfarmvets.com
Katie: 07507 656747 katie@haywoodfarmvets.com
Open hours: M-F | 8:30 - 16:00
Out of Hours: 01630 810016