Q&A: Bovine tuberculosis explained
Every year, farmers and vets go through the pain and sometimes misery of doing TB tests on a herd, but it’s all for a good reason.
We don’t think about the good this preventive measure does, because many of us have never seen a symptomatic animal.
To help answer some of the questions farmers have asked about the TB and the TB test, the Irish Cattle Breeding Federation has put together a TB fact sheet (see icbf.com/wp/?p=6810#more-6810), with help from the Department of Agriculture.
The TB page includes answers on what causes TB, what the symptoms of TB are, why you’ve most likely never seen an animal with symptoms, why it seems to just pop up out of the blue, and a few others.
What is bovine tuberculosis (BTB)?
Bovine tuberculosis is a disease caused by the mycobacterium bovis bacteria.
The bacteria are killed by sunlight (ultraviolet rays) but cannot be killed by drying out, and survive in both acidic and basic environments. Reports on length of survival of M bovis in the environment vary from 18 to 332 days, depending on temperature and sunlight.
The scary thing about M bovis is that it can survive for long periods of time in warm moist areas like cattle faeces. In fact, it can survive one to eight weeks in cattle faeces!
Bovine TB can infect a large range of mammals, though when compared to rates in cattle, it rarely infects sheep and horses.
These bacteria are considered zoonotic, meaning they can infect and cause disease in people too.
Typically, people contract BTB through drinking raw milk from infected cattle, which is why it’s important to pasteurise milk before drinking it.
But farmers and those who work directly with cattle or cattle carcases are also at risk in other ways.
How do cattle get BTB?
A cow can pick up BTB many different ways, but the most common transmission scenario is when they’re exposed to another cow that is already infected.
Once the bacteria get into the cow, they breed and infect many different organs, primarily the lungs, digestive tract, and udder, which means that they can be expelled through breathing, infected wounds, saliva, milk, urine, and faeces.
This is why testing once a year is so important.
Once a cow begins shedding the bacteria, it can spread through your herd very rapidly.
While testing your animals every year can be costly, the loss of your whole herd would be much more devastating.
Cows can also pick up the bacteria by accessing contaminated badger setts, badger latrines, and open streams, ingesting bacteria left from an infected badger’s visit to feeding or drinking troughs, or rarely by socialising with infected deer.
What if I haven’t had a reactor in my herd for a long time?
Unfortunately, that is the tricky part about BTB.
Since it lives in wildlife reservoirs like badgers, and to a much more limited extent deer, animals that don’t respect fences or property lines, they can introduce BTB into your herd even after you’ve gone many years without having a reactor.
Unfortunately, once it gets into a herd, BTB can spread rapidly, which is why it’s so important to test yearly and cull any reactors quickly.
By culling the reactor, you decrease the chance that the infection will spread through the rest of your herd.
But this is also why farms get locked up, after a reactor has been found.
It is important to do the follow-up tests after the reactor has been removed, but also after the immune reaction from the first test has decreased, or else you might get false negatives or positives.
This is why the two tests following the removal of a reactor must be done at least 42 days apart from one another. These follow-up tests ensure other cattle that may have been socialising with the positive reactor have not become infected.
Due to the slow growth of the bacteria, the Department has now brought in follow-up tests to ensure that no cattle have become reactors since the removal of the last.
Again, it may not be fun to do these tests, but it is better to test and remove than to lose a whole herd to the disease!
Sometimes a test will come back inconclusive and the animals are “doubtful”.
In this case, and especially if the animal doesn’t have TB, it is important to wait before doing a follow-up test, because the immune response of the animal may still be overly sensitive.
There are a few reasons this animal may be doubtful and not actually have TB. One reason is it may be co-infected with Johne’s disease. In this case, both of the injection sites will be raised. Another reason it might be doubtful would be that the animal was recently exposed to a similar bacteria.
