Tuberculosis can be caused by three different types of bacterium, all of which are related:
The bacteria do not just affect the species they are named after, which is why bTB is seen as a threat to human health and why badgers also carry, and spread, M.bovis.
Cattle is by far the most susceptible domestic species to the M.bovis bacteria, although farmed deer, boar, bison, buffalo, goats, llamas and alpacas can also be affected.
It is possible, although rare, for cats and dogs to contract bTB. Sheep and horses are resistant. Rare cases of TB in pigs are more commonly caused by M.avium.
Is the TB bacterium related to anything else?
Interestingly, M.tuberculosis, bovis and avium belong to the same family of bacterium that cause leprosy in humans and Johne’s disease in cattle.
This means cattle vaccinated for Johne’s could test positive for TB even if they do not have the disease. Conversely, an animal could test positive for Johne’s disease if it has very recently been tested for TB.
Once a bovine is infected, however, it does not immediately start spreading the disease. TB develops very slowly and it takes time for lesions to grow in the lungs – and these lesions have to open up before cattle start coughing out the bacteria.
Very few cases of TB are reported in cattle. This is possibly because the symptoms are very similar to other respiratory diseases but also because regular TB testing catches the infection long before it becomes a chronic disease visually affecting the animal.
As lesions are most common in the lungs (called tubercules) a hard, dry, short cough is usually the first symptom, leading to more frequent coughing and laboured/painful breathing. As this continues cattle will lose condition and later cough up blood, and scour.
If lesions develop in the udder then, over time, it will become solid and hard lumps may be felt. But the milk will look normal until the disease is very developed, when it will eventually become watery and bluish in colour.
In the advanced stages of TB an animal becomes ‘desensitised’ by the heavy challenge of infection and so will not react to testing – this is called anergy and is one reason for false-positive test results.
But others may develop open lesions much more quickly, possibly before they are identified through testing, especially if the herd is on four-year testing.
It is impossible to predict how TB will affect an individual, as it depends on a number of factors including its immune system, disease status, diet and age.
Although the risk is smaller, cattle can also ingest the bacteria (for example, through eating food infected cattle have coughed on, or that has come in contact with badger urine) or contract it via the teat canal.
TB lesions can grow in parts of the body other than the lungs, including the digestive tract, udder and liver. If these lesions open M.bovis can be shed in the milk (which would put suckling calves at risk) and urine/faeces.
It is because of the risk posed by open lesions in the udder that milk is pasteurised. Milk from reactors is condemned and it is not recommended to use raw milk in the family home or feed it to other livestock (or farm cats).
The risk of spreading Johne’s disease by applied slurry/FYM to grazing ground is perhaps more widely known and the advice there is not to graze a field too soon after muck spreading.
Milk from TB reactors can be spread on farmland with slurry if the appropriate waste management exemption (free of change) or a waste management licence (not free) is applied for.
Some farmers have put TB breakdowns down to a newly purchased bull but the chances of this being via the semen is incredibly small, as in the unlikely event of lesions forming in the scrotum the bull would probably become infertile.
A bull who has had TB long enough to develop open lesions in its reproductive organs would present a major risk via breathing/coughing on females it comes into contact with.
This means knowing the disease status of the herd is worthwhile, especially to find out if BVD is a problem, as this particularly affects the immune system.
Older animals tend to have weaker immune systems and are also exposed to the disease for longer if the herd is not on regular testing.
The Department of Health still views TB as a ‘major public health problem’ and of the 9.2 million new worldwide cases of TB in 2007 (resulting in 1.7 million deaths) around 7,750 were in the UK.
In the UK and across the world, more than 99 per cent of new cases in humans are caused by M.tuberculosis and not M.bovis.
The risk is still there and so TB is a notifiable disease in all farmed animals.
TB in humans presents with the same symptoms whether it is caused by M.tuberculosis and not M.bovis. And in the same way that cattle can spread TB to humans, a human carrying TB could technically infect cattle.
If you are concerned you, or a family member, has contracted TB (either from drinking raw milk or coming in direct contact with cows) you can contact your GP or local community TB nurse to request a test.
The young, old and immuno-suppressed are most at risk.
Despite being developed against M.tuberculosis, the BCG vaccine offer humans some protection against M.bovis.