The bacteria that cause Johnes disease (Mycobacterium avium subspecies paratuberculosis or M. avium) are closely related to those that cause bovine TB (Mycobacterium bovis or M. bovis).

Therefore, vaccination against Johnes disease can affect how quickly farmers can eliminate TB from your herd, advises the TB Advisory Service.

Johnes disease is extremely common in cattle in the UK and has a severe financial and welfare impact in both dairy and beef herds. Eliminating the disease and preventing reintroduction is possible, although difficult, and requires continued effort over a long period.

Johnes vaccines are available and are sometimes used to assist in this battle.

The problem arises because the two diseases are so closely related. If an animal has been vaccinated against Johnes disease, it is less likely that the skin test will detect TB.

When the TB skin test is carried out, two injections are made into the skin of the neck, one of avian tuberculin and one of bovine tuberculin. Avian tuberculin is an extract of M. avium and bovine tuberculin is an extract of M. bovis.

The test result depends on the comparison of the swelling in the skin measured at the site of each injection. Normally, TB infected animals develop a larger swelling at the site of the bovine injection which indicates a positive result for TB.

However, animals that have been vaccinated against Johnes will have immunity to M. avium and are likely to develop a larger swelling at the site of the avian injection which may therefore result in a negative result to the TB skin test.

This means that TB infected animals are less likely to be identified and may be left in the herd posing a risk of spread to others and preventing the herd going clear.

Getting rid of Johnes disease is possible without vaccination and farmers are advised to speak to their vet before considering using it.