TB testing

Farmers throughout Scotland are accustomed to the four yearly tuberculosis (TB) test being carried out by their vet.   And with Scotland achieving Officially Tuberculosis Free (OTF) status in 2009 most of you will be expecting a clear test when it is your turn to be tested.  But what happens when the test is not clear?  What happens after a reactor is found?  And the big question most farmers have is when will I be able to sell cattle again?

TB breakdowns can come about in various ways.  The one people will be most likely to anticipate is vets finding a reaction to the bovine tuberculin injected during  a TB test.  If the vet finds that the reaction lump at the bovine site is larger than the lump at the avian injection site then the animal is considered to have a reaction.  If the difference in size between the avian and bovine reactions is relatively small then the animal is an Inconclusive Reactor (IR) but a larger size difference results in the animal being classed as a Reactor.  These animals are treated in different ways depending on this reaction.

Inconclusive reactors

If your vet has found one or more Inconclusive reactors you will be required to isolate these animals from all other stock on the farm.  Once isolated you will usually be allowed to trade the other animals on the farm.  Inconclusive reactors will have movement restrictions placed on them.

You are then faced with a choice.  You can wait for 60 days and have the inconclusive animal(s) retested.  If the animal tests clear at the second test all restrictions will be lifted.  If however the animal tests as an Inconclusive reactor or Reactor at the second test then the animal will be taken for slaughter. The whole herd will then be placed under movement restrictions and the Official TB Free status of the herd is suspended (OFTS).  In this case the animal will be valued and compensation paid for the value of the animal.  Assuming that TB is not found at slaughter or cultured from a sample, one whole herd test is required to be negative before any cattle can move again.

An alternative route is to voluntarily slaughter the animal after the first inconclusive test.  If you are considering this you should check first with APHA if this will trigger any extra herd tests.  Arrangements need to be made with APHA  for the animal to be slaughtered under supervision. The animal will be examined specifically for TB at slaughter and samples collected for bacterial culture.  In this situation no compensation will be paid to the farmer and the farmer will also have to pay any costs associated with transport and slaughter of the animal.

Reactors

If one or more reactor is found in the herd then the whole herd will be placed under movement restrictions and Official TB Free status suspended (OFTS).  Milk from reactors must not enter the bulk tank and advice on disposal of milk should be sought from APHA.  An APHA vet will carry out a disease investigation to attempt to identify how disease entered the herd.  The reactor(s) will be valued and compensation paid.  The animal(s) will then be taken and slaughtered.  Usually animals will be slaughtered at an abattoir but if they are under meat withhold due to drug treatment, in the last 10% of pregnancy or otherwise unfit to transport or go for food consumption they will need to be slaughtered on farm and sampled usually at an SAC Consulting Disease Surveillance Centre.  APHA will cover all costs associated with the transportation and testing of a reactor animal.

If there is evidence of TB infection at slaughter the affected areas will be cultured to try to grow the bacteria.  If no evidence of infection is seen then a standard selection of samples will be taken for culture.  As the bacterium involved is particularly slow growing you can expect to wait 8-14 weeks for results from the culture.  This can be a long and anxious wait until the results are ready.  If no evidence of TB is found in the carcass or on culture then a single whole herd test is required before the movement restrictions can be lifted.  This will result in movement restrictions being in place for a minimum of 10 weeks.

It can be frustrating when a reactor is slaughtered and no TB lesions are found at post mortem and TB is not cultured.  However it is important to remember that TB can cause lesions too small to be seen with the naked eye and the bacteria is difficult to grow so the animal may well have been incubating TB that was not found.

If TB is proved to be present then the Official TB free status of the herd is withdrawn (OFTW).  This means the whole herd is under restrictions until two clear whole herd TB tests can be carried out at least 60 days apart.  The first clear test has to be carried out at severe interpretation.  This means a smaller tuberculin reaction will result in the animal being classified as a reactor compared to the standard test procedure.  The second test can be carried out at standard interpretation.  In Scotland it is also likely that the herd will receive a gamma-interferon blood test.  This test can detect animals exposed to the infection at an earlier stage than the skin test.

After the herd has had two clear tests and adequate cleansing and disinfection of the farm has been completed, cattle can be moved again. You can expect the herd to be under movement restrictions for a minimum of 4½  months.  Some herds where further reactors are found undergo multiple whole herd tests at 2 month intervals and are under restrictions for several months. Another test will be required 12 months later before the herd can go back to the standard 4 yearly testing interval.

Slaughter house detection

All cattle that are sent for slaughter in the UK are examined for evidence of TB as a matter of routine.  If the Official Veterinary Surgeon finds any lesions in a carcass that are suspicious of TB this is referred to APHA.  The whole herd is under restrictions until the culture results are received.  If positive then testing of the herd will then be carried out in the same way as if the animal had been found at a herd skin test.

Buying or Selling cattle whilst under movement restrictions

You may be permitted to move some cattle onto your premises under license from APHA whilst under movement restrictions.  Cattle may move off your premises under licence direct to slaughter, to an approved slaughter gathering or to an approved finishing unit.  There are approved slaughter gatherings and finishing units in England and Wales only

Effects on others

If a farm is proved to have TB present in their cattle their farm boundaries will be checked to see if their cattle have contact with any neighbouring cattle.  If it is considered that there is a risk of disease spreading to neighbouring herds these will be tested under contiguous testing rules.  In addition any cattle moved off the farm since 2 months before the last clear test will be traced and tested if they are still alive.

If a reactor has been purchased and has evidence of TB in the carcass or a positive culture then the herd of origin will be traced and tested if it has not been tested recently

Recent Importation

Animals imported from the Republic of Ireland must be isolated and detained at the first destination premises until all animals in the consignment have a clear post import test.  No compensation will be paid if reactors are detected at the post import test.  Before considering importation you should consider the practicalities of isolating the imported cattle.  This can be challenging in a milking herd.

Marion McMillan, Veterinary Investigation Officer