What are BrBME tests?
The test procedure is known as the Enzyme Linked ImmunoSorbent Assay, or ELISA for short. This test type may be used to detect a number of diseases in humans and animals using blood serum or milk. When used to test milk for brucellosis we know it as the Brucellosis Bulk Milk ELISA or BrBME. It has been used in Great Britain and in the Republic of Ireland for some time to detect brucellosis and is capable of detecting one infected animal in a herd of around 300 milking cows.
As with any other biological test, there may be false positive and false negative results. In other words not every herd reacting positively to the test is infected, and not every infected herd will give a positive reaction to the test. For this reason the test is not considered to be a definitive indicator of disease. Rather, a positive or inconclusive result is an indicator that individual blood testing of the herd should be carried out to determine if an infected animal is present.
Value of BrBME tests
Over four years from March 2000-2004, 254 non-negative herds were identified and subjected to blood testing. Of these, 106 had already been identified by other means as being at special risk and blood tests had been arranged. The remaining 148 were on a routine blood-testing regime when they showed non-negative to the BrBME. Of these, 41 were found to be infected with brucellosis.
Thus the BrBME led to the detection of infected herds which might otherwise have been left to spread disease to their neighbours and further afield for up to two years before the presence of the disease would have been detected through routine testing. The test has therefore proved its worth beyond doubt.
The main advantage of the BrBME is that the brucellosis status of all milking cows in a herd can be assessed by taking one milk sample from the bulk tank. This means cattle do not need to be individually restrained and tested. Indeed Department of Agriculture, Environment and Rural Affairs (DAERA) staff do not even need to visit the farm as the milk samples are supplied by the dairy which collects milk from the farm.
This makes sampling very cost effective. Because they are undergoing monthly BrBME, dairy herds in clear areas can be accorded a lower priority with regard to blood testing. This allows DAERA staff to concentrate their efforts on the blood sampling of other herds. The test is recognised by the European Union as an official test for the purpose of the brucellosis control programme.
The test does not, of course, identify suspect individuals within the herd. Indeed, it is not a definitive indicator of infection within the herd. However, it does highlight herds in which there may be a problem and allows us to conduct further investigations in these herds. A few herds throw up repeated positive results even though there is no infection in these herds.
Sample collection for BrBME tests
The vast majority of dairy farms have their milk collected by one of the large dairies which supply the Department of Agriculture, Environment and Rural Affairs (DAERA) with monthly milk samples for Brucellosis testing. The majority of these draw off a sample of milk while the milk tanker pumps milk out of the bulk tank. These samples, contained in small snap-top containers, are sent by the Dairies to AFBI laboratories at Stormont.
Dairies based in the Republic of Ireland decant milk samples from their Northern Ireland suppliers into 'deep well blocks' which take up much less space than the original containers. These deep well blocks are then sent to the AFBI labs.
A few dairy farms do not have their milk collected by the larger dairies. In such cases, DAERA staff visit these farms monthly to collect samples which are then processed in the usual way.
Interpretation of BrBME test results
Results are provided by the lab to the Veterinary Service Animal Health Group, and are categorised into negative, inconclusive, low positive, medium positive, and high positive. Currently, we take identical action in all cases where the result is 'non-negative'. With increasing experience in the use of this test, differentiation may be made. In the case of larger herds, the inherent dilution factor means that negative results must be regarded with caution.
Follow-up for non-negative BrBME results
A herd restriction is placed on all the herds giving positive or inconclusive readings to the Brucellosis Bulk Milk ELISA (Enzyme Linked ImmunoSorbent Assay)(BrBME). Veterinary Service Animal Health Group staff will take a repeat sample from the bulk tank as a check on the original result. A herd blood test will be arranged and carried out as soon as possible. The results of this test are interpreted in the normal way and follow-up action is as for any other blood test.
Repeat BrBME positive results
Some herds yield positive results to bulk milk tests month after month despite the fact that blood tests reveal no sign of disease in the herd. The assumption is that this is caused by interference from antibodies to other diseases. While this situation continues, the test is, of course, of no value in detecting the introduction of Brucellosis to such herds. In such cases we may carry out individual milk sampling of cows in these herds, to identify the offending animals.