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Farm Animal December Newsletter

You will all have received letters from Animal Health to explain the news rules for TB testing. To put these simply. In future an animal, which is inconclusive at two consecutive tests, will be deemed to be a reactor and will be taken.

We will now start each test reading under standard interpretation. However if we find a reactor (in those herds under TB restriction for disease rather than for time) we will then change to severe interpretation. This might lead us to giving you false information at the conclusion of the test so please bear with us if we have to phone you up with different results once we have written up the test and put the figures through the computer.

Paul and I recently joined a TB roadshow that was going across the country and while there was nothing new there were a few interesting facts that came out. I need to emphasise again that the skin test is very specific although not highly sensitive; in other words if the skin test shows a reactor it is more than 99% certain that the cow has TB but the test only shows up 77-95% of cases. The reason for this can be explained by the graph below.

The time between picking upthe infection and showing up of disease, through testing, is dependant on the number of bacteria that infected the animal to begin with. Basically the skin test picks up disease very quickly while the presence of lesions or bacteria takes some time to build up.

In 2008, there were 619 reactors in cattle under 6 months of age.

One of the reasons we will not be using a TB vaccine in cattle (besides the fact there isn't one) is the EU regulations. First there can be no trade in TB vaccinated cattle and, more important, under hygiene regulations no product (ie meat and milk) can be consumed from vaccinated animals. Animal health is working to try and change these regulations.

And finally a lovely bit of medical history. Do you know what BCG stands for and how the vaccine was originally made? BCG is Bacille Calmette-Guerin. Dr Calmette was a doctor while Muerin was a vet who were trying to find a vaccine against TB in 1913. In vaccine manufacture it is usual to passage the bug through various media until it has lost its strength to cause disease but still has the antigens to allow the body to react against it. For some reason Calmette and Guerin grew the TB bacteria on potatoes soaked in bile and gylcerine. The culture was changed every few months but meanwhile Dr Calmette went to the trenches in the first World War and Guerin carried on with the passaging as noone told him to stop. When Dr Calmette came back from the war he found that Guerin had passaged the bacteria 230 times so decided to stop and the vaccine was created and has been the same ever since through billions of doses worldwide in humans.

Next week Naxcel will be available. This is a long acting Excenel which is given into the base of the ear. It still has a zero milk withhold time but a 9 day meat withhold when given into the base of the ear. The antibiotic lasts 8 days when given by this method. It is remarkably easy to administer it: we have had a few bottles to try and have not had a problem when we, students or clients have given it. We are not entirely sure of the cost yet but get the impression one shot will cost about the same as three shots of the present solution so there will be a factor of convenience and also a longer course of action for the same price.