By Stuart Anderson, Director of the Mycoplasma bovis Programme
A question asked frequently of the Mycoplasma bovis Programme after a detect result from our bulk tank milk screening, is around the sample sizes when on-farm blood testing is carried out.
Bulk tank milk samples are taken at the pick-up of milk from each dairy supplier and routinely screened for indications of Mycoplasma bovis (M. bovis). The initial screening test looks for M. bovis antibodies and a detect result signals the need for a closer look to determine if the farm does have infected cattle.
A closer look means understanding the farm’s operation and cattle management, and on-farm blood testing. M. bovis is tricky to find, especially in individual animals, so our testing has been developed to find it at herd level. Sample sizes for testing are determined by herd size. For herds of 220 cattle or less, we take blood from each animal. For herds greater than 220 we take blood from up to 250 animals. At the lab the blood is tested, and results are interpreted at the whole herd level. The science is that if the disease is present, we expect at least 1 in every 20 cattle (5%), to have M. bovis antibodies.
We expect at least a 5% prevalence of antibodies through a random sample. As an analogy – you decide to re-sow a pasture with a rye and clover mix. You put in 95% rye and 5% clover. Once it’s established, you want to see if the clover has come through. You’d reasonably expect that about 5% of the pasture is clover. To check, you’ll look in a few spots randomly through the paddock. If the clover is there, you’ll find it. You don’t need to go through every blade of grass, on your hands and knees.
Part of our national surveillance programme, Bulk Tank Milk screening is a powerful tool, helping to find instances of M. bovis outside the network of infected farms we’ve tracked down already.
Take the recent cluster of properties confirmed infected with M. bovis in mid-Canterbury. They were found through the August screening. We had expected to find more infected farms over spring, it’s a time we have more samples to test, animals are under stress from calving and we have 2018 heifers entering the milking platform for the first time.
After initial results detected M. bovis antibodies, blood testing was carried out and dairies were confirmed infected M. bovis. Then the systematic work of tracing movements of cattle began. So far (1 Dec), this essential work has identified a further 4 infected properties.
The properties infected are all connected by animal movements and that’s why good NAIT records are so important. They help trace infected animals faster and stop the spread of the disease to other herds and other farms.
Investigation into the source of infection for the newly confirmed properties are ongoing. Much like looking for an electric fence fault, sometimes the smoking gun is glaringly obvious and other times it’s systematically tracing each different line and looking at different possibilities until you find the source.
- The MPI Mycoplasma bovis Programme has a new website which has been running for a few months now. It’s a really good source of information and easy to navigate: www.mbovis.govt.nz. To keep an eye on the numbers, there’s a regular update you can sign up to under News & Events.