Animals – Still battling bovis

During the autumn, to screen for the bacteria Mycoplasma bovis, bulk milk samples were collected every fortnight from milking herds throughout New Zealand. Testing of these samples was finally concluded in the last couple of weeks. Of the 693 dairy farms tested in Northland, 18 tested positive for M. bovis, which is 2.6 per cent of the herds tested.

However, the disease is unlikely to be as widespread as these initial results suggest. The test that is undertaken in the milk at this stage of the eradication process is called a screening test. To identify a small number of bacteria in a bulk milk sample requires a screening test that is extremely sensitive. The down side to such a sensitive test is that it can provide false positive results. These are caused by bacteria in the milk sample that are similar in shape and size to M. bovis. Therefore, a positive screening test doesn’t necessarily mean the herd is actually positive for M. bovis. Of the 18 Northland farms which tested positive for the bacteria, only a small number of them will actually have the disease. MPI suspects that of these 18 farms, only one or two (if any) will have true M. bovis infections or 0.2 per cent.

Because it is impossible to tell which herds have false positives without further testing, these 18 farms have all been put on movement restrictions while a second test is performed to confirm whether the disease is present or not. This second test has very few false positive results, meaning that if they test positive at this second test then the farm has a true M. bovis infection. This second test involves blood testing every single animal on these properties and if a certain percentage of these animals test positive for the disease, then they are declared an infected herd.

As you can imagine, the process of testing every animal on these properties is time consuming, so it may be a little while before these farms are declared free from the disease and taken off movement restrictions. MPI suggests this may take up to 12 weeks.

To continue the monitoring process, MPI plans to take a bulk milk sample every month from every farm throughout New Zealand. For this reason, it is vital that farmers keep their National Animal Identification & Tracing (NAIT) records up to date so that in the unlikely event your farm tests positive at one of the screening tests, you have accurate records for animals entering and leaving the property to help with the secondary testing process.

If you have any further questions regarding the testing process and what implications it may have for yourself, please don’t hesitate to contact us at Wellsford Vets.

Neil Warnock, Wellsford Vet Clinic
www.vetsonline.co.nz/wellsfordvet