Testing is carried out to identify M. bovis infected herds. Mycoplasma bovis (M. bovis) is difficult to diagnose in an individual animal, as clinical signs are not seen in all infected animals, and collecting of samples for diagnostic testing is necessary to identify infected animals and groups of animals.

The tests used within the Programme identify infected groups of cattle. To ensure the testing is accurate, we test a large number of animals, multiple times.

What tests do we use?

There are two tests we use to detect M. bovis:

• ELISA test (Enzyme Linked Immunosorbent Assay test)
• PCR test (Polymerase Chain Reaction test)

The number and groups of cattle that samples for testing are taken from varies from property to property, depending on a number of factors such as the:

• number of cattle on farm
• way in which cattle are organised into groups
• age of the cattle
• way the property was identified as being at risk of M. bovis infection.

How the ELISA test works

The ELISA test works by detecting antibodies to M. bovis bacteria in blood or milk. It looks for the immune response to the bacteria, rather than the bacteria itself.

An individual animal that returns a positive ELISA test result is referred to as a ‘reactor’.

ELISA test results are interpreted across the group of cattle tested, often referred to as herd-level interpretation. If a percentage of the cattle tested in a group are ‘reactors’ the herd-level result is positive.

How the PCR test works

The PCR test detects M. bovis DNA in a sample taken from the animal (e.g. tissue, milk, or swabs from tonsils). It is the same type of test used to determine COVID-19 infection in humans.

A positive PCR test is conclusive evidence that the disease is present in that animal because we have found the DNA of the bacteria.

However, a negative result is less reliable. The PCR test relies on the bacteria being captured on the swab, and unfortunately an infected animal will not always be shedding the bacteria, meaning we can’t get it on the swab.

This means a significant proportion of infected animals will test negative in a PCR test. Therefore we can’t use the test to know for sure that an individual animal isn’t infected.

If any sample from a group of cattle returns a positive PCR result, then that is a conclusive determination that the disease is present in that group.

Testing rounds and time frames

In order to be sure that we are finding all infected cattle, there will be instances where we require multiple rounds of testing.

We expect that the results for each round will be provided to you within two to three weeks of the samples being taken.

Results are initially provided over the phone, and followed up with written confirmation.

1st Round

All management groups at risk of infection will be identified and blood samples taken from a sample group of the cattle. All sampled animals must be identified individually with a NAIT approved RFID ear tag.

A nasal swab will be collected from any trace cattle less than a year old.

All trace animals (that have come from a herd infected with M. bovis) will be raddled with blue stock paint and tagged with a blue ear tag to be sent for slaughter sampling after this round of testing.

Where the ELISA test detects M. bovis antibodies present in a group of less than 40 cattle with trace animals present, the entire group will need to be sent to slaughter for sampling.

2nd Round

Properties that will require more than one round of testing include those where:

• the ELISA test detects M. bovis antibodies in round 1
• trace animals were present in the group
• the group was fewer than 40 animals (and round 1 was negative)

Round 2 testing needs to take place at least three weeks after round 1, and at least two weeks after slaughter sampling, to allow any possible M. bovis antibodies to build up.

All management groups that require a 2nd round of testing will be identified by our epidemiologists.

3+ Rounds

It is uncommon for properties to require more than two rounds to determine their infection status, however it does happen on occasion

Properties that require 3+ rounds include those:

• where we have been unable to determine the disease status through two rounds of testing due to irregular results
• the Programme has identified additional trace animals during the farm census, and those cattle are still on farm

The management groups that require further rounds of testing will be identified by our epidemiologists.

Slaughter sampling 

Slaughter sampling All trace animals are considered high risk and will be sampled at slaughter under Section 121 of the Biosecurity Act 1993.

After round 1 of sampling all trace animals are sent to slaughter and have a blood sample and tonsillar swab collected. This is an essential element of determining the infection status of a property and removing the risk trace animals present.

Census

The Programme will carry out a ‘census’ of all cattle on properties. This is a headcount of all cattle on the farm and how they are organised into groups.The census helps us:

• identify any other trace animals which need to be culled
• get details of each animal’s lifetime history
• confirm farm records such as MINDA and NAIT.