Finding Mycoplasma bovis (M. bovis)

National background surveillance

National background surveillance of M. bovis involves:

• beef and drystock cattle surveillance
• bulk tank milk screening.

We test a range of animals, including:

• cattle bred for meat production
• beef-breeding herds
• dairy heifers at grazing properties
• cattle at small and non-commercial farming enterprises.

We regularly test for M. bovis to:

• discover any unknown pockets of infection
• build confidence that the disease is absent in New Zealand’s cattle herd.

Tracing M. bovis

When infection is confirmed on a property, we start tracing or mapping out cattle movements on and off that property to identify farms at risk.

We look at cattle movements:

• onto the property to try to work out how the infection arrived on the farm
• off the property during and just before what we think the infection risk period is, so we can track if the infection has spread off the farm.

We do most of our tracing through:

• NAIT records
• records kept at confirmed infected properties
• reports from farmers who have a movement that isn't recorded in NAIT.

Network surveillance of M. bovis

Network surveillance investigates properties traced or linked to the network of properties with confirmed M. bovis. We sample and test cattle on properties we think are at risk of infection.

This includes properties that:

• have sent cattle to an infected property
• have received cattle from an infected property
• are near a confirmed property and/or operated by the same owner or enterprise.

Testing methods

We test to:

• screen for M. bovis (national surveillance)
• confirm the presence or absence of M. bovis on farm.

M. bovis is difficult to diagnose in an individual animal because some animals don't show clinical signs of illness. This means we use tests that identify infected groups of cattle. To detect low levels of infection, we may test many animals, often multiple times.

Two tests for M. bovis

We use 2 tests to detect M. bovis:

• Enzyme-linked immunosorbent assay (ELISA) test.
• Polymerase chain reaction (PCR) test.

How the ELISA test works

The ELISA test detects antibodies for M. bovis bacteria in blood or milk. Antibodies help to fight infection. This means the test looks for an immune response to the bacteria, rather than the bacteria itself.

ELISA test results are interpreted across the group of cattle tested, often referred to as herd-level interpretation.

How the PCR test works

The PCR test detects M. bovis DNA in a sample taken from the animal (for example, tissue, milk or swabs from tonsils). If any sample from a group of cattle returns a positive PCR result, that is conclusive evidence the disease is present in that group.

However, a negative PCR result is less reliable as the test relies on M. bovis bacteria being captured on the swab, and an infected animal isn't always shedding bacteria. As a significant proportion of infected animals test negative in a PCR test, we can't use the test to know for sure that an individual animal isn't infected.

Determining confirmed infection

For a property to become a confirmed property, our epidemiologists must decide that it meets the definition of having M. bovis. This means it has:

• returned a positive PCR test result, or
• returned ELISA test results with serology (blood serum study) levels that indicate the disease is, or was, present, or
• returned repeated marginal serology results, and other risk factors indicate the disease is present. This means culling the animals is the only responsible decision to achieve confidence of absence.