Testing ofÃ‚ Mycoplasma bovis
With cattle on the move, farmers are concerned about purchasing animals that might be infected with Mycoplasma bovis and are looking to test for reassurance. Unfortunately, there is no 100%-guaranteed test to detect Mycoplasma bovis infected animals. Serology tests for antibodies produce both false-positive and false-negative results in NZ cattle. False-positive animals are attributed to cattle carrying other innocuous organisms that cross-react with the tests. False-negative animals are seen when animals fail to seroconvert. PCR testing of milk, nasopharyngeal, preputial and semen samples is more reliable, rarely producing false-positive results but may produce false-negatives in animals that are not shedding organisms at the time of testing or in which sampling doesn’t collect infected material. Please refer to the MPI guidelines to learn how to properly collect nasopharyngeal samples (Click here for guidelines).
Further information can be found on the MPI website but a summary of the pertinent points regarding testing follows.
From MPI Guidelines for Testing of Mycoplasma bovis
General Test Limitations and Unknowns: Mycoplasma bovis is notoriously difficult to detect, and there is little information about test use for detection of the agent to a nearly 100% level of certainty. This is especially true of subclinical carrier animals.
- PCR Limitations and Unknowns: Real time M. bovis PCR has high sensitivity (approaching 99%), however, diagnostic sensitivity is compromised by intermittent shedding in milk, semen and nasal secretions. Bulk milk and colostrum can be useful since pooled milk may allow for detection even if some infected animals are not shedding at the time of collection, but it may possibly lose sensitivity over the course of a milking season.
- ELISA Limitations and Unknowns: NOTE: Currently, MPI does not recommend ELISA testing due to poor specificity and resultant detection of false-positive cattle. Evaluation of new ELISA tests continues to try to find one or more with better sensitivity and specificity.
- Selection of mobs to test: Because M. bovis spreads variably between animals and requires close, prolonged contact to spread, the status of one mob on a farm, saleyard, etc CANNOT be used to determine the status of other management groups. In addition, if animals have mixed with other mobs recently (e.g. at sale or grazier), some time may be required for animals to seroconvert or begin shedding these factors should be taken into consideration when results/interpretations are reported.
Sampling Plans/Diagnostic Approaches (general scenarios):
- Sick animals PCR of exudates/dry swabs of lesions from animals showing lesions consistent with M. bovis infection (e.g. joint fluid, lung lesion, serosal surface, mastitic milk). Single samples are appropriate for PCR in these cases.
- Milking herds PCR bulk milk + PCR colostrum and/or mastitic milk, could be combined with ELISA blood testing if desired (Note: At present, ELISA cannot be carried out by commercial laboratories). Intermittent shedding can require multiple samples collected over time to detect infected animals
- Dry stock Non-stressed dry stock may carry M. bovis without clinical disease for months. PCR testing of nasopharyngeal swabs or tonsillar crypts can be done, however, this is a low-yield test and not recommended unless there is concern for the origin of animals (in which case contact MPI to report this as a possible trace). Note that tonsillar samples are difficult to obtain in live animals, however, deep oro- or nasopharyngeal samples have been relatively sensitive in some cases.
- Bulls For small mobs/individual bulls, the provenance of the animals and disease status of farm of origin or last farms serviced is preferred over single-animal testing.
- Semen PCR is useful and is appropriate for single samples intermittent shedding should be taken into consideration however.
MPI Notification Process
A PCR test returning a positive or suspicious result is retested at the laboratory and a sample sent to MPI.
A herd reported to MPI will likely enter the M. bovis surveillance stream, which in the least serious case will result in resampling of animals (rebleeding, PCR) and in the most serious case could result in the property coming under movement control.
Reporting of negative results to submitters
Because M. bovis is evasive, and can shed intermittently or remain silent in carrier animals, negative results do not guarantee absence of infection but instead reflect Mycoplasma bovis not detected in this sample.
Mycoplasma bovis testing guidelines sample sizes and types
Suggested sample sizes for different mob sizes
|Mob size||Suggested sample size|