Routine serological control of breeder flocks is usually carried out by Rapid Plate Agglutination. This is a simple and cheap test, but has some limitations:
1. false negative reactions can occur; especially in young flocks, till 12 weeks of age, the test is not reliable.
2. false positive reactions can also occur, caused by:
- infections with other Mycoplasma’s (e.g. M.S.)
- frozen sera
- shortly after vaccination with oil-based vaccines
- bacterial contamination of sera.
False positive reactions can be differentiated from real positives by:
- Plate dilution/tube agglutination test: repeat the agglutination with diluted sera
- Haemagglutination Inhibition Test (H.I.). This test is more specific, but becomes later positive than the agglutination test.
We diagnose a flock as MG infected if 3% of the samples have agglutination titres >1:8 and/or an H.I. titre >1:16.
Another option for routine serology is ELISA. This test is far more expensive than the agglutination test and gives at least as many false reactions.
It is possible to confirm a serological diagnosis by isolation of the Mycoplasma. However, this is a rather complicated technique. A positive result gives a definite diagnosis, but negative results are not significant.
A more modern technique is using DNA probes for M.G. These are very specific, but also rather expensive. The test can be made more sensitive by using a Polymerase Chain Reaction (P.C.R.) prior to the probe, to multiply the DNA.
A further test kit can also differentiate vaccine strain (F strain) of M.G. from field strain.