Swine Health and Production

November and December, 1999

Response to Critical review of "Profiling Mycoplasma hyopneumoniae in farms using serology and a nested PCR technique.

There are three comments I would like to add to the critical review:

  1. Nested-PCR is a highly sensitive technique, and care must be taken to avoid false positive results due to contamination during sample processing and PCR preparation. The reviewer mentions Stark's et al. work, where the authors have detected Mycoplasma hyopneumoniae from air samples. In that trial, the authors obtained positive results not from swabs exposed to air, but from filters obtained from a vacuum pressure pump. Using a vacuum pump the authors were able to detect the microorganisms in rooms where the pigs coughed intensely. I would think that it is less likely to obtain a positive result by exposing a swab to the environment. However, it has to be tested.
  2. The reviewer mentions the lack of correlation between PCR and serology as a shortcoming. We did not expect to see such correlation, since both tests measure different things: PCR detects the antigen, and serology seroconversion. Although seroconversion is expected after antigen exposure, the timing is different. Seroconversion occurs after 4-8 weeks after exposure. We did observe seroconversion in all farms, and in all cases antigen detection preceeded seroconversion.
  3. I agree with the comment that the presence of the microorganism in nasal swabs and clinical pneumonia should be studied. It would also be interesting to determine for how long after infection the microorganims remains in the nasal cavity, and if nasal colonization is constant during that period of time.