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We appreciate Dr. Wiwanitkit’s interest in our manuscript regarding Mycobacterium avium subsp. paratuberculosis (MAP) in the environment of dairy herds (1). We agree that the effect of seasonal variations, or fluctuations in temperature and humidity on the shedding and survival of MAP in the environment needs further elucidation. Assessing seasonal differences in the MAP environmental contamination on these farms was not one of our objectives in the current study. In this study we collected environmental samples every 6 months which resulted in farms being consistently sampled during the same 2 seasons each year. Similarly, all adult cows in the herds were tested only once annually, generally during the same season each year, to determine the within herd Johne’s disease prevalence, precluding any inferences regarding MAP shedding and season. We can report that environmental samples were collected in each of the 4 seasons of the year, although samples were not necessarily collected all 4 seasons on each individual herd. MAP-positive cultures were identified during each season, with no season having an apparent higher or lower proportion of culture positive samples. In the one study we found that specifically looked at seasonal climatic conditions on the diagnosis of MAP in dairy cattle, there was no significant seasonal effect on individual cows testing positive for MAP on serum ELISA or fecal culture (2).
We also caution that care be taken when interpreting literature regarding shedding and contamination of MAP. Individual test results, and interpretation of those results, will vary greatly depending on type of sample (individual vs. pooled; feces vs. milk vs. serum vs. tissue), sample handling and preparation, and the particular test used. In this study we were investigating MAP contamination in the environmental environment of dairy farms. This contamination was primarily fecal in origin and was detected using either solid culture on Herrold’s egg yolk medium or an automated liquid culture system (TREK ESP II). The 2 studies cited by Dr. Wiwanitkit involved detecting MAP shed in milk, either in pooled waste milk samples (3) or individual (goat) milk samples (4). In these two studies alone there were 3 different culture systems and 2 different PCR tests used, each with a different threshold for detecting the bacterium. Moreover, studies comparing the results of different diagnostic tests for MAP have found very little agreement between concurrent milk and serum ELISA’s (5,6) and milk and fecal cultures and direct or nested PCR (7). MAP is an extremely challenging pathogen to research. Until some standard is set for diagnosing MAP, it is important to understand the limitations of each study before making broad comparisons.