In a recent retrospective study of the bulk tank milk samples cultured for mycoplasma from a central California creamery over the period of 1989 through 1995, the yearly percentage of mycoplasma-positive bulk tank samples for all species of mycoplasma ranged from 1.8% to 5.8%. When the non-pathogenic mycoplasma were removed, the yearly percentage mycoplasma positive bulk tank samples was between 1.2% and 3.1% for the study period. The bulk tank milk samples were cultured using direct plating at the creamery and were speciated at the Veterinary Medical Teaching and Research Center, Milk Quality Laboratory.
Considering only incident cases (the first cases or the next case when over a year had gone by since the first case for a given species), only one species of mycoplasma was isolated from 74% of the dairies having a positive case. Nearly 20% of the positive dairies had 2 species of mycoplasma isolated as incident cases. The extreme was a dairy which had 6 different mycoplasma species isolated and four dairies which had 5 different species found.
Mycoplasma bovis was the most common pathogenic species isolated and was the species most likely to be isolated multiple times on any one dairy. Incident cases of M. bovis accounted for 49% of the isolations. Other pathogenic mycoplasma isolated were M. bovigenitalium, M. alkalenscens, M. canadense and M. californicum (most - least isolations). Acholeplasma laidlawii, a non-pathogenic mycoplasma, was also found. It was particularly common in 1989 and 1995, however, it was usually found at less than 10 colonies per isolation (95%).
A seasonal trend in numbers of isolations was apparent with M. bovigenitalium, M. californicum and Acholeplasma laidlawii. They were seen more often from December through March of each year. No seasonal pattern was noted with the other pathogenic mycoplasma species.
Most of the isolates had fewer than 50 colonies isolated from each bulk tank milk sample. Only 24% would have been classified as 1+ or having greater than 101-200 colonies per sample. However, with M. bovis, over 30% of the samples had over 100 colonies isolated. Only 9.2% of the isolates had over 400 colonies per sample.
This study reconfirms the necessity for routine bulk tank milk sampling as monitoring method for mycoplasma in dairy herds. Routinely sampling the bulk tank milk for mycoplasma will provide a means for early detection of mycoplasma before it becomes a serious herd problem. Speciation or determination of the species of mycoplasma is important as all mycoplasma species are not thought to be equally pathogenic. Total herd cultures or extensive culling should not be attempted prior to speciation. Keep in mind that the use of enrichment media for bulk tank sampling may result in the detection of more non-pathogenic mycoplasma. Bulk tank testing, whether by the creamery or local veterinarian, for mycoplasma should be used along with culture of fresh cows, culture of clinical cases of mastitis, bulk tank somatic cell counts and culture of non-responsive mastitis cases.