Veterinary Extension, School of Veterinary Medicine, University of California Davis, Veterinary Medicine Teaching and Research Center, Tulare, CA 93274

Culturing milk samples for mastitis pathogens can provide a great deal of valuable information for a dairyman. The amount of information will depend on how many samples are taken, the distribution of the samples over the lactation, the type of milk that is sampled, and the laboratory methods that are used to test the samples. The long-term goal of a culture program should be to prevent new infections while monitoring the control program. The short-term goal may be to evaluate the current treatment plan or determine the cause of an outbreak. In any cases, consultation with the herd veterinarian will yield insight into the proper interpretation of the bacterial isolation results.

Number of samples taken — A single milk sample from an individual cow may provide significant information for that cow in question, however, multiple samples from many cows will provide much more information for mastitis prevention and control within the herd. During a period of increased clinical cases, the results from at least 10 or more samples should be considered. In all instances, the milk culture results should be permanently recorded as a basis for historical analysis at some later date. Usefulness of the culture data can be maximized when linked to the treatment protocol. This allows an evaluation of the various treatment antibiotics based on the bacteria that caused the infection. The key is to use milk culture results to do more than just decide what seems to be the appropriate antibiotic for mastitis treatment. Always keep in mind that the antibiotic sensitivity report gives only an indication of what antibiotic might cure the infection, however, many other cow factors determine if the antibiotic will really work in the cow to cure mastitis.

Distribution of samples over the lactation — When clinical cases of mastitis are cultured, this provides an opportunity to determine the times during lactation when most observable mastitis cases occur. This information permits a close inspection of the mastitis control measures during the periods of highest risk of new infections. For instance when clinical cases seem to be occurring early in lactation, particularly those bacteria considered to be environmental in source, the dry cow lot, transition pens and freshening pen sanitation should be checked out. The dry cow, antibiotic infusion techniques should also be re-evaluated especially when contagious pathogens predominate. When a wide distribution of cases happen over the entire lactation period, a review of the milking procedures might be indicated. The DHIA somatic cell count reports can also help to determine when subclinical cases of mastitis are occurring and will complement the use of culture results.

Source of milk samples — Several sources of milk samples should be considered to include cows with clinical cases of mastitis, fresh cows, new herd additions or any cows that have been off the dairy, cows with high somatic cell counts and string or bulk tank samples. A complete preventive and control program would utilize all of these samples. This would provide information about the cause of the new infections as well as chronic infections. Chronic infections can also be found through the DHIA somatic cell counts or review of written records for repeated treatments in the same cow. In addition, it will suggest when the infections are occurring and the outcome of treatments. Isolation of mastitis pathogens from the fresh cows indicates problems with dry cow antibiotic treatment or sanitation in the dry pens. String or bulk tank cultures provide an early warning system for the invasion of Strep ag, Staph aureus and mycoplasma into the herd. Finding a single colony of any of these highly contagious pathogens in the bulk tank milk may be an early warning that a dangerous pathogen has arrived in your dairy herd. A good biosecurity measure would be to culture all new additions to the herd whether home-raised or purchased before they are mixed with the general milking herd.

Laboratory methods — It is possible to culture for specific organisms or to do a general test. To gain the most information, the laboratory should be looking for Strep ag, Staph aureus, environmental streps such as Strep uberis and dysgalactiae, other staph than Staph aureus, E coli and other coliforms and mycoplasma. The mycoplasma isolates should be identified to species such as M. bovis, M. californicum and others. This is important, as all mycoplasmas isolated from milk are not thought to cause mastitis. Laboratories familiar with cow milk culturing procedures such as those suggested by the National Mastitis Council should also be on the lookout for Serratia, Prototheca, Pseudomonas and yeasts (Candida or Nocardia) that occasionally cause mastitis in cows. When ever possible, use a laboratory that accepts veterinary samples rather than a human hospital laboratory. Procedures for human pathogens often differ from those for cows and may complicate the interpretation of the milk sample results. Consult with your veterinarian to find out about the laboratory where your samples are being tested.

Remember that milk samples that cannot be taken to the mastitis laboratory within 12-24 hours should be frozen for later delivery to the mastitis laboratory. All samples should be refrigerated or kept on ice until delivered to the laboratory or frozen. Routinely identify the date, cow, and quarter on each sample and in the written treatment record.

The more milk samples that are cultured, the more information that will be available to prevent and control mastitis within the herd. Culture often. Ask your veterinarian to help you setup a mastitis culture program as well as a written mastitis treatment plan. Mastitis does not have to be a problem within your herd.