This video series will guide you through the principles of on farm culturing and selective mastitis treatment. You will learn what you need to get started, how to collect sterile milk samples, culture bacteria and diagnose results. Throughout this series, too, we will discuss which mastitis cases are treatable with antibiotics (and which are not) and how to use antibiotics appropriately to limit residue in milk.
Episode #1: Treatment Decisions for Clinical Mastitis
Episode #3: Selecting Culture Media
Episode #4: Mastitis Severity Scoring
Episode #5: How to Collect an Aseptic Milk Sample
Episode #6: How to Set Up Culture Plates
Episode #7: How to Read Culture Plates
Episode #10: How to Identify Gram Negative Species
Episode #11: On Farm Culturing: What Can Go Wrong?
In spite of considerable improvements in milk quality, mastitis continues to be the most frequent and costly disease of dairy cows. Mastitis is caused by bacterial infection of the mammary gland and appropriate treatment and control programs are based on understanding the etiology of the infection. Historically, mastitis was primarily caused by Staphylococcus aureus and Streptococcus agalactiae, but in many regions, emphasis on improved milk quality has resulted in effective control of these pathogens. However, as dairy farms have increased in size and adopted intensive management strategies, an increasingly diverse group of pathogens have been associated with the occurrence of mastitis. Of 793 cases of clinical mastitis that were cultured from 50 Wisconsin dairy herds, the most common bacteriological diagnoses were no bacterial growth (25% of cases) and E coli (21% of cases); a further 11% of cases were caused by a variety of opportunistic organisms for which no approved antimicrobials can be expected to be effective (Oliveira et al., 2013). Regardless of etiology, almost all of the cases were treated symptomatically for 4-5 days using intramammary products (Oliveira and Ruegg, 2014). Many of the treatments were not appropriate but without diagnosis of etiology it is difficult to know when antimicrobial therapy is indicated. While clinical signs may be suggestive of some pathogens, detection of mastitis is based on observation of non-specific signs of inflammation and it is impossible to diagnose the etiology based on observation of the milk, gland or animal. Thus, almost all mastitis experts recommend the use of milk culturing to direct mastitis control programs.
Milk samples may be individually collected from affected quarters (quarter milk samples) or combined from all four glands into a single vial (composite milk samples). To maximize the possibility of recovering bacteria from the sample, milk should be collected after the teats have been prepared for milking (application of pre-milking disinfectant and drying) but before the units are attached. After collection, milk samples need to be cooled immediately and should be cultured on farm as soon as possible or submitted to the laboratory within 24 hours of collection. In general, growth plates that contains nutrients required for bacterial growth are inoculated, and the media is placed in an incubator that contains the appropriate atmosphere and temperature to encourage growth of the target organisms.
After 24 hours of incubation, culture plates are observed and the treatment protocol is specified based on the culture outcome. The techniques used in on farm culturing programs are simple but it is quite possible for mistakes to occur and veterinarians should be involved in reviewing plates and records of microbiological results. Remember, the most common mistake is related to failure to aseptically collect the milk sample.
The use of OFC to direct treatment of clinical mastitis gives farmers the opportunity to make better treatment decisions and reduce costs associated with milk discard and treatment of microbiologically negative cases.