The case definition on a dairy is a way to define what, exactly, constitutes an intervention point for cow care employees. Monitoring of mastitis rates and events is common on dairies given the prevalence of the disease, and having a plan for developing case definitions and protocols is critical. Greg Goodell, DVM, The Dairy Authority, LLC, Greeley, Colo., says mastitis is a good, albeit complicated, example of a disease that should be monitored.
“The veterinarian must combine the health of the cow, ability of the farm personnel to identify disease and the most prevalent presentation of the disease with goals of the dairy,” Goodell says. As far as identifying disease, establishing case definitions is important. “The case definition on a dairy is a fancy way of defining to the farm personnel what, exactly, constitutes an intervention point for them.”
Some dairies define mastitis as any visible clots, flakes or otherwise abnormal milk (true medical case definition) where other dairies define a case of mastitis as a cow who presents to the milker during the milking process a second shift or a second day with signs of clots or flakes.
“At first glance these sort of definitions may raise the eyebrows of some until it is realized that a barn of astute milkers in a herd where the most common mastitis cases presented are mild coliform cases and can quickly fill a hospital pen with cows who were seen with a single flake in the parlor but spend two days in the hospital pen with no clinical signs and a negative culture report.”
Greg Goodell, DVM. Goodell says an easier case definition would be retained placenta. “True medical case definition is described as any placenta retained after 12 hours. This isn’t practical on most dairies so we define the case definition as any cow with placenta still present 24 hours after calving. In this instance the case definition is simply based on time.”
If the case definition of mastitis is a laterally recumbent cow in the milking pen then mastitis treatments in this herd will be much different from a herd where the case definition is a single flake in a cow’s udder at milking time. “These are two extremes that wouldn’t typically be used, but it makes the point,” Goodell explains.
The case definition is different enough from dairy-to-dairy that Goodell often finds it unreliable to compare disease rates between dairies without knowing case definitions. “Some of that issue is iatrogenic due to how cases are recorded. If a cow arrives with a case of mastitis, first line therapy fails and progresses to the second line therapy it is still only one mastitis event. If the cow is recorded with a second mastitis event it would be interpreted that she is a chronic mastitis cow whereas more correctly the treatment we prescribed did not work. She may indeed turn into a chronic mastitis cow, however we need to evaluate therapies in these cases to see if what we are using works best for the disease at hand.” Also, he adds, if dairies record the second treatment event as a second case of mastitis, it can falsely inflate the mastitis rate on the dairy.