"Bacteria must be kept away from the teat ends. Fundamentally, 99 percent of the time mastitis occurs when the ability of the teat end sphincter to control bacteria is exceeded. To control mastitis, the cause must be known," noted Pam Ruegg, UW-Madison, at the Wisconsin Dairy Field Representatives Conference. While we can detect mastitis, it is not at the moment of infection; we never know exactly when the infection occurred. We detect mastitis based on the results of the cow's immune response. Yet, when detection occurs, are bacteria still present in the udder? We often use antibiotics, but if bacteria aren't present, we don't need to treat that case of mastitis.
Most times, a cow's immune response is successful in curing the infection. Therefore, tests don't always detect bacterial infection. Sometimes we see the results of a bacterial response. Detection after the immune response is why samples are often culture negative.
Bacteria will infect different parts of the udder and spread among cows in varying ways. They also require different treatments and specific control plans.
Ruegg noted that there are three key reasons to culture:
- To identify the cause of mastitis,
- Surveillance
- Make a diagnosis to decide on treatment
This test is based on the concept that the nucleus of bacterial cells contains DNA with unique sequences of nucleotides or DNA building blocks. PCR will identify bacteria that have known sequences present in a master library.
DNA is extracted from the milk and strands of DNA are multiplied until they can be identified. This refers to the cycling threshold (CT) number found on PCR reports. The CT indicates how many copies had to be made before diagnosis. Lower numbers are better.
PCR tests detect both living and dead bacteria. You cannot differentiate between the two. "We don't know how long DNA will stay in the udder," Ruegg added. A traditional culture will only identify live bacteria.
As it is a new technology, decision-making based on PCR testing is unknown. You must always refer to the cow's history to guide your decisions.
Ruegg concluded that the use of molecular methods would be more common as it becomes cheaper. Molecular tests give us different information than we previously used. Like any other test, though, its value is based on the value of the intervention.
The author , Amanda Smith, was an associate editor and an animal science graduate of Cornell University. Smith covered feeding, milk quality and headed up the World Dairy Expo Supplement. She grew up on a Medina, N.Y., dairy, and interned at a 1,700-cow western New York dairy, a large New York calf and heifer farm, and studied in New Zealand for one semester.
