By Robert Tremblay
Staphylococcus aureus (SA, for short) is a well-known cause of contagious mastitis in cows. SA is difficult to diagnose because milk samples from cows infected with SA are often negative when the milk is sent for culture. Not only is SA infection difficult to diagnose, it is difficult to treat too.
Staphylococci (staph for short), as a group of bacteria, have a large number of other species. When they are found in milk from a cow with mastitis, they are usually reported as coagulase-negative Staphylococcus (CNS). When a milk sample is reported as having CNS, it is difficult to know how important that is. Over the last few years mastitis researchers have been expanding our knowledge of CNS.
It is important to be able to accurately tell the different CNS bacteria apart. By knowing if there are specific CNS bacteria found in cows with mastitis, then we can begin to look for patterns of infection. It also helps in trying to find out where the bacteria come from. Knowing where they come from helps us figure out how cows get infected, useful when identifying how to prevent new infections. Being able to identify individual CNS bacteria could help us figure out the best ways to treat CNS infections too.
It turns out that there are about 20 to 40 different CNS bacteria that are found in cows with clinical mastitis but the majority of cases (about 80 %) are caused by only three bacteria. These are Staph chromogenes, Staph simulans and Staph xylosus. The animal health laboratory in Guelph will now report if they find either of the first two bacteria. They will also report if they find Staph epidermidis, a CNS bacterium that is often found on people. AHL decided to identify these specific CNS bacteria because there is a concern that these CNS could cause long-term mastitis infections with an increase in the somatic cell count (SCC) in the infected cows.
Staph chromogenes accounts for close to half of the CNS found in cows with clinical mastitis in Canada. Other research shows that it lives on the skin of cattle. It appears that it does not live in the udder itself or streak canal but can invade and grow in the udder when it gets a chance. In other words, it is an opportunist. We dont know if it takes advantage of specific circumstances to invade the udder so good general milking hygiene is likely the best prevention.
Staph simulans is a little different. Some studies show that it is an opportunist like Staph chromogenes. Other studies find that it acts as if it is contagious, meaning that it is being passed from an infected quarter to another cow or quarter. Even though another infected quarter is the source on farms where Staph simulans is spread contagiously, we dont really know where it comes from in herds where it acts as an opportunist. Because it can be contagious, it is valuable to know which cows are infected with Staph simulans.
Staph xylosus is the least understood of the major CNS found in cows with mastitis. It is commonly found in the dairy barns that suggests that it could very well be an environmental opportunist like the coliform bacteria. There are conflicting findings about whether it also lives on the cows skin. Some research found it in skin but other studies didnt detect it.
Now that we have a better ability to separate out the different CNS bacteria, it will help us better understand how they cause disease. The ultimate objective, of course, is to stop them from causing disease at all.
Dr. Robert Tremblay is a veterinarian for Boehringer-Ingelheim and lives near Guelph.