Ask the Vet: The Case for Monthly Bulk Tank Cultures

By: Dr. Elizabeth Martens, DVM, Valleywide Veterinary Services, Bridport, VT

The bulk tank milk culture is a powerful tool to make sure your herd doesn’t become infected with any of the really bad mastitis bugs and allows you to do something about it before it affects SCC and mastitis case numbers. The main things we look for on a bulk tank culture are: Mycoplasma, Prototheca and Staph aureus. These are the big 3 contagious, non-treatable bacteria that can become a big problem if ignored. They can pop up suddenly even in 100% closed herds. Prototheca and Mycoplasma are considered newly emerging issues on dairy farms in the Northeast, while Staph aureus is a relatively well known ongoing cause of contagious mastitis.

A bulk tank culture will also tell you about the prevalence of more common mastitis causing bacteria, like environmental staphs and streps and coliforms such as E. coli and Kelbsiella. It can help you narrow down potential sources for mastitis infection in your cows, though I won’t go into that in depth in this article.

Staph aureus forms tiny abscesses throughout the udder that cause ongoing inflammation and elevated SCC, but the actually bacteria is shed intermittently. This means that a negative culture doesn’t necessarily mean the cow or farm is free of the bacteria, but routine repeated negative culture can give you fairly high confidence of true negative status.

Prototheca mastitis in cattle is the only known example of an algae causing infection in a mammal. Pretty cool, right? Given the way it can slowly spread through a dairy herd and not get detected until a large portion of the cattle are infected, most farmers don’t think so. Algae normally live in dirty water – very frequently seen in lakes and ponds, and if you were to look under a microscope you would find it in dirty water puddles in many dairy barns as well. Once a cow on a farm is infected, she can pass the infection to other cows even without the ideal warm, wet conditions. Dairies in the southeastern US have been dealing with Prototheca for a while but in the past couple years we’ve started finding it in New York and Vermont.

Mycoplasma was first discovered on California dairy farms about 60 years ago and is now recognized worldwide as one of the most difficult bacteria to deal with on dairy farms. It causes chronic disease (pneumonia, ear infections, arthritis and mastitis) and also plays a role in calfhood pneumonia. Antibiotics are highly ineffective against this bacteria. If you are feeding non-pasteurized milk to calves, it’s extra important to make sure none of your cows are shedding mycoplasma in their milk.

The approach we take if a tank culture comes back positive for any of these is similar: don’t panic, do a repeat tank sample a month later, and identify a reasonable sized group of cows to sample that will give us the best chances of finding some of the culprit cows. Or in some states, such as NY, Quality Milk Production Service can do a whole herd survey and sample all cows during one milking.

We need to come up with a plan that meets the farm’s goals and will actually work for the long term. Some farms sample all fresh or mastitis cows for all bacteria or only looking for specific bacteria based on their bulk tank culture results. You can cut down cost if, for example, you are only interested in checking cows for staph aureus. Some farms sample only repeat mastitis cows or chronic high SCC. A cow with mastitis that clears up completely after treatment probably doesn’t have any of these 3 bacteria. Any new cows entering your herd should be cultured, and if you can ask the source farm for a bulk tank culture too, even better.

The key is to always keep up with the monthly bulk tank culture to keep tabs on the size of the problem (or lack thereof). It’s ok to tolerate a few cows carrying these bacteria as long as you know who they are and they aren’t spreading rapidly. If you wait until herd SCC is increasing due to a primarily contagious mastitis pathogen, 20% or more of your herd may already be infected. The cost of monthly bulk tank cultures is miniscule compared to the benefits of preventing contagious mastitis from affecting your bottom line.

Dr. Marten is a dairy veterinarian at Valleywide Veterinary Services in Bridport, VT. The practice runs a private milk quality laboratory with proficiency certification from Cornell’s Quality Milk Production Service. They culture thousands of milk samples every year and offer consulting on milk quality issues for cow and goat dairies. She can be reached at Valleywide Veterinary Services, 2562 Vermont, VT-22A, Bridport, VT 05734, (802) 758-6888

Posted: to Organic Production on Fri, May 8, 2026
Updated: Sat, May 9, 2026