After visiting and analyzing data from thousands of dairies in her career, Pamela Ruegg, DVM, MPVM, Dipl. ABVP (Dairy Practice), can attest that there is no better way to deal with mastitis than through accurate records of detection and treatment.

An associate professor and extension milk quality specialist at the University of Wisconsin – Madison, Ruegg says, “There is no way to understand the amount of mastitis that is being treated without recording information.”

There are two types of mastitis: subclinical and clinical.

Subclinical mastitis is milk that appears normal but has the presence of inflammation caused by a bacterial infection.

“SCC (somatic cell count) is an indirect indicator of bacterial infection and is a better indicator than the actual recovery of bacteria,” Ruegg says, adding, “Our methods are not great at finding bacteria. The cows’ methods are better than the lab tests.”


In clinical mastitis, there are visual abnormalities in the milk or udder.

Both types of mastitis can be caused by the same pathogens.

“Mastitis is caused by bacteria 99.9 percent of the time,” Ruegg says. “There is only one route of infection – the teat end.” Therefore, in order to reduce mastitis one must reduce exposure at the teat end.

Ruegg says she is a huge advocate of on-farm culture systems to determine the pathogens affecting the herd. However, these systems must be used with caution.

“Farmers don’t seem to understand they’re growing pathogens on those plates,” she says, noting that this practice typically occurs on the counter of the breakroom. “They’re usually growing bacteria in one hand and eating lunch in the other.”

Proper disposal is also important to avoid the pathogens from spreading further on the dairy. This means destroying the bacteria and not just placing the plate in the garbage.

SCC as an indicator
If a herd has a very high level of SCC (around 700,000), there are a lot of infected cows, Ruegg says. If the same herd claims there are no clinical cases it is a sign that detection is very poor.

“In the past 30 to 40 years as herd sizes have increased, we haven’t found a way to replace the expert person who knows and loves cows,” Ruegg says. “The expert system of a single herdsperson cannot be replaced.”

Today, the responsibility typically rests on many shoulders, some are better trained or more devoted to the cause than others. It can also be more difficult to spot the visual signs of clinical mastitis compared to flagging high SCC levels for subclinical mastitis on a milk quality report.

All cows have somatic cells present at low levels in a normal mammary gland. When infection occurs the number of somatic cells increases and the udder begins to fight the infection instead of producing milk. Less production and poorer quality milk occurs at 250,000 SCC.

The somatic cells cause injury to the secretory cells, reducing the synthesis of lactose, fat and protein. They also allow leakage of blood components into milk and result in a reduced shelf life of the final product.

Subclinical mastitis
Without any visual signs of subclinical mastitis, detection does require testing for SCC.

Almost any pathogen can cause mastitis. Subclinical mastitis often occurs because of host-adaption of the bacteria and most cases are from gram-positive bacteria.

Culture data is needed in order to put in place a treatment and prevention plan. Ruegg recommends taking a minimum of 25 quarter samples from the herd, preferably from high-SCC cows.

Based on her research, she also cautions that a single negative milk culture from a high-SCC cow is not sufficient evidence that the cow is not infected.

In a study across lactations, Ruegg found that older cows have more opportunities for exposure to pathogens, and any cow infected with mastitis once is at a higher risk for future infections.

As SCC grows in the herd, Ruegg discovered that the total number of cows culled per year increased, the rates of new infection were higher, the number of clinical cases grew and milk production declined. In her latest study, she saw a correlation with first-test milk yield and SCC during the first week postpartum.

Clinical mastitis
The prevalence of clinical mastitis is more problematic on larger dairies, she says, noting that without proper detection many of these cases go unnoticed.

In a 2007 study, Ruegg found six to eight cases occurring in every 100 cows per month. “That’s not a goal,” she says. “It’s just what we see out there.”

A goal for well-performing (and detecting) herds could be less than four cases treated per month per 100 cows.

It can be very difficult to know the actual clinical case rate, she says. Very few cases are recorded and, if so, the records are often difficult to summarize.

At the very least, producers should use severity scoring to track clinical mastitis cases. She uses the following method.

Score of 1: Abnormal milk only

Score of 2: Abnormal milk and a swollen quarter

Score of 3: Abnormal milk, swollen quarter and sick cow

“That data should be recorded for every case,” she recommends. Not only for herd health information, but also to identify if the dairy has a decent detection system in place.

She says that a score of 3 should occur in 5 to 20 percent of all cases recorded on the farm. If that percentage is higher, it is likely mild cases are not being detected.

Almost all mastitis is treated using intramammary tubes and the treatment may not always be recorded. Veterinarians have treatment records; however, they are usually presented with only the most severe cases.

The treatment of mastitis is the major reason for antibiotic use on dairies. According to Ruegg, 80 percent of all antibiotic usage on the farm is for mastitis.

Wisconsin farmers estimated that each case of clinical mastitis costs them $91 in medicine, vet and labor and milk discard. Sixty percent of that cost is in milk discard alone. Ruegg says the actual cost per case is probably double when culling is considered.

To further develop a good record- keeping system, treatment records should also be included, listing the cow and date, severity score, case number, the infected quarter, teat condition, gram-positive or -negative diagnosis, treatment and date of saleable milk.

Clinical mastitis can occur throughout a lactation. Ruegg’s thumb rule is 10 percent in the first week post-calving, 30 percent in the first month and 60 percent in the rest of the lactation.

As with subclinical mastitis, cows with a history of clinical mastitis in a previous lactation were four times more likely to have an infection in the first 120 days of the following lactation even if they received a treatment while dry.

Older cows are also less likely to respond to treatment.

Risk factors for mastitis to occur in older cows include a history of previous infections, greater milk yields and impaired teat end health.

“Age needs to be accounted for when assessing mastitis rates,” Ruegg says. Younger herds should have less mastitis and herds that cull based on recurrence cannot be compared to herds that do not.

Comparisons and assessments are not possible without accurate records of diagnosis and treatment. PD