Subclinical Mastitis: The Stealthy Intruder

Due to its presentation of symptoms, most producers are familiar with clinical mastitis and its occurrence on their farms. By definition, cows with clinical mastitis display outward symptoms of the infection: red, warm, swollen quarter(s); flakes or clots in milk; sudden decrease in production and feed intake coupled with other behavioral changes. When detected, these cows are managed according to the farm’s protocol. Clinical mastitis infections directly affect profitability through treatment costs and lost production.

However, on many farms, clinical mastitis is just the tip of the iceberg. Subclinical mastitis can be up to 20 times more prevalent than clinical mastitis and presents a unique challenge for farms because the milk, the udder, and the cow all appear normal, despite an infection. Thus, visual, day-to-day observation is insufficient for detection of a subclinical mastitis infection.

The economic impact of subclinical mastitis is difficult to quantify because most of the effects are indirect and are not always be apparent. As such, subclinical mastitis can have a substantial impact on profitability because it 1) often goes undetected for an extended period of time; 2) reduces the productive potential of a cow by causing subtle decreases in milk yield during and after an infection; 3) can increase overall bulk tank SCC if enough cows are affected; and 4) is often caused by contagious pathogens which can spread easily from cow to cow.

Subclinical mastitis can be caused by environmental pathogens (coliforms, non-agalactiae Streptococcus, and coagulase-negative Staphylococcus) or contagious pathogens (Streptococcus agalactiae, Staphylococcus aureus, or Mycoplasma species). Subclinical infections caused by environmental pathogens are generally short-lived and may cure spontaneously. In contrast, contagious pathogens often induce long-term infections that do not typically cure spontaneously. Performing a milk culture can help identify the pathogen causing the infection which provides information to help direct treatment and management changes that may be needed to address the problem.

Prolonged, elevated bulk tank SCC can be indicative of subclinical infections at the herd-level, but this information does little to help solve problems at the cow-level. Because there are no outward symptoms of subclinical mastitis, the most reliable, consistent way to monitor its prevalence is to examine individual cow SCC on a routine basis. Oftentimes this information is obtained through monthly DHIA testing. These tests can help producers identify high-SCC cows who should be evaluated more closely for additional indicators of infection. Once suspect cows are identified, a California mastitis test (CMT) can help identify the problem quarter(s) and a milk sample should be collected and cultured for pathogen identification. Individual SCC data can also be used to identify whether subclinical mastitis is an issue at the farm-level. If the SCC of more than 15% of cows is above 200,000 cells/mL on any given test day, it could indicate that subclinical mastitis is an issue for the farm.

It is important to recognize that the SCC and CMT are just tools to help identify cows and quarters that may be infected with a pathogen. Cows should not be treated based on this information alone and should only be treated if indicated by results from a milk culture test. A milk culture test can be performed by your veterinarian or on-farm using a commercially available milk culturing kit. Producers should contact their veterinarian to discuss treatment strategies for various pathogens.

There are several approaches that can be taken to help reduce the prevalence of subclinical mastitis in the herd. An effective control strategy will utilize multiple aspects of a good mastitis management program.

1. Identify infected cows. This is best accomplished through routine monitoring of individual SCC. Identification of infected cows is also important for managing the spread of contagious pathogens. Analysis of SCC over time of individual cows can also indicate if treatments are effective and if there is a specific time (season or stage of lactation) when SCC increases.
    
2. Identify the mastitis pathogen. Pathogen identification can direct treatment and prevention strategies. Depending on the pathogen, certain treatments may or may not be effective in clearing the infection and sometimes culling is warranted to irradiate certain pathogens, as is often the case for Mycoplasma infections.
    
3. Practice good milking hygiene. Cleanliness at milking time is always of critical importance in the context of mastitis prevention, but it is especially important when dealing with contagious pathogens. To mitigate risk for cross-contamination, milkers should always wear gloves when milking and gloves should be changed after working with an infected cow. The one-towel-per-cow approach for cleaning/drying teats and adequate cleaning of the milking unit between cows also can help prevent cross-contamination.
    
4. Milk sick cows last. Contagious pathogens are often the culprits behind subclinical mastitis infections. Milking cows with an identified or suspected subclinical infection last in the milking order can help prevent spread to healthy herd mates.

Subclinical mastitis is a costly disease that affects dairy herds of all sizes and types. However, routine monitoring of individual-cow SCC, performance of targeted milk culture tests, and utilization of good milking procedures can help producers manage and minimize its effects in the herd.