Winning the Battle of Subclinical Mastitis Detection

Mastitis can occur in both clinical and subclinical forms. Clinical mastitis is readily detected by observable abnormalities in the milk (e.g., clots, discoloration) or changes in the udder (swelling, heat). Subclinical mastitis is more difficult to detect because overt clinical signs are absent, so indirect tests such as somatic cell count (SCC) or enzyme-based assays are required (A. Pakrashi, 2023).

When diagnosing subclinical mastitis, one must assess the cow's immune response through measures such as elevated SCC, which reflects leukocyte infiltration into the mammary gland in response to infection (A. Pakrashi, 2023). Subclinical mastitis often goes undetected and exerts the greatest economic impact through its long-term negative effects on milk yield (M. Shafeeq, 2021). The presence of cows with subclinical mastitis within a herd can raise the overall bulk tank SCC, which may reduce the price paid to the producer for milk. Failure to treat subclinical infections may allow chronic inflammation to establish, which is less likely to respond to antibiotic therapy (L. Martins, 2020).

Monitoring subclinical mastitis status should be done at the herd and individual cow levels. Reviewing the bulk tank SCC (monthly or more frequently) at the herd level is a first-line indicator of subclinical infection (G. E. Shook, 2017).

Milk from subclinically infected quarters typically appears normal even when millions of somatic cells are present. A quarter with an SCC exceeding 200,000 cells/mL is often considered evidence of subclinical mastitis (M. Shafeeq, 2021). Routine individual cow monitoring for subclinical infection is therefore critical. One method is to run monthly SCC testing through the Dairy Herd Improvement (DHIA) program. Another is cow-side SCC or inflammation testing (e.g., CMT, electronic devices). The long-standing California Mastitis Test (CMT) remains an acceptable first-line screening method. Several cow-side electronic devices and inline measurement systems (e.g., electrical conductivity) also provide useful detection options.

What tools do you have available on your farm to help detect subclinical mastitis?

The CMT is a valuable tool—inexpensive, easy to use, and effective as a screening test (Roberts, 2024). Early identification of new infections is crucial to maximizing cure rates and preventing them from becoming chronic. It is good practice to apply the CMT to fresh cows during the first few days after calving to catch subclinical cases early. Identifying mastitis cases early in lactation helps pinpoint where infections occur and supports the prevention of future cases.

As mentioned, individual cow SCC results from DHIA test-day data provide a powerful tool for monitoring mastitis status. The “hot sheet” reports typically include data such as individual cow SCC, stage of lactation, lactation number, milk yield, and each cow's contribution to the bulk tank. Cows with SCC above 200,000 cells/mL often warrant further evaluation (e.g., CMT of individual quarters). To fully benefit from the DHIA data, it is advisable to collect bacteriological culture samples from cows above the threshold (or at least from the top 10–20 cows in SCC ranking). Doing so offers information that can guide prevention, treatment, or culling decisions (G. E. Shook, 2017).

Producers must combine multiple tools and approaches to detect subclinical mastitis successfully. Without individual cow records or cow-side SCC testing, effectively identifying and managing subclinical infections becomes exceedingly difficult.

References

Pakrashi, C. R. (2023). Early detection of subclinical mastitis in lactating dairy cows using cow-level features. Journal of Dairy Science, 4978-4990.

E. Shook, R. B. (2017). Relationship between intramammary infection prevalence and somatic cell score in commercial dairy herds. Journal of Dairy Science, 9691-9701.

Martins, M. M. (2020). Chronic subclinical mastitis reduces milk and component yield at the cow level. Journal of Dairy Science, 298-305.

Shafeeq, A. M. (2021, December). Economic Impacts of Clinical and Subclinical Mastitis on Dairy Farms. Veterinary Science Research, 31-39.

Roberts, J. (2024). The California Mastitis Test: What is its value? Mastitis Matters, 29(5); 184-193