Subclinical mastitis is an inflammation of the mammary gland in dairy cows that presents without visible signs of illness. Unlike clinical mastitis, where noticeable symptoms like udder swelling or changes in milk are evident, subclinical cases remain hidden. This lack of overt signs makes it a widespread and economically impactful problem at the herd level, often going undetected without specific testing.
Identifying an Invisible Infection
Detecting subclinical mastitis relies on testing the milk rather than observing the cow for outward symptoms. The primary method is the Somatic Cell Count (SCC). Somatic cells are primarily white blood cells, and their presence indicates an immune response to an infection or inflammation within the mammary gland.
SCC can be measured in bulk tank milk, providing an overall indicator of udder health for the entire herd. Individual cow SCC measurements are also taken, often monthly, to pinpoint specific animals with elevated cell counts. A count above 200,000 cells/mL suggests the presence of subclinical mastitis in an individual cow.
Another practical, cow-side diagnostic tool is the California Mastitis Test (CMT). This test involves mixing milk from each quarter with a reagent in a paddle. The reagent reacts with the DNA in somatic cells, causing the mixture to thicken or form a gel. The degree of gel formation corresponds to the approximate SCC and the severity of inflammation.
Primary Causes and Risk Factors
Subclinical mastitis is primarily caused by bacterial pathogens, categorized by origin and transmission. Contagious pathogens, like Staphylococcus aureus and Streptococcus agalactiae, reside on udder skin and in infected glands. They spread during milking via contaminated equipment, hands, or washcloths, moving from an infected cow to a healthy one.
Environmental pathogens, including Escherichia coli and Klebsiella species, originate from the cow’s surroundings. These bacteria are found in bedding, soil, and manure. Cows become infected when teats contact a contaminated environment, especially between milkings.
Several risk factors increase a cow’s susceptibility to subclinical mastitis. Damaged teats are more vulnerable to bacterial invasion. Older cows have a higher likelihood of infection. The stage of lactation influences susceptibility, as cows are more prone to new infections during the early dry period and around calving. Poor milking machine function, such as vacuum fluctuations or improper pulsation, can damage teat tissue and compromise the udder’s natural defenses, increasing the risk of infection.
Economic and Production Consequences
The hidden nature of subclinical mastitis has a significant economic impact on dairy operations. The largest economic loss is reduced milk production, with infected cows experiencing a 10% to 30% decrease in yield. This reduction occurs because inflammation damages milk-producing tissues.
Subclinical mastitis also decreases milk quality. Elevated somatic cell counts can lead to financial penalties or loss of quality premiums from milk processors. Many processors have strict SCC limits, and exceeding these can result in reduced payment per hundredweight of milk. High SCC alters milk composition, affecting components like fat and protein, which impacts processing and product yield.
The disease incurs increased operational costs, including diagnostic testing (SCC, CMT) and treatments for cows transitioning to clinical mastitis. Labor costs also rise due to time spent managing affected animals.
Chronically infected cows that do not respond to treatment are often removed prematurely. This premature culling represents a substantial loss of a productive asset, as the investment in the animal is not fully recouped. Replacing these culled animals adds further economic burden.
Management and Prevention Strategies
Effective management and prevention of subclinical mastitis involve proactive measures. Maintaining strict milking hygiene is a primary element of prevention. A consistent milking routine includes forestripping to stimulate milk let-down and identify abnormal milk. Pre-dipping teats with disinfectant before milking reduces surface bacteria. Post-milking teat dipping with a germicidal solution seals the teat canal, preventing new infections.
Environmental cleanliness reduces exposure to environmental pathogens. Providing clean, dry bedding and well-ventilated housing minimizes manure and moisture accumulation, which are breeding grounds for bacteria. Regular cleaning of alleys and resting areas reduces the daily pathogen load.
Dry cow therapy is another strategy, treating all quarters at the end of lactation. This practice aims to clear existing subclinical infections and prevent new ones during the vulnerable dry period. Internal teat sealants are often used with antibiotics to provide a physical barrier against bacterial entry.
Regular testing and maintenance of milking equipment are also important. Milking machines should be tested periodically by a qualified technician to ensure proper vacuum levels, pulsation rates, and liner integrity. Malfunctioning equipment can cause teat damage, increasing susceptibility to infection. Maintaining equipment minimizes stress on teats and preserves the udder’s natural defenses.