Mastitis

What is Mastitis?

Mastitis is an inflammation of the mammary gland (udder) caused by bacterial infection entering through the teat canal. It is the most prevalent and economically devastating disease in dairy cattle worldwide, accounting for 35–40% of all veterinary costs and an estimated $2 billion in annual losses in the US alone.

Mastitis is classified as clinical (visible signs: swollen udder, abnormal milk, reduced appetite) or subclinical (no visible signs but elevated somatic cell count and reduced milk production). Subclinical mastitis is far more common — for every clinical case, there are typically 3–4 subclinical cases silently reducing production.

The primary pathogens include contagious organisms (Staphylococcus aureus, Streptococcus agalactiae) that spread during milking, and environmental organisms (E. coli, Klebsiella, Streptococcus uberis) that inhabit dirty bedding and housing. Environmental mastitis has become the dominant form in well-managed herds.

Prevention focuses on: proper pre- and post-milking teat disinfection, milking infected cows last, maintaining clean bedding, dry cow therapy, functional milking equipment, and regular monitoring through milk recording and somatic cell count testing.

Clinical vs Subclinical Mastitis

Mastitis presents in two forms, and understanding the distinction is critical for management. Clinical mastitis is the visible form — the cow shows obvious signs: swollen, hot udder; abnormal milk (clots, watery, discolored); reduced appetite; and sometimes fever. Clinical cases are acute and require immediate treatment. The cost per clinical case ranges from $150–$300 when accounting for treatment (antibiotics, veterinary fees), milk withdrawal (cull milk for 5–7 days), lost production during the episode, and potential culling of chronically infected cows. Subclinical mastitis has no visible signs — the milk looks normal and the udder appears healthy — but somatic cell count (SCC) is elevated above 200,000 cells/mL, indicating immune response to infection. Subclinical mastitis is far more damaging economically because it affects 3–4 times more cows than clinical mastitis and goes undetected without SCC testing. The cost per subclinical case is $50–$100 in lost production (reduced milk yield, lower components). The ratio of clinical to subclinical cases in a typical herd is 1:3 to 1:4 — meaning for every 10 clinical cases, there are 30–40 subclinical cases silently reducing production. A herd with 200 cows and a bulk tank SCC of 300,000 likely has 40–60 cows with subclinical mastitis, losing $2,000–$6,000/month in production. Regular milk recording (monthly or more frequent) and individual cow SCC testing are essential for identifying subclinical cases. Cows with SCC consistently above 400,000 should be evaluated for treatment or culling.

Mastitis Pathogen Groups

Mastitis-causing bacteria fall into two main groups, and prevention strategies differ for each. Contagious pathogens — primarily Staphylococcus aureus and Streptococcus agalactiae — are spread from cow to cow during the milking process via contaminated milking equipment, liners, or milkers' hands. Staph aureus is particularly difficult to cure because it can form deep-seated infections in udder tissue, with cure rates of only 20–40% even with aggressive treatment. Strep agalactiae is more responsive to treatment (cure rates 80–90%) but spreads rapidly if not controlled. Prevention of contagious mastitis focuses on milking hygiene: proper pre-milking teat disinfection (30-second soak), post-milking teat dipping (full coverage), unit attachment within 60–90 seconds of stimulation, and milking infected cows last. Environmental pathogens — including E. coli, Klebsiella species, and Streptococcus uberis — are shed in manure and contaminate the cows' environment (bedding, flooring, water). E. coli mastitis can be severe and acute, sometimes causing endotoxemia and death. Klebsiella is increasingly common in sawdust and sand bedded barns, with cure rates of only 30–50%. Strep uberis is the most common environmental pathogen in grazing systems. Environmental mastitis prevention centers on clean, dry bedding (replace or add fresh bedding regularly), proper drainage in loafing areas, pre-milking teat cleaning (wash and dry, not just disinfect), and adequate stall comfort to minimize udder contact with contaminated surfaces. In herds where environmental mastitis dominates (common in well-managed herds with low contagious prevalence), bedding management is the single most impactful intervention.

The 5-Point Mastitis Prevention Plan

The 5-point plan is the industry-standard framework for mastitis prevention, developed from decades of research. Point 1: Pre-milking teat disinfection — clean and sanitize teats before every milking using an approved teat disinfectant (iodine, chlorine dioxide, or peracetic acid based). Soak teats for 30 seconds, then dry with single-use paper towels (never cloth towels, which harbor bacteria). This reduces environmental pathogen load on teats by 90–99%. Point 2: Post-milking teat disinfection — dip or spray all teats immediately after unit removal with an approved disinfectant. This is the single most effective mastitis prevention measure, reducing new intramammary infections by 50–60%. Ensure full teat coverage, including the teat end. Point 3: Milking infected cows last — identify cows with clinical mastitis or elevated SCC (>400,000) and milk them last in the milking order. Use separate cloth towels or, preferably, single-use towels for these cows. This prevents contagious pathogens from spreading to healthy cows. Point 4: Clean, dry bedding — maintain clean, dry stall surfaces. For sand bedding, add fresh sand regularly. For organic bedding (sawdust, shavings), replace or add fresh material at least weekly. Target stall surface moisture below 30%. Clean bedding reduces environmental mastitis incidence by 30–50%. Point 5: Milking equipment maintenance — inspect liners monthly for wear (replace when overextended), maintain vacuum levels within manufacturer specifications (typically 12.5–15.0 inches Hg), ensure pulsator ratios are correct (60:40 or 70:30), and check take-off sensors. Worn liners and malfunctioning equipment increase teat end impact and mastitis risk. Implementing all 5 points consistently reduces new mastitis infections by 60–80% and bulk tank SCC by 30–50% within 6–12 months.