Overview
Bovine mastitis is broadly classified into clinical mastitis (visible changes in milk, udder, or cow — clots, swelling, fever) and subclinical mastitis (no visible signs, but elevated somatic cell count). The primary cause is bacterial infection of the udder. Contagious mastitis pathogens (Staphylococcus aureus, Streptococcus agalactiae, Streptococcus dysgalactiae, Corynebacterium bovis) spread cow-to-cow during milking via contaminated milking equipment and hands. Environmental mastitis pathogens (E. coli, Streptococcus uberis, Klebsiella, Enterococcus) are acquired from the environment between milkings — associated with poor bedding, muddy conditions, and inadequate pre-milking teat preparation. The California Mastitis Test (CMT) is the standard field-screening tool for subclinical mastitis, detecting somatic cells in milk as a gel reaction. Bulk tank somatic cell count (BTSCC) monitoring at the herd level is a key indicator of mastitis burden.
Common Clinical Signs
Diagnostic Approach
| Diagnostic Test | Expected Findings & Interpretation |
|---|---|
| California Mastitis Test (CMT) | Gel formation correlates with somatic cell count (SCC). Scoring: N (negative), T (trace), 1 (mild reaction), 2 (moderate), 3 (severe). CMT ≥1 indicates elevated SCC and likely subclinical mastitis. Practical herd-screening tool for all 4 quarters. Results should guide culture sampling. |
| Individual Quarter Milk Somatic Cell Count | SCC >200,000 cells/mL in an individual quarter = mastitis. Normal lactating cow: <100,000/mL per quarter. SCC of >1,000,000 suggests active infection. Individual SCC is more useful for management than bulk SCC for identifying the affected quarter. |
| Bulk Tank Somatic Cell Count (BTSCC) | Indicator of herd-level mastitis burden. <200,000 cells/mL = excellent. 200,000-400,000 = acceptable but room for improvement. >400,000 cells/mL = significant herd mastitis problem requiring investigation. BTSCC is a management tool, not a diagnostic tool. |
| Milk Bacterial Culture | Identify causative organism before selecting treatment. Swab of aseptically collected milk from the affected quarter. 3-5 mL milk on blood agar, incubated 24-48h. Results guide antibiotic selection. Essential for chronic mastitis or treatment failures. |
| Antimicrobial Susceptibility Testing (AST) | Performed on culture isolates when initial treatment fails. Selects appropriate antibiotic for persistent infections — especially important for Staph. aureus and chronic environmental mastitis cases. Follow culture + AST before second-line treatment decisions. |
| CBC / Blood Chemistry | For systemic mastitis (toxic mastitis): neutrophilia with left shift, hyperfibrinogenemia, hemoconcentration. Elevated acute phase proteins (haptoglobin) in severe cases. |
Major Mastitis Pathogens & Implications
| Pathogen | Type | Clinical Notes | Treatment Prognosis |
|---|---|---|---|
| Staphylococcus aureus | Contagious | Chronic, subclinical, fibrosis of quarter. Most important contagious pathogen. Difficult to cure. Spreads during milking. Requires whole-herd approach. | Poor cure rate (~30%). Often cull the cow. |
| Streptococcus agalactiae | Contagious | Highly contagious, causes clinical mastitis. Responds well to antibiotics. Eradication possible with aggressive protocol. | Good cure rate with penicillin/ampicillin. |
| Escherichia coli | Environmental | Peracute toxic mastitis. Systemic endotoxemia. High fever, shock, rapid death if untreated. | Variable — depends on speed of treatment. NSAID + fluids critical. |
| Streptococcus uberis | Environmental | Most common environmental pathogen in many regions. Clinical or subclinical. Persists in bedding, manure. | Moderate cure rate (~60-70%). |
| Klebsiella spp. | Environmental | Severe, often peracute. Environmental source: soiled bedding, wet hutches. High CFR. | Guarded. Environmental control is key. |
| Streptococcus dysgalactiae | Contagious/Environmental | Intermediate — causes clinical mastitis, moderate severity. Spreads cow-to-cow but also environmental reservoir. | Good cure rate with antibiotics. |
Differential Diagnoses
- Traumatic mastitis — Injury from hardware, kicks, or blunt force. Typically one-quarter, no infectious agent. History of injury, sudden onset. Milk may appear normal or slightly altered. Local inflammation but no fever unless secondary infection develops.
- Udder edema (physiologic or pathologic) — Non-inflammatory swelling of udder in periparturient cows. Bilateral, symmetrical. No heat, pain, or milk changes. Normal milk. Management: reduced sodium intake pre-calving, diuretics in severe cases.
- Contagious ecthyma (ORF) — Viral papular disease of teat skin causing scabs and ulcers. Secondary bacterial infection of teat or udder possible. Zoonotic. Characteristic proliferative lesions on teats.
Control Program Essentials
The gold standard mastitis control program is the Five-Point Plan (National Mastitis Council): (1) Treat all clinical cases promptly with appropriate antibiotics + NSAIDs + frequent milking. (2) Dry off all cows using dry cow therapy (intramammary antibiotic + internal teat sealant) to cure existing infections and prevent new infections during the dry period. (3) Maintain milking hygiene — pre-milking teat disinfection (post-dip with iodine or chlorhexidine), wear gloves, milking equipment maintenance. (4) Cull chronically infected cows — Staph. aureus carriers with recurrent clinical mastitis or persistently high SCC. (5) Monitor BTSCC monthly and CMT individual quarters quarterly to track progress.