Bovine mastitis is the most economically important disease in dairy cattle worldwide. It accounts for more antibiotic use in dairy herds than any other condition and drives significant culling decisions. On the NAVLE, mastitis questions test your ability to distinguish contagious from environmental pathogens, interpret CMT and SCC results, and make clinically sound treatment and control recommendations.
Classification: Contagious vs Environmental
Contagious Pathogens
Spread cow-to-cow, primarily during milking
- Staphylococcus aureus — most common; chronic subclinical
- Streptococcus agalactiae — obligate udder pathogen; eradicable
- Mycoplasma bovis — no treatment; cull
Control: milking hygiene, teat dipping, dry cow therapy
Environmental Pathogens
Acquired from bedding, soil, feces between milkings
- Streptococcus uberis — most common environmental strep
- Escherichia coli — endotoxin; peracute toxemia
- Klebsiella — worse prognosis than E. coli
- Trueperella pyogenes — summer mastitis; fly vector
Control: bedding management, pre-milking disinfection
Key Pathogens in Detail
Staphylococcus aureus
The most common contagious mastitis pathogen. S. aureus forms biofilms within mammary tissue that physically protect bacteria from antibiotics and phagocytic cells. This is why chronic S. aureus infections have a cure rate of only <30% after repeated treatments. The organism is coagulase-positive and produces β-toxins that cause tissue necrosis.
Clinical forms range from subclinical (elevated SCC, CMT positive, no visible changes) to gangrenous mastitis (wet gangrene, cold black quarter—rare but fatal without aggressive intervention).
Streptococcus agalactiae
S. agalactiae is the ONLY bovine mastitis pathogen that is an obligate udder inhabitant—it cannot survive for meaningful periods outside mammary tissue. This has two critical implications: (1) it can be eradicated from a herd with aggressive treatment; (2) its presence always indicates cow-to-cow transmission during milking.
CAMP test positive. Highly sensitive to penicillin G (near 100% cure rate when treated appropriately). Eradication program: culture all cows → treat lactating positives with penicillin → blanket dry cow therapy at dry-off → strict post-milking teat dipping → retest to confirm clearance.
Mycoplasma bovis
No effective treatment exists for Mycoplasma mastitis. It causes severe agalactia (complete milk loss), is extremely contagious at milking, and can cause concurrent respiratory disease and arthritis in the herd. Affected cows should be culled and any new animals should be screened before introduction. Mycoplasma requires special culture media (PPLO broth/agar) and slow growth (7–14 days)—it will NOT grow on standard blood agar.
Escherichia coli (Coliform Mastitis)
Classic presentation: periparturient dairy cow (within 2 weeks of freshening), peracute onset, severe toxemia. The quarter produces watery, serous or hemorrhagic secretions rather than pus. The cow is febrile (41–42°C), depressed, anorectic, and may be recumbent. Endotoxin (LPS) drives the systemic inflammatory response.
Treatment: IV fluid resuscitation (isotonic fluids, often with calcium supplementation—periparturient cows are commonly hypocalcemic concurrently), flunixin meglumine 2.2 mg/kg IV (reduces endotoxin-mediated inflammation), frequent milking of the affected quarter, oxytocin to facilitate milk letdown. Systemic antibiotics: controversial—E. coli is eliminated by the immune system in most cases; antibiotics are indicated when bacteremia is suspected.
Diagnosis
California Mastitis Test (CMT)
The CMT uses a detergent (sodium lauryl sulfate) that causes DNA from somatic cells to form a gel precipitate. It is a cow-side screening test, not a pathogen identification tool.
Somatic Cell Count (SCC)
SCC is the gold standard for monitoring udder health at the herd level.
- Individual cow threshold: >200,000 cells/mL = likely subclinical mastitis
- Bulk tank SCC: <200,000 = excellent control; 200,000–400,000 = moderate; >400,000 = poor control
- US regulatory limit for Grade A milk: <750,000 cells/mL
- EU regulatory limit: <400,000 cells/mL
Culture and Identification
Aseptic milk sampling (clean teat end, discard foremilk) is required for accurate culture. Key identifiers:
- S. aureus: golden-yellow colonies on blood agar, β-hemolysis, coagulase-positive
- S. agalactiae: CAMP test positive, esculin-negative (distinguishes from S. uberis which is esculin-positive)
- E. coli: pink colonies on MacConkey agar, Gram-negative rods, lactose-fermenter
- Mycoplasma bovis: requires PPLO broth; slow growth 7–14 days; “fried egg” colonies
Treatment Principles
Intramammary (IMM) Antibiotics
IMM therapy delivers antibiotic directly into the gland. Milk and meat withholding times are mandatory—violations are federal offenses. Key lactating cow products: pirlimycin, ceftiofur, amoxicillin. Label compliance is not optional.
Dry Cow Therapy (DCT)
Administering IMM antibiotics at dry-off serves two purposes: (1) cure existing subclinical infections, (2) prevent new infections during the dry period when the teat canal is vulnerable. Traditional blanket DCT (all cows treated) is being replaced by selective dry cow therapy (SDCT) under antimicrobial stewardship guidelines—only culture-positive or high-SCC cows are treated with antibiotics; all cows receive a teat sealant.
The Five-Point Mastitis Control Plan
The classic herd-level mastitis control framework, proven effective since the 1960s:
- Teat disinfection post-milking (post-dip with effective germicide — iodine or chlorhexidine)
- Dry cow therapy at dry-off for all quarters (or selective DCT)
- Early detection and treatment of clinical cases
- Culling of chronic and incurable cows (especially chronic S. aureus)
- Milking machine maintenance (vacuum levels, pulsation rates, liner condition)