Microbiology Exam 3 Review - Mastitis

Microbiology Exam 3 Review Lecture 14: Mastitis

Definition of Mastitis

  • Mastitis refers to the inflammation of one or more quarters of the mammary gland.

Causes

  • Bacterial Infections (70-80%)

  • Yeast/Mold Infections (2-4%)

  • Unknown Causes (20-30%)
      - Including trauma, weather, etc.

Significance

  • Prevalence: All dairy herds in the U.S. contain subclinical mastitis (inapparent)

  • Clinical Disease: Seen in 10-12% of herds

  • Economic Impact:
      - Decreased milk production
      - Costs associated with discarding milk
      - Expenses related to animal replacement
      - Veterinary fees

Health Concerns

  • Animal Health:
      - Loss of functional quarters or milk production, potentially leading to death

  • Human Health:
      - Poor quality milk
      - Potential antibiotic residue in milk
      - Possible transmission of zoonoses

Anatomy of the Bovine Mammary Gland

  • The bovine mammary gland consists of four quarters that have separate ductal systems.
      - Importance: Drugs and infectious agents do not spread between these quarters.

Mammary Gland Defense Mechanisms
Physical Barriers

  • Teat Sphincter: Closes the teat canal to prevent bacterial entry.

  • Keratin Plug:
      - Obstructs and prevents bacterial ascension.
      - Contains antimicrobial agents, such as long-chain fatty acids (LCFA).

  • Keratinized Squamous Epithelium: Prevents bacterial adherence.

  • Flushing of Milk: Removes bacteria and reduces the risk of bacteria ascending the ductal system.

Immune Response

  • Macrophages: Less than 100,000 cells/mL in healthy mammary glands.

  • Neutrophils: Increase to >5 x 10^6 cells/mL in mastitis cases; about 90% of these are neutrophils.

  • Humoral Response: Involves antibodies that neutralize bacteria and toxins, promoting opsonization.

  • Cell-Mediated Response: Involves T-cells enhancing the immune response and killing infected cells.

  • Soluble Mediators:
      - Lactoferrin: Binds iron, preventing its use by bacteria.
      - Lysozyme: Cleaves the peptidoglycan layer in Gram-positive bacterial cell walls.

Risk Factors for Mastitis
Host Factors

  • Older cows (more than 4 lactations).

  • Periparturient cows.

  • Teat lesions that harbor bacteria or cause inadequate milking.

  • Sphincter dynamics where it requires about 2 hours to return to its contracted position post-milking.

  • Low heritability with increased milk production correlating to increased risk.

Environmental Factors

  • Poor milking techniques or hygiene.

  • Malfunctioning milking machines.

  • Contaminated environments such as housing and pasture.

  • Coliform Mastitis: More frequent in housed cows.

  • Negative energy balance in high-yield cows, especially early postpartum.

Microbial Factors

  • Type of infectious agent involved.

  • Virulence factors of the pathogens (contagious vs environmental).

Encounter: Host Source

  • Contagious Pathogens:
      - Obligate parasites requiring a host for replication, such as S. aureus and S. agalactiae.
      - Common source: infected mammary gland from carrier animals during milking.
      - Rarely causes symptoms in infected animals, leading to frequent clinical episodes.

Specific Pathogens

  • Staphylococcus aureus:
      - Gram-positive (G+) cocci in clusters.
      - Facultative anaerobe.
      - Catalase positive, coagulase positive.
      - Produces hemolysins leading to tissue damage and necrosis.
      - Synthesis of a superantigen triggering systemic shock.

  • Streptococcus agalactiae:
      - G+ cocci in chains.
      - Produces B-hemolysins; catalase negative.

- CAMP positive (if pathogenic Strep)

  • Mycoplasma bovis:
      - No cell wall, pleomorphic shape, and facultative anaerobe.
      - Identified using PCR for speciation.

Environment Source of Infections

  • Main Opportunistic Pathogens: E. coli, Klebsiella, Streptococcus.

  • Commonly sourced from bedding or pasture exposure occurring between milking.

  • Most frequent cause of clinical mastitis (80-90% infections).

  • E. coli characteristics:
      - Gram-negative (G-) rod.
      - Does not adhere to epithelium; it proliferates in ducts leading to endotoxin release, causing systemic signs.

Key Virulence Factors

  • LPS (lipopolysaccharide): Causes endotoxemia during infections.

Entry of Pathogens

  • Most common mode of entry is the breakdown of the teat sphincter barrier during milking.

  • The keratin plug flushes out, the canal distends, and the sphincter remains open for approximately 2 hours, allowing bacteria to ascend via the canal.

Multiplication and Spread of Mastitis Bacteria

  • If not eliminated by host defenses, pathogens will multiply in the gland.

Virulence Factors Involved

  • Adhesins: Allow pathogens to attach to epithelial cells.

  • Capsules: Prevent phagocytosis by immune cells.

  • Exotoxins and Endotoxins: Contribute to virulence (particularly in G- bacteria).

Host Immune Response Induction

  • Bacteria stimulate the release of chemoattractants and cytokines, such as:
      - TNF-α
      - IL-1
      - IL-8
      - Eicosanides
      - Reactive oxygen radicals (O2 radicals)

  • Polymorphonuclear neutrophils (PMNs) infiltrate the infection site and engulf bacteria, releasing enzymes that may damage host epithelial cells and reduce milk production.

Damage from Infection

  • No Residual Damage: Complete recovery is possible, returning the mammary gland to function.

  • Persistent Infection: May cause damage to mammary gland alveoli, leading to swelling and possible leakage of extracellular components into milk.

  • Chronic Disease: Ongoing infection leads to continuous damage, potential abscessation, gangrenous reactions, atrophy, and fibrosis of mammary tissue.

Diagnosis of Mastitis
Clinical Signs Indicative of Mastitis

  • Clinical Mastitis: Characterized by inflammation and infection in the udder leading to:
      - Red, swollen, tender, and hard udder
      - Abnormal milk (e.g., flecks, clots, discoloration)
      - Goal to maintain <2% clinical mastitis per month in lactating cows.

  • Subclinical Mastitis: No visible udder signs; requires specific tests for detection.

Direct Diagnostic Tests

  • Direct Microscopy / Gram Stain:
      - Milk sample needs to be centrifuged and stained. Effective for diagnosing mainly G+ infections.

  • Bacteriologic Culture:
      - Highly effective for clinical and subclinical mastitis, dependent on the level of shedding.
      - At least 100 CFU/mL required for detection from the culture.
      - Minimize false negatives by repeated sampling over multiple days.

Results Interpretation of Diagnostic Testing

  • Typical Shedding Pattern: High during acute infection, then intermittent submissions.

  • Subclinical/Chronic Manifestations: Often exhibit peaks followed by cyclic shedding within the detection threshold, possibly resulting in false negatives.

Indirect Diagnostic Tests

  • Somatic Cell Count (SCC): Assess bulk milk; acceptable standard is <200,000 mL.

  • California Mastitis Test (CMT): Provides an indication of leukocyte levels.

  • Electrical Conductivity of Milk: Increases Na and Cl ion concentration during mastitis.

Treatment of Mastitis
Intra-Mammary Therapy

  • Intravenous infusion of antibiotics (abx) into the udder, particularly for:
      - Strep: Penicillin shows a 90-100% cure rate.
      - Staph: Penicillin or cephalosporin shows a 50% cure rate.
      - Mycoplasma: Limited options due to resistance to antibiotics.

Systemic Therapy

  • Involves IV or IM antibiotics, essential if systemic signs of infection are present (e.g., caused by exotoxins of Staph or endotoxins from E. coli).

Supportive Therapy

  • Removal of toxins through frequent milking, coupled with treatment for dehydration and swelling relief.

Dry Cow Therapy

  • Effective for treating contagious mastitis.

  • Involves administering a larger dose of antibiotics prior to dry-off, which reduces infections present at the time and decreases future infection rates.

Prevention of Mastitis
Strategies for Contagious Infections

  • Uphold milking parlor hygiene to prevent transmission.

  • Maintain udder hygiene:
      - Pre- and post-milking teat disinfection is critical for reducing mastitis risk.

  • Identify and segregate chronic or subclinical cases:
      - Conduct routine testing for SCC, CMT, and microbiology.

  • Implement quarantine protocols for new animals and ensure adequate nutrition.

  • Use vaccines against specific pathogens (e.g., Staph aureus and Mycoplasma).

Strategies for Environmental Infections

  • Enforce good environmental hygiene practices, including cleanliness in bedding and feeding areas, while reducing overcrowding.

  • Vaccination against E. coli through products like the J5 bacterin to lower the severity of clinical signs originating from endotoxemia.