lecture #31

Course Introduction and Resources

• Lecture Title: Infection of musculoskeletal system - II (Lecture #31).

• Instructor: Dr. Dipak Kathayat, BVSc & AH, MS, PhD, Assistant Professor of Microbiology.

• Institution: Richard A. Gillespie College of Veterinary Medicine, Lincoln Memorial University, Harrogate, TN.

• Resource Bibliography: Material for this presentation is derived from the following core veterinary microbiology texts:
    - Veterinary Microbiology and Microbial Diseases, Quinn et al., 2nd ed.
    - Veterinary Microbiology, Songer and Post.
    - Clinical Veterinary Microbiology, Markey et al., 2nd ed.
    - Infectious Diseases of the Dog and Cat, Greene, 4th ed.
    - Veterinary Microbiology, Hirsh et al., 2nd ed.
    - Veterinary Microbiology, McVey et al., 4th ed.

Malignant Edema (Gas Gangrene)

• General Definition: Malignant edema is defined as an acute, generally fatal toxemia that affects all species and ages of animals. It is primarily characterized as a gas gangrene.

• Etiology:
    - Primary Agent: Clostridium septicum is the most common cause.
    - Mixed Infections: Other clostridial species may be isolated alongside C. septicum, including:
      - C. chauvoei
      - C. perfringens type A
      - C. novyi type A
      - C. sordellii

• Epidemiology and Pathogenesis:
    - Habitat: C. septicum is ubiquitous in soil and the intestinal contents of all animal species worldwide.
    - Modes of Infection:
      - Contamination of wounds containing soil, devitalized tissue, or other tissue debilitants.
      - Activation of dormant spores within muscle tissues, which is common in horses and likely in cattle.
    - Associated Events/Risk Factors:
      - Accidental wounds.
      - Husbandry procedures: Castration, docking, shearing, and lambing (specifically in sheep).
      - Medical procedures: Insanitary vaccinations and intramuscular (IM) injections in horses.
      - Obstetrical issues: Parturition and traumatic parturition in cattle.
    - Pathogenic Action: Potent clostridial exotoxins cause excessive localized inflammation, severe edema, necrosis, and gangrene, spreading systemically to cause fatal toxemia.

• Clinical Findings:
    - Onset: Signs typically develop within $6-48 \, ext{hours}$ post-injury or spore activation.
    - Systemic Signs: High fever, anorexia, and severe systemic toxemia.
    - Local Lesions:
      - Initially appear as soft swellings that pit on pressure.
      - Rapidly extend due to infiltrating exudate in subcutaneous and intermuscular connective tissues.
      - Affected muscles appear dark brown to black with possible gas accumulations.
      - Lesions are painful and may progress to extensive local sloughing of skin and tissue.
    - Species-Specific Manifestations:
      - Rams: Severe head edema from fighting wounds.
      - Parturient Animals: Infection with vulvar lacerations leads to vulvar edema, toxemia, and potential death within $24-48 \, ext{hours}$.
      - Pigs: Characteristic neck swelling and blue gangrene coloration.

• Diagnosis:
    - Field/Visual Analysis: Observations of clostridial myonecrosis or gas gangrene. Clinical distinction: Malignant edema affects horses and pigs, but not blackleg.
    - Laboratory Confirmation:
      - Fluorescent-Antibody (FA) Staining: Rapid confirmation using tissue smears for C. septicum.
      - Gram Stain: Identification of Gram-positive rods from fine-needle aspirates.
      - PCR Assay: Direct clostridia species identification.
      - Anaerobic Culture: Definitive confirmation.
      - Caveat: C. septicum is a postmortem invader. Its detection in specimens from animals dead for $ext{≥} 24 \, ext{hours}$ is not significant.
      - Differential Diagnosis: Rule out anthrax in horses and pigs.

• Treatment and Control:
    - Treatment: Early administration of high-dose penicillin, tetracyclines, or other broad-spectrum antimicrobials.
    - Surgical Intervention: Relief incisions (fasciotomies) for clostridial myonecrosis management.
    - Immunization: Use of multivalent bacterins is vital; C. septicum is usually included in combined vaccines (e.g., blackleg/malignant edema).
    - Sanitation: Proper carcass disposal is essential to prevent soil contamination.

Blackleg (Black Quarter)

• General Definition: An acute, febrile, highly fatal, worldwide disease of cattle and sheep characterized by necrotizing myositis and emphysematous swelling of large muscles.

• Etiological Agent: Clostridium chauvoei.

• Etiology and Spore Characteristics:
    - C. chauvoei is naturally found in the intestinal tract.
    - Spores are resilient and can remain viable in soil for years.
    - Microscopic Appearance: Spores are oval, central to subterminal, and may bulge the mother cell, displaying characteristic citron forms in tissue smears.

• Epidemiology:
    - Cattle (Endogenous Infection): Infection occurs without wounds; organisms likely traverse the GI wall, enter the bloodstream, and deposit in muscle, spleen, liver, or gut.
      - Activation triggers: bruising or excessive exercise.
      - Predisposed animals: Often healthy beef breeds, $6-24 \, ext{months}$ old, but can affect calves as young as $6 \, ext{weeks}$ and adults up to $10-12 \, ext{years}$. Outbreaks often follow soil excavations or flooding.
    - Sheep (Wound Infection): Almost always follows injury (e.g., shearing cuts, docking). More frequent in New Zealand than in cattle.
    - Mortality: Case fatality rate approaches 100 ext{%}.

• Clinical Findings:
    - Sudden Onset: Animals may be found dead without prior signs.
    - Initial Signs: Severe lameness (most common in hind legs), marked depression, high fever.
    - Progression: Body temperature may drop to normal or subnormal.
    - Physical Lesions:
      - Crepitant/edematous swellings in large muscle groups, initially focal, hot, and painful. Later, skin becomes cold and insensitive with diminished blood supply and detectable crepitation.
    - Internal Lesions: Muscle appears dry, black, with visible gas bubbles; lesions can be restricted to myocardium and diaphragm in some cases.
    - Terminal Signs: Prostration, tremors, abnormal breathing, and pericardial friction rub (if cardiac involvement); death occurs within $12-48 \, ext{hours}$.

• Diagnosis:
    - Clinical Suspicion: Rapid fatality in well-nourished young beef cattle with crepitant muscle swellings.
    - Laboratory Methods:
      - FAT (Fluorescent Antibody Test): Rapid identification of C. chauvoei.
      - Standard Culture: Anaerobic culture followed by biochemical identification.
      - Immunohistochemistry: Conducted on formalin-fixed tissue.
      - PCR Assay: Reliable for clinical samples (not as much for environmental samples).

• Control and Management:
    - Vaccination: Multivalent vaccines with C. chauvoei, C. septicum, and optionally C. novyi are effective.
    - Cattle Protocol:
      - Vaccinate calves at $2 \, ext{months}$ of age (two doses, $4 \, ext{weeks}$ apart, then annual boosters before danger period).
    - Sheep/Goats: May have a weaker immune response than cattle.
    - Outbreak Management:
      - Vaccinate all susceptible cattle and provide prophylactic penicillin.
      - Relocate cattle from affected pastures; custom