Veterinary Bacteriology & Mycology Study Notes

Actinomyces

  • General Characteristics of Actinomyces

    • Most species produce true mycelium that fragments into elements of irregular size.
    • May exhibit angular branching.
    • Non-acid fast, carboxyphilic, non-motile, and non-sporeforming.
    • Microaerophilic or facultative anaerobic.
    • Cell wall does not contain diaminopimelic acid or arabinose.

  • Key Species: A. bovis

    1. Causes actinomycosis and "lumpy jaw."

  • Clinical Presentation

    • True actinomycosis in cattle primarily affects bony structures, particularly the mandible or lower jaw.
    • Morphology and Staining:
    • Sulfur granules observed in a tangled mass of filaments, surrounded by a considerable mass of acidophilic capsular material.
    • Filaments stain positively with Gram's stain and also retain usual basophilic stains.
    • Crushed granules show a great diversity of forms resembling a mixed infection presentation.
    • Forms observed include:
    • Cocci, rods of varying size, filaments, branching forms, club-shaped or spiral.
    • A. bovis appears in the form of diphtheroid bacilli when young, while old cultures show filaments of all types.
    • In presence of CO2, takes the forms of branching filaments and clubs.

  • Cultural and Biochemical Features

    • Facultative anaerobe that grows well in the presence of 10% to 15% CO2.
    • Serophilic, catalase negative, requiring $37^{ ext{o}}$C for growth.

  • On Blood Agar Plates (BAP)

    • Colonies are small and non-hemolytic.
    • Organism does not grow in milk and gelatin unless serum is added.

  • Serotypes

    • Belongs to Group B of Slack and Gerencser’s classification (1970) of Actinomyces.
    • Contains two serotypes.

  • Epidemiology and Pathogenesis

    • A. bovis is a normal and obligate parasite of oropharynx and digestive tract.
    • Enters tissues of the jaw through dental alveoli or in association with foreign material including wood or wire.
    • In the mandible, it causes rarefying osteomyelitis; soft granulation tissue forms in the mandible and along the lower esophagus and reticulum, which develops necrotic areas with pus that discharges to the surface through fistulous tracts.
    • Over time, the affected area hardens into dense tumor-like masses that are thick, mucoid, tenacious, and greenish-yellow.

  • Diagnosis

    1. Direct Examination:
    • Small amount of pus in a Petri dish is washed to expose small 1-3 mm sulfur granules, which are larger than grey-white granules in Actinobacillus.
    • Transfer granule to slide, add drop of 10% sodium hydroxide, crush with coverslip: characteristic "ray fungus" with club-shaped margins.
    • Clubs are caused by gelatinous sheath and deposition of calcium phosphate around terminal filaments.
    • Gram stain shows delicate, intertwined, branching Gram-positive filaments.
    1. Isolation and Identification:
    • Grows well on BAP, Brain Heart Infusion (BHI) agar, and thioglycolate broth in anaerobic atmosphere containing 5-10% CO2.
    • Colonies are white, rough, nodular and difficult to remove with radiating mycelia; Gram-positive, coryneform rods and slightly branched, slender filaments observed.
    • Some filaments fragmented into bacillary and coccoid forms.
    • Presumptive identification is based on characteristic sulfur granules.

  • Treatment

    1. Drainage of abscesses.
    2. Antibiotics: Penicillin or clindamycin.
    3. Oral iodides or intravenous therapy.

  • Other Species of Actinomyces

    • A. viscosus: Aerobic and catalase positive, found in human and canine oral cavity, related to periodontal disease.
    • A. hordeovulnaris: Causes localized abscesses and systemic infections in dogs.
    • A. suis: Associated with pyogranulomatous mastitis in sows.
    • A. israelii: Causes actinomycosis in humans.
    • Other conditions related to Actinomyces spp.: Supra-atlantal bursitis (poll evil) and supraspinous bursitis (fistulous withers).

Dermatophilus

  • Characteristics of Dermatophilus

    • D. congolenis: Gram-positive, branching, filamentous segmented rod.
    • Aerobic, non-sporeforming, non-acid fast, motile zoospores.

  • Habitat and Transmission

    • Dermatophilus species are obligate parasites.
    • Spread by contact, fomites, and biting insects.
    • Moist conditions promote their dissemination.

  • Pathogenicity

    • Results in streptothricosis or dermatophilosis in horses, cattle, sheep, goats, dogs, cats, deer, squirrels, and humans.
    • Affected areas include superficial layers of skin characterized by formation of crusts or scabs of varying sizes.
    • Removal of scabs leads to moist, depressed areas.

  • Specific Conditions Related to Dermatophilus

    • Mycotic dermatitis in sheep manifests through:
    1. Lumpy wool: dermatitis of wool-covered areas.
    2. Dermatitis of face and scrotum.
    3. Dermatitis of lower leg and foot; ulcerative dermatitis known as "strawberry foot rot."

  • Immune Response

    • Both humoral and cell-mediated responses to infection are not clearly understood.

  • Diagnosis

    1. Direct Examination:
    • Smears from softened scabs with distilled water stained using Giemsa or Gram method.
    • Segmenting longitudinal and transverse filaments and coccoid spores stain deep purple; spores are found in packets.
    1. Isolation and Identification:
    • Grown on BAP, Tryptose agar; produces small, rough, greyish-white colonies after 24-48 hours with fimbricated, lace-like borders that become yellow-orange.
    • Motile zoospores formed as a result of septation of hyphae can be observed, with polar flagella visible in wet mounts.

  • Treatment and Control

    1. Combination treatment with penicillin and streptomycin, tetracycline, and chloramphenicol.
    2. Removal of scabs with brush and mild soap before application of iodine compounds and copper sulfate.
    3. Regular grooming and isolation in dry quarters.
    4. Infections can last for long periods, but reinfection does not occur.

Nocardia

  • Characteristics of Nocardia

    • Non-motile, non-sporeforming, gram-positive, shows branching and aerial hyphae.
    • Aerobic, can split sugars by oxidation, and is partially acid-fast.
    • Nocardia asteroides is a complex made up of N. asteroides, N. farcinica, N. nova.
    • N. asteroides are soil saprophytes, with infection often occurring through inhalation or wounds.
    • Not considered contagious (exogenous).

  • Pathogenesis

    • Infections begin as nodules or pustules that rupture and cause suppuration with subsequent induration.
    • Can present in localized forms (skin) or thoracic and systemic forms.

  • Virulence Factors

    • The cell wall contains mycolic acid which contributes to virulence by resisting phagocyte killing.
    • They are facultative intracellular parasites, catalase positive, and produce superoxide dismutase.

  • Clinical Manifestations

    1. Cattle: Acute or chronic mastitis with granulomatous lesions and draining fistulous tracts; bovine farcy (N. farcinica) manifests as a suppurative infection involving the neck and limbs, usually starting on skin and including superficial lymph nodes and vessels.
    2. Dogs and Cats: Localized forms present as subcutaneous lesions (mycetomas) or lymph node involvement in both species; thoracic forms in dogs can extend to the abdominal cavity, leading to suppurative pleuritis or peritonitis along with abdominal abscesses. Symptoms may include severe halitosis, gingivitis, and ulceration of the oral cavity.
    3. Horses: Infrequent occurrences, but respiratory nocardiosis can occur in immunosuppressed horses. Granules are not found in infections by N. asteroides.
    4. Humans: Most common is pulmonary nocardiosis presenting as subcutaneous forms, with possible CNS involvement. The case fatality rate can be very high.

  • Zoonotic Potential

    • Nocardiosis is not considered zoonotic.

  • Diagnosis

    1. Direct Examination:
    • Pus reveals rods to coccoid forms of gram-positive filaments; most strains retain carbol fuchsin.
    1. Isolation and Identification:
    • Grows on unenriched media, BAP, and Sabouraud Dextrose Agar (SDA) at $25-37^{ ext{o}}$C.
    • Colonies may appear irregularly folded, raised, and granular, with colors ranging from white to yellow to deep orange.
    • Gram-positive, partially acid-fast, with branching mycelial filaments that may break into bacillary and coccoid forms, resulting in cultures giving a powdery appearance.

  • Presumptive Identification

    • Based on demonstration of organism in clinical material and colonial, cultural, morphologic, and staining characteristics.

  • Treatment

    • Surgical debridement and drainage of lesions may be necessary.
    • Antimicrobial treatment such as Trimethoprim-sulfamide; may last for as long as 12 weeks.