Bacterial Skin Diseases: Flashcards
Mycobacterium Spp. Morphology & Staining Characteristics:
-Rod shaped (bacilli); Acid-fast stain
-Acid fast staining due to mycolic acids (lipids) retention of dye (carbol fusion) despite acid decolorizing wash; Thus acid-fast staining is used to aid diagnosis
-Hydrophobic outer layer (rich in mycolic acids) protects against host killing and interferes with gram stain reaction
-Divided into 3 Subgroups:
-Mycobacterium-tuberculosis Complex (MTBC)
-Mycobacteria or Non-tuberculosis mycobacteria (NTM)
-Fastidious mycobacteria
Mycobacterium-Tuberculosis Complex (MTBC):
-10 MTBC species including: M. tuberculosis, M. bovis, M. microti; These 3 are the main agents of TB in animals!!
-Slow growing. It causes disease in immune competent hosts but survive poorly outside of the host reservoir
-Facultative intracellular pathogens of macrophages
-All cause clinical TB
-Zoonotic potential!!!!
Mycobacteria or Non-tuberculosis Mycobacteria (NTM):
-Opportunistic environmental saprophytes
-Some are slow growing like M. avium complex (MAC)
-Some are Rapidly Growing Mycobacteria (RGMs) like M. smegmatis, M. fortuitum, and M. abscessus
Fastidious Mycobacteria:
-Lepromatous (Leprosy)
-Difficult or impossible to culture using standard means
Mycobacterium Spp. Transmission:
-Inhalation of M. tuberculosis containing respiratory droplets
-Inhalation of M. bovis respiratory droplets, ingestion of raw milk or meat from infected cattle
-Ingestion of MAC in water and feedstuff
-Contamination of wound with environmental mycobacteria (MAC and RGMs); This is the common route in cats but less so in dogs
-Ingestion or bite of M. microti infected vole/mouse; This is seen in cats/dogs in Europe and is also endemic to Michigan
Mycobacterium Spp. Disease Presentation & Role of Host:
-Host Granuloma Formation:
-Granuloma formation or granulomatous/pyogranulomatous host response is characteristic!
-Granuloma formation functions to contain bacterial spread
-Lesions due to intracellular and extracellular growth of the bacterium as well as host responses
-Cutaneous Disease:
-More common in cats than dogs
-Outdoor lifestyle, wound contamination by bacteria in environment
-Should suspect this will be present in animals with chronic granulomatous/pyogranulomatous skin lesions unresponsive to antibiotics, especially if animal is immunosuppressed
-MTBC causes nodular lesions +/- draining tracts in outdoor cats in Europe. Not in USA
-Slow growing NTB MAC organisms can cause ulcerated nodular lesions +/- draining tracts +/- local LN enlargement; May disseminate
-FIV/Immunosuppressed drugs may predispose
-Suspect RGM in cats with chronic draining, nodular/ulcerated lesions with subQ tissue involvement in inguinal area (panniculitis)
-Fastidious mycobacteria: M. lepraemurium and others can’t be cultured. Can cause single or multiple subQ nodules of feline leprosy
Mycobacterium Spp. diagnosis, treatment, and prevention:
-Diagnosis: Aspirate or biopsy of lesion; ID of acid-fast bacteria; ALWAYS acid-fast stain if cytology/histology shows granuloma, granulomatous, or pyogranulomatous inflammation; PCR or culture of biopsy tissue
-Treatment: Months-long, 2-3 drug combination therapy (Fluoroquinolone + macrolide + rifampin); Reportable if MTBC organism but zoonotic risk of cutaneous lesions is fairly low!!!
Nocardia Spp. Morphology & Staining characteristics:
-Gram positive, filamentous rods
-Widely distributed in the soil and water; Not considered normal flora of animals
-Opportunistic disease: Results in pyogranulomatous inflammation
-Nocardia asteroides complex includes many species of veterinary significance (often times the specific species is not determined)
Nocardia Spp. Transmission:
-Most often via contamination of wounds
-Rarely by inhalation or ingestion
Nocardia Spp. Clinical Disease:
-Disease may reflect underlying immunosuppression
-3 Disease forms: Localized skin form; Thoracic form (pneumonia, pleuritis); And septicemic (generalized form)
-All 3 forms occur in dogs and cats
-Localized subQ lesions (Mycetomas) with or without LN enlargement
-Infection begins as a pustule or nodule that ruptures and forms a draining tract
Nocardia Spp. Diagnosis & Treatment:
-Diagnosis: Cytological prep of tissue biopsy with gram stain of lesion shows gram + branching filamentous rods
-Treatment: Surgical debridement with systemic antibiotics; Treatment of thoracic and systemic forms is often unrewarding
Dermatophilus Congolensis Morphology and staining characteristics:
-Gram positive, filamentous, branching
-Hyphal form divides longitudinally and then transversely to form parallel rows of motile, coccoid organisms called zoospores (Look like railroad tracks)
-Replicates only on animals
-Carrier animals are primary source of infection
Dermatophilus Congolensis Transmission:
-Carriers maintain organisms in populations
-Direct contact with affected animals by contaminated fomites and mechanically by flies
-Can survive for up to 3 years in crusts in the environment
-Prolonged rainfall, high humidity, higher temperatures, and trauma to skin are all factors that favor transmission
-Can see epidemics during rainy seasons
Dermatophilus Congolensis Pathogenesis:
-Soaking or trauma to the skin favors germination of motile coccoid zoospores which form “hyphal-like” branches in the epithelium and invade hair follicles
-Neutrophilic inflammation occurs and scab-like crusts develop
-Organisms eliminated as epithelium regenerates and crusts are shed
-Scabs are easily lifted by grasping the hair
Dermatophilus Congolensis Clinical Disease:
-Called “Dermatophilosis”
-In horses may be called “rain scald” and in sheep is called “lumpy wool”
-Affects cattle, horses, sheep, and goats
-Rare in pigs, dogs, cats, and humans
-Purulent crusts, matted hair
-More common in animals exposed to chronic moisture, young and immunocompromised hosts
-Removal of crusts reveals a moist, gray-to-pink surface with hair roots protruding through the undersurface of the crust
Dermatophilus Congolensis diagnosis:
-Most often done by clinical signs
-Stained, cytological prep of crusts for characteristic bacterial morphology
Dermatophilus Congolensis Treatment and Control:
-Remove crusts by grooming. Do not discard crusts in environment of animal. Wear gloves due to zoonotic potential!!!
-Topical chlorhexidine-based shampoo +/- systemic antibiotics
-Shelter for prolonged rain exposure is most important!!
-Fly control may help as well
Erysipelothrix Spp. Morphology & Staining Characteristics:
-Gram Positive rod (sometimes filamentous)
-Primary pathogen is E. rhusiopathiae
-Widespread in nature and can survive for long periods in water and soil
-Normal flora on skin of fish
-Also found on mucous membranes and in GI tract of carrier swine
-E. rhusiopathiae is a pathogen of swine and humans (Erysipeloid)
Transmission of E. rhusiopathiae:
-Mostly by ingestion of contaminated material (Food, soil, water, feces)
-Found on skin of fresh and saltwater fish and may be transmitted by ingestion of fishmeal (dietary supplement)
-Carrier pigs shed organism in nasal secretions, saliva, and feces. Infection can be acquired by direct contact with carriers, infected pigs, and fomites
E. Rhusiopathiae Swine Clinical Disease:
-Younger animals (3-18 months most susceptible)
-Disease (Erysipelaas): “Diamond skin disease,” appears as a reddish-purple rhomboid shaped skin infarct
-Can develop acute septicemia, endocarditis, and arthritis
E. rhusiopathiae Diagnosis, treatment, and control:
-Blood culture and culture/PCR of tissues taken at necropsy confirms septicemia
-Antibiotics: Penicillin effective in controlling outbreaks
-Treatment not effective in chronic forms (arthritis)
-Vaccines available (live-attenuated strains; formalin fixed killed vaccines) to protect pigs
-Humoral immunity (antibody) is protective
Erysipeloid:
-Zoonotic disease in humans
-Enters via broken skin
-Usually localized erythematous swelling at entry site (hands/fingers)
-Occasional systemic complications observed
-Penicillin treatment usually resolves infection
Staphylococcus Hyicus Morphology & Staining Characteristics:
-Resides on skin and mucosal surfaces of ALL animals and people
-Invades upon epithelial damage & are opportunistic invaders
-Diseases characterized by neutrophilic inflammatory response (pus)
-Many make beta-lactamase
-Gram positive Cocci
Staphylococcus Hyicus Transmission:
-Organism seeds skin during birth or transfers from skin of sow
-Can’t penetrate intact skin
-Enters following injuries caused by fighting, rough concrete floors, and mites
-Virulent strains produce exfoliative toxin, which damages adhesions between keratinocytes
