JPC SYSTEMIC PATHOLOGY
INTEGUMENTARY SYSTEM
August 2022
I-F03 (NP)

Signalment (JPC# 1597220):  Young alligator.

HISTORY:  This alligator died unexpectedly.  At necropsy, a necrotic skin lesion was present in the flank area in the region where the skin of the leg meets the abdominal skin.

HISTOPATHOLOGIC DESCRIPTION:  Scaled skin: There is a focally extensive area of epithelial loss (ulceration) with replacement by abundant eosinophilic cellular and karyorrhectic debris (necrosis), large numbers of macrophages, fewer heterophils and lymphocytes (serocellular crust), and small basophilic colonies of 2 x 4 μm bacilli.  Macrophages often contain intracytoplasmic, brown, granular pigment (melanomacrophages) and there are increased numbers within the deep dermis (pigmentary incontinence). The inflammatory infiltrate extends into the deep dermis where there is increased amounts of fibrous connective tissue. The underlying skeletal myocytes are slightly shrunken and hypereosinophilic (atrophy), and there is expansion of the perimysium and endomysium by increased clear space (edema) and fibrosis.  The adjacent epidermis is moderately hyperplastic, and keratinocytes are occasionally shrunken and hypereosinophilic with pyknotic nuclei (necrotic), or are swollen and rounded with clear cytoplasm (intracellular edema).  There is mild ortho- and parakeratotic hyperkeratosis. 

Scaled skin, periodic acid-Schiff (PAS):  Within the epidermis and extending into the dermis, there are numerous PAS-positive, 3-5 μm diameter fungal hyphae with frequent septations; parallel walls; acute angle, dichotomous branching; and terminal bulbous swellings.

Lung, trachea, esophagus: Essentially normal tissue.

MORPHOLOGIC DIAGNOSIS:  Scaled skin:  Dermatitis, necrotizing, subacute, focally extensive, moderate, with ulceration, superficial bacilli, and many fungal hyphae, etiology consistent with Aspergillus spp., alligator, reptile.

ETIOLOGIC DIAGNOSIS:  Cutaneous aspergillosis

CAUSE:  Aspergillus spp.

GENERAL DISCUSSION:

• Ubiquitous, saprophytic, spore-forming fungus
• Opportunistic infection associated with immunosuppression, impairment of phagocytosis, chemotherapy, or prolonged corticosteroid therapy
  • Excessive exposure to fungus, vitamin A deficiency, poor husbandry or poor adaptability to captivity in reptiles and psitticines, or prolonged antibiotic therapy
• Angioinvasive fungus that colonizes and invades vessels in the mucosae and skin leading to systemic disease
• Cutaneous infection is generally rare among domestic animals, but a can be more common in reptiles

PATHOGENESIS:

• Primary route of infection in most animals is through inhalation of conidia (~2 to 3 μm in diameter) which are ultimately phagocytosed by neutrophils and alveolar and mucosal macrophages.
  • If phagocytes are unable to kill conidia (i.e., defective neutrophil functions), conidia germinate in the mucus layer and mucosae and begin to colonize mucosae of the nasal cavities and sinuses
  • May also enter through the gastrointestinal tract; in reptiles, also entry into skin through abrasions or other wounds
• Gastrointestinal infections are usually secondary and are associated with damage to mucous membranes, viral erosive diseases, stress, and metabolic disturbances
• Aspergillus contain numerous proteases, lipid compounds including aflatoxins, gliotoxins, ochratoxins, and complement activation inhibitors which play a role in evasion of host defenses
  • Proteinases play a role in the ability of the organism to invade tissues; hematogenous dissemination can occur
• An angioinvasive fungus that has the ability to colonize and invade barrier systems such as mucosae and skin, invade the vascular system (through vessel walls and endothelial cells) within these barrier systems, break off into the bloodstream, circulate, and attach to and invade endothelium and ECM of other organ systems, and cause disease.
• fumigatus uses β-glucans, melanin, and other molecules to block killing by reactive oxygen species, phagolysosomal acidification, and other mechanisms in macrophages and neutrophils.
  • Leukocyte trafficking may be used in systemic spread
• flavus and A. niger can produce large quantities of oxalates in feed (D-T03) (renal damage)
• Protection is mediated by macrophages that phagocytize spores and by neutrophils that attach to hyphae
• Resistance or recovery from a systemic fungal infection requires strong innate (phagocytosis by neutrophils and macrophages) and adaptive (T‑cell-mediated) immunity

TYPICAL CLINICAL FINDINGS:

• Most commonly pulmonary and intestinal lesions (e.g. chronic rhinitis)
• Variable depending on species and route of infection and if it has spread systemically

TYPICAL GROSS FINDINGS: 

• Mucosa: multifocal to coalescing, necrohemorrhagic foci; epithelialized plaques, or occasionally raised nodules (e.g. can present as a nasal mass)
• Skin: multifocal to coalescing, raised, irregular, crusty plaques; subcutaneous granulomas have been reported in horses and cattle
• Respiratory tract: Acute pseudomembranous rhinitis and sinusitis.
  • Lung (P-F06): Non-invasive pulmonary form is characterized by formation of aspergilloma (granulomatous pneumonia), consisting of a tangled mycelial mass, usually located within a preformed pulmonary cavity; may be mistaken for a neoplasm

TYPICAL LIGHT MICROSCOPIC FINDINGS:

• In tissues, 5-7 um wide, regularly septate hyphae with thick parallel walls and dichotomous, acute angle branching, and occasionally have bulbous apical dilations.
• Primary lesion in invasive aspergillosis is usually pulmonary, with hematogenous spread
• Multiple septate branching hyphae can radiate from the center of vessels
• In pulmonary lesions, fungal hyphae are generally visible in the core of the lesion and in the walls of blood vessels.
• Well-aerated necrotic cavities in lungs, air sacs of birds, pleura, or nasal mucosa may develop conidiophores (fruiting bodies, these may provide definitive diagnosis but are rarely seen in tissue sections); extensive deposition of birefringent calcium oxalate crystals may be present in these areas
  • Conidiophore stalks are enlarged at the uppermost point to form a globose, hemispherical, flask-shaped vesicle; peg-like sterigmata form in a single layer (uniseriate) or two layers (biseriate) with the second row arising from the first; unbranched chains of conidia form from the distal ends
• Superficial infections mainly in the epidermis with infrequent invasion of the hair follicles and/or dermis
• Fibrinohemorrhagic granulomas extending through all layers of gastrointestinal tract

ADDITIONAL DIAGNOSTIC TESTS: 

• Isolation by culture
• Immunohistochemical staining using monoclonal and polyclonal antibodies
• ELISA, AGDD
• Cytology: 5-7µm hyphae, regularly septate with branching

DIFFERENTIAL DIAGNOSIS:

• Angioinvasive fungi: Aspergillus spp., Candida spp., Fusarium spp., and zygomycetes (e.g., Absidia spp., Rhizopus spp., and Mucor spp.)
• Fungal hyphae in tissue:
  • Aspergillus , Mucor spp., Rhizopus spp., Candida spp., Fusarium spp., and Trichophyton spp., have been isolated from cutaneous lesions in crocodilians
    • Zygomycetes (Mucor , Rhizopus spp.): Hyphae (up to 15 um in diameter) are infrequently septate, have nonparallel walls, often appear collapsed or twisted; stain less uniformly
    • Candida – produce hyphae, pseudohyphae, and budding yeast in tissue
    • Pseudallescheria boydii – tissue phase is similar; must culture to differentiate

COMPARATIVE PATHOLOGY:

• Cattle: Rumenitis, omasitis, and abomasitis; gastroenteritis (usually secondary (e.g rumen acidosis caused by grain overload); mastitis; abortions (R-F01); bronchopneumonia and hyperkeratotic dermatitis (fetal lesions; aborted fetus lesions resemble ichthyosis or extensive ringworm)
• Horses: Guttural pouch and pulmonary mycosis; dissemination often results from primary intestinal lesion (e.g. colic) (reported as embolic aspergillosis pneumonia); keratitis; abortions, endocarditis; persistent diarrhea in foals
• Sheep and goats: Mastitis in ewes; respiratory, gastrointestinal, and systemic lesions in lambs
• Dogs: Most problematic in German Shepherd dogs. Usually limited to nasal cavity, paransal sinuses, or respiratory system. (e.g. granulomatous rhinitis or rarely cause chronic destructive bronchitis. Rare cause of endocarditis, myocarditits, or splenitis. Can also cause infection of kidneys (U-F02) and other viscera. terreus can be involved (German shepherd dogs). Can occasionally cause osteomyelitis or diskospondylitis. Dogs with disseminated disease may develop cutaneous lesions
• Cats: Pulmonary (e.g. nasal) and cutaneous infections, mycotic colitis has been reported in cats as a hemorrhagic ulcerative colitis, secondary to colonic damage and leukopenia caused by FPLV
• Poultry: Known as “brooder pneumonia”; all stages of life can be affected including embryonating eggs ( fumigatus), chicks and poults (typically >50% mortality even with low level contamination), adults (respiratory, digestive, or CNS; chronic infections more common in older birds); lesions often present as fuzzy gray, blue, green, or black material or pale yellow plaques; pulmonary granulomas must be differentiated from Mycoplasma gallisepticum infections; fungal infections in young chicks and turkey poults must be differentiated from the fungus Ochroconis gallopava by culture
• ZEW: Osteomyelitis, encephalitis, cardiomyositis, ocular disease, cutaneous lesions (including dermatitis, feather abnormalities, pododermatitis) or disseminated systemic disease in multiple species
  • Granulomatous or nodular pneumonia and/or airsacculitis in avians
  • Obstructive tracheitis in captive bottlenose dolphins
  • Reported in cervids, camelids, monkeys, perissodactyls (e.g. equids or rhinos), lagomorphs, cetaceans (often secondary to morbillivirus co-infection), a variety of avians, a variety of reptiles (chellonians, crocodilians)
  • Most common cause of death in captive penguins and raptors
  • In NHPs, causes allergic bronchopulmonary aspergillosis (a type of an extrinsic asthma (associated with a type I hypersensitivity reaction)) which is associated with the Aspergillus colonization of airways followed by development of Aspergillus-specific IgE
  • Reported as a cause of mortality in corals

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