JPC SYSTEMIC PATHOLOGY

INTEGUMENTARY SYSTEM

August 2025

I-F01

Signalment (JPC# 21474-5/6): 2-year-old dog

HISTORY: This dog from Florida had an expanding, 3 cm, circular, indurated granuloma with numerous sinuses and fistulas in the skin over the right flank.

HISTOPATHOLOGIC DESCRIPTION: SLIDE A: Haired skin: Affecting 75% of the deep dermis and subcutis and extending into and separating and surrounding skeletal myofibers of the panniculus carnosus are multiple, coalescing, 0.5-2 mm diameter inflammatory nodules centered on eosinophilic cellular and karyorrhectic debris (lytic necrosis) with embedded faint negative images of fungal hyphae that have variably visible eosinophilic hyphae tubular cell walls. Necrotic foci and fungal hyphae are surrounded by numerous epithelioid macrophages, lymphocytes, plasma cells, few neutrophils, and occasional multinucleated giant cells (foreign body and Langhans type), admixed with fibroblasts and scant fibrous connective tissue. Inflammation extends into the adjacent adipose tissue and skeletal muscle. Remaining skeletal myocytes are often shrunken with fragmented, hypereosinophilic sarcoplasm and nuclear pyknosis (necrosis), or pale, swollen, and vacuolated sarcoplasm (degeneration). Multifocally, scattered macrophages contain brown, intracytoplasmic pigment (hemosiderin). Diffusely, there is mild orthokeratotic hyperkeratosis.

Slide B: Gomori methenamine silver (GMS): Within the pyogranulomas are many argyrophilic hyphae and hyphal fragments. Hyphae are 4‑15 um in width, occasionally septate; have thin, non-parallel walls; and display non‑dichotomous branching.

MORPHOLOGIC DIAGNOSIS: Haired skin: Dermatitis and panniculitis, nodular, granulomatous, multifocal to coalescing, marked, with numerous fungal hyphae, breed not specified, canine.

ETIOLOGIC DIAGNOSIS: Cutaneous zygomycosis

GENERAL DISCUSSION:

• Rare disease in dogs, cats, horses, llamas, sheep, goats, pigs, and birds: except Conidiobolus spp. common rhinitis agent in sheep
• Class Zygomycetes has two orders:
  1. Order Mucorales:
     1. Systemic disease called “mucormycosis”
     2. Genera: Absidia, Mucor, Rhizopus, Rhizomucor, Mortierella
  2. Order Entomophthorales
     1. Subcutaneous disease called “entomophthoromycosis”
     2. Genera: Basidiobolus, Conidiobolus
• Non-contagious (environment is the source), ubiquitous, saprophytic fungi
• Immunosuppression may increase the risk of infection
• Three syndromes have been described:
  1. Rhinocerebral
  2. Cutaneous/subcutaneous (most frequently in horses) – Basidiobolus haptosporus (horse) & Conidiobolus coronatus (dogs) most common
  3. Systemic
     1. Pulmonary
     2. Gastrointestinal
• Immune compromise usually not a feature as it is in humans, rather exposure to large numbers of organisms or implantation via penetrating injury

PATHOGENESIS:

• Gains entry through cutaneous wounds, oral cavity, respiratory tract, GI tract
• Systemic dissemination through angioinvasion by hyphae; common with Mucorales, but uncommon feature of Entomophthorales

TYPICAL CLINICAL FINDINGS:

• Pruritis is a common feature
• Chronic and rapidly progressive

TYPICAL GROSS FINDINGS:

• Skin lesion on lateral head, neck, trunk: Nodular, ulcerated, draining, with small yellow/white gritty masses (kunkers, leeches)
• Can reach 30 cm in diameter
• Surface is alopecic, scarred, and ulcerated
• GI disease characterized by necrosis/hemorrhage or granulomatous inflammation

TYPICAL LIGHT MICROSCOPIC FINDINGS:

• Necrotic core surrounded by a zone of eosinophils, neutrophils, epithelioid macrophages, and multinucleated giant cells are separated by fibrovascular connective tissue.
• Broad (5-20.5um), sparsely septate fungal hyphae with non-dichotomous branching and non-parallel walls that stain poorly with routine (H&E) stains
• Folded, twisted, or compressed hyphae may be seen
• Some hyphae are surrounded by eosinophilic granular material or “eosinophilic halo” (Splendore-Hoeppli phenomenon)
• Will see fungal hyphae within vessel walls commonly with Mucorales, but not with Entomophthorales

ADDITIONAL DIAGNOSTIC TESTS:

• Genus and species can only be diagnosed by culture
• Good staining with GMS, poor staining with PAS
• Need immunohistochemistry, culture, or PCR to differentiate from pythiosis

DIFFERENTIAL DIAGNOSIS:

Gross (cutaneous granulomas):

• Pythiosis (I-F02): skin lesions are most common in horses (less common in dogs), generally located on lower limbs and ventrum; have larger, coral-shaped kunkers
• Bacterial granulomas (actinomyces, mycobacteria, nocardia)
• Foreign-body granulomas

Histopathologic (subcutaneous mycoses, algae, and other infective agents)

• Pythium sp. (I-F02): Lesions are very similar to zygomycosis, but hyphae are narrower (2-6um) and thicker walled; need culture to definitively differentiate
• Lagenidium sp.: Reported only in dogs, more clinically aggressive than pythiosis, tends to go to distant visceral sites, hyphae stain with H&E
• Aspergillus sp.: Parallel walls, dichotomous branching, hyphae with frequent septations
• Candida sp.: Parallel septate hyphae along with yeast-like cells and pseudo hyphae
• Phaeohyphomycosis (I-F04): Pigmented (dematiaceous) fungi
• Eumycotic mycetoma: Presence of macroscopic granules in the exudates
• Prototheca wickerhami (S-M03: Usually a systemic disease in dogs and cats
• Sporothrix schenckii (I-F07): Oval or cigar shaped yeasts
• Rhinosporidium seeberi (P-F01): Large (100‑300 um) endosporulating sporangia
• Coccidioides immitis (P-F03): Zygomycetes have been confused with this organism when there are transected hyphae-like spherules present in section

COMPARATIVE PATHOLOGY:

• Horse: Basidiobolus haptosporus ‑ subcutaneous swellings of lateral head, chest, and neck
• Non‑human primate and horse: Conidiobolus coronatus – subcutaneous nasofacial swelling
• Sheep: Conidiobolus incongruus ‑ rhinocerebral and nasal zygomycosis, loss of condition, and death within 7-10 days
• Bovine: Mucormycosis (Mucor, Rhizopus, Absidia) ruminal ulceration and mastitis secondary to grain overload; Zygomycetes cause abortions, second only to Aspergillus fumigatus; a differential diagnosis for granulomatous lymphadenitis in feedlot steers
• Basidiobolus sp. has been associated with the digestive tracts of a wide variety of amphibians and reptiles
• Porpoise: Mucormycosis (Neophocaena spp) systemically with secondary granulomatous pericarditis
• Canaries: M. ramosissimus associated with feather loss in canaries
• Deer: Conidiobolus incongruus associated with systemic infection and osteopathy
• Amphibians: Disseminated mycosis due to Mucor amphibiorum in anurans; granulomas in various organs

REFERENCES:

1. Caswell JL, Williams KJ. Infectious diseases of the respiratory system. ln: Maxie MG ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 2. 6th ed. Philadelphia, PA: Elsevier Saunders. 2016:573.
2. Cullen JM, Stalker MJ. Infectious diseases of the liver. ln: Maxie MG,ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 2. 6th ed. Philadelphia, PA: Elsevier Saunders. 2016:324.
3. Evans DE, Kawabata A, Wilson LD, Kim K, Dehghanpir SD, Gaunt SD, Welborn M, Grasperge B, Gill MS. Entomophthoromycosis and mucormycosis as causes of pneumonia in Vietnamese potbellied pigs. J Vet Diagn Invest. 2018;30(1):161-164.
4. Ginn PE, Mansell JEKL, Rakich PM. Integumentary system. ln: Maxie MG ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 1. 6th ed. Philadelphia, PA: Elsevier Saunders. 2016:659-660.
5. Pessier AP. Amphibia. In: Terio K, McAloose D, Leger J, eds. Pathology of Wildlife and Zoo Animals, San Diego, CA: Elsevier 2018:939-940.
6. Schmidt RE, Reavill DR, Phalen DN. Pathology of Pet and Aviary Birds. 2^nd ed. Ames, IA: Wiley Blackwell; 2015:25, 39, 48, 71, 75, 80.
7. Simmons J, Gibson SV. Bacterial and mycotic diseases. In: Bennett BT, Abee CR, and Henrickson R. Nonhuman Primates in Biomedical Research. 2nd ed. London, UK: Academic Press; 2012:157-159.
8. St. Leger J, Raverty S, Mena A. Cetacea. In: Terio K, McAloose D, Leger J, eds. Pathology of Wildlife and Zoo Animals, San Diego, CA: Elsevier 2018:560-561.
9. Rodrigues Hoffmann A, Ramos MG, Walker RT, Stranahan LW. Hyphae, pseudohyphae, yeasts, spherules, spores, and more: A review on the morphology and pathology of fungal and oomycete infections in the skin of domestic animals. Vet Pathol. 2023;60(6):812-828.
10. Uzal FA, Plattner BL, Hostetter JM. Forestomachs. ln: Maxie MG,ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 2. 6th ed. Philadelphia, PA: Elsevier Saunders. 2016:42.
11. Welle MM, Linder KE. The Integument. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 7th ed. St. Louis, MO: Elsevier; 2022:1178.

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