JPC SYSTEMIC PATHOLOGY

INTEGUMENTARY SYSTEM

September 2025

I-P01

Signalment (JPC# 1823588): Age and breed unspecified, horse

HISTORY: This horse had generalized dermatitis. Several randomly scattered nodular cutaneous lesions were also present in and around the cervical region.

MICROSCOPIC DESCRIPTION: Haired skin, site not specified: Multifocally within the superficial and deep dermis, predominantly surrounding blood vessels and adnexa, there are variably sized nodular aggregates of numerous eosinophils, lymphocytes, plasma cells, and histiocytes, rare multinucleated giant cells, and fewer mast cells. Within the nodules and surrounding dermis, there are numerous tangential and longitudinal sections of microfilariae. Microfilariae are 3-4 µm wide with a thin cuticle and are filled with myriad 1 µm x 2 µm basophilic nuclei. The overlying epidermis is moderately hyperplastic with orthokeratotic hyperkeratosis.

MORPHOLOGIC DIAGNOSIS: Haired skin, site not specified: Dermatitis, perivascular and periadnexal, eosinophilic and lymphohistiocytic, subacute, multifocal, moderate, with numerous microfilariae, breed unspecified, equine.

ETIOLOGY: Onchocerca sp. (most likely Onchocerca cervicalis)

ETIOLOGIC DIAGNOSIS: Cutaneous onchocerciasis

GENERAL DISCUSSION:

• Onchocerca spp. are filarial nematodes that principally affect horses but can also infect cattle, canids, and other mammals worldwide; cause cutaneous and ocular lesions; incidence has greatly decreased with the use of avermectins
  • O. cervicalis – horses; worldwide
  • O. reticulata – horses; Europe and Asia
  • O. gutturosa – horses and cattle; N. America, Africa, Australia, Europe
  • O. gibsoni – cattle; Africa, Asia, Australia
  • O. lienalis – cattle; Australia, North America (potentially ≈ O. gutturosa)
  • O. lupi – dogs, cats, humans; Asia, Southwestern US
  • O. volvulus – humans; causes River Blindness
• Most horses are asymptomatic; prevalence increases with age (>4 years); only certain horses develop clinical signs
• The presence of microfilariae does not necessarily confirm the diagnosis of cutaneous onchocerciasis; they may be found in the skin of clinically normal horses and in lesions from any equine dermatosis
• Previously thought to cause fistulous withers, poll evil, and recurrent equine uveitis, but no longer associated with these diseases

PATHOGENESIS:

• Microfilaria sucked up by blood-sucking intermediate hosts (Simuliidae spp. black flies, gnats, Culicoides spp. biting midges)
• Develop from microfilarid to L3 larvae in intermediate host
• Transferred to new host where L3 migrates through SQ tissues, adults encyst near connective tissues, generate microfilariae that migrate through SQ tissue back to the skin (not through the blood stream as with other filarids)
• Cutaneous lesions thought to be due to type I and type III hypersensitivity reactions to antigens from dead and dying microfilariae or to arthropod bites
• Ocular lesions due to aberrant migration of microfilariae
• Inflammatory response partly due to presence of Wolbachia spp. bacteria (as with other filarids)

TYPICAL CLINICAL FINDINGS:

• Incidence of O. cervicalis in clinically asymptomatic horses in endemic areas was 25–100% (before widespread avermectin [ivermectin] use)
• Adult filarids generally do not cause clinical signs; microfilariae are responsible for the ocular and cutaneous lesions, and are usually seen in older horses
• Pruritus is mild to severe
• Non-seasonal

TYPICAL GROSS FINDINGS:

• Adults are found encased in nodular, fibrotic, caseous, and calcifying lesions
  • O. cervicalis – funicular portion of ligamentum nuchae over shoulder and neck
  • O. reticulata – tissue of tendon sheaths adjacent to carpus or suspensory ligaments, fetlock
  • O. gutturosa – lamellar part of ligamentum nuchae
• Microfilariae localize along the ventral midline (especially near umbilicus), face, base of mane, proximal forelimbs, pectoral region
• Cutaneous lesions: Patchy alopecia, erythema, scaling, depigmentation; annular lesion in the center of the forehead is characteristic
• Ocular lesions: Uveitis, conjunctivitis, keratitis, depigmentation of lateral limbus

TYPICAL MICROSCOPIC FINDINGS:

Nematode:

• Adult Onchocerca sp.:
  • Filarid nematodes have characteristic very small intestine on cross-section (the most important diagnostic feature of filarids)
  • Microfilariae within the uterus of adult female worms
  • Polymyarian-coelomyarian musculature often becomes atrophied and replaced by hypodermis, thus in cross-section there is an outer cuticle with inner hypodermal layer (unique to Onchocerca sp.)
  • Females have cuticular rings or annulations that encircle the worm and thus appear as cuticular bumps in longitudinal section (unique to Onchocerca sp.)
• Microfilariae (all filarid species): Cuticle, appear to lack musculature, and are filled with myriad 1x2µm basophilic nuclei (“bag of nuclei”)

Cutaneous lesions:

• Adult filarid nematodes are typically found in tangled masses in discrete nodules of dense connective tissue within deep subcutaneous tissues, ligaments, and aponeuroses
• Microfilariae:
• Dead microfilariae induce a pyogranulomatous reaction with marked fibrosis and mineralization
• Microfilariae are found in nests and pockets in the dermis just under the epidermis and adjacent to adnexal structures; sparse to abundant numbers
• Mononuclear and eosinophilic perivascular dermatitis
• No correlation between number of microfilariae and severity of lesions

Ocular lesions: dogs, cats, horses

• Dogs/cats: Adults worms; conjunctival or episcleral inflammatory nodules, exophthalmos, eosinophilic and granulomatous inflammation, fibrosis
• Horses: Microfilaria; eosinophilic and granulomatous conjunctivitis and peripheral stromal keratitis, uveitis

DIFFERENTIAL DIAGNOSIS:

For gross findings:

• Fly bite dermatoses
• Dermatophytosis
• Sarcoptic mange, psoroptic mange
• Pelodera dermatitis (strongyle)
• Trombiculiasis
• Atopy

For histologic findings:

• Dirofilaria spp. filariasis in dogs/cats
• Stephanofilaria spp. and Setaria spp. in cattle

COMPARATIVE PATHOLOGY:

• Cattle
  • Onchocerca gutturosa: Connective tissue adjacent to nuchal ligament and fascia adjacent to bones in limbs
  • O. gibsoni (Africa, Asia, Australia): Brisket, stifle, hip; 2-9cm SQ nodules; economically significant due to time required for trimming nodules out of carcasses
  • O. lienalis (Australia, North America) (may be same species as O. gutturosa): gastrosplenic ligament and splenic capsule
• Cervids:
  • Onchocerca cervipedis; leg; black fly is intermediate host; white worm up to 8 inches long in skin of legs or brisket; usually nonpathogenic; moose, caribou, Columbian black-tailed deer
• Sheep: Elaeophora schneideri adults damage leptomeningeal arteries of domestic sheep, microfilariae cause dermatosis, retinal and nasal/oral mucosal irritation.
• Canids: Considered an abnormal host for this parasite; O. lupi reported to cause granulomatous nodules in the eye, periocular tissues, and skin (S-P03)
• Gorilla: Onchocerca volvulus; parasite of humans; located in a subcutaneous fibrous nodule
• Donkeys: Recent case report of suspensory ligament desmitis caused by Onchocerca sp. (Paraschou, 2021)

REFERENCES:

1. Strait K, Else JG, Eberhard ML. Parasitic Diseases of Nonhuman Primates. In: Abee CR, Mansfield K, Tardif S, Morris T, eds. Nonhuman Primates in Biomedical Research: Volume 2: Diseases. 2nd ed. San Diego, CA: Elsevier; 2012:243.
2. Cooper BJ, Valentine BA. Muscle and tendon. In: Maxie MG, ed. Jubb, Kennedy,and Palmer’s Pathology of Domestic Animals. Vol 1. 6th ed. St. Louis, MO: Elsevier; 2016:247.
3. Deschuillers PL, Raskin RE. Chapter 16: Eyes and Ears. In: Raskin RE, Meyer DJ, and Boes KM eds. Canine and Feline Cytopathology: A Color Atlas and Interpretation Guide. 4th ed. St. Louis, MO: Elsevier; 2022:566.
4. Gardiner CH, Poynton SL. An Atlas of Metazoan Parasites in Animal Tissues. Washington, DC: Armed Forces Institute of Pathology; 2006:3, 35, 38.
5. Howerth EW, Nemeth NM, Ryser-Degiorgis MP. Cervidae. In: Terio KA, McAloose D, St. Leger J, eds. Pathology of Wildlife and Zoo Animals. London, UK: Academic Press; 2018:176.e10,
6. Labelle P. The Eye. In: In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 7th ed. St. Louis, MO: Elsevier; 2022:1407-1408.
7. Mauldin EA, Peters-Kennedy J. Integumentary system. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 1. 6th ed. St. Louis, MO: Elsevier; 2016:687-688.
8. Paraschou G, Adako GM, Priestnall SL, Burden FA. Suspensory Ligament Desmitis Caused by Onchocerca sp. in Three Donkeys. Vet Pathol. 2021;58(2):401-404.
9. Welle MM, Linder KE. The integument. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 7th ed. St. Louis, MO: Elsevier; 2022:1184.
10. Wilcock BP, Njaa BL. Special senses. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 1. 6th ed. St. Louis, MO: Elsevier; 2016: 452.
11. Young KM, Teixeira LBC. Eyes and Associated Structures. In: Valenciano AC, Cowell RL, eds. Diagnostic Cytology and Hematology of the Dog and Cat. 5th ed. St. Louis, MO: Elsevier Mosby; 2014:159.

Click the slide to view.

---