JPC SYSTEMIC PATHOLOGY
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: Within the superficial and deep dermis, predominantly surrounding blood vessels and adnexa, there are variably sized, nodular, aggregates of high numbers of eosinophils, fewer lymphocytes, plasma cells, mast cells, and occasional melanomacrophages (pigmentary incontinence). There are numerous tangential and longitudinal sections of 3-4um wide microfilaria scattered throughout areas of dermal inflammation. The overlying epidermis is mildly hyperplastic with mild orthokeratotic hyperkeratosis.
MORPHOLOGIC DIAGNOSIS: Haired skin, site not specified: Dermatitis, perivascular and periadnexal, eosinophilic, subacute, multifocal, moderate, with numerous microfilariae, breed unspecified, equine
ETIOLOGY: Onchocerca cervicalis
ETIOLOGIC DIAGNOSIS: Cutaneous onchocerciasis
- Onchocerca are filarial nematodes that affect horses, cattle, canids and other mammals worldwide causing cutaneous and ocular lesions; incidence has greatly decreased with the use of avermectins
- Most horses are asymptomatic and prevalence increases with age; only certain horses develop clinical signs
- Onchocerca cervicalis or reticulata are the most common pathogenic species affecting horses
- O. gutturosa infects horses but is thought to be nonpathogenic; infect the connective tissue of the flexor tendons and suspensory ligaments
- The presence of microfilaria 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
- Microfilaria transmitted by intermediate hosts (black flies, gnats, biting midges) and migrate through tissue, not through the blood stream as other filarids
- Cutaneous lesions thought to be due to type I and type III hypersensitivity reactions to antigens from dead and dying microfilaria or to arthropod bites
- Ocular lesions due to aberrant migration of microfilaria
TYPICAL CLINICAL FINDINGS:
- Incidence of cervicalis in clinically normal horses is 25–100%
- Adult filarids generally do not cause clinical signs
- Microfilaria are responsible for ocular and cutaneous lesions, and are usually seen in older horses
- Patchy alopecia, erythema, scaling, depigmentation
- Uveitis, conjunctivitis, keratitis, depigmentation of lateral limbus
- Recent case report of aberrant migration within the intervertebral joint capsule causing cervical vertebral stenotic myelopathy in a horse
TYPICAL GROSS FINDINGS:
- Adults are found encased in nodular, fibrotic, caseous, and calcifying lesions in the ligamentum nuchae and the skin
- Microfilaria localize along the ventral midline (especially near umbilicus), face, base of mane, proximal forelimbs, pectoral region
- The presence of a pigmented circular area (“bull’s eye”, annular lesion) or plaques in the center of the forehead is highly suggestive of the disease
TYPICAL MICROSCOPIC FINDINGS:
- Adult nematodes are typically found in tangled masses in discrete nodules of dense connective tissue within deep subcutaneous tissues, ligaments and aponeurosis of large animals
- Filarid nematode morphological characteristics in tissue section:
- Presence of a very small intestine—the most important diagnostic feature of filarids
- Microfilariae within the uterus of adult female worms
- Coelomyarian musculature often becomes atrophied and replaced by hypodermis thus in cross-section there is an outer cutica with inner hypodermal layer (unique to Onchocerca)
- Females have cuticular rings or annulations that encircle the worm and thus appear as cuticular bumps in longitudinal section (unique to Onchocerca)
- Dead nematodes induce a pyogranulomatous reaction with marked fibrosis and mineralization
- Microfilaria are found in nests and pockets in the dermis just under the epidermis and adjacent to adnexal structures
- Mononuclear and eosinophilic perivascular dermatitis
- No correlation between number of microfilaria and severity of lesions
Ocular lesions (caused by microfilaria):
- Keratitis, corneal vascularization, pigmentation, collagen degeneration, calcification
- Eosinophilic, neutrophilic, and lymphoplasmacytic conjunctivitis
For gross findings:
- Fly bite dermatoses
- Sarcoptic mange, psoroptic mange
- Pelodera dermatitis
- Food hypersensitivity
- Onchocerca gutturosa: Connective tissue adjacent to nuchal ligament and fascia adjacent to bones in limbs
- Onchocerca lienalis: Gastrosplenic ligament
- Canids: Onchocerca lienalis or lupi reported to cause granulomatous nodules in the eye, periocular tissues, and skin
- 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.
- Gardiner CH, Poynton SL. An Atlas of Metazoan Parasites in Animal Tissues. Washington, DC: Armed Forces Institute of Pathology; 2006: 35, 38.
- Hestvik G, Ekman S, Lindberg R. Onchocercosis of an intervertebral joint capsule causing cervical vertebral stenotic myelopathy in a horse. J Vet Diagn Invest. 2006; 18:307-310.
- Jones TC. Diseases caused by parasitic helminths and arthropods. In: Jones TC, ed. Veterinary Pathology. 6th ed. Baltimore, MD: Williams and Wilkins; 1997: 642-644.
- 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: 687-8.
- Radostits OM. Other skin conditions in horses and ruminants. In: Radostits OM, ed. Veterinary Medicine: A Textbook of the Diseases of Cattle, Sheep, Pigs, Goats and Horses. 9th ed. Philadelphia, PA: W. B. Saunders Company; 2000:1375.
- Rooney JR. Eyes and ears. In: Rooney JR, ed. Equine Pathology. 1st ed. Ames, IA: Iowa State University Press; 1996: 125-126,162, 294-295.
- Saenchez MD, Orita VM, Nolan TJ. Pathology in practice. J Am Vet Med Assoc. 2012;240(4):385-387.
- Scott D. Parasitic diseases. In: Scott D ed. Large Animal Dermatology. 1st ed. Philadelphia, PA: W.B. Saunders Company; 1988: 255-259.
- Smith BP. Skin diseases. In: Large Animal Internal Medicine. Smith BP, ed. 2nd ed. St. Louis, MO: Mosby; 1996: 1388-1391, 1427-1428.
- Sreter T, Szell Z, Egyed Z, Varga I. Ocular onchocerciasis in dogs: A review. Vet Rec. 2002; 151:176-180.
- 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.
- Zarfoss MK, Dubielzig RR, Eberhard ML, Schmidt KS. Canine ocular onchocerciasis in the United States: Two new cases and a review of the literature. Veterinary Ophthalmology. 2005: 8:51-57.