JPC SYSTEMIC PATHOLOGY
SPECIAL SENSES 
April 2021
S-P03

SIGNALMENT (JPC #4084734): Six-year-old male castrated mixed-breed dog.

HISTORY: The dog presented with suppurative conjunctivitis and epiphora, and during examination, a 2 cm long worm was found within the conjunctival sac.

HISTOPATHOLOGIC DESCRIPTION: Eye: Multifocally expanding the sclera, there are extensive areas of coalescing, granulomatous inflammation composed of large numbers of macrophages, nodular aggregates of lymphocytes and plasma cells, small amounts of cellular debris, plump fibroblasts, and small amounts of mature collagen. Multifocally, areas of granulomatous inflammation contain numerous cross and tangential sections of adult and few larval nematodes which range up to 150um in diameter and have a cuticle with  ring-like circumferential cuticular ridges, a pseudocoelom, atrophied polymyarian-coelomyarian musculature which is multifocally replaced by hypodermis, multiple cross sections of an eccentrically placed small intestine, and reproductive organs.  Uteri occasionally contain abundant cross sections of microfilariae. The conjunctiva is multifocally ulcerated and expanded by large numbers of lymphocytes, plasma cells, and fewer macrophages and neutrophils, multifocally abundant eosinophils, as well as granulation tissue admixed with small amounts of hemorrhage, fibrin, edema, and cellular debris. On one side, the granulation tissue matures into fibrous connective tissue that blends with the ocular fibrous tunic and obscures the ipsilateral drainage angle.

MORPHOLOGIC DIAGNOSIS: Globe: Scleritis, granulomatous and eosinophilic, multifocal to coalescing, severe, chronic, with lymphoplasmacytic conjunctivitis, pre-iridial fibrovascular membrane, and numerous adult filarid nematodes, mixed-breed dog, canine.

ETIOLOGIC DIAGNOSIS: Ocular onchocerciasis

CAUSE:  Onchocerca lupi

GENERAL DISCUSSION:

• Onchocerca are vector-borne filarid nematodes with a relatively narrow host range
• Emerging zoonotic disease of increasing importance
• Ocular disease seen in dogs, cats, and humans in the southwest United States and in dogs from Europe

PATHOGENESIS/LIFE CYCLE:

• Onchocerca have endosymbiotic bacteria, Wohlbachia sp., required for survival
• Full life cycle and definitive hosts of lupi are unknown
• Most likely similar to other Onchocerca: intermediate hosts (simuliid flies [black flies] or Culicoides spp. [gnats/midges]) transmit larvae to definitive host via blood feeding > larvae mature into adults, mate, and produce microfilariae > intermediate host ingests microfilariae

TYPICAL CLINICAL FINDINGS:

• Unilateral or bilateral lesions
• Acute: Conjunctivitis, chemosis, conjunctival erythema, periocular swelling, exophthalmos, protrusion of the nictitans, epiphora, +/- small, thin adult worm within conjunctival sac
• Chronic: Granulomatous nodules in various parts of the eye and periocular tissues
• May be asymptomatic, incidental finding

TYPICAL GROSS FINDINGS: 

• Nodular lesions, single or multiple, raised, red, 2mm-2cm masses of the sclera, third eyelid, conjunctiva, cornea, and/or in retrobulbar tissues centered on adult worms
• Granulomatous episcleritis, conjunctivitis
• +/- Anterior segment involvement, +/- corneal edema, corneal vascularization, anterior uveitis

TYPICAL LIGHT MICROSCOPIC FINDINGS:

• Granulation tissue, fibrosis, lymphoplasmacytic to granulomatous inflammation +/- eosinophils in the conjunctival substantia propria, episcleral tissue, and orbital fascia with cavities lined by epithelioid macrophages containing adult male and female filarial nematodes often seen together
• Parasite: cuticle with ring-like circumferential cuticular ridges (annulations) seen in longitudinal/oblique sections; atrophied coelomyarian-polymyarian musculature replaced by hypodermal tissue; small intestine (key filarid feature); reproductive structures (in adult females, uterus contains microfilariae)

DIFFERENTIAL DIAGNOSIS:

Ocular parasitic infections in dogs:

• Toxocara canis
• Angiostrongylus vasorum
• Dirofilaria immitis
• Ancylostoma caninum
• Trichinella spiralis
• Thelazia
  • Only true ocular parasite
  • Found in the conjunctival sac and lacrimal duct

 

COMPARATIVE PATHOLOGY:

Onchocerca spp. in other animals:

• Onchocerca : donkeys
• O. gibsoni: cattle (Africa, Asia, and Australia)
  • Forms a fibrous “worm nodule” on the brisket or hindlimbs
• O. gutturosa: cattle, horses (North America, Africa, Australia, and Europe)
  • Paired adult worms on surface of the ligamentum nuchae
  • Does not form nodules
• O. lienalis: cattle (Australia and North America)
  • Found within the gastrosplenic ligament and splenic capsule
• O. cervicalis: horses (worldwide) – I-P03
  • Within the ligamentum nuchae over the shoulder or neck
• O. reticulata: horses (Europe and Asia)
  • Around tendon sheaths adjacent to the carpus, suspensory ligaments, or fetlock
  • Adult worms incite a mild inflammatory reaction; mineral and fibrosis present in chronic cases; usually asymptomatic infections
  • Recent report of suspensory ligament desmitis of multiple limbs in 3 donkeys from the UK; fibrosing, eosinophilic, lymphoplasmacytic desmitis with osseous and cartilaginous metaplasia and intralesional coiled adult nematodes; not speciated beyond Onchocerca (Paraschou, Vet Pathol. 2021)
• O. volvulus: humans (Africa and Central America)
  • Common cause of blindness
  • Transmitted by simuliid flies
  • Microfilariae within all segments of eye à diffuse, sclerosing, superficial stromal keratitis à anterior uveitis with synechiae à eventual glaucoma à blindness

Other parasites that can cause ocular disease due to aberrant migration:

• Echinococcus – primates
• Cysticercus – pigs
• Elaeophora schneideri – elk
• Baylisascaris procyonis / other ascarids – many species
• Onchocerca cervicalis – horses (microfilaria cause the ocular lesions)
• Setaria – horses
• Diplostomum spathaceum – fish
• Various fly larvae (Gedoelstia, Cuterebra

References:

1. Cooper BJ, Valentine BA. Muscle and Tendon. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals, Vol 1, 6^th St. Louis, MO: Elsevier; 2016:247-248.
2. Cullen CL, Webb AA. Ocular manifestations of systemic disease. In: Gelatt KN, Gilger BC, Kern TJ, eds. Veterinary Ophthalmology. Vol 2. 5^th Ames, IA: Blackwell Publishing; 2013:1931.
3. Dubielzig RR, Ketring KL, McLellan GJ, Albert DM. Veterinary Ocular Pathology: A comparative review. St. Louis, MO: Saunders Elsevier; 2010:117-119.
4. Gardiner CH, Poynton SL. An Atlas of Metazoan Parasites in Animal Tissues. Washington DC: Armed Forces Institute of Pathology, American Registry of Pathology; 1999: 35-38.
5. Labelle P. The Eye. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 6^th St. Louis, MO: Elsevier; 2017: 1293-1294.
6. Paraschou G, Adako GM, Priestnall SL, Burden FA. Suspensory Ligament Desmitis Caused by Onchocerca in Three Donkeys. Vet Pathol. 2021;58(2):401-404.
7. Wilcock BP, Njaa BL. Special Senses. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals, Vol 1, 6^th St. Louis, MO: Elsevier; 2016: 425-426, 451-452, 478.

Click the slide to view.

---