An emerging ophthalmic parasite of the Southwestern USA (NM, CO, AZ, UT, NV and southern CA)
Steve Dugan, DVM, MS, Diplomate of the ACVO
Canine ocular parasites, although uncommon, do occur and the three most common are Thelaziasis, Dirofilariasis, and Onchocerciasis. Canine thelaziasis is caused by Thelazia callipaeda and Thelazia californiensis. The vectors for these thin, creamy white 1 cm threadlike nematodes are flies. The adult worms live within the conjunctival fornices and cause mechanical irritation exhibited by conjunctivitis and occasionally keratitis. Treatment consists of flushing and manually extracting those nematodes identified followed by ivermectin parenterally or topical imidacloprid with moxidectin or topical milbemycin.
Dirofilaria immitis is the most commonly reported canine intraocular parasite. Aberrantly migrated anterior chamber Dirofilaria fourth and fifth stage larvae (up to 13.9 mm in length) induced anterior uveitis via the release of parasitic metabolites, direct mechanical injury, and/or antigen-antibody reactions. Resolution of the anterior uveitis is rapid upon the successful surgical extraction of the worm(s) through a corneal or limbal incision.
Onchocerca species are filarial nematodes that have worldwide distribution and infect various ungulates including horses and cattle. Cases of ocular onchocerciasis in dogs have been reported predominantly in the western US and southern and central Europe. Onchocerca linealis, whose natural host is cattle, is considered the most likely species in the cases of American canine ocular onchocerciasis based upon parasite morphology and geographic distribution (western US). The life cycle of Onchocerca requires microfilarial maturation within an insect intermediate host, black flies or gnats. The mature larvae are transmitted to the definitive host during insect blood feeding, the larvae migrate to specific sites within the definitive host where the larvae mature. Adult Onchocerca species usually form nodules in deep subcutaneous tissues, tendons and ligaments. The adults mate and release microfilariae that migrate to the dermis and accumulate in the skin and occasionally the conjunctiva. The microfilariae are subsequently ingested by the biting insects and the cycle begins once again.
Canine ocular Onchocerciasis should be considered as a differential when the clinician is presented with a patient (German Shepherds are over-represented) originating from the western US with uni- or bilateral firm episcleral nodule(s) or possibly periorbital swelling with lacrimation, photophobia, corneal edema, and elevation of the nictitating membrane. Other differential considerations should include: immune-mediated nodular granulomatous episclerokeratitis (IMNGEK); immune-mediated nodular episcleritis, neoplasia (e.g., histiocytoma, hemangioma, and hemangiosarcoma); and, foreign body. Following a comprehensive ophthalmic examination, offering a complete blood profile (hemogram which may unveil an absolute eosinophilia, chemistry profile and heartworm) is indicated. Initial treatment may include topical and subconjunctival corticosteroids and doxycycline per os. Doxycycline has immunomodulatory effects and as such may assist in eliminating IMNGE. In addition, Onchocerca filarioids contain intracytoplasmic endosymbiotic bacteria of the order Rickettsiaceae (Wolbachia species) which are believed to be necessary for successful molting and reproduction. These Wolbachia endosymbionts, like other Rickettsiae bacteria, are susceptible to doxycycline and as such doxycycline may interrupt the parasite’s life cycle.
If a follow-up examination 10 to 14 days after the initiation of treatment fails to reveal improvement, then pursuing general anesthesia with an excisional biopsy of the ocular mass(es) if feasible and adjunctive cryotherapy is indicated. Removal of an episcleral mass may reveal long (1.4 cm), threadlike, coiled, white nematodes within granulomas. Histopathology definitively differentiates between immune-mediated disease, neoplasia, foreign body and parasitism. Ochocerciasis is characterized by worms surrounded by an intense pyogranulomatous inflammatory response that may contain a few to moderate numbers of eosinophils. Eosinophils function to kill parasites via direct contact.
Following a definitive diagnosis via histopathology, the most successful treatment regimen reported consists of prednisone @ 0.5 mg/kg per os BID for 10 to 14 days; doxycycline @ 5 mg/kg per os BID for 1 month; the adulticide melarsomine @ 2.5 mg/kg intramuscularly q 24 h for 2 days; and, ivermectin for its filaricide activity@ 50 micrograms/kg subcutaneously 1 month after surgery. Periorbital edema accompanied by intense pruritis have been reported to commonly occur a few days following melarsomine administration. This inflammatory response is likely an allergic reaction to the death of the remaining adult parasites. Anthelmintic treatment is considered necessary in all affected dogs in order to achieve a disease free state.
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