A challenging case: A dog with ocular masses - Veterinary Medicine
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A challenging case: A dog with ocular masses
First thought to be trauma-related scarring, the lesions in this patient's eye stemmed from a somewhat more surprising cause.


VETERINARY MEDICINE


Ocular lesions may be the only clinical signs in dogs infected with this parasite. Canine ocular onchocerciasis has been identified worldwide. Forty-three cases were reported in Greece,5,10 five in Hungary,11,12 and seven in the United States.13-16 The morphologies of the parasites obtained from the ocular tissue samples from dogs from all these countries were similar, and it was thought that these parasites were all the same species.10,13,14,16,17 To confirm this, polymerase chain reaction was used to amplify the cytochrome oxidase gene from the microfilariae and the 16S ribosomal RNA gene from their Wolbachia endosymbiont. These gene sequences were identified and compared, and they appeared to be identical to the Onchocerca species isolated from the dogs in Greece and Hungary.10 The exact Onchocerca species infecting the eyes of dogs is under debate, but a recent study suggested that it is not an aberrant infection with Onchocerca lienalis (the parasite that infects cattle) but is Onchocerca lupi, a species first discovered in a wolf's eye.12,17,18 Other investigators think that, based on parasite morphology and geographic distribution, O. lienalis is most likely the species that infects dogs in the United States.16

Clinical signs of ocular onchocerciasis in dogs include periorbital swelling, exophthalmos, lacrimation, photophobia, conjunctival congestion, corneal edema, protrusion of the nictitating membrane, and subconjunctival granuloma or cyst formation.5,10 The mass lesions may be single or multiple.10 The results of complete blood counts and serum chemistry profiles of affected dogs may be normal or, as in the dog presented here, may reveal an absolute eosinophilia.10

Diagnosing ocular onchocerciasis in dogs is based on clinical signs and a history of travel to the western United States, Hungary, or Greece, where the disease is most commonly found. Interestingly, in the reported cases, German shepherds are over-represented.5 A definitive diagnosis of the disease is based on a histopathologic evaluation of the subconjunctival masses formed by the worms.15 These parasites induce an intense proliferative pyogranulomatous inflammation surrounding the nematodes.15 The Onchocerca species nematode is identified by its thick, multilayered cuticle and circumferential external ridges. It also possesses prominent hypodermal tissue between the cuticle and muscles.12,15

The mainstay of treatment for ocular onchocerciasis in dogs is the complete removal of the nematode and inflammatory masses and a follow-up anthelmintic protocol.5 In one study, researchers treated 23 reported cases with postoperative systemic and topical antibiotics for nine days after surgery.5 To achieve complete removal of possible remaining parasites, systemic anthelmintic treatment was begun on the ninth day after surgery. This treatment consisted of melarsomine (2.5 mg/kg intramuscularly) given once a day for two days and ivermectin (50 g/kg subcutaneously) given once one month after that.5 The dogs had marked edema of the periorbital tissues after melarsomine therapy, but the edema resolved in seven days. Lesions did not recur in any of the dogs after this protocol.5

Although uncommon, more and more cases of canine ocular onchocerciasis are being reported worldwide, especially in the western United States.15 Recent increases in the number of diagnosed cases in the United States may be due to an increased awareness of the disease, to increased exposure of dogs to an aberrant form of the disease that affects cattle, or to the establishment of a previously unidentified canine-specific species in the affected areas.10,11,15,17 Ocular onchocerciasis should be included in the differential diagnoses of subconjunctival masses, particularly in dogs that have been in California or Nevada or in Hungary or Greece. The prognosis appears to be good with appropriate therapy, and complete resolution is likely.

Juliet R. Gionfriddo, DVM, MS, DACVO
Brendan Mangan, DVM, MS
Department of Clinical Sciences
Greg Wilkerson, DVM
Department of Microbiology, Immunology, and Pathology
Cynthia C. Powell, DVM, MS, DACVO
Department of Clinical Sciences
College of Veterinary Medicine and Biomedical Sciences
Colorado State University
Fort Collins, CO 80523

Deborah S. Friedman, DVM, DACVO
Animal Eye Care
1612 Washington Blvd.
Fremont, CA 94539

E.J. Ehrhart, DVM, PhD, DACVP
Department of Microbiology, Immunology, and Pathology
College of Veterinary Medicine and Biomedical Sciences
Colorado State University
Fort Collins, CO 80523

REFERENCES

1. Chitwood M, Lichtenfels JR. Identification of parasitic metazoan in tissue sections. USDA. Beltsville, Md 1973.

2. World Health organization. Onchocerciasis (river blindness). Fact Sheet. No. 59 1990.


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Source: VETERINARY MEDICINE,
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