Human Ophthalmomyiasis Interna Caused by Hypoderma tarandi, Northern Canada

Human myiasis caused by bot flies of nonhuman animals is rare but may be increasing. The treatment of choice is laser photocoagulation or vitrectomy with larva removal and intraocular steroids. Ophthalmomyiasis caused by Hypoderma spp. should be recognized as a potentially reversible cause of vision loss.

by Hypoderma spp. is not known. Eyebrows and eyelashes have been suggested as possible targets for oviposition (3). Oviposition on human scalp hair has been achieved experimentally and could be the preferential site in humans (3). An alternative explanation is transfer of the larvae directly from the guard hairs of the caribou to the human eye or skin through close contact with animal pelts. The parasite does not appear to complete its life cycle in humans (1,2).
We present the fi rst, to our knowledge, 2 published cases of ophthalmomyiasis interna caused by H. tarandi in Canada. Furthermore, we present the fi rst published use of Hypoderma spp. serologic testing to assist in the diagnosis of myiasis in humans.

The Cases and Literature Review
The fi rst patient was a 41-year-old woman from Rankin Inlet, Nunavut, Canada, who noticed fl oaters (objects in the fi eld of vision that originate in the vitreous) in her right eye in August 2006. Initial funduscopic examination showed posterior vitreous detachment. Two weeks later, her vision was more impaired; repeat funduscopy showed panuveitis. Pretreatment blood count was within normal limits with no eosinophilia. Topical steroids were ineffective. At a third assessment, her visual acuity was 20/400; funduscopic examination showed an intraocular larva ( Figure 1). The parasite appeared to recede behind the retina and could not be photocoagulated. A pars plana vitrectomy and intraocular laser treatment of the entry and exit sites were performed. Triamcinolone (0.4 mg) was administered for intraocular infl ammation, and antibiotics were given as prophylaxis. The larva was not recovered. Postoperative magnetic resonance imaging demonstrated no parasite or abnormality. The larva was assumed to be that of H. tarandi because of its appearance (shape, size, and segments), the late summer timing, and the patient's residence in the subArctic. Serologic testing for H. tarandi by Western blot, as described by Baron and Colwell,was performed (6). Six weeks after symptom onset, serum was positive for immunoglobulin (Ig) G, IgE, and IgM to hypodermin C, a larval collagenase of H. tarandi (7). No seroreactivity to the hypodermins A and B of H. lineatum was observed. Because the larva was unrecoverable, the patient was treated with 1 oral dose (9 mg) of ivermectin (Merck & Co., Kirkland, Quebec, Canada) 1 week after vitrectomy. After 6 months, her visual acuity had improved to 20/30. The second patient was an 11-year-old Inuit boy from Chesterfi eld Inlet, Nunavut, Canada, who was examined in October 1997 because of a painful right eye, scleral redness, and blurry vision. Examination found uveitis and glaucoma. Topical steroid therapy was begun, after which retinal detachment, hemorrhages, and a 4-mm larva in the subretinal space were noted. Blood work showed marginal eosinophilia; computed tomographic examination of the We searched the literature, using PubMed, for the terms "ophthalmomyiasis" with limits "human" and "English." We also reviewed references of selected publications. We reviewed only cases of ophthalmomyiasis interna caused by oestrid fl ies found in North America, which were confi rmed by visible larvae (online Appendix Table, available from www.cdc.gov/EID/content/14/1/64-appT.htm). Demographics and clinical presentation of patients are in the Table. For statistical analysis, outcomes were separated into good (vision unchanged from baseline or better than 20/80), moderate (able to see shapes or fi ngers or vision worse than 20/80 but not blind), and severe (able only to see light or movement, completely blind or enucleated). Although moderate or severe vision loss appeared to occur more commonly with Hypoderma than Cuterebra infestations (53.3% vs 14.3%), this frequency was not statistically signifi cant (p = 0.08). Photocoagulation appeared  to produce better outcomes (80% good outcomes) than no intervention (73.7% good outcomes) or surgical removal/ vitrectomy (41.2% good outcomes); however, χ 2 analysis showed no statistically signifi cant difference (p = 0.121). This observation may suggest that by the time the patient seeks treatment, damage to ocular structures has already occurred. Recent cases have been treated with photocoagulation or vitrectomy and intraocular steroid administration. Although no specifi c intervention is associated with better outcomes, given the diffi culty in studying outcomes in this rare condition and the success of this therapy for other foreign bodies in the eye, this course of action is reasonable. Ivermectin as medical therapy for ophthalmomyiasis interna has not been reported, but it is effective as therapy and prophylaxis for bot fl y infestation of livestock (9). Ivermectin as adjunctive therapy in select cases of ophthalmomyiasis may be of benefi t, but evidence is lacking. Only 1 case of ophthalmomyiasis externa caused by D. hominis and treated with ivermectin has been reported (10).

Conclusions
Human myiasis caused by bot fl ies of nonhuman animals is rarely reported. However, diagnoses may increase as a result of increasing population in northern latitudes, encroachment into habitats with natural hosts like caribou, and better access to ophthalmologists. The treatment of choice seems to be laser photocoagulation or vitrectomy with larva removal and coadministration of intraocular steroids. In areas where Hypoderma spp. are common, health-care providers should consider this condition and promptly refer patients to an ophthalmologist. P.R.S.L.-W. received honoraria from Sanofi -Pasteur for providing educational sessions.
Dr Lagacé-Wiens is a fellow in the department of Medical Microbiology and Infectious Diseases at the University of Manitoba in Winnipeg, Canada. His interests are parasitic and tropical diseases, antimicrobial drug resistance, and infectious diseases.