Purpose: Several nematodes have been postulated as etiologic agents in diffuse unilateral subacute neuroretinitis (DUSN), but the cause of this condition remains uncertain. The authors report the first case of DUSN from the western United States (northern California), along with morphometric, serologic, and epidemiologic evidence supporting Baylisascaris procyonis as its cause. Methods: One patient was examined and evaluated for disc edema and transient obscurations of vision. A diagnosis of DUSN was confirmed when a motile nematode was identified within the substance of the patient's retina. Morphometric analysis of the nematode was done from projected fundus photographs. Serologic evidence of Baylisascaris infection was suggested by Western blot analysis. A necropsy was done on 12 raccoons from the area. They were examined for evidence of Baylisascaris infection, previously believed to be nonendemic in the region. Results: The intraocular nematode measured 1727 X 67 µm, most consistent with Baylisascaris. The patient had considerable exposure to raccoons, and was seropositive for B. procyonis infection on Western blot analysis. Necropsy evaluation showed B. procyonis infection in 8 of 12 raccoons examined from the area. Conclusions: The morphometric, serologic, and epidemiologic findings in this case provide evidence that the raccoon ascarid, B. procyonis, is a cause of the large nematode variant of DUSN.
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