The First South American Case of Diffuse Unilateral Subacute Neuroretinitis Caused by a Large Nematode

Diffuse unilateral subacute neuroretinitis (DUSN) is characterized by early visual complaints, vitritis, papillitis, and recurrence of evanescent gray-white outer retinal lesions and later by progressive visual loss, optic atrophy, retinal vessel narrowing, and diffuse retinal pigment epithelium (RPE) degeneration occurring unilaterally in healthy patients.¹ Classically, a motile, white, glistening nematode may be found during any disease stage, and should be suspected even in patients with advanced optic atrophy and degenerative RPE changes. At least 2 unidentified nematodes are associated with the syndrome. In endemic areas of the southeastern United States and South America, the nematode is 400 to 700 µm; in the northern midwestern United States and Germany the nematode is 1500 to 2000 µm.^1-6 We report the first South American case of DUSN caused by the larger nematode.

A healthy 15-year-old boy with a history of amblyopia in the right eye had acute visual loss in his left eye in June 1995. Visual acuity was 20/40 OS and 20/200 OD. Biomicroscopy revealed a normal anterior segment and fundus in the right eye and trace cells in the anterior chamber and anterior vitreous of the left eye. Signs of mild vitritis and papillitis associated with diffuse RPE alterations were present in the left eye. Early-stage DUSN was diagnosed, and argon laser treatment was applied to the superior temporal retina where a small worm was presumed to be present. After 4 weeks of laser treatment, visual acuity was 20/20 OS and the ocular inflammation had lessened. The patient returned 2 years later with severe visual loss in the left eye. Visual acuity was counting fingers and an afferent pupillary defect was observed. Examination through a slightly cloudy vitreous showed a 1500- to 2000-µm, white, motile nematode in the nasal retina (Figure 1). The optic nerve was pale and narrowing of the retinal vessels associated with evidence of more RPE involvement was observed. Despite no history of exposure to raccoons, a similar second-stage larvae of Baylisascaris procyonis was suggested as a probable cause of DUSN. The patient had no other systemic complaints, and the unidentified large worm was destroyed by strong photocoagulation (Figure 2). Visual acuity improved to 20/200 OS and had not changed at the final examination (August 1998) (Figure 3).

In Brazil, DUSN is increasingly considered an important cause of posterior uveitis in children and young healthy adults. Most patients are unaware of the disease until ocular examination performed in school or an ocular examination later in life.^2,3 The typical late signs of DUSN predominate in these cases, and a solitary 400- to 700-µm nematode is frequently present at this stage, even years after disease onset. According to Gass,¹ laser treatment of the nematode at any disease stage may improve visual acuity and inflammatory ocular signs. However, in our experience this improvement may be temporary, even if laser is applied to retinal areas simulating the presence of a worm. It is possible that some laser response in the RPE may interfere temporarily with the activity of the subretinal worm.

In 1984, Kazacos et al⁴ showed that at least some DUSN cases are caused by B procyonis (Nematoda, family Ascarididae larvae), which are common intestinal roundworms of lower carnivores, including raccoons and skunks. Those authors experimentally produced DUSN in primates that were fed B procyonis eggs.⁶ Additionally, the size of the intraretinal larvae and previous patients' contact with raccoons made the hypothesis that B procyonis was the probable cause of the disease even stronger. In their opinion, DUSN is caused by 2 species of nematodes or 2 sizes of a single species, reflecting different ages of larvae.^4,5 The latter seems to apply to our patient. Unlike Toxocara species, which do not exceed 700 µm, Baylisascaris larvae grow considerably, from about 300 µm at hatching to 2000 µm or larger, the size most frequently recovered from clinically affected animals.⁴ Adult B procyonis worms may also infect rats, squirrels, and dogs. Humans and several other animals are potential intermediate hosts and become infected by ingesting B procyonis eggs from raccoon feces. The larvae hatch in the small intestine, enter the systemic circulation, and are distributed to various organs, including the eye.^4,5 In humans, B procyonis may cause visceral larva migrans, cerebrospinal nematodiasis, and ocular larva migrans. Fatal cases of B procyonis larva migrans have been reported.^1,4,5 Although Brazil is not an area endemic for raccoons, and cases of B procyonis ocular infection have not been reported in South America, we believe that our case could be the first. The presence of skunks in the peridomiciliary area of our patient makes this possibility likely.^4,5 Dogs and rats should also be considered potential sources of infection. It is important to emphasize that other species of nematodes should be considered as potential candidates for the cause of our patient's symptoms. As more local clinicians and veterinarians become aware of these larger ocular nematode infections, other important epidemiologic findings will be reported.

Reprints: Arnaldo Cialdini, MD, Avenue T2, Number 401, Alto Setor Bueno, CEP 74210-010, Goiânia, Goiás, Brazil (e-mail: retina@cbco.com.br).