Case Report: Management of Dead Intraocular Helminth Parasites in Asymptomatic Patients

Sparganosis.

Spirometra species are globally distributed intestinal parasites of dogs, cats, and other mammals with a complex life cycle. Humans may get infected by 1) drinking water containing infected copepods (first intermediate host), 2) ingesting raw or undercooked flesh of a second intermediate host (e.g., frogs and snakes), or 3) applying flesh of a second intermediate host to wounds or mucous membranes. In the accidental human dead-end host, the larvae are capable of tissue invasive migration and may end up in any organ, including the eye. Spargana larvae may measure from millimeters up to 50 cm and remain viable up to several years (possibly decades).³ Mostly human infections present as subcutaneous nodules, and the diagnosis is only made after surgical resection.⁴ Drug treatment of sparganosis is restricted to cases where surgical removal of larvae is impossible, for example, in cerebral sparganosis. In such cases, antiparasitic treatment with praziquantel has shown to be effective.⁵ Ocular sparganosis is a rare entity primarily reported from Southeast Asia in the past, when applying poultice made of frog or snake meat on open wounds or sore eyes was a common practice in traditional medicine.⁶ This practice, allowing direct tissue invasion by migratory spargana larvae, has fortunately become rare. However, spargana larvae may also migrate to and invade the eye in patients who are orally infected.⁷ The source of infection in our patients is unclear. The prevalence and natural hosts of Spirometra species is currently not well described in Nepal. Because our patients did not report suggestive food habits or high-risk traditional medicine practices, we suspect that they acquired their parasites via unfiltered water containing copepods harboring infective procercoid larvae. Reviewing the literature on ocular sparganosis, we found only case reports describing symptomatic infections with considerable pathology requiring surgical intervention.

Angiostrongyliasis.

The rat lungworm (A. cantonensis) is a parasite whose main host is the rat in whose pulmonary arteries it lives. After hatching from their eggs in the rat’s lung, larvae are passed in the feces and infect slugs or snails which serve the parasite as intermediate hosts. Humans get infected by ingesting 1) raw or undercooked snails or slugs, 2) paratenic hosts (crabs, frogs, and freshwater shrimps), or 3) vegetables contaminated with infective larvae. In the accidental human dead-end host, the parasite’s larvae have a strong tropism for the central nervous system and are a worldwide leading cause of eosinophilic meningitis. Ocular invasion by an Angiostrongylus larva (2.6–12.6 mm in length) is reported in 1.1% of diagnosed angiostrongyliasis cases.⁸ Interestingly, we found no description of an ocular infection where more than one larva was found. Within the eye, the larva is primarily found in the vitreous cavity (39%), subretinal space (33%), and anterior chamber (22%).⁹ Live larvae in the subretinal space are treated by laser photocoagulation, whereas live intra-cameral and intra-vitreal larvae are treated by laser photocoagulation followed by surgical removal.⁹ Reviewing the literature on ocular angiostrongyliasis, we found only case reports describing symptomatic infection with considerable pathology requiring intervention. The only case report we identified describing the finding of a dead intraocular Angiostrongylus larva was a 20-month-old child who had just received antiparasitic treatment for Angiostrongylus-related eosinophilic meningitis when additionally unilateral retinal pathologies and a dead larva in the vitreous cavity were discovered.¹⁰

Why in our four cases no local signs of concomitant or past inflammation was present remains unclear. Considering that tissue invasive parasites will only be recognized and diagnosed once they have reached a detectable size (macroscopically or by imaging methods) and/or cause clinical apparent pathology, we speculate that infections with still small and prematurely dying parasites are more likely to remain asymptomatic or oligosymptomatic and thus undiagnosed. This assumption is supported by the small size of the plerozercoid larvae in our three cases when compared with the reported size of procercoid larvae resected from the periocular soft tissue.^11–13 In addition, once inside the eye, parasites find themselves in a relatively protected site from the host immune response.¹⁴

A major limitation of our study is that we can neither provide definitive proof of the diagnosis nor a species identification. A positive serological test could have supported the morphological diagnosis. However, serologies for tissue-invasive parasites are currently not available in Nepal, and in our experience, a negative serological test would not have refuted the diagnosis because following treatment or the natural death of a parasite seroreversion is more the rule than the exeption.

Our report is the first case series examining the role of observation, without intervention, in asymptomatic patients with reported dead intraocular helminth larvae not causing obvious pathology. We conclude that in asymptomatic patients, dead intraocular helminth larvae not causing apparent pathology do not require surgical removal and patients may be followed up under a watch-and-wait strategy.