What is Toxocariasis?
Ocular Toxocariasis is a rare infection from common roundworms Toxocara canis and Toxocara cati. It typically affects children and can lead to a significant loss of vision despite medical and surgical therapies. Toxocara canis and Toxocara cati are among the most widely distributed helminthic species in the world with a high zoonotic impact. Millions of people have been infected and hundreds of thousands are suffering from toxocariasis.
The disease has two major categorizations: visceral larva migrans (VLM) and ocular larva migrans (OLM). Toxocara antibody levels in the United States are estimated at 13.9%, but symptomatic infections are significantly less common, especially OLM.
- Ocular Toxocariasis is an infection caused by roundworms that results in unilateral vision loss.
- The roundworms live in dog and cat intestines and contact leads to infection.
- Proper hygiene and public awareness are key to prevention.
Toxocariasis can be found worldwide. The nematodes live and mature in dog or cat intestines. As a mature adult, the organism releases eggs which are passed in the stool. Human contact with infected materials leads to human infection. The clinical course in humans varies from asymptomatic infection to severe organ injury and depends on the parasite load, the sites of larval migration, and the host’s inflammatory response.
The clinical presentation can be classified into one of four forms:
- Posterior pole granuloma
- Peripheral granuloma
- Nematode endophthalmitis
- Atypical presentation
Risk Factors for Toxocariasis
Geophagia (deliberate consumption of earth, soil, or clay) at a young age, playing in sandboxes, and exposure to and ownership of puppies and kittens are risk factors for Toxocariasis. In puppies 2 to 6 months old, the prevalence of Toxocara Canis has been reported to be over 80%; however, in dogs older than one year this number drops to 20%. Water or food contaminated with the Toxocara eggs is another possible exposure.
The symptoms of toxocariasis usually present unilaterally and include:
- Reduced vision
- Leukocoria (white pupil)
An eye doctor will take a comprehensive history and perform a dilated eye exam. In the back of one eye, they may view vitritis which is common in 90% of patients. If the infection is acute, Toxocara retinochoroiditis appears as a hazy, ill-defined white lesion with overlying vitritis. As it resolves, the lesion becomes a more distinct, demarcated, elevated white mass ranging from one-half to four-disc diameters in size. Atypical presentations may include inflammation and swelling of the optic nerve head, motile subretinal larvae, and diffuse chorioretinitis. Conjunctivitis, keratitis, iridocyclitis, focal iris nodules, and cataracts can also be observed.
Management includes quieting the inflammation, eliminating the offending organism, and repairing vitreoretinal findings. Blood work like an ELISA may be useful for ocular toxocariasis. Several imaging modalities are now used to visualize the manifestations of larvae and assist in the diagnosis and management of this infection.
Medical therapy includes topical steroids to limit inflammation and prevent the development of tractional membranes and retinal detachments. Other options include periocular injections and oral corticosteroids at 0.5-1 mg/kg. In the case of anterior segment inflammation, cycloplegics are also used to prevent the formation of synechiae. The use of anti-parasitic therapy is unproven in the case of ocular toxocariasis. There is some support for the use of albendazole or thiabendazole to eradicate the organism.
The CDC reports that approximately 25% of new cases of ocular toxocariasis require surgery. Vitrectomy is the most common surgery performed for ocular toxocariasis. Individuals may need surgery for persistent vitreous opacification, hemorrhage, tractional retinal detachment, and epiretinal membranes.
Prevention of Toxocariasis
For the most part, the public is unaware of Toxocariasis. Increasing public awareness and reducing human exposure is the key to prevention. As the disease is more common in children, it’s important that they practice good hygiene, like handwashing after exposure to sandboxes, parks, and playgrounds. In adults, ingestion is most common, and an association has been found with eating raw meat especially raw cow liver.