Ocular toxocariasis is a rare infection caused by roundworms, Toxocara canis and Toxocara cati. It was first recognized to be associated with dogs in the 1940s. It typically affects children and can lead to profound monocular loss of vision despite known medical and surgical therapies. Its prevalence has been estimated in certain populations and found to be rare. Presentations typically include posterior uvietis with symptoms and signs such as reduced vision, photophobia, floaters, leukocoria, and retinal granulomatous disease. Management includes quieting inflammation, eliminating the offending organism, and repairing vitreoretinal sequelae. Prognosis is often correlated to presentation and the degree to which sequelae are present. Vision typically ranges from 20/40 to 20/400 depending on these factors.
Toxocariasis ICD9 128.0. This article is specific to the ophthalmic variant, ocular larva migrans (OLM).
Toxocariasis is a zoonosis, which results from infection with common roundworms Toxocara canis and Toxocara cati. It exists as two major categorizations, visceral larva migans (VLM) and ocular larva migrans (OLM). Although seroprevalence as measured by Toxocara antibody levels in the United States has been estimated at 13.9%, symptomatic infection is significantly less common, especially OLM.
Prevalence and Incidence
The prevelance of this disease has been measured in certain subpopulations as well as the general population. In a North Californian uveitic population, 1% of patients were found to have ocular toxocariasis. According to Morbidity and Mortality Weekly Report released by the CDC, from September 2009 until September 2010 there were 68 new cases of ocular toxocariasis in the country with an emphasis of new cases from the South (57%).
There is a concern of underreporting and misdiagnosis. One study performed at a large academic institution found that 26% of misdiagnosed retinoblastomas were later identified as toxocariasis.
Typically, this disease manifests in children and early adolescents but 23 case studies found afflicted patients ranging from 8 to 45 years of age. In 2010 the CDC reported an average age in the United States of 8.1 years with a range from 1 to 60 years of age.
Infection by roundworms, Toxocara canis or Toxocara cati. These nematodes live and mature in dog or cat intestine, respectively. As a mature adult the organism releases eggs which are passed in the stool. Contact with infected materials leads to human infection.
Geophagia, exposure to and ownership of puppies and kittens (specifically young dogs and cats). In puppies 2 to 6 months old the prevalence of Toxocara canis has been reported to be over 80%. In dogs older than one year this number drops to 20%.
In humans, after contact with infected fecal material, the roundworm migrates throughout various organs but is unable to sexually mature. Reactive inflammatory processes lead to the organism's encapsulation. Ocular larva migrans is a result of the ingested egg developing and migrating to the eye causing local disease.
Avoidance of exposure to risk factors. By some studies, over 90% of patients have an identifiable history of exposure.
The majority of patients will either own or have exposure to a dog or cat. Patients without this history likely were unknowningly in contact with contaminated surfaces or are reported as geophagic by their parents. The vast majority of patients report unilateral reduced vision as well as other symptoms typical of uveitis, such as photophobia.
OLM is unilateral in 90% of cases. Ocular toxocariasis typically presents as posterior uvietis, namely, granulomatous disease of the peripheral (up to 50%) or central (up to 25%) retina. Other common presentations include endophthalmitis (up to 25%) and intermediate uveitis. It is also rarely associated with granulomas of the optic disc.
The most common sign associated with ocular toxocariasis is vitritis as it is identified in over 90% of patient. Other presenting signs include leukocoria, ocular injection, and strabismus.
Decreased vision, pain, photophobia, and floaters.
OLM is largely a clinical diagnosis based on history and examination as described in this article.
Unlike in VLM, OLM patients do not typically have marked eosinophilia. The most useful test for ocular toxacariasis is an ELISA.
ELISA for Toxocara execretory secretory (TES) antigen has a 90% sensitivity and specificity of VLM. OLM is not as easily detectable in peripheral blood tests and so this test is supportive but not the gold standard of diagnosis in ocular presentations. Specificity is lowered due to cross reaction with other helminthic infections. In addition, the value of this test is compromised secondary to the high prevalence of seropositivity to Toxocara without symptomatic infection in the general population. In one study this ELISA was found positive in 50% of patients while negative in 36.4% of patients and unknown in 13.6% of patients (N=22). In particular, one patient in this study with a negative serum ELISA was later found to have a positive aqueous ELISA.
- Retinoblastoma (in this case B scans typically find calcifications which are extremely uncommon in OLM. Noninflamed eyes without cataract are also suggestive of Retinoblastoma.)
- Coats Disease
- Persistent Fetal Vasculature
- Retinopathy of Prematurity
- Familial Exudative Vitreoretinopathy
- Idiopathic Peripheral Uveoretinitis
- Optic Neuritis
Management of this disease focuses on three main points:
- Mminimizing inflammation
- Eliminating the offending organism
- Addressing vitreal and retinal complications secondary to infection
Topical steroids are typically used to limit inflammation in order to prevent the development of trational membranes and resulting retinal detachments. Other options include periocular injections and oral medication. In the case of anterior segment inflammation, cylcoplegics are also used to prevent the formation of synechiae.
The use of these drugs is unproven in the case of ocular toxocariasis. There is some support for the use of albendazole or thiabendazole to eradicate the organism. Albendazole is the preference of some physicians as it has increased blood brain barrier penetration.
Medical follow up
There is no standard protocol for medical follow up specific to ocular toxocariasis. Patients are typically followed as those with other forms of uveitic disease.
The CDC reports that 25% of patients presenting with new cases of ocular toxocariasis require surgery. Vitrectomy is the most common surgical therapy for ocular toxocariasis. The most common indications for surgical intervention are persistent vitreous opacification, hemorrhage, tractional retinal detachment, and epiretinal membranes. Other indications for retinal surgery include rhegmatogenous retinal detachments. Tractional retinal detachments are more common in cases that include granulomas located in the peripheral retina.
Surgical follow up
There is no established protocol for surgical follow up of toxocariasis patients. In one study, 87% of patients requiring surgery had visual acuities worse than 20/400 with a mean age at the time of surgery being 8.1 years. This limited functional success was found despite each surgery being deemed an anatomical success with rare complications.
A list of sequelae of this disease more commonly includes cystoid macular edema, traction retinal detachment, epiretinal membrane, and cataract. With more infrequency OLM has also been associated with retrolenticular membrane, rhegmatogenous retinal detachment, anterior lens capsule membrane, and macular holes.
Prognosis is typically excellent for those patients without sequelae. These cases are mostly self limited or controlled with medical management. It has been identified that median visual acuity depends on presentation. A median visual acuity of 20/50 has been associated with patient presentations with posterior pole granulomas. Patients with peripheral granulomas had a median visual acuity of 20/70 and presentations with endophalmitis were associated with a median visual acuity between 20/200 and 20/400.
Those that require surgical intervention typically have a poorer prognosis and visual acuity. In very rare cases ocular toxocara has been associated with no light perception.
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