TOXOCARIASIS

Ocular toxocariasis is a common parasitic infestation in children and young adults caused by Toxocara canis. A common ascarid of dogs, T canis is the most frequent cause of visceral larval migrans and ocular toxocariasis. Interestingly, both diseases rarely occur together.

Symptoms

Patients typically present with unilateral visual loss, strabismus, or leukokoria. There is no discomfort. A delay between onset of symptoms and diagnosis is common and may average 1 year.

Clinical Features

Toxocara endophthalmitis classically presents in one of three ways: (1) chronic endophthalmitis, (2) posterior pole “granuloma,” or (3) peripheral granulomatous inflammatory mass. Common to all presentations is a white and quiet eye. The degree of intraocular inflammation varies from mild, with a hazy vitreous, to severe, with granulomatous anterior uveitis, hypopyon, and dense vitreous membranes. It should be noted that the causative larva is 300 µm to 400 µm in size and cannot be seen clinically.

Ancillary Testing

In eyes with media opacification, ultrasonography is used to rule out other conditions in the differential diagnosis. An ultrasound can rule out short axial length in persistent hyperplastic primary vitreous, calcifications in retinoblastoma, and retinal detachment. If invasive testing is warranted, an anterior chamber tap can be used to test aqueous for enzyme-linked immunosorbent assay (ELISA) to T canis. When a hypopyon is aspirated, the presence of eosinophils is suggestive of the disease.

Pathology/Pathogenesis

Humans contract the disease primarily by ingestion of contaminated soil and not by direct contact with dogs. Infectious larvae migrate through the wall of the small intestine and are available for hematogenous spread to the eye. Damage to intraocular structures may occur from migration of the motile larva, toxicity due to secretory products of the worm, or the host inflammatory response. Histopathologically, the intraocular larvae incites an intense inflammatory reaction in which the larva is surrounded by eosinophils, mononuclear cells, histiocytes, epithelioid cells, and giant cells, forming a granuloma or abscess.

Treatment/Prognosis

Most cases of ocular toxocariasis present without significant inflammation or vitreoretinal pathology and require no treatment. Medical therapy is directed at controlling acute inflammation with local and systemic steroids. Because T canis larvae cannot replicate in humans, infection is not exacerbated by immune suppression. The benefit of antihelminthic agents is unproven. Vitreoretinal surgery can be used to manage complications such as vitreous opacification, retinal detachment, or epiretinal membrane. The visual prognosis largely depends on macular involvement and the potential for amblyopia.

Systemic Evaluation

Peripheral eosinophilia is rare in patients with ocular toxocariasis. A serum ELISA titer for T canis of 1:8 provides 90% specificity and 91% sensitivity and is highly suggestive (but not diagnostic) of ocular toxocariasis in the right clinical setting.

Toxocariasis may present with an inflammatory lesion in the posterior fundus. Peripapillary lesions are common. The associated vitritis may be mild or severe.

Peripheral Toxocara lesions are usually larger than the posterior lesions and often have fibrous extension with retinal folds extending to the posterior fundus.

Macular lesions may result in visual loss. There may be associated subretinal fluid and subretinal deposits.

This patient had an old inflammatory scar with a fibrous band extending to the optic disc. The differential diagnosis includes toxoplasmosis.

An unusual complication of ocular Toxocara infection is choroidal neovascularization. Note the grayish green subretinal lesion superotemporal to the optic disc.

Fluorescein angiogram of the same patient reveals the area of classic choroidal neovascularization.

TOXOPLASMOSIS

Infection with Toxoplasma gondii is congenital or acquired. Ocular toxoplasmosis is usually a recurrent manifestation of congenital disease. Toxoplasmosis is the most common protozoal eye infection and the most frequent cause of focal necrotizing retinitis in healthy adults.

Symptoms

Symptoms are present in more than 90% of patients with active Toxoplasma retinochoroiditis. Most patients report floaters and reduced central vision.

Clinical Features

Focal necrotizing retinitis of the inner retina presents as a unilateral white-yellow retinal lesion with overlying vitritis (classic “headlight in the fog”). If the retinitis involves primarily the outer retina, a serous neurosensory retinal detachment may be present. A hyperpigmented chorioretinal scar can often be seen adjacent to the lesion or in the fellow eye. Associated retinal findings are hemorrhage or localized vasculitis. In some patients (often immunocompromised individuals), acute retinal necrosis with diffuse toxoplasmosis retinitis may be seen. Associated ocular findings are papillitis, mild granulomatous iritis, and scleritis.

Ancillary Testing

Ancillary testing is usually not necessary as the diagnosis of acute Toxoplasma retinitis is very likely in patients who demonstrate a focal retinitis in one eye and one or more chorioretinal scars. A fluorescein angiogram will show staining in the area of retinitis and late hyperfluorescence of the optic nerve. Visual field testing may reveal large defects due to optic nerve involvement.

Pathology/Pathogenesis

Human infection occurs by ingestion of T gondii oocysts from cats (acquired) or by maternal transplacental spread if the mother is infected during pregnancy (congenital).

If the parasite reaches the eye, infection progresses from the retina to the choroid, producing a true retinochoroiditis. The host’s immune response controls the infection as tachyzoites give way to bradyzoites and encystment. Tissue cysts may remain dormant for years. Ultimately, cysts rupture, releasing organisms into surrounding retina, with a recurrence of necrotic retinochoroiditis. Histopathologically, the inflammatory response is mononuclear with lymphocytes, macrophages, epithelioid cells, and plasma cells.

Treatment/Prognosis

In immunocompetent hosts, ocular toxoplasmosis is a self-limited disease. However, most active lesions that are 2 to 3 mm from the disc or fovea, which threaten or affect vision, or extramacular lesions with severe vitritis are treated. Classically, therapy consists of pyrimethamine, sulfadiazine, folinic acid, and corticosteroids. Clindamycin and trimethoprim-sulfamethoxazole have also been used. Oral corticosteroids should never be given without antibiotics. Topical corticosteroids and cycloplegics are used for anterior uveitis. The prognosis is favorable for most patients.

Systemic Evaluation

The enzyme-linked immunosorbent assay test can be used to detect IgG or IgM anti-Toxoplasma antibodies. Because of a high prevalence of seropositivity in the population, this is not a diagnostic test. The diagnosis of ocular toxoplasmosis is still presumptive, based on

typical ocular findings in the setting of positive serology. It is important to rule out infection with human immunodeficiency virus as toxoplasmosis in patients with acquired immunodeficiency syndrome is a leading cause of death. Hematologic monitoring must be performed if treating with pyrimethamine.

Ocular toxoplasmosis is characterized by full-thickness retinitis, usually located adjacent to a chorioretinal scar from previous infection. It is one of the most common causes of posterior uveitis.

This pair of fundus photographs demonstrates the natural course of toxoplasmosis retinitis. There is active retinitis adjacent to pigmented chorioretinal scars.

The description “headlight in the fog” refers to the appearance of the active toxoplasmosis retinitis through dense vitritis. Toxoplasmosis retinitis may be associated with vasculitis, as seen in this patient.

This photograph of the same patient taken 1 year later reveals an atrophic scar with mild pigmentation in the area of previous infection.

Congenital toxoplasmosis often involves the macula. The chororetinal scars may be large and pigmented with prominent scleral show.

The fellow eye of the same patient reveals a focal area of active toxoplasmosis retinitis temporal to the macula.