Toxocara canis is a commonly occurring roundworm (nematode) found in dogs. Encysted larva in the adult dog may reactivate during pregnancy and cross the placenta to infest their offspring. The ova are shed in the animal’s feces and humans become infected by ingesting contaminated soil or food. Very young children who consume infected soil are most at risk.
- Systemic toxocara infection is known as visceral larva migrans, where larvae migrate via the bloodstream leading to granulomatous reactions particularly in the liver, lungs and brain. Symptoms of this illness may include a cough and lymphadenopathy with the child being febrile, but it is usually mild and escapes recognition.
- Ocular toxocariasis is typically unilateral and may occur in the setting of visceral larva migrans, but is more often seen in older healthy children with no systemic signs or history, possibly due to their increased hygiene. Ocular involvement is usually in the form of granulomas and chronic inflammation simulating endophthalmitis, with a type of pars planitis, anterior uveitis, optic neuritis and neuroretinitis being less commonly seen.
Patients may be symptom free and the condition discovered on routine examination or they may complain of a unilateral loss of vision and flashes and floaters. The eye may appear quiet externally, even in the presence of active intraocular inflammation.
Active infection may manifest as unilateral iritis and vitritis or appear as a chronic endophthalmitis. A white pupil (leukocoria) and strabismus may be presenting signs.
Chronic infection commonly shows as a solitary inflammatory white granuloma either at the posterior pole or in the retinal periphery. Typically an elevated lesion about the size of the optic disc, a granuloma may be accompanied by tractional bands in the vitreous that can cause a significant distortion of the retina and its blood vessels leading to a marked loss of vision. Retinal detachment, hemorrhage, exudative reactions, chronic intraocular inflammation and phthisis bulbi are potential sequelae.
After the active infection has resolved, grey-white chorioretinal scars remain.
Ocular toxocariasis is an uncommon to rare disease, most often occurring in children.
Inflammatory conditions such as unilateral pars planitis and acute endophthalmitis of other etiologies. The causes of a white pupil include retinoblastoma, Coat’s disease, persistent hyperplastic primary vitreous, retinal astrocytoma, retinopathy of prematurity and congenital cataract.
A high enzyme-linked immunosorbent assay (ELISA) titre is indicative of toxocariasis in the appropriate clinical setting. If intensive vitritis is present, ultrasound and ultrasonographic biomicroscopy are useful diagnostic techniques.
Treatment with systemic or periocular steroids is usually undertaken in patients who have an active inflammation. In unresponsive cases and those where there is systemic involvement, antihelminthic preparations may be prescribed, although there is a risk of increased inflammation.
A pars plana vitrectomy may be helpful although the overall prognosis for vision is generally poor.
It is essential that family pets are regularly wormed.
Active acute granuloma, adjacent to the optic disc, associated with toxocara. Image courtesy of Dr Anthony Hall, Melbourne.
Healed toxocara, in the left eye of an 18 year old female, with profound loss of vision for as long as she could remember. There is a fibrous scar between macula and disc, and areas of chorioretinal atrophy and pigment migration at the posterior pole. The other eye was normal.