Ocular toxocariasis, though rare, causes blindness in seven out of 10 of those affected by it, typically children, according to a national survey conducted by the CDC.

The agency identified 68 cases during a one-year period, and of the 30 for whom clinical data were available, vision loss occurred in 25 patients (83%) and was permanent in 17 (68%), the agency reported in the June 10 issue of Morbidity and Mortality Weekly Report.

Toxocariasis can occur following the ingestion of embryonated eggs of Toxocara canis and Toxocara cati -- the roundworms that occur in dogs and cats.
These eggs can be found in the faeces of infected animals and are un-embryonated when first shed.
The eggs become viable embryos after about two to four weeks in the environment -- usually in dirt or sand. If ingested at this point, the larvae can spread through the body and cause systemic reactions. Migration to the eye causes ocular toxocariasis.
In an editorial note, the agency emphasized that most cases of Toxocara infection result from ingestion of soil, such as in playgrounds and sandboxes, that has been contaminated by dog or cat feces.
Among the 68 cases identified in the CDC report, which was based on surveys sent to more than 3,000 ophthalmologists, demographic information was available for 44.
Patients' median age was 8½, slightly more than half were male, and the majority were white.

Advice from UK NHS site…

Toxocariasis

Toxocariasis is an infection caused by small worms found in the poo of some dogs, cats and foxes. It's rare but can sometimes cause serious problems.
1. How you get toxocariasis
Toxocariasis is caused by worms found in some dog, cat and fox poo. Animals with these worms in their digestive system can pass out worm eggs in their poo. You can become infected if poo, soil or sand containing these eggs gets in your mouth. Toxocariasis is not caught from fresh poo. It's usually caught from soil or sand that has contained worm eggs for a few weeks or months. Toxocariasis cannot be spread between people.
2. Is toxocariasis serious?
Most people who swallow the worm eggs that cause toxocariasis do not become ill. The worms usually die or pass out in your poo within a few months without you noticing. Rarely, the worms can spread to parts of the body such as the liver, lungs or eyes. This can lead to serious problems like difficulty breathing or loss of vision.
3. Symptoms of toxocariasis
Toxocariasis only causes symptoms if the worms spread in your body. Depending on where the worms spread, symptoms can include:
• a high temperature
• coughing or wheezing
• stomach ache
• an itchy rash
• eye pain or redness
• changes to your sight, such as seeing small dots or lines (floaters) or flashes of light
• loss of vision (usually in 1 eye)