Not common. This parasitic infection can get into the brain (neurocysticercosis) . In many cases, people will be asymptomatic since the immune system will kill it. However, a few people will have seizures (from brain swelling), and blindness (inflammation from parasites in the eye), especially when treatment with an anti-parasitic is initiated. An evaluation by an ophthalmologist before tx is recommended.
Worms of eye. Cystocircosis is very rare in the us but more common in less developed world. It is the larval stage of various parasites that find their way into the eye's conjunctiva or retina and thus exert the effect on vision. During exam they can be seen to move away from the light. Treatment with anti helmiths " worm medicine" helps as does simple direct removal in case of surface of the eye.
Cysticercosis. Cysticercosis is a parasitic infestation with humans as an inappropriate host. The parasites can set up residence anywhere in the body although they prefer the liver and the cns. If they get into the eye, they set up a terrific inflammation which can destroy some or all of the vision. It is a statistical issue but many infected have no symptoms so the actual percentage is not known.