Contact lens cases are often contaminated with Acanthamoeba which cannot be killed by normal contact lens solution, according to a new Spanish study.

Published in the Journal of Medical Microbiology, the study report describes Acanthamoeba as one of the most common types of protozoa in soil, which is often found in fresh water.

The researchers behind the study say that most species eat bacteria, and some can cause infections in humans.

They have revealed that one of the diseases caused by Acanthamoeba is called amoebic keratitis, an infection of the eye which is very painful and can cause blindness.

According to them, about 85 per cent of all amoebic keratitis cases occur in people who wear contact lenses.

The team say that people who wear lenses while swimming or use tap water to rinse their lenses have an increased risk of infection, as amoeba is usually present in chlorinated swimming pools and domestic tap water.

"The prevalence of this infection has risen in the past twenty years worldwide, mainly because more people are wearing contact lenses," said Dr Basilio Valladares from the University Institute of Tropical Diseases and Public Health of the Canary Islands, University of La Laguna.

"When people rinse their contact lens cases in tap water, they become contaminated with amoebae that feed on bacteria. They are then transferred onto the lenses and can live between the contact lens and the eye. This is particularly worrying because commercial contact lens solutions do not kill the amoebae," the researcher added.