A disease carried by dogs that can harm eyesight in humans can be picked up by simply stroking the animal, say researchers.
Dogs may carry the worms in their coat
Toxocara canis is a worm that grows in the guts of dogs, and can be found in their faeces.
If the worm gets into the human body, it can threaten eyesight, and cause toxocariasis, an illness similar in symptoms to glandular fever.
Young children are considered particularly at risk from dog faeces deposited in public parks and playing areas.
It had been previously assumed that people were only at risk if they came into direct contact with faeces, prompting campaigns to encourage dog owners to clean up after their pets when they take them for walks.
Eggs in fur
However, research by a vet from Somerset, reports New Scientist magazine, suggests that even stroking and petting dogs could transfer the worms from dog to owner.
The vet, Ian Wright, looked at 60 dogs, and found a quarter had millimetre-long worm eggs in their fur, particularly in the area around the back of their legs.
Although only three of the 71 eggs they recovered contained mature embryos capable of infecting humans, embryos had begun to develop in a quarter of them.
The quantity of eggs found was equivalent to 180 per gram of dog hair - much higher than the amount found in a gram of soil.
Mr Wright said: "It has probably got very little to do with dogs fouling public places."
Toxocara canis is not a massive problem in the UK - there are fewer than 20 people a year infected despite the existence of millions of pet dogs.
This is partly due to more efforts at worm control in the UK compared to other countries which have higher rates.
Owners are urged to worm their dogs regularly and wash their hand before meals, and after petting the animal.
A spokesman for the Chartered Institute of Environmental Health reiterated this advice, saying that the finding, published in the journal The Veterinary Record, should be "kept in proportion".