SCIENTISTS INVESTIGATING the health of Guide Dogs have made a startling discovery: sometimes the blind are leading the blind.
According to a new study, at least one in ten working guide dogs is seriously shortsighted. Some of the so-called seeing dogs have such poor vision that they would be prescribed glasses if they were human, researchers found.
However, the dogs' blurred vision appeared to make no difference to their day-to-day duties. The scientists, who carried out the tests on dogs in New Zealand, suspect that the animals compensated by relying more on smell and hearing.
Rates of myopia are unlikely to be as high in Britain, where young dogs are given eye tests, as part of a rigorous health screening programme, before beginning their training.
But because canine eyesight deteriorates with age, some older British guide dogs may be struggling to see properly.
The findings were presented last week at a conference in Cambridge. Dr John Phillips, of the University of Auckland, said: "The reason why we are interested in this is because you are providing a blind person with a surrogate visual system.
"If we gave them an electronic device you would want to know how well it worked. It seemed to us that there was a gap that needed to be looked at."
Past studies have suggested that around 15 per cent of Labradors are myopic.
Dr Phillips examined the eyesight of 61 dogs, including 39 Labradors, from the Royal New Zealand Foundation of the Blind Guide Dog centre.
Using a standard vision test in which a dog's eye is examined using an ophthalmoscope, they found around 11 per cent of the dogs were short-sighted, the scientists are due to report today at the International Myopia Conference.
One of the dogs has such bad vision that, if it were a child, it would be unable to see the largest letter on a standard sight test. "Some guide dogs guide adults and children through traffic," said Dr Phillips. "Dogs use hearing and smell to compensate, but that may not help them detect a car coming from the side."
Although the dogs were shortsighted, their owners had not reported any problems, as the dogs probably compensated for their own lack of visual input by the use of the acute senses of smell and hearing. Because examining dog eyesight is notorious difficult, the researchers also devised and tried out a new test.
To get the animal's attention, an image of a barking dog was projected on to a wall. Once the dog was staring at the right place, a moving target appeared on the wall. A video camera linked to a computer tracked the dog's eyes as they followed the object. The target was made of lines that became thinner and thinner. The computer recorded the point at which the dog could no longer see the target and worked out how shortsighted it was.
The test is also being adapted to detect eye problems in babies.
There are around 5,000 working guide dogs in Britain, all bred by the Guide Dogs for the Blind. Around 1,200 puppies are born each year to brood bitches chosen for intelligence and temperament. The charity said all breeding dogs were given annual eye inspections. Guide dogs also had eye tests 12 and 16 months before they entered service.
"This screening shows there is no familial myopia in the UK's stock of guide dogs," said Vicky Morley, a spokesman for the charity. "If any ailment caused problems with a dog's workability, we would have to consider retiring the animal.
"It is essential that they are completely alert to act as mobility aids for their visually impaired owners."