Children run less risk of being sensitive to allergens if there is a dog in the house in the early years of their lives, scientists have found.

The conclusion, based on a six-year study of 9,000 children, adds weight to the theory that growing up with a pet trains the immune system to be less sensitive to potential triggers for allergies such as asthma, eczema and hay fever.

The “hygiene theory” of allergy holds that modern life has simply become too clean, meaning that babies’ immune systems are not exposed to enough germs to develop normally.

Having a dog provides enough dirt of the right kind, the new German study suggests. But it may be important that baby meets dog early enough to affect the immune system as it develops. “Our results show clearly that the presence of a dog in the home during subjects’ infancy is associated with a significantly low level of sensitisation to pollens and inhaled allergens,” said Joachim Heinrich of the National Research Centre for Environmental Health in Munich.

The same protective effect was not seen in children who had frequent contact with dogs but none at home.

Previous studies have suggested that exposure to pets may have a protective effect against allergies but many of these studies were based on retrospective questioning of subjects about their exposure.

The new study did not require anybody to remember anything. The children were followed from birth to the age of six. This is likely to make for more reliable results.

In the European Respiratory Journal, Professor Heinrich and colleagues say that the blood of children raised in households with dogs contained fewer markers for allergy, such as antibodies to pollen, house dust mites, cat and dog dander, and mould spores. But actual experience was rather less encouraging. Those children raised alongside a dog were no less likely to develop asthma or other allergies than were the other children. So while their blood samples suggested they were not susceptible, their experience suggested they were.

“It is not crystal clear why this is so,” Professor Heinrich said. He hopes that the protective effect may show up later in life and is continuing to follow the children’s progress. Further assessments will be made when they reach the age of 10.

In the meantime, he does not recommend that parents get a puppy. “Until we understand the mechanisms underlying this protective effect from dogs, we will not be able to draw any further conclusions or make any recommendations,” Dr Heinrich said.

Doctors who specialise in allergy have found advising parents difficult. Where children already have allergies, cats and dogs tend to make them worse by exposing them to allergens from the pets’ coats.

But more recent evidence has tended to show that early exposure to cats, dogs, and to farm animals is neutral or even protective. Children raised on farms appear to be protected against all sorts of allergens, not just those produced by farm animals.

Other studies similar in design to Professor Heinrich’s have produced equivocal findings. Some suggest early exposure to cats increases the risk, others that it diminishes it. Yet others find no effect one way or the other.

But one study published in the Journal of Allergy and Clinical Immunology in 2002 found that asthma symptoms were reduced in homes that owned a dog, and probably also in those that owned a cat.

Dr Guy Marks, of the Institute for Respiratory Medicine in New South Wales, concluded in 2002 that parents should neither be advised to rid their homes of pets, nor acquire them as a prevention against asthma. Further research was needed, he concluded.

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