Thursday, May 01, 2008

Are dogs a vaccine against allergies?

The study reported below looks pretty sound but it is a typical example of how medical research tends to be conducted in its own little bubble without reference to other evidence on the question examined. There could, for instance, be no group that disconfirms the "dirty environment" hypothesis more strikingly than Australian Aborigines. They commonly live in appallingly dirty environments that shock outsiders and they also live in VERY close contact with dogs.

So what is their incidence of autoimmune diseases? Is it low? Far from it. We read, for instance: "Contrary to popular belief, Indigenous Australians are more likely to have asthma than non-Indigenous Australians. This difference exists across all age groups but it is most pronounced in older adults, especially women aged over 35 in whom the prevalence for Indigenous Australians is double that for non-Indigenous Australians". Beat that! Another great theory stubs its toe on pesky facts

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.


Sun lamps help unborn babies beat osteoporosis?

This is epidemiology again but it has a reasonable basis in theory. What else characterizes women who give birth at the most favourable time should be investigated, however

Women due to give birth in winter should use a sun lamp during the final three months of pregnancy to protect their child from osteoporosis in later life, doctors have suggested. They made their recommendation as research found that children born to mothers whose final three months of pregnancy included a summer month were 40% less likely to suffer the bone-wasting condition in adult-hood. A mother's exposure to sunlight in that final period ensures the developing baby receives enough vitamin D to form strong bones.

Doctors suggest that women whose last trimester of pregnancy does not fall between May and September should consider taking a holiday in the Mediterranean. As flying is not advised in the late stages of pregnancy, however, they suggest that women may need to settle for a sun lamp or vitamin D supplements.

Dr Marwan Bukhari, a consultant rheumatologist at the Royal Lancaster Infirmary and author of the study presented to the British Society for Rheumatology, said: "You only get good sunlight [when you make vitamin D] between May and September in this country. Pregnant women should have vitamin D supplements or should have lots of good sunshine in somewhere like north Africa or the southern Mediterranean [in winter]." Bukhari added: "Sun lamps are an option. It needs to be the right kind of sun lamp to convert fat under the skin to vitamin D." The doctors are not recommending sunbeds, which give a far higher dose of ultraviolet light than lamps.

Bukhari and colleagues studied 17,000 patients, mostly women and 95% of whom were white. They had all had scans carried out at the Royal Lancaster Infirmary between 1992 and 2004. They found that patients under 50 were 40% less likely to have developed osteoporosis if their mother's last trimester of pregnancy included a summer month. Older patients were 20%-40% less likely to have osteoporosis if their mothers' late stages of pregnancy were in the summer.

The study will revive the debate over whether excessive caution about exposure to sunshine is creating other health problems. Michael Holick, professor of medicine at Boston University in America, said a lack of vitamin D, caused by overzealous avoidance of the sun, was leading to thousands of unnecessary cancer deaths each year and increasing vulnerability to rickets. Bukhari said: "You could get skin cancer from a sun lamp but not if you use a judicious amount. An hour a month will not give you skin cancer."


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