While dustmites have long been cited as the cause of childhood allergies, new research evidence suggests that early exposure to bacteria in house dust is protective, not damaging. The results of the study throw into question all the conventional wisdom of how to 'protect' babies and, indeed, suggest that the aggressive hygiene measures of modern societies may be partly responsible for the increase in atopic diseases.

Investigators analysed the housedust from the homes of 61 babies aged 9-24 months, all of whom had had at least three episodes of wheezing and were considered to be at risk of developing asthma. The housedust samples from the house holds whose children had asthma had significantly less endotoxin than the dust from healthy babies' homes. The more endotoxin they found in the housedust, the more immune activity was identified in the baby's blood (Lancet, 2000; 35: 1680-3).Endotoxin is a component of the cell wall of certain bacteria that may cause protective changes in the infant's immune system. Likewise, a recent Italian study found that respiratory allergy was less frequent in people heavily exposed to orofaecal and foodborne microbes, such as Helicobacter pylori, Toxoplasma gondii and hepatitis A virus. These pathogens, say the researchers, stimulate lymphoid tissue in the gut to produce chemicals which fight atopy. Moreover, these organisms need not cause disease to exert a protective effect (BMJ, 2000; 320: 412-7).

These findings call into question the sterilisation, using antibacterial soaps and keeping children away from sick friends. However, the one proviso is that early exposure to bacteria is most likely to be health enhancing when the child's health is well supported in other ways. This may include breastfeeding, since early exposure to cow's milk predisposes a child to atopy (BMJ, 1999; 319: 815-9), and a staged diet that does not introduce other known food allergens, such as cow's milk and wheat, too early in life.