Having older siblings and a dog that lives in the home could reduce the likelihood of infants developing egg allergies, according to a new study published in the latest edition of the journal Allergy.
Food allergy, along with its most concerning manifestation – anaphylaxis (a life-threatening allergic reaction) – is an increasingly common problem among children in many industrialised countries including Australia.
Egg allergy is the most common food allergy in infants and young children, affecting around 8% of 12-month-old infants in Melbourne. Although egg allergy itself often resolves, it’s a potential early marker for the later development of other allergic conditions such as asthma and allergic rhinitis (hay fever).
Testing the environment
In this study of over 5,000 infants, my colleagues and I investigated the role of a wide range of environmental and demographic factors have on the development of egg allergy. This included the number of older siblings, contact with other children during childcare, exposure to pets, caesarean delivery, infant diet, parents’ country of birth, family history of allergy and the use of antibiotics in infancy.
We recruited infants at 12 months of age from council-led immunisation programs across Melbourne. Every child underwent a skin prick test – a screening test to identify which infants were at risk of food allergy. About half of those testing positive are expected to be truly allergic.
Any child with a positive screening test then underwent an oral food challenge. This involves giving small amounts of the food that caused a reaction on the skin prick test in increasing doses in a medical setting and monitoring for any reactions. This is the gold standard for diagnosis of food allergy, particularly in those who have never eaten the food in question.
We found that infants with siblings and infants with a pet dog inside the home were less likely to be allergic to egg at one year of age. Of the infants with no siblings, 10.8% were allergic to egg, however as the number of siblings increased, the incidence of egg allergy decreased; the rate of egg allergies in infants who had three or more siblings was almost three-fold lower.
We also found a link to dog ownership, with infants in families with no dogs almost twice as likely to have an egg allergy than those who had a dog who was allowed inside the house. This protective effect of dog exposure was present even among those with no family history of allergic disease, suggesting the finding couldn’t be explained by allergic families choosing to avoid dogs.
The hygiene hypothesis
Our findings provide support for the hygiene hypothesis in the development of food allergy. The hygiene hypothesis has previously been implicated in other allergic diseases, particularly allergic rhinitis. However, few studies have investigated whether early life exposures to siblings and pets may play a role in the development of food allergy.
Broadly, the hygiene hypothesis is an evolving theory which states that the immune system needs to be exposed to appropriate stimulation during development so that it is “trained” to attack things that might cause us harm (such as bacterial or viral infections) while ignoring harmless things such as foods.
This stimulation could come from infections (by bacteria, viruses or parasites such as worms) or from harmless bacteria found in our environment.
So where do siblings and pets come in to this story?
Studies have shown that children who have siblings have more infections early in life, which might help to train the developing immune system. Having a pet dog may act in a similar way to prevent allergy. Environments with pets tend to have more endotoxins (toxins which are found in some bacteria), which are very effective at stimulating the immune system.
Interestingly, although we know that childcare also increases exposure to infections, this did not seem to protect against the development of food allergy. This might be because infants generally did not attend childcare in the first few months of life. Perhaps only exposures that take place very early on in life are protective.
So what would we advise parents to do to prevent food allergy?
Before we can answer that, there is still more work to be done. In particular, we need to identify which exposures are important in immune development.
Improved hygiene overall is a good thing – cleaner food and water supplies, for example, lead to a reduction in serious diseases. But ideally, we would like to identify some exposures that could prevent allergies without putting children at risk of infectious illnesses.
We have previously shown that later introduction of egg is strongly associated with risk of egg allergy. Combined with this knowledge, our new findings add to a growing body of evidence to identify possible causes of allergies.
The study was led by Professor Katrina Allen and a team of investigators from the Murdoch Childrens Research Institute and the University of Melbourne.