Growing up with a dog 'reduces childhood asthma risk'

Behind the Headlines

Tuesday November 3 2015

The study's findings seem to support the hygiene hypothesis

Exposure to animals may protect against allergies

"Children who grow up with a pet dog in the family home have a lower risk of developing asthma," The Times reports. 

A large Swedish study found an association between pet ownership and reduced risk of asthma. Living on a farm was also found to reduce this risk.

The study found exposure to dogs reduced the risk of both preschool (by 10%) and school-age children having asthma by 13%. And living on a farm as a child – not just visiting one – also appeared to reduce asthma risk by an estimated 31% for preschool children and 52% for school-age children.

Some commentators have argued these results add weight to what is known as the hygiene hypothesis. This is the idea that children who grow up in sterile environments have reduced exposure to infectious agents, such as those carried by dogs, so they have an underdeveloped immune system. This may then make them more vulnerable to allergic conditions such as asthma.

However, one of this study's limitations is the findings can only highlight a potential link: it cannot categorically prove living with or around animals reduces the risk of childhood asthma.

The research has attempted to adjust for various potential confounders, including parental asthma, but other factors may still have had an influence.

One proven way to reduce the risk of childhood asthma is to never expose your children to tobacco smoke (secondhand smoke) both during pregnancy and when they are growing up.

Where did the story come from?

The study was carried out by researchers from Uppsala University in Sweden.

It was funded by the Swedish Research Council, Stockholm County Council, the Strategic Research Program in Epidemiology at Karolinska Institutet, and the Swedish Heart Lung Foundation. There were no significant conflicts of interest.

The study was published in the peer-reviewed JAMA Pediatrics.

The UK media has generally reported the findings accurately. The Independent quoted one of the authors of the study, who said: "These kinds of epidemiological studies look for associations in large populations, but do not provide answers on whether and how animals could protect children from developing asthma.

"We know that children with established allergy to cats or dogs should avoid them, but our results also indicate that children who grow up with dogs have reduced risks of asthma later in life."

What kind of research was this?

This cohort study aimed to investigate the association between exposure to dogs and farm animals during the first year of life – as in living with or around them – and having asthma as a preschool (around three years old) or school-age child (around six years old).

This study design is able to suggest links for further investigation, but is unable to prove cause and effect. There may be a number of other factors influencing risk, such as parental asthma, other allergies, air pollution, or other environmental exposures.

The only way to establish a causal link would be to run a randomised controlled trial (RCT), but realistically such a trial would be both expensive and impracticable – it would be difficult to convince thousands of families to adopt a dog at random or move to a farm, for example. 

What did the research involve?

The researchers included all children born in Sweden over a 10-year period from 2001-10, who were identified through the Swedish Register of the Total Population and the Medical Birth Register.

The need for informed consent and parental permission was waived by the regional ethical board in Stockholm.

The study population was split into two groups:

  • children born between January 1 2001 and December 31 2004 (school-age children)
  • children born between July 1 2005 and December 31 2010 (preschool-age children)

Children were excluded if their parents moved to Sweden after the child was 15 years of age or if there was incomplete information on parental identity or migration.

For school-age children, their asthma status was assessed during the seventh year of life. For preschoolers, this was assessed from the age of one and then throughout the study period.

Four different asthma definitions were explored:

  • an asthma diagnosis obtained only from the National Patient Register (NPR)
  • asthma medications noted in the Swedish Prescribed Drug Register (SPDR)
  • having both the NPR diagnosis and asthma medications noted in the SPDR
  • having either one or both NPR diagnosis and asthma medications noted in the SPDR

The researchers selected having either one or both NPR diagnosis and asthma medications noted in the SPDR as the most appropriate outcome measure.

Exposure to dogs was defined as having a parent registered as a dog owner during the child's whole first year of life. Exposure to farm animals was defined as parents who were animal producers and related workers in the child's first year of life.

A number of statistical analyses were performed to assess different levels of exposure to dogs and farm animals. The analyses were adjusted for potential confounders, including parental age, educational level, country of birth, and asthma status.  

What were the basic results?

During the 10-year study period, there were 1,011,051 children born in Sweden. Researchers included 376,638 preschool-age children, of whom 53,460 (14.2%) were exposed to dogs and 1,729 (0.5%) were exposed to farm animals. They included 276,298 school-age children, where 22,629 (8.2%) of whom were exposed to dogs and 958 (0.3%) were exposed to farm animals.

After controlling for potential confounders, having a dog during the first year of life was associated with a decreased risk of asthma:

When analysed by parental asthma status, school-age children had a reduced risk regardless of whether their parent had asthma or not. However, when dividing up preschool children, exposure to dogs no longer had any effect on asthma risk, either for those with parental asthma or without.

Living with or around farm animals was also associated with a reduced risk of asthma in both school-age children (OR 0.48, 95% CI 0.31 to 0.76) and preschool-age children (HR 0.69, 95% CI 0.56 to 0.84) after adjusting for confounders. 

However, again, the results changed when divided by parental asthma status. For both school-age and preschool children, those who had a parent free from asthma had a reduced risk, but those with a parent with asthma did not.

Dog or farm animal exposure had no significant effect on risk of asthma in children under the age of three.  

How did the researchers interpret the results?

The researchers concluded that: "The data support the hypothesis that exposure to dogs and farm animals during the first year of life reduces the risk of asthma in children at age six years.

"This information might be helpful in decision making for families and physicians on the appropriateness and timing of early animal exposure." 

Conclusion

This cohort study aimed to study the association between living with or around dogs or farm animals during the first year of life and the risk of asthma in preschool children and school-aged children. The results suggest early exposure to dogs and farm animals may reduce the risk of childhood asthma.

However, there are a number of limitations and caveats to consider. This study type can suggest an association, but it cannot prove cause and effect. The researchers adjusted their analysis for various potential confounders, including parental age, education level and country of birth. But it was not possible to account for all confounding factors, and other factors could have had an influence.

Importantly, the researchers did take parental asthma status into account, but adjusting for this gave inconsistent results, with some links remaining significant, while others did not. For example, school-age children with early dog exposure had a reduced risk regardless of whether their parent had asthma.

But when the two groups were divided in two according to parental asthma status, no risk reduction was found for either. When it came to farm animal exposure, risk was reduced in children of parents without asthma, but not in those who had parental asthma, for both groups.

This slightly clouds the picture and makes it difficult to give a clear, consistent message on whether animal exposure has a direct effect on risk, or whether it is influenced by other factors, such as parental or child eczema, hay fever, or dust mite or animal fur allergies. These things may influence both the decision to live with an animal and the child's risk of developing asthma.

That said, the study has strengths: it included a large sample, followed participants for a number of years, and also used medical registers to identify child asthma, rather than relying on parental report.

However, as the researchers used official registers, there may be a problem with missing data for dog ownership or parental asthma status, for example. The study was also unable to account for exposure to other animals, particularly at close family members' homes, where there may be high levels of exposure that would not have been linked.

It is not exactly clear what causes asthma, although it is thought to be a combination of factors, including genetic and environmental. Modern hygiene standards are often considered to be one of these factors, and the researchers suggest this may be why exposure to animals could have a protective effect.

However, this cannot be confirmed at this stage. More research is needed before we can consider giving any official advice to parents about the benefits – or otherwise – of having a pet.  

Analysis by Bazian. Edited by NHS Choices. Follow NHS Choices on Twitter. Join the Healthy Evidence forum.

Analysis by Bazian

Edited by NHS Choices

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