Do childhood pets affect allergy risk?

Behind the Headlines

Tuesday June 14 2011

Having a pet as a child is not likely to give you an allergy

“Having a pet in the house during the first year of a child's life could halve the risk of them becoming allergic to the animals, a study suggests,” reported the Daily Mail.

This story is based on a study that followed 566 children from birth up to the age of 18 years. It found that exposure to a cat in the first year of life was associated with a halved risk of having an immune system that was sensitised to cat allergens. The findings for dogs were more complicated, with the link between exposure and reduced risk of later sensitisation found in boys only.

This study used an appropriate design for investigating the link, but it also has some limitations that make it difficult to state conclusively  that childhood pet exposure reduces the risk of allergies later. Only about half of those eligible participated, and the numbers analysed were relatively small. The way the researchers did their analyses also made it difficult to assess whether other factors might be influencing the results.

Although the results of this study are not conclusive, they do suggest that early childhood exposure to a dog or cat is not likely to make a person more allergic to these animals as an adult. However, much larger studies will be needed to confirm the findings.

 

Where did the story come from?

The study was carried out by researchers from the Henry Ford Hospital and the Medical College of Georgia in the US. The work was funded by the Fund for Henry Ford Hospital and the National Institute of Allergy and Infectious Diseases (NIAID). The study was published in the peer-reviewed medical journal Clinical & Experimental Allergy.

The Daily Mail has reported this story in an appropriate way.

 

What kind of research was this?

This prospective cohort study assessed whether there was a link between having a cat or dog in early life and the risk of allergies to cats or dogs in adulthood. The researchers say that most studies that have investigated whether there is a link between having pets in childhood and allergies have just looked at the allergies found in childhood, not those identified in adults.

This type of study is ideal for looking at this sort of question about whether a particular exposure might increase or decrease the risk of a later outcome.

 

What did the research involve?

The researchers recruited pregnant women from one area of Michigan who were due to give birth between April 15 1987 and August 31 1989. The mothers reported whether they kept pets in the house up to when the children were age six, and at age 18 the children were tested for pet allergies.

Of the 1,194 pregnant women who were eligible to participate, 835 were enrolled and filled in annual questionnaires about their children’s health until they were age six. This included reporting the number and type of any current household pets, and whether they were kept mostly indoors or outdoors.
 
At age 18 the children were asked to complete a telephone interview and to attend a clinic where they would provide a blood sample for allergy testing. Of the 835 eligible teenagers, 671 agreed to participate. They were asked about past pet keeping, whether pets had ever been removed from the home because of allergies or other reasons, family history of allergies and other factors. Blood samples were tested for levels of dog- and cat-specific antibodies (IgE), and those with levels at or above 0.35kU/L were considered to have been “sensitised” to dogs or cats.

The final analyses were carried out on 566 individuals who provided blood samples and information about their pet histories. The researchers carried out analyses to assess the effects of the following exposures on the risk of dog or cat sensitisation at age 18:

  • exposure to an indoor dog or cat for at least two weeks in the first year of life
  • exposure to an indoor dog or cat for at least one year between the ages of one and five, six and twelve, and thirteen or older
  • total number of years the child had been exposed to an indoor dog or cat

They carried out overall analyses first, and then looked at the results by gender, parental allergy history, whether the child was a firstborn and delivery type (vaginal birth or Caesarean) to see if the effects were different in any of these groups.

 

What were the basic results?

About a third of the 18-year-olds (32.5%) had had an indoor dog and 19.4% an indoor cat in the first year of their life. The researchers found that 17.8% of the 18-year-olds were sensitised to dogs and 20.5% to cats. The study did not present what proportion of teens exposed to an indoor cat or dog in the first year of life was sensitised.

Overall, teens who had been exposed to an indoor dog in the first year of life were similarly likely to have a dog sensitivity at age 18 as those who did not have this early dog exposure. This was the case regardless of whether the teen’s parents had a history of allergies. When analysed by gender, boys who had been exposed to an indoor dog in the first year of life were half as likely to have a dog sensitivity at age 18 compared with those who had not (relative risk [RR] 0.50, 95% confidence interval [CI] 0.27 to 0.92). This difference was not found in girls.

Overall, teens who had been exposed to an indoor cat in the first year of life were about half as likely to have a cat sensitivity at age 18 (relative risk [RR] 0.52, 95% confidence interval [CI] 0.31 to 0.90). This link almost reached significance in those with a parental history of allergy, but not those with no parental history of allergy.
 
Exposure at other ages (one to five, six to twelve, and thirteen or older) and total exposure to indoor dogs or cats were not associated with sensitisation to dogs or cats at age 18.

 

How did the researchers interpret the results?

The researchers conclude that “the first year of life is the critical period during childhood when indoor exposure to dogs or cats influences sensitization to these animals”. They say that “exposure to a dog or cat in the first year of life was associated with a decreased likelihood of sensitization to the allergens of that animal”. They add that the effect for dogs appears to be restricted to boys.

 

Conclusion

This study suggests that having a cat in the first year of life may reduce allergic sensitivity to cats at age 18. The results for dogs are less clear. The study used an appropriate study design for addressing this question, but there are several limitations that need to be taken into account when interpreting its results:

  • Less than half of the offspring from the 1,194 eligible pregnancies were included in the final analyses. The results might have been different if all offspring had been followed up.
  • The number of individuals analysed was relatively small. Studies in larger samples of people will be needed to confirm the results.
  • The study relied on mothers and children to report parental allergies, and on teens to recall pet exposure from age six to 18 years. These reports may have some inaccuracies.
  • The researchers carried out separate analyses for offspring with and without a parental history of allergy, but this did not have to be specifically a dog or cat allergy. Homes where parents had a cat or dog allergy might have been less likely to have a pet, and this could influence the results, particularly if a tendency to have allergies is to some extent inherited.
  • As with all studies of this type, there may be some unknown or unmeasured factors that may be influencing results. The study did not directly take into account in the analyses the factors that might influence results, such as parental allergy. Instead, the researchers repeated the analyses in different subgroups of people to see if they found different effects. This makes it difficult to know whether the links found would still be significant if these factors were taken into account.

Although the results of this study are not conclusive, they suggest that early childhood exposure to a dog or cat is not likely to make a person more allergic to these animals as an adult. Much larger studies will be needed to confirm this finding.

Analysis by Bazian

Edited by NHS Choices

Links to the headlines

How to beat pet allergy: A cat or dog at home could halve risk. Daily Mail, June 14 2011

Links to the science

Wegienka G, Johnson CC, Havstad S, et al. Lifetime dog and cat exposure and dog- and cat-specific sensitization at age 18 years. Clinical and Experimental Allergy 2011; 41: 979–986

Further reading

Kilburn SA, Lasserson TJ, McKean MC. Pet allergen control measures for allergic asthma in children and adults. Cochrane Database of Systematic Reviews 2001, Issue 1

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