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People with a family history of Crohn鈥檚 disease who grow up with a dog seem less likely to develop the condition.

Crohn鈥檚 is a type of inflammatory bowel disease (IBD). About 1.3 per cent of US adults have IBD, according to the . Prevalence of the condition is increasing in the US and other Western countries for unknown reasons. One potential explanation is the so-called hygiene hypothesis.

鈥淭he hygiene hypothesis is related to the fact that in the Western world, there are less exposures to microbes. A potential effect of this lack of microbial exposure is an increased risk of chronic disease later in life,鈥� says at the University of Toronto.

To look into this further, Turpin and his colleagues analysed data from 4289 participants of the . Since 2008, the project has followed more than 5000 people who have first-degree relatives with Crohn鈥檚 disease, a risk factor for developing the condition. Researchers have collected information every six months on participants鈥� physical health and lifestyle. Turpin鈥檚 team found that those who lived with a dog between the ages of 5 and 15 were 40 per cent less likely to develop Crohn鈥檚 than those who didn鈥檛.

The researchers then examined faecal and urine samples from all participants and found that those who grew up with a dog had a slightly more diverse microbiome and greater amounts of a bacterium called Adlercreutzia.

The bacterium might be one mechanism by which owning a dog protects people against developing IBD, says Turpin, who presented the research at the Digestive Disease Week Conference on 23 May in San Diego, California. Previous research has found that people who develop another form of IBD known as ulcerative colitis have fewer Adlercreutzia bacteria in their gut. And, the bacteria are commonly found in dogs鈥� microbiomes.

Those who grew up with a dog also had greater gut barrier function, meaning their intestines better prevented unwanted substances, like pathogens, from infiltrating the rest of the body.

While these findings offer support for the hygiene hypothesis, Turpin says there are some significant limitations. For one, the results only point to an association, meaning the reduced risk could be due to a related factor of owning a dog, such as higher physical activity, he says. Additionally, only 86 participants reported whether they had a dog between the ages of 5 and 15, with 32 saying they did, limiting the sample size.

鈥淏ased on what we have right now, I wouldn鈥檛 feel comfortable saying if you get a dog, you鈥檒l be protected,鈥� says Turpin. 鈥淚 think [the findings] raise awareness that we need to start looking at measures of environmental exposure to better understand the development of Crohn鈥檚 disease.鈥�

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