Are pets good or bad for your child’s allergies?

For most of my childhood, my parents were reluctant to adopt whatever barked or meowed. Part of it was because of my dad’s unadmitted phobia of dogs; another part had to do with the fear my pediatrician had struck in my mom. If she wanted to keep me and my sister allergy-free, he told her, then no pets should be allowed in our house.

“For a while, this was the common recommendation: the child should not be exposed to allergens,” says Erika von Mutius, a pediatric allergist at the University of Munich. And that included any kind of furry animal.

The idea that pets were bad for kids gained traction with a 1997 study that followed the progression of allergies in 1,314 German newborns in their first three years of life. Its results strongly suggested that babies who had tested positive for IgE antibodies, defensive proteins produced by the immune system during an allergic reaction, had been in contact with higher concentrations of an allergen mostly found in cats and kittens. These findings led to the belief that avoiding any kind of early-life exposure to pets could spare children from developing allergies to animals.

In the early 2000s, however, an opposing trend emerged. A number of studies and systematic reviews started to suggest what at first seemed counterintuitive: exposure to pets in the very early stages of life might confer protective benefits and prevent the development of allergic rhinitis, asthma and a skin condition called eczema.

This exposure hypothesis is “not an exact science yet,” says Clifford Bassett, an allergist and professor at NYU Langone Health in New York City. So, which side is right? Here’s what the evidence actually says.

What is the state of science on pet exposure and allergies?

It’s not crystal clear. To date, no randomized controlled trials (RCTs) exist on whether doggos and kitties are either risk factor or protect you against allergies. RCTs — which are regarded as the highest-quality standard in scientific studies — test how effective an intervention is by randomly giving patients the real thing or a phony treatment that just appears to be the real thing. Since we still can’t be fooled by a fake dog, this leaves us with the next best evidence in line, which are cohort studies, like the one of German kids. Cohort studies follow up on a group of people to see how they react to a certain agent (pets, in this case) over time.

But the problem is that some of these studies have not been able to find the protective benefits of pets. A large 2011 review of nine papers attributed this to the fact that the studies were not all designed the same way. Varying methods can introduce unwanted noise and lead to conflicting results, as the authors found.

Of the nine studies analyzed, six detected lower levels of IgE antibodies and 15 to 21 percent less eczema in children who had been exposed to cats or dogs around the time they were born. Another saw no association whatsoever, and two discovered an increased risk for babies who either had a family history of allergies or a genetic tendency to develop eczema.

One of the biggest efforts to examine whether owning a pet in early childhood influenced asthma and allergy development in children came in 2012 with the establishment of the Global Allergy and Asthma European Network, a research group on allergic diseases funded by the European Union. That network worked with researchers on a large-scale study to get a better picture of the relationship between pets — including cats, dogs, birds and rodents — and the development of asthma and allergies in kids.

“We focused on having a huge cohort. I mean, 22 thousand children? That’s quite a big number,” says Karin Lødrup Carlsen, lead author of the study and a professor of pediatrics at the University of Oslo. “What we ended up with was that there was hardly any signal whatsoever, either beneficial or harmful, related to pets.”

The fact that contradicting results of this kind abound in the research means that scientists are still missing the pieces of the puzzle, according to von Mutius. “Controversies occur because there is something hidden that you’re not measuring,” she adds.

Why do some scientists think pets help reduce allergy risks?

The whole idea that your pooch might lower allergy risks in children comes from the idea that when babies are raised in a pristine environment and lack exposure to germs, they’re more susceptible to allergic diseases. This proposal is called “the hygiene hypothesis.”

But for years, the underlying cause of the hypothetical protective pet effect has been disputed. Some attribute it to pet allergens, which are specific proteins present in dog and cat dander that we inhale. But more and more studies are starting to point towards the hygiene hypothesis: It may not be the pets themselves that are beneficial, but instead the multitudes of microbes that travel inside them.

When a 2010 study examined the types of bacteria and fungi in the dust of U.S. households, researchers were surprised to see that homes with dogs had a far greater abundance of bacteria than those with no pets. The discovery “told us for the first time that there really is a microbial signal that coincides with pet ownership,” says microbiologist Susan Lynch, from the University of California, San Francisco: pets bring germs to your house that wouldn’t be there otherwise.

Four years later, she and her team published another paper to test whether this house dust rich in dog microbes would protect mice against allergens. And it did. Compared to those feeding on dust from dog-less houses or no dust at all, mice that ate the dog-associated house dust were less likely to develop allergic reactions. “This is all great,” says Lynch. “But it’s all in mice, right?”

Right. So, in 2016, she and her team decided to take a step further and look at something even more enlightening: human poop. After examining the stool of 308 American babies that were less than a year old, they found three different groups of bacterial communities. Each had a different abundance of certain species and, what’s more, produced different molecules that related to various allergic responses later in life. The results, which were published in Nature Medicine, fitted with what scientists suspected: babies with the highest risk of developing allergies and asthma significantly lacked dog exposure.

Lynch, however, prefers to remain cautious before making too much of her results. Microbes may well be important, but the evidence is still a work in progress. “I would say that we are a long way from even understanding what happens,” she says. “You don’t want to play around with that until you know exactly what you’re doing.”

In what cases does keeping a pet hurt your kid?

If your child is already allergic to animals, then you don’t really have a lot of options. “If you have children who are allergic to dogs, in general, you don’t want to have a dog in the home,” says James Gern, a pediatrician at the School of Medicine and Public Health at the University of Wisconsin-Madison. Bassett agrees. If a child has evidence of a cat allergy, mainly wheezing, itching or nasal symptoms, he says, then putting a cat in the household is probably not a good idea. So that’s that.

Studies have also found that children who are born in families with a high prevalence of pet allergies or who have specific mutations that make them more vulnerable to skin irritations don’t do very well around furry pets.

A 2008 PLOS Medicine paper from Denmark with 411 newborns showed, for example, that infants with mutations in the FLG gene, which produces a protein on the outer layer of the skin, were twice as likely to develop eczema than kids with healthy copies of the gene. And when they were exposed to cats (but not dogs,) that risk greatly increased.

The downside to these high-risk genetic mutations is that you won’t know for sure whether your child has them unless you test for them. And in the case of some labs, such as the Mayo Clinic Molecular Genetics Laboratory, in Rochester, the cost of such analysis can go as high as $1,717. Gern, however, remains skeptical. “At present, I would advise against this sort of testing until there is better information about which genes to test and what the results actually mean with respect to pets and health,” he says.

What’s the bottom line?

You may need to wait awhile before this issue is settled for good. So far, some medical societies have decided not to take a stand. Australian and British guidelines, for example, warn that “no recommendations can be made” and that “healthcare professionals should not offer advice on pet ownership as a strategy for preventing childhood asthma.”

Bassett, however, says that “having an early exposure to pets is probably a good thing, but it’s not something we put on a [to-do] list for young parents.”

To Lødrup Carlsen, unless you have a child who has already developed allergies, science can’t tell you how to live your life. “Whether or not you have a pet, that is the choice you make yourself,” she says. “But you shouldn’t make that choice based on the assumption that it will reduce or increase your [baby’s] risk of allergies.”