A Strong Case for Having More Than One Pet

Analysis by Dr. Karen Shaw Becker

pets and allergies

Story at-a-glance -

  • A recent Swedish study suggests exposure to pets in infancy has a protective effect against allergies later on, and the more pets in the home, the better
  • According to researchers at the University of Wisconsin, exposure to pets around the time of birth is optimal
  • Another study concludes babies exposed to pets while in the womb and up to 3 months of age have high levels of immune boosting gut bacteria
  • Recent research from Denmark suggests cat ownership reduces the risk of childhood asthma in kids who are genetically predisposed

A recent study conducted in Sweden adds to a growing body of evidence that exposure to pets in infancy may have a protective effect against allergies later in childhood.1 The study, published in the journal PLOS One, also suggests that the more pets, the better.

A total of 1,278 children were involved in the study. About 80 percent of the parents completed questionnaires on pet ownership and the incidence of allergic rhinitis (runny nose, itchy eyes), asthma and eczema in their children. Researchers interviewed the remaining 20 percent of parents when their children were 6 to 12 months old, and had clinical evaluations done at 18 months, 3 years and 8 to 9 years.

In both groups, reports of allergies declined as the number of pets in the household increased. About a third of the children without pets reported allergies, while no kids in homes with five or more cats or dogs did. The researchers controlled for many factors for their study, but the link between pets in the home and decreased risk for allergies remained consistent.

“This is the hygiene hypothesis at work,” lead study author Bill Hesselmar, an associate professor at the University of Gothenburg, told The New York Times.2

Allergies and the Hygiene Hypothesis

The theory of the hygiene hypothesis, born in the late 1980s, is that the huge increase in allergic disorders in the last century is due in part to our somewhat obsessive cleanliness standards. Since the advent of antibacterial products for every conceivable use, we’ve become hyper vigilant in trying to avoid every germ in the environment. On the surface this might seem like a sensible approach, but not everyone is convinced.

It is thought that early exposure to bacteria and parasites prepares immature immune systems to fight dangerous infections. Further, this “priming” of the immune system also helps it learn the difference between serious health hazards like a pneumonia infection, and harmless irritants like pet dander and pollen.

According to the hygiene hypothesis, when the immune system remains naïve from lack of exposure to real pathogens, it is more likely to mount attacks against benign environmental triggers. It’s possible then, that exposure to pets in the home during the first years of life helps a child’s immune system become balanced in terms of recognizing the difference between harmless and potentially pathogenic organisms.

Newborn Exposure to Family Pets Is Key in Preventing Allergies

Research conducted by professors in the Department of Pediatrics at the University of Wisconsin suggests that exposure to dogs in infancy, especially around the time of birth, influences immune development and reduces the likelihood of certain allergies.3

The researchers evaluated 275 children who had at least one parent with respiratory allergies or asthma. On an annual basis for three years, the children’s families were asked if they had a dog at home and if the kids had allergic symptoms. In addition, the researchers ordered blood tests to check the children’s immune responses.

The study results showed that children with a dog at home as newborns were much less likely to have atopic dermatitis (itchy skin — 12 percent versus 27 percent) and wheezing (19 versus 36 percent) by their third birthday. And it was all about early exposure, as kids whose families got a dog after they were born didn’t show the same health benefits.

The UW researchers believe exposure to dogs “… may contribute to a critical step in a child’s rapidly developing immune system — a step that may occur shortly after birth.”

Exposure to Pets While in the Womb Increases Healthy Gut Bacteria

A study published in the journal Microbiome suggests babies born into families with pets have higher levels of two types of gut microbes linked to a lower incidence of both allergies and obesity.4

“There's definitely a critical window of time when gut immunity and microbes co-develop and when disruptions to the process result in changes to gut immunity," Anita Kozyrskyj, Ph.D., study co-author, University of Alberta pediatric epidemiologist, and a leading researcher on gut microbes, told ScienceDaily.5

These findings are part of ongoing research using fecal samples from 746 babies registered in the Canadian Healthy Infant Longitudinal Development study that shows kids who grow up with dogs have less asthma than those who grow up in homes without pets.

For the study, the researchers evaluated infants whose mothers were pregnant between 2009 and 2012. The mothers were asked if they had a pet at home during their second and third trimester, what kind (most had dogs, cats came in second) and whether it was still in the home during the baby’s first 3 months.

More than half the infants were exposed to at least one pet with fur during either the mother’s pregnancy or after birth. Eight percent were exposed during pregnancy only, and around 47 percent were exposed before and after birth. The researchers discovered that exposure to pets while in the womb or up to 3 months of age increased the levels of two strains of gut bacteria — one linked to a reduction in childhood allergies, and the other to childhood obesity.

The levels of these two bacteria were twice as high in babies with exposure to a pet in the home. The route of exposure is indirect, from dog to mother to baby during pregnancy as well as the first 3 months of life. This means that even if the dog was no longer around at the time of the baby’s birth, his or her microbiome would still benefit from the exchange of health-giving bacteria.

In addition, the exchange occurred even in higher risk birth situations including a C-section delivery, when the mother was taking antibiotics at the time of birth and when the mother didn’t breastfeed. Yet another protective benefit is that the presence of pets in the home reduced the incidence of transmission of vaginal GPS (group B strep) during birth, which causes pneumonia in newborn infants (and is the reason mothers are sometimes given antibiotics during delivery).

Exposure to Cats Reduces Asthma Risk in Genetically Predisposed Kids

Another recent study concludes that young children with a cat at home have a reduced risk for asthma. The study was conducted in Denmark and involved 377 toddlers born to asthmatic mothers, and was published in the Journal of Allergy and Clinical Immunology.6 Historically, studies evaluating the effect of exposure to cats and dogs early in life on the incidence of childhood asthma have frequently contradicted each other. Some researchers believe exposure is a risk factor, while others conclude it has a protective effect.

These conflicting outcomes point to the likelihood that a genetic component in some cat-owning children triggers a protective effect against asthma. Genetic variation at the chromosome 17q21 locus is the strongest known genetic risk factor for childhood asthma, and kids with this genotype often develop the disease and suffer with frequent episodes and acute severe attacks.7

Almost one in three children in the Denmark study carries the genetic variant, which is thought to be proportionate to the population in general. When the variant is activated, it doubles the risk the child will develop asthma before age 12 and also plays a role in bronchitis and pneumonia. Per the American Academy of Allergy Asthma & Immunology, which reported on the study:

“Cat exposure attenuated the risk of asthma development during the first 12 years of life in children with the high-risk genotype, but had no effect in children with the low-risk genotype. Increasing levels of cat allergens collected in the children’s homes were associated with increased protection from asthma in these high-risk children.

The researchers found no such effects of exposure to dog. The high-risk asthma 17q21 genotype was also associated with a higher risk of pneumonia and bronchiolitis, and this risk was likewise decreased in children exposed to cat. Replication studies showed similar results on asthma risk.”8

Lead researcher Dr. Jakob Stokholm believes the explanation for the protective effect of exposure to cats may be related to bacteria the animals carry, as well as fungi or viruses they bring into the home.

"It’s very exciting that they find this connection because other studies have struggled to conclude anything final,” said Dr. Arne Høst, study co-author. “Now it looks like the effect is linked to a particular gene-variant, which goes to show just how complex the development of asthma and allergies are. It’s not only about genes and the environment, but how the two interact, and there’s so much that we still don’t know.”9