Pets Infants

Relationship Between Pets in the Home and Allergy in Infants

Page created November 2002; modified November 2003.

Recently, a series of published epidemiologic studies have described an association between pets in the home during infancy and a decreased level of asthma, sensitization to the animals in the children at a later age and possible atopy itself. These studies mostly follow a basic design in which a cohort of infants are identified and the presence or absence of pets in their homes at infancy and at a later age is determined. The presence of atopy is usually defined by skin test or blood work and the children are grouped according to the number of pets in the home as infants. The data is consistent in showing a relationship between pet ownership and the decreased development of allergy to the pet but inconsistent as to the type of pet and the numbers of pets.

I am updating this page a year after it was created because this observation is such an important one and it is now difficult to give advice to parents with infants who are at risk for allergy. It is not being suggested that keeping a pet suppresses all allergy to the pet but rather in a given population, there is a decrease in the amount of allergy to animals in children growing up in a home where there was a pet in their infancy compared to growing up without pet animals. In fact at the end of this page, I present cases seen in my office where keeping a pet in infancy failed to suppress allergy to the pet and the child is at risk for asthma as a result.

The publications since I first wrote this page have not sorted out any of the contradictions and in fact some of them call attention to selection bias in the populations studied that could have influenced the results. It must be remembered that these studies are epidemiologic in nature and rely on mathematical analyses to suggest relationships. A great deal of skepticism must be used when approaching epidemiologic studies since they simply show associations and despite the claims of epidemiologists can never show causality independent of the strength of the association. Such studies are useful as "hypotheses generators". In order to determine confounding variables, regression analysis is applied to show that the confounder did not bias the result. That means that all confounders must be known in order to apply mathematical models. However in reality all confounding variables are not known. Moreover the results obtained can vary depending on which mathematical model is applied to the data.

With that in mind, I will review the published data and then provide case examples that did not behave according to the studies.

Introduction

Allergy is the major mechanism for creating airway inflammation involving eosinophils and inflammation is thought to be the cause of asthma. A number of studies have shown that sensitivity to indoor year-round allergens are the important allergic cause of asthma and pet sensitivity plays a significant role. The CAMP study which is a long-term study examining childhood asthma found that children who were sensitized to a dog or cat in their home had a greater risk for night-time awakenings from asthma, a marker for more severe asthma. Abstract Other studies have confirmed that sensitization to a pet in the home is associated with more severe asthma. Abstract Abstract

There has also been a well documented doubling in the prevalence of allergy and asthma in the last 25 years in industrialized countries. Abstract Abstract A number of theories have been proposed to account for this increase, including poor indoor air quality, since houses are now insulated to a greater degree for energy conservation. Another theory is referred to as the "hygiene" theory and suggests that we have sterilized the environment surrounding infants to the point that their immune system is deflected towards the allergic immune mechanism. Partial support for this theory has come from epidemiologic studies performed in several countries that have found a lower prevalence of allergy in children living on farms compared to children living in cities.Go to Farms and Allergy

Until recently we have thought that prevention of allergy might be achieved by modifying infants' diets (prolonged breast feeding with maternal avoidance of certain allergenic foods) and the practice of allergy control, including the absence of furry or feathered pet animals in the home of infants. The results, at least in the short term, seemed to support the efficacy of primary prevention in the reduction of allergy. Abstract Abstract Abstract Abstract Abstract

Bjorksten 1999:
Retrospective collection of animal data in a population being studied. Cat keeping yielded less skin-test positivity to cat. Animal keeping yielded less asthma at age 12-13. In contrast dog-keeping did not influence a positive skin-test to dog in later life.
Roost 1999:
Current cat ownership increases the likelihood of sensitization to cat. Cat presence in infancy reduced the likelihood of adult sensitization to cat.
Svanes European 1999:
Dog ownership in infancy but not cat ownership was associated with less atopy in adulthood.
Platts Mills, Sporik 2001:
Sentization to cat dander was strongly associated with asthma. But highest levels of cat dander were associated with less IgE antibody sensitization to cat and higher IgG antibody to cat.
Martinez:
Indoor dogs in infancy were associated with lower prevalence of frequent wheeze while indoor cats were not.
Nafstad 2001 Norwegian:
There was a negative association between pet-keeping in infancy and atopic diseases at age 4 that is, the presence of a pet reduced the likelihood of allergy.
Apelberg 2001 systematic review:
Exposure to pets in older children was associated with an increased risk of asthma but exposure in infancy seemed to have a protective effect. However they also felt the result could come from selection bias in the children studied.
Ownby 2002:
Full article
The prevalence of skin test positivity (atopy) at age 6 to 7 years was 33.6% with no dog or cat exposure in the first year of life, 34.3% with exposure to 1 dog or cat and 15.4% with exposure to 2 or more dogs or cats. The prevalence of any positive allergen-specific IgE test result (seroatopy) was 38.5% with no exposure to dog or cat, 41.2% with exposure to one cat or dog and 17.9% with exposure to 2 or more dogs or cats. In this study, there was a dose effect and the protective response seemed to require exposure to more than one animal in infancy.
Custovic 2001:
Sensitization to cat was less in homes with the least and most cat dander concentrations. Sensitization was greatest in the presence of medium concentrations of cat dander (Fel d1).
Perzanowski 2002:
High exposure to cat allergen can induce both immunoglobulin (Ig) G and IgG4 antibodies specific for Fel d 1 without IgE antibodies. Living with a cat was inversely related both to having a positive skin test to cat and incidence of physician-diagnosed asthma. This effect on incident asthma was most pronounced among children with a family history of asthma. The evidence also suggests that many of the children exposed to cats at home can develop an immune response that does not include immunoglobulin E. Weaker protective trends were seen with dog ownership.

Studies added in December 2003.

Anyo et al. 2002
These results suggest that the inverse association between current pet ownership and sensitization and hayfever symptoms was partly due to the removal of pets in families with sensitized and/or symptomatic children. Pet ownership in the first two years of life only seemed to offer some protection against sensitization to pollen.
Caledon 2002
among children whose mothers had a history of asthma, exposures to cat allergen were associated with an increased risk of wheezing at or after the age of 3 years. There was no association between wheezing and exposure to dog or dog allergen.
Remes 2001
Dog exposure in early life might prevent the development of asthma-like symptoms, at least in low-risk children with no family history of asthma. Nevertheless, early pet exposure does not seem to significantly influence the development of allergic sensitization.
Oberle 2003
Allowing cats to be in the child's bedroom from the first year of life onwards may be an indicator of intensive exposure to cats and appears to prevent the development of childhood asthma.
Melen 2001
Wheezy children with reported exposure to cats during the first 2 years of life were more likely to have developed sensitization to cat by 4 years of age than unexposed children. High levels of cat allergen (Fel d 1> or =8 microg/g dust) were associated with an increased risk of sensitization to cat and, in combination with tobacco smoke, also with the development of more severe asthma. In young asthmatic children, early exposure to cat and tobacco smoke increased the risk of allergic sensitization and further development of more severe asthma later in childhood.
Almqvist 2003
Cats were less frequently kept in families with parental asthma, rhinoconjunctivitis, or pet or pollen allergy (3.5% to 5.8%) than in families without parental allergic disease (10.8% to 11.8%). Dogs were less common in families with (3.3%) than in families without (5.9%) parental atopic eczema/dermatitis syndrome. Families with smoking mothers and those with low socioeconomic index kept cats and dogs more frequently. Cat allergen levels were lower in homes with than in homes without maternal pet allergy, and this tended to hold true even for homes without a cat. Cat ownership decreased from birth to 2 years of age, especially in families with parental history of allergic diseases. There seems to be a selection of pet exposure based on parental history of allergy, maternal smoking, and socioeconomic factors. This has to be taken into consideration in evaluations of risk associations between pet exposure and allergic disease in childhood.
Almqvist 2003
Early exposure to cat seems to increase the risk of sensitization to cat but not of asthma at 4 years of age. Dog ownership, on the other hand, appears to be associated with lowered risk of sensitization to airborne allergens and asthma. Both aetiological relationships and selection effects have to be considered in the interpretation of these findings.

There are two theories proposed to explain how the presence of pets in homes with infants could prevent the development of allergy. The first theory I will call the "endotoxin theory". Endotoxin is a component of gram-negative bacteria that stimulates a particular immune response (Th1 response) and that response may turn off or at least prevent the development of an allergic response (Th2 response). Theory of endotoxin Braun-Farlander

A second theory proposes that high levels of dander in the infant's environment leads to the formation of an IgG (non-allergic antibody governed by Th1 cells) response to the dander and shuts down an IgE antibody allergic response (governed by Th2 cells) to dander. Platts mills editorial

There is considerable basic research data to support such mechanisms but the observation that pets in the homes of infants may deflect the immune system away from either sensitization to the pet or even atopy, would be the first clinically relevant demonstration that such mechanisms can contribute to human immune responses.

However nature can be quite confusing and even perverse. When infants have been studied in cross-section, data has been published demonstrating a relationship between animal exposure in the home and sensitization to the pet animal. Such sensitization is associated with asthma and more difficult asthma. Abstract Abstract abstract Abstract

CASES SEEN IN MY OFFICE
1. THREE-YEAR-OLD BOY with 2 pet cats from birth.

HISTORY:

When this patient was very young he tried peanut butter and was found to have itching of the tongue with scratching at it. His mother put peanut butter on his tongue again and he seemed to have the same reaction. He never had hives, swelling or respiratory symptoms. He has had no problem with nuts or fish or seeds. He had some eczema as an infant but no other symptoms of allergy and no symptoms around the two pet cats. He has no symptoms of asthma.

FAMILY HISTORY: No asthma or allergy in the immediate family.

ENVIRONMENTAL HISTORY: No one smokes in the home. There are the two pet cats since before he was born. Dust mite control is not being done in the bedroom. No contact with mould, cockroach or mice but he has a feather duvet.

SKIN TESTING: Moderate positive to two preparations of peanut but negative to a series of nuts. Positive to two preparations of cat dander and also positive to tree, birch pollen and ragweed.

FINAL DIAGNOSES AND ASSESSMENT:

1. Atopic: He is allergic and sensitized to peanut and cat. His reactions to peanut have been trivial but with peanut we take no chances. He will strictly avoid peanut and we taught the use of an Epi-pen. He is not allergic to nuts and in the home he can have nuts. He is sensitized to the pet cats, although he is not having much in the way of symptoms from that sensitivity. Certainly, he should be watched very carefully for asthma. The cats have been in the home prior to his birth and there is epidemiologic data now suggesting that the presence of animals in the home of infants can help prevent sensitivity to the animals and perhaps atopy itself. It is not clear how this occurs but may not be due to the presence of the animal danders, but possibly due to endotoxin from the animals and the effect of the endotoxin stimulating T-Cell responses. There are several epidemiologic studies which suggest that the amount of atopy is reduced to half in the population of children who have had animals in their home from infancy compared to those without the presence of animals. Unfortunately, I see the allergic children in whom there is a failure to decrease allergy. This patient is clearly that kind of failure. He presented very early with allergy to peanut and he is sensitized to the cats. So far he has no symptoms of asthma but he needs to be watched carefully. Several of the studies found that even though there is a decrease in sensitivity to the pet animals, when allergy occurs, it is associated with asthma. Dust mite control should be done in the bedroom. There probably should be no pets in the home but at this point he is not having difficulty with it. The home should be checked for any sources of mould.

CASE 2

2. TWO-YEAR-OLD FEMALE with 2 pet dogs and a cat since birth

RELEVANT HISTORY:

On a few occasions when this child has been around horse she seems to develop itching of the eyes and rhinorrhea or runny eyes. About one month prior to this visit, she was in a stable for two hours, and began wheezing and coughing. This cleared over a few days. However, two or three weeks later with a respiratory tract infection she became quite wheezy. She is now using Fluticasone and salbutamol. She seemed to clear her symptoms fairly quickly and at this visit is free of any cough. She now has no problem with exercise and currently is taking Fluticasone 50mcg two puffs twice per day. She has no other symptoms of allergy and does not seem to have symptoms around her two pet cats and dog. She has had no problem with milk, egg and fish but has never had peanut, nuts, seeds or shellfish.

FAMILY HISTORY: Father has had asthma and father is allergic.

ENVIRONMENTAL HISTORY: No one smokes in the home. There are two pet cats and a dog. Dust mite control is not being done in the bedroom.

SKIN TESTS: Positive to cat and dog.

FINAL DIAGNOSIS AND ASSESSMENTS:

1. Atopic: The patient is allergic and is sensitized to animal dander, but apparently not horse. It would be extremely unusual for a youngster this age to be sensitized to horse and not be sensitized to the animals that are in the home. Having pets in the home from birth onward has been suggested to reduce the incidence of allergy to the pet, allergy generally and asthma in children. There are approximately eight published epidemiologic studies suggesting this phenomenon. The reduction in the amount of allergy is probably to about half the amount that occurs in homes without pets. However, I would obviously see the failures and in this case having pets in the home has failed to prevent sensitivity in this child. It has been suggested that endotoxin associated with the animals stimulates an immune response in the child that turns off an allergic immune response. Alternatively high levels of dander in the home might promote an IgG immune response to the dander while failing to elicit IgE. That clearly has not happened here. As a result of the allergy, this child likely has chronic eosinophilic inflammation in her airways and is developing asthma. The pets must go. Even though it is possible to reduce the amount of dander in the home with the pets present, it is not possible to completely eliminate it unless the pets are gone. That will be very important for her asthma and the faster the stimulus for developing asthma is eliminated, the better the outcome. Dust mite control can be done in the bedroom and that will also help to reduce the amount of dander.

2. Asthma: She has had episodes of asthma that seemed to occur when she was around horses in stables but there are usually cats in barns as well. Certainly she had wheezing in the absence of a cold which would suggest that allergy is playing a significant role in the development of the asthma. At this point, she is free of symptoms and after she has been on the Fluticasone 50mcg two puffs bid for three weeks it can be reduced to one puff twice per day. If she stays clear over seven days, Flovent can be reduced to one puff per day as maintenance. At the first sign of a cold the Flovent should be increased to full doses and Ventolin added as needed for symptom relief. Even when the pets are eliminated, it can take up to six months for the dander to be cleared from the home. During that time she would probably benefit from low dose maintenance inhaled steroid. While her symptoms might seem intermittent, there is a very high likelihood that the inflammation is actually chronic. Certainly when I studied children who were allergic to their pet, using the technique of induced sputum, the children had chronic eosinophilic inflammation in their airways even when their symptoms seemed intermittent.

CASE 3

3. TEN YEAR-OLD BOY with 3 cats in the home since birth

RELEVANT HISTORY:

The patient was first seen as a 1 year-old after he developed hives when tasting egg. He has had some mild eczematous patches but did not show any symptoms around the 3 pet cats. He had some cough with his first respiratory tract infection but no other problems. Skin tests were positive to egg, peanut and slightly to cat. Mother smoked in the home.

The patient was seen again at age ten and was able to eat eggs but avoids peanut and nuts. He had no symptoms around the 3 pet cats but has had asthma for many years. He was supposed to be taking Fluticasone 50 mcg, 1 puff bid but usually forgets. The past 3 - 4 months his asthma has increased and he has awakened during the night with asthma symptoms once or twice per week. At that time he takes salbutamol (albuterol) and has commonly had a poor response. He uses the salbutamol daily, 4 - 5 times per day. He becomes short of breath easily with exercise. He had been given a course of prednisone 2 months before the present visit and he had been started on Advair 250 mcg, 1 puff bid (combination of Fluticasone and Salmeterol), and Monteleukast qhs but despite that has continued to need frequent salbutamol. A few days prior to this visit he had been seen in the emergency room at the Hospital For Sick Children.

He readily admitted that he took the Advair infrequently.

Skin tests: Strongly positive to peanut with small positives to a series of nuts. Positive to cat, dog, horse, mouse, mites, alternaria, cladosporium, aspergillus, indoor moulds, tree, grass and ragweed.

Spirometry: Spirometry was within normal limits but he had taken ventolin and Advair prior to the visit.

Assessment:The patient is broadly atopic and sensitized to the pet cats. He has asthma that is at a high level that could easily threaten him (awakening during the night with poor response to salbutamol and using salbutamol frquently during the day).

Management Plan:A discussion ensued with the patient and his mother on the threatening nature of his asthma including the possibility of death from asthma, the mechanism of asthma, the need for environmental control (especially cats and environmental tobacco smoke) and the need for adherence to medication usage. Oral prednisone 50 mg daily for 5 days was started. Flovent Discus 250 mcg, 2 puffs bid twice per day was undertaken with maternal supervision. It was explained that the oral prednisone was the most powerful therapy for initiating control of asthma but a 5 day schedule was chosen to reduce the potential for side-effects (which were outlined) but would not be sufficient to achieve full control. It would be critical that he take the inhaled steroid and it would take at least three weeks to achieve control and longer to improve exercise tolerance completely, if possible. The goal will be to bring him to a symptom-free state as quickly as possible, eliminate the night-time awakenings (a sign of threatening asthma) and eliminate the need for short-acting bronchodilator during the day. I do not believe that he should be managed with a combination of inhaled steroid and long-acting bronchodilator until he has been controlled with inhaled steroid alone. The presence of the long-acting bronchodilator (either Salmeterol or Formoterol) has the potential to mask continuing inflammation in the airways. Go to Abstract. The Canadian Guidelines on the management of asthma suggest that combined medication should be introduced only if full control of the asthma cannot be achieved with reasonable doses of inhaled steroid alone. Go to the Canadian Asthma Consensus Guidelines. Go to Guideline Update This patient has had poorly controlled asthma for many years and ultimately will likely demonstrate a need for combined therapy either Symbicort or Advair.

HOW TO RECONCILE THE CONFLICTING OBSERVATIONS:

There are a number of epidemiologic studies that suggest that having pets in the home of infants can lead to less sensitivity to the animals and perhaps less allergy and asthma in older children.
More recent studies have not made the evidence any clearer and have often called attention to bias in the epidemiologic data since parents who are atopic and have asthma are less likely to keep pets in the home because of their own allergies.
The data on pet ownership in infancy seems to correlate with the studies that have found less allergy in children who have been raised on a farm compared to city children. There is also some evidence, that is less convincing at present, suggesting that children who attend day-care early in life have less allergy and therefore less asthma later in childhood. This all relates to the theory that there has been an increase in allergy and asthma in children because their environment has become too clean (hygiene hypothesis).
Unfortunately studies done examining children in cross-section have shown that infants and toddlers who are allergic to a pet animal have significant probability of developing asthma.
This contradiction in data can be understood by selection bias. If an infant lives in a home with pets and develops an allergic reaction to a food, then his/her atopic immune response has not been suppressed and they are at risk for sensitivity to the pet and therefore asthma. Clearly suppressing atopy by the presence of pet animals is a crude treatment with a less than absolute outcome. If an infant who lives in a home with pet animals is sent to an allergist because of a reaction to a food (usually milk, egg or peanut) they are selected from the group who potentially have failed to have their atopic immune response suppressed by the pet animals. They then may be at higher risk for becoming allergic to the pets and developing asthma. The allergist is likely to see the failure of the suppression phenomenon observed when the entire population is studied. However these failures are extremely important because sensitivity to a pet early in life is likely to create chronic high-grade asthma. The failures must be identified early in order to prevent the progression to chronic asthma. The cases presented above illustrate the failures.
Most asthma in infants is viral-induced non-atopic asthma that the children will likely outgrow. It is not necessary for those families to eliminate the pet animals. However once an infant manifests allergy, the risk equation shifts and they are at higher risk for becoming allergic to the pet and developing chronic, difficult, atopic asthma.
It is probably true that having a pet in the home does not predispose to becoming sensitized to the pet but if sensitization occurs, there is a higher risk for developing asthma and asthma that is more severe.

TOP
RETURN TO HOME PAGE
GO TO FARMS, ALLERGY