INTRODUCTION — Patients ask a variety of questions regarding the impact of pets on allergic rhinitis and asthma. These relatively simple-sounding questions expose a challenging area of research, and the answers to these questions are still being investigated. This topic will review several concepts that have taken shape in recent decades and discuss how these apply to common patient concerns.
The measures that patients with pet allergies can take to reduce exposure to dog and cat allergens in the home are discussed separately. (See "Allergen avoidance in the treatment of asthma and allergic rhinitis", section on 'Pets'.)
OVERVIEW — In most epidemiologic studies, a strong positive correlation is found between sensitization to cat or dog allergens and asthma [1-4] and, to a lesser degree, allergic rhinitis [5]. In different cohorts, the odds ratio for asthma associated with sensitization to cat or dog ranges from 3 to 9.2, and the association is consistently highly significant. Therefore, it is important to understand whether living with a pet increases or decreases the likelihood of sensitization to pet allergens and, ultimately, the risk of developing allergic disease.
When considering studies of allergic disease, it is important to understand the distinction between the terms "sensitization" and "allergy." Sensitization refers to the production of allergen-specific immunoglobulin E (IgE). Sensitization is usually demonstrated by skin testing or in vitro immunoassays for IgE to specific allergens (sometimes referred to as radioallergosorbent testing [RAST], although this term describes an antiquated form of the test).
Being sensitized to an allergen is not synonymous with being allergic to that allergen, because individuals may produce IgE to allergens in a given substance but not develop symptoms upon exposure to that substance. Individuals are considered to have clinically significant allergy or allergic disease when they have allergen-specific IgE and develop symptoms upon exposure to substances containing that allergen. Thus, greater numbers of people are sensitized to an allergen than are clinically allergic to it. The variables that determine why some sensitized individuals have clinically meaningful allergic disease while others do not have not been fully identified. This is discussed in greater detail separately. (See "The relationship between IgE and allergic disease", section on 'Terminology'.)
IMPACT OF CAT EXPOSURE ON SENSITIZATION AND ASTHMA — In the late 1990s, researchers showed that children living in a house with a cat were less likely to be sensitized to cat allergens (ie, have cat-specific IgE) [6-9]. That work contradicted the accepted view at the time that more allergen exposure correlated with a greater likelihood of sensitization, which had been demonstrated with dust mite and cockroach [10]. Since then, many groups have confirmed that the highest levels of cat exposure are associated with decreased sensitization [11-15], and there are very few studies that show any increase in sensitization among children who live in a home with a cat [16]. The dose response observed in different studies varies between a true bell-shaped curve [11,12] and a plateau where there is no increase with the highest exposure [17]. Evidence from a birth cohort in an area where having IgE antibodies to pet allergens is a major risk factor for asthma showed that most of the cat-allergic subjects with asthma (85 percent) did not live with a cat [18]. Also in this cohort, high-titer IgE antibodies to cat and distinct molecular allergens from cat were associated with higher odds of asthma.
Immunologic changes — The reasons why exposure to cat results in different immunologic changes compared with exposure to dust mite or cockroach are not clear [10]. There is some evidence that living with a cat results in the development of cat-specific tolerance. In one study of 11-year-old schoolchildren in the United States, the prevalence of sensitization to cat rose as exposure increased from very low levels to moderate levels but decreased at the highest levels of exposure [12]. By contrast, immunoglobulin G (IgG) antibodies to the major cat allergen (Fel d 1) increased as the level of exposure to cat allergen increased. These immunologic changes are similar to those seen in patients undergoing allergen immunotherapy with inhalant allergens for the treatment of allergic rhinitis and asthma. Thus, some children and adults living in a house with a cat developed an antibody profile characterized by IgG antibodies to the major cat allergen but no IgE antibodies or positive skin tests to cat. This immune response, in which a significant portion of the IgG antibodies are of the IgG4 subclass, has been called a modified T helper type 2 (Th2) response because class-switching of B cells to produce IgG4 requires interleukin (IL) 4, similar to the production of IgE [7,12]. Individuals with this response do not seem to have symptoms on exposure to cats. Furthermore, in vitro exposure of T cells from these subjects to distinct peptides of Fel d 1 results in increased IL-10 production, again supporting the view that this is a form of tolerance [19,20]. (See "Allergen immunotherapy for allergic disease: Therapeutic mechanisms".)
Possible mechanisms — One theory about why high levels of exposure to Fel d 1 may result in lower rates of sensitization has to do with the fact that Fel d 1 can reach airborne levels that are exceptionally high (eg, 1 mcg Fel d 1 inhaled per day) and similar to the quantities used for sublingual immunotherapy. These high levels are possible because Fel d 1 is associated with small particles and remains suspended in the air for relatively long periods of time. In comparison, allergens from dust mite or cockroach are carried on larger particles that readily settle to the ground and reach high airborne levels only briefly during disturbance [21-23].
Fel d 1 is easily dispersed in the environment as well and is found in public places and homes without cats at levels that are sufficient to maintain measurable airborne Fel d 1 [8,24-26]. In one study, the authors identified a home without a cat where the house dust contained 80 mcg Fel d 1 per gram of dust, which is a value common in homes with a cat [27]. Indeed, levels of exposure in schools or homes without a cat are sufficient to sensitize since, in some studies, the majority of cat-allergic children have never lived in a house with a cat [14].
Additional theories about why exposure to high levels of Fel d 1 results in lower rates of sensitization, although less developed than the first, concern biologic properties of the allergen itself. Fel d 1 is a protein (a uteroglobin) that is homologous to Clara cell secretory protein, which is found in the lungs of mice [28,29]. These proteins have been shown to have intrinsic antiinflammatory effects. Glycosylation of the major allergen Fel d 1 may also facilitate gastrointestinal absorption and a different immune response.
IMPACT OF DOG EXPOSURE ON SENSITIZATION AND ASTHMA — Several studies have found that early dog exposure (ie, the first few years of life) in the home is protective for the development of asthma or wheezing [1,14,15]. However, the immunologic effects of dog exposure appear to be somewhat different from those of cat exposure in that dog exposure appears to have immunologic effects that extend to other immune diseases, as evidenced by the finding that children who live with a dog have protection from Crohn disease [30]. The effects of dog exposure also apply to other allergens besides dog allergens. Several studies have found that early dog exposure in the home lowers the risk of sensitization to an array of allergens, as though dog exposure has a more general protective effect against allergic disease [15,31,32]. Living with a dog (at age three months) has been shown to be protective against developing a food allergy from 12 to 36 months of age, with a 90 percent reduction in odds [33]. However, study results have varied depending on the age of the children studied and other exposures, suggesting possible effect modification related to these factors [34].
There is evidence that the mechanism in dog exposure relates to microbes. One study speculated that the presence of a dog in the house can alter the developing diversity of bacteria in the gastrointestinal tract of babies in the house [35]. In keeping with a greater microbial presence, higher levels of endotoxin have been documented in homes with a dog [26,36]. Several studies have examined endotoxin exposure in relation to dogs with inconsistent results [37-39]. However, studies of household dust samples have shown that endotoxin represents only a narrow measure of exposure to microbial products [40]. Evidence seems to increasingly suggest that the total quantity of microbial exposures and/or microbial diversity may be more relevant [40]. Microbial exposures in houses with a cat have not been extensively studied, although the available data do not suggest that cats increase either floor dust or airborne endotoxin levels as dogs do or have this same protective effect [26,41].
The presence of particular microbes or microbial products influences the type of immune response the body develops to a specific antigen, in some cases protecting against the development of allergic responses. As an example, immunizing rodents with antigen in alum induces IgE antibody production, while using the same antigen with complete Freund's adjuvant, which includes inactivated mycobacterial cell walls, does not result in IgE production [42,43]. Exposure to allergens in the context of microbial products is the basic mechanism on which allergen immunotherapy is based. It is also the likely explanation for observations made in the 1870s that hay fever (ie, IgE-mediated seasonal allergic rhinitis) was less common in English farmers than in people of other occupations or the fact that decreased rates of allergic sensitization are seen in modern studies of Bavarian farm children [44,45]. This response to environmental exposure to microbial products may be more likely to occur in children with specific genotypes. Gene by environment interaction was noted in European birth cohorts such that having asthma risk (G) allele rs2305480 genotype was not associated with increased risk of asthma among those living with a dog during infancy [46]. In summary, it is possible that children living with dogs may be exposed to pet allergens in combination with microbial products, such as endotoxin, which act as adjuvants. Exposure to allergens in the presence of adjuvants may be acting as a kind of allergen immunotherapy. (See "Increasing prevalence of asthma and allergic rhinitis and the role of environmental factors".)
TIMING AND DOSE OF INITIAL EXPOSURE — The time of life and number of pets to which an individual is regularly exposed appear to influence the effect of that exposure:
●First years of life – Animal studies suggest that early exposure to allergens can have highly significant effects relevant to sensitization or tolerance, although very few human studies have been able to document this as clearly because of confounding factors. Most birth cohort studies are lacking either accurate documentation of early exposure or lacking sufficient numbers of children who are exposed to an animal for the first year of life but not subsequently. Despite these issues, relatively convincing results were seen in a birth cohort study that showed that cat exposure in the first year of life was associated with decreased sensitization to cat allergens at age 18 years [32]. In the same study, the long-term effects of early dog exposure were less clear. Children from a birth cohort in Sweden were found to have greater reduction in allergic diseases as the number of pets increased. The prevalence of any allergic disease decreased from 47 to 35 percent with one cat or dog, and it decreased to 26 percent with two or more cats or dogs [47].
●Adulthood – There are few studies examining the effect of newly acquiring a pet in adulthood, although one found that exposure seemed to correlate directly with sensitization. In a population-based cohort study of 6292 adults, the risk of becoming sensitized to cat and of developing symptoms when near cats was significantly higher in adults who newly acquired a pet cat, especially those who allowed the cat in the bedroom [48]. The risk of becoming sensitized was highest in those who were already sensitized to other allergens and in those with preexisting allergic diseases, such as asthma, allergic rhinitis, or eczema. Having owned a cat in childhood was protective against sensitization as a result of acquiring a cat in adulthood. This study did not examine levels of Fel d 1 in house dust to determine if those with the highest exposure were less likely to be sensitized [49]. Overall, there is only limited evidence about the impact of new pet exposure in adult life.
IMPACT OF ONGOING EXPOSURE — Assuming it is true that early exposure to an animal can result in tolerance, an important question is whether the tolerant response acquired by growing up in the presence of a pet requires ongoing exposure. In the United States, allergists regularly encounter young adults who experience new-onset or more severe symptoms related to an animal when they return home from college and are reunited with their pet. Specifically, they report severe symptoms upon returning to home after being away for several months, even though the home environment did not appear to bother them just a few months earlier. This phenomenon is often referred to as the "Thanksgiving effect" because this American holiday in late November is often the first occasion when students return home. The true prevalence of this phenomenon is not known, and the potential immunologic events have not been investigated, although at least one study suggests that some students may experience a rapid fall in IgG antibodies during a year living in a relatively allergen-free college dormitory [50].
CONFOUNDING FACTORS — One problem in research of pet ownership and allergic disease is that well-performed studies regularly appear that contradict the findings of most others. Several confounding issues that have been identified over time are described below:
●In most populations, there is a positive correlation between sensitization to cat or dog allergens and the development of asthma. However, this is not true in communities in which the prevalence of pet ownership is very low, such as African American communities in Atlanta, Georgia, in which cat ownership is less than 5 percent. In these areas, sensitization to cat is not significantly associated with asthma [51]. This is probably because exposure to animal allergens at school or in homes without a pet may be too low to induce sensitization [52].
●Researchers have questioned whether families with a history of allergy tend to avoid keeping pets, which could confuse the results of studies [53]. Evidence supporting this has been reported in several European studies [16,54]. In contrast, in studies from the United States, New Zealand, and the northern region of Sweden, this pattern of behavior has not been consistently found [12-15,31]. The effects of this behavior would only be relevant if children with an atopic family history were more likely to become sensitized if they lived in a house without a cat. This is the effect that was originally described. Thus, the influence of family history remains an unresolved issue. It is possible that parental choice related to allergy in the family may influence the effect of pet ownership in some areas of Europe but is unlikely to be relevant in other areas of the world.
●The presence of a pet in the home and the reported ownership of a domestic animal do not always confer the same exposure. Most of the birth cohorts have not measured exposure in the home. In many communities, dogs are kept outside, while, in some communities, cats are also kept outside [51]. These differences will have a major effect on allergen levels in the home.
COMMON PATIENT QUESTIONS
Primary prevention — Sometimes, adults will ask about the impact of pets in the home when they are considering or expecting a child. A 2012 meta-analysis concluded that "advice from health care practitioners to avoid or to specifically acquire pets for primary prevention of asthma or allergic rhinitis in children should not be given" [16]. The evidence does not provide a basis for advising patients to remove animals from the home because they are expecting a child and are hoping to reduce that child's risk of allergic disease. Likewise, it would not be appropriate to suggest that a family obtain pets for the singular purpose of preventing allergic disease. Thus, the risk of allergic disease in unborn children should not impact a family's decision to own pets.
Established allergic disease in a patient with pets — Patients who present with symptoms of rhinitis or asthma and are living in a house with a cat or a dog may ask whether the animal is contributing to the patient's symptoms. In this situation, it is logical to determine whether the patient is sensitized to the animal in question before suggesting a course of action. There is no evidence that reducing pet exposure will decrease symptoms in an asthmatic patient who is not actually allergic to that pet. What controlled evidence there is relates to patients with positive skin tests to animal dander. Our approach in patients with year-round symptoms is to perform skin testing to animal danders, dust mites, cockroaches, and a sampling of common molds.
If referral to an allergist for skin testing is not feasible, in vitro allergy testing to common inhalant allergens that are important in the geographic area can be performed. These panels, which are available commercially and typically test for IgE antibodies to common seasonal and perennial inhalant allergens, are intended for use by generalists [55]. However, this approach can be costly if excessive numbers of tests are included or if the allergens selected are not relevant to the geographic area in question. A logical approach would involve consulting an allergist in the area initially to identify a small number of important perennial allergens for the area (eg, animal danders, dust mites, cockroach, if relevant, and several prevalent molds) [56].
●If the skin tests are convincingly positive to the type of animal already in the patient's home, it is reasonable to assume that the animal is contributing to the patient's symptoms. In this case, removal is the most effective course of action. However, disease severity and patient preference should also be considered. If the patient has chronic rhinitis symptoms that are easily controlled with low doses of medication, then it may be reasonable simply to inform the patient that the pet is likely contributing and leave the decision to them.
●If there are other allergens to which the patient is also sensitized, then reducing exposure to all relevant allergens should be recommended. Steps to reduce overall allergen exposure in the home, such as removal of carpeting and use of high-efficiency particular air (HEPA) filters, are reviewed elsewhere. (See "Allergen avoidance in the treatment of asthma and allergic rhinitis".)
Other scenarios — There are several other scenarios that arise with some regularity and sometimes lead to consults with an allergy specialist. However, the outcomes of these scenarios are difficult to predict, allergy testing is usually not helpful, and the consultation may prove disappointing to both patient and clinician. The best advice for patients in these situations is not clear, although the answers the authors would give, based on their research and clinical experience, are provided.
●An adult patient with longstanding allergic rhinitis or asthma, who is managing the symptoms successfully with medication, asks about getting a cat or dog for their child. How likely is it that the adult will experience worsening symptoms?
•In this situation, it is useful to ask the patient if they experience symptoms when intermittently exposed to cats or dogs. If intermittent exposure causes symptoms, then it is likely that the patient would have symptoms if that type of animal were introduced into the home.
●A patient with allergic rhinitis and asthma has recently introduced a pet into their home and is increasingly symptomatic. Is there any logic to trying to wait and see if tolerance develops?
•Anecdotally, some people report that they become less symptomatic after living with a pet, although there is no method for distinguishing patients who become tolerant from those who remain symptomatic. Of note, the "wait and see" approach is not recommended if worsening symptoms include asthma exacerbations or increased need for medications to control asthma.
●If a child has symptoms with intermittent exposure to cats or dogs and yet the family obtains a pet after the first few years of the child's life, is it possible that the child will become tolerant to that pet?
•Most children with reactions with intermittent exposure will continue to have symptoms with persistent exposure. However, some children do appear to become more tolerant over time or to experience more low-grade, chronic symptoms, such as persistent nasal congestion, rather than the more obvious symptoms they had previously experienced with intermittent exposure. This can also vary dramatically depending on the specific pet and on precautions that the family may choose to take, such as keeping the pet out of the child's bedroom.
●Would obtaining a "hypoallergenic pet" be advisable?
•There is no convincing evidence that certain breeds of dogs are less allergenic than others. A line of cats has been commercially bred that are said to have lower levels of Fel d 1, although the company has not submitted data to a peer-reviewed journal to support this claim. Furthermore, these cats are very costly. Hypoallergenic pets are discussed elsewhere. (See "Allergen avoidance in the treatment of asthma and allergic rhinitis", section on '"Hypoallergenic" animals'.)
SUMMARY
●Household pet exposure and asthma – In epidemiologic studies, sensitization to cat or dog allergens is strongly associated with asthma. Thus, it would be desirable to know with certainty whether living with cats or dogs increases or decreases the risk of sensitization and whether this is consistently translated into a lower risk of allergic disorders. That certainty is lacking. (See 'Overview' above.)
●Cat exposure in early life – Some studies show that exposure to cats early in childhood results in lower rates of sensitization (production of cat-specific immunoglobulin E [IgE] antibodies) to cat allergens, possibly by inducing cat-specific tolerance, which is generally associated with immunoglobulin G (IgG) antibodies to cat allergens. This may be a result of the very high levels of cat allergen that exist in homes with cats. (See 'Impact of cat exposure on sensitization and asthma' above.)
●Dog exposure in early life – Early exposure to a dog appears to protect against the development of asthma and against sensitization to dog allergens, as well as to other allergens. The best explanation is that keeping a dog in the home increases the density and/or diversity of microbes present, and exposure to allergens in the context of an array of microbial products results in lower rates of sensitization. (See 'Impact of dog exposure on sensitization and asthma' above.)
●Advice to patients and families:
•Given that the impact of pets in the home in early childhood is not fully understood, we do not advise patients either to obtain a pet or remove an existing pet from the home for the sole purpose of reducing the risk of allergic disease in future children. (See 'Primary prevention' above.)
•In patients who present with symptoms of rhinitis or asthma and are living in a house with a cat or a dog and are reluctant to remove the animal, we perform an allergy evaluation to assess if they are sensitized to that animal rather than empirically recommending removal of the pet. (See 'Established allergic disease in a patient with pets' above.)
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