Current Allergy and Asthma Reports

, Volume 12, Issue 5, pp 413–423

Exposure to Cats: Update on Risks for Sensitization and Allergic Diseases

Authors

    • Centre for Molecular, Environmental, Genetic and Analytic (MEGA) Epidemiology, Melbourne School of Population HealthThe University of Melbourne
    • Murdoch Childrens Research InstituteRoyal Children’s Hospital
    • Centre for Molecular, Environmental, Genetic & Analytic Epidemiology, School of Population Health, Dentistry & Health SciencesThe University of Melbourne
  • Caroline L. Lodge
    • Centre for Molecular, Environmental, Genetic and Analytic (MEGA) Epidemiology, Melbourne School of Population HealthThe University of Melbourne
  • Melanie C. Matheson
    • Centre for Molecular, Environmental, Genetic and Analytic (MEGA) Epidemiology, Melbourne School of Population HealthThe University of Melbourne
  • Brittany Campbell
    • Centre for Molecular, Environmental, Genetic and Analytic (MEGA) Epidemiology, Melbourne School of Population HealthThe University of Melbourne
  • Adrian J. Lowe
    • Centre for Molecular, Environmental, Genetic and Analytic (MEGA) Epidemiology, Melbourne School of Population HealthThe University of Melbourne
    • Murdoch Childrens Research InstituteRoyal Children’s Hospital
ALLERGENS (RK BUSH, SECTION EDITOR)

DOI: 10.1007/s11882-012-0288-x

Cite this article as:
Dharmage, S.C., Lodge, C.L., Matheson, M.C. et al. Curr Allergy Asthma Rep (2012) 12: 413. doi:10.1007/s11882-012-0288-x

Abstract

Cats are the pets most commonly implicated in the etiology of asthma and allergic disease. However, systematic reviews have concluded that there is a lack of evidence to support the idea that cat exposure in early life increases the risk of allergic disease. Indeed, it appears most likely that cat exposure is protective against allergic diseases. Recent large prospective studies have shown that living with a cat during childhood, especially during the first year of a child’s life, could be protective. However, any advice given to the parents should also incorporate how new acquisition of cats can affect other family members, especially those who are already sensitized. Research is urgently needed to determine whether the suggested impact of acquisition of cats in adult life is modified by the person’s childhood pet ownership, to help parents who seek advice on whether or not to get a cat.

Keywords

Cat exposureCat allergensFel d 1AsthmaEczemaAtopic dermatitisAllergicReviewRhinitisConjunctivitisSensitizationAllergiesPetsCatsRisks

Introduction

Allergic diseases have increased substantially in the past few decades and have become major public health problems worldwide [15]. Ecological studies suggest that the rise in allergies is linked to western lifestyle and urbanization [1, 6]. Among the main factors related to the westernized environment that have been postulated as contributing to the allergy epidemic are increased sources of indoor allergens, including pets [7]. Given that exposure to pets can be modified, its role in the etiology of allergic disease has received much attention over the last decade.

While cats are the pets most commonly implicated in the etiology of asthma and allergic disease, cat ownership also brings many benefits, particularly in western society. Studies have shown cat keeping to be associated with reduced risk of death from cardiovascular disease [8] and improvement in health and wellbeing [9]. Current guidelines across the globe agree that there is insufficient evidence to provide any recommendations in relation to cat keeping and asthma and allergic disease [10]. Given that in many countries, cats are found in 20 %–50 % of households [11, 12], clarifying the role of cats in allergies will have substantial public health implications. This review summarizes the biological mechanisms that have been proposed to explain the association between exposure to cats and allergic disease, outlines the specific methodological challenges in investigating this association, evaluates the evidence presented in systematic reviews to date, and updates this evidence on the basis of the most recently published studies.

Biological Mechanisms Behind the Relationship Between Cats and Allergic Disease

Both the allergen hypothesis [13] and the hygiene hypothesis [14] have been put forward to explain the link between cats and allergic diseases

The allergen hypothesis initially suggested that increased exposure to allergens leads to increased allergic diseases [13]. This hypothesis has subsequently been modified to suggest that early exposure to allergens may help induce tolerance [15]. The initial evidence that allergen exposure may lead to immune tolerance came from investigations into the role of early life exposure to cats. For many years, it was assumed that exposure to furry pets, including cats, was associated with an increased risk of allergies [16]. In 1999, Hesselmar et al. reported results for a Swedish cohort showing that children raised in a house with a cat were less likely to become allergic to cats [15]. This publication was followed by multiple investigations focusing on the impact of cat allergens on allergies in both children and adults, including our work [1720], and the results have been conflicting. Interestingly, a U-shaped relationship between cat allergens and cat sensitization has been reported, with very low and very high exposures protecting participants from being sensitized to cats [18]. Lack of account of such a nonlinear relationship in the data analyses may have contributed to the inconsistent results.

Clinically, the most important cat allergen is Fel d 1, which becomes airborne at high concentrations in homes with a cat [21, 22]. Fel d 1 is found mainly in the feline sebaceous glands of the skin, but also in the salivary glands. It is known to be a very heat-stable protein and, thus, resists degradation. It is carried in different-sized particles with a diameter of less than 5 μm, with some less than 2.5 μm. Hence, they remain suspended in the air for extended periods of time [23], are thrown back into the air with minimal room disturbance [24], and are easily inhaled [21]. It is estimated that a cat carries 60–130 mg of the major cat allergen Fel d 1 on its coat and sheds this allergen at a rate of 0.1 mg per day [25].

Similar to the other allergens, cat allergens can induce an immunoglobulin E (IgE) response in humans, as well as other isotypes such as IgG, IgA, and IgG4 [26]. These responses usually lead to induction of T-helper 2 (Th2) responses that promote allergic disease. However, the immunological responses depend on the individual’s degree of susceptibility to allergens, the immunological properties of the allergen, and the level of exposure. Interestingly, a high prevalence of the IgG and IgG4 antibodies to Fel d 1 has been detected among children who both are exposed to cats and have developed tolerance. This IgG/IgG4 response has been labeled a modified Th 2 response [27].

Only a few studies have directly explored the relevance of the hygiene hypothesis in the link between cat exposure and allergic disease. It has been proposed that increased exposure to bacterial compounds, such as endotoxin, associated with pet keeping, may help reduce allergic disease by shifting the immune system from a Th2 to a Th1 type pattern [18]. A recent study of adolescents has confirmed the association between lower atopic status and higher microbiodiversity in the surroundings of their homes, as well as on their skin [28]. However, only some studies have found higher endotoxin levels in homes with domestic cats, as compared with those without, while the others have found no association between cat ownership and home endotoxin levels [29]. It has also been observed that the microbial composition of house dust is associated with dog ownership, but not cat ownership [30]. Overall, there is only weak evidence that microbial exposure mediates the link between cats and allergic diseases, especially when compared with the evidence for the mediation effect by cat allergens.

Major Methodological Challenges That Lead to Inconsistent Results

One of the major challenges in this field is the high rate of misclassification of exposure to cats and cat allergens. Cat allergens are ubiquitous in communities with a high level of cat ownership, due to the transport of allergens via cat owners. Cat allergens have been observed in homes that do not have cats as pets [3133] and also in schools [34, 35] and hospitals [36]. This is compounded by the difficulty of removing cat allergens, which adhere to inert surfaces and are often present long after the removal of the cat, even with regular cleaning of the house [37]. This means that even individuals who do not own or come in direct contact with a cat can be exposed to high levels of cat allergens for prolonged periods if the prevalence of cat ownership is high in the community. This misclassification of exposure will inevitably bias any associations between exposure to cats and allergic disease outcomes toward the null.

Another methodological challenge posed in this field is related to selective avoidance or removal of cats because of allergic symptoms or family history of allergies. Due to counseling from physicians and messages disseminated by the lay media, there is widespread belief in many communities that cats may either induce or exacerbate allergic diseases. This belief, regardless of its accuracy, can persuade individuals and families who have or are at significant risk of developing these conditions to avoid cats or remove them from the household. Such behavior could lead to spurious protective effects of pets. We observed selective avoidance of pets following the development of asthma and allergy in both childhood and adulthood, and the findings were consistent for cats, although not for dogs [38]. However, the removal of pets was relatively uncommon. Those who already keep pets seem to continue to do so, particularly if they have decided to keep a pet in spite of their own or a family member's allergy [38]. We also showed that selective avoidance accounts for only a part of the protective effects of pets presented in the literature; the protective association was still evident once the selective avoidance was taken into account [38].

Recollection of pet ownership in early life is the major methodological issue when cat ownership is ascertained retrospectively. We have found substantial agreement when adults were asked about their childhood pets on two separate occasions 9 years apart [39]. This suggests that information is repeatable, but it does not follow that the recall is necessarily accurate. Others have found high accuracy when young adults recall early childhood pet keeping, with exposure to cats between 0 and 6 years of age being correctly reported, on average, 86.3 % of the time (95 % confidence interval: 85.0–87.5) [40]. However, those with a history of asthma recalled cats more accurately than did those without a history of asthma, suggesting that retrospective evaluation of childhood pet exposure may spuriously lead to a positive association between cat keeping and increased risk of asthma [40].

What Have the Systematic Reviews of Past Studies Taught Us?

It is important to consider all available information in a systematic manner to understand the current state of the field and whether the existing evidence can inform practice and policy. To date, all the studies conducted in this area have been observational, and they have included various study designs. Prospective cohorts are the best observational epidemiological design for measuring the timing of exposures and outcomes. This study design allows the researcher to confirm that the exposure precedes the outcome, which is one of the most important factors in establishing causation. In contrast, both cross-sectional and case–control studies are subject to recall bias.

Within the birth cohort studies in this field, however, there are methodological differences that hinder a clear summary of the evidence. First, the timing of exposure measurement is not uniform across studies. Exposure to cats in the perinatal period, when the immune system is undergoing rapid development, is arguably the most important window for immune system education. Hence, exposure during the perinatal period may have effects on allergic outcomes that are different from the effects of exposures at other times. Additionally, outcomes are measured at different times, and this is a key methodological challenge, since allergic outcomes are time sensitive. Approximately 75 % of all wheeze measured before the age of six is likely to resolve [41, 42]. The rate of sensitization may be higher in cat-exposed children in early life but similar to that in non-cat-exposed children if measured later in life [43]. Other differences between studies that hinder an overall analysis are the differences in community prevalence of pet keeping, which is known to affect the risk of sensitization and allergic disease [12].

Over recent decades, eight major systematic reviews have summarized the evidence on the relationship between exposure to cats and allergic diseases. Over time, the focus in these reviews has shifted to emphasize results of the longitudinal studies, rather than trying to condense the information from all study designs. With this shift in focus, the message from systematic reviews concerning cats and the risk of allergic disease has altered. Although early reviews suggested that cat exposure in early life increases the risk of allergic disease, later reviews indicated that there was no good evidence to support this conclusion. Indeed, as summarized below, it appears most likely that early life cat exposure is protective against allergic diseases.

The first major systematic review, which included studies up to 1997 [44], retrieved a total of 89 studies: 56 of cross-sectional design, 14 case–control studies, and 17 prospective cohorts. Findings differed between study designs, with cross-sectional and case–control studies providing weak evidence that pet exposure increased the risk of allergic disease and prospective cohort studies generally finding no association. Despite these differences, the authors concluded that all pet exposure carried a risk for subsequent sensitization and that the relationship was stronger for cats than for other pets. Pet avoidance was recommended for families with atopic individuals under the age of 2 years. Although the scope of this review was wide-ranging, incorporating all available evidence on pet exposure and allergic disease, there was insufficient credence paid to the hierarchy of evidence that is inherent in different study designs.

Pearce et al. restricted their review to studies up to 2000 that had a quantitative measure of aeroallergen exposure and examined whether allergen exposure including Fel d 1 is related to asthma [45]. Only four of the included studies measured cat allergen levels. This systematic review concluded “that allergen exposure is at most a minor factor for the development of asthma in children,” with most of the evidence being “relatively weak and far from convincing.”

A systematic review and meta-analysis of studies published between 1966 and 1999 concentrated on exposure to domestic animals in the home and the subsequent occurrence of wheeze or asthma [46]. Only articles where pet exposure was clearly shown to predate wheeze/asthma were included. Thirty-two articles fulfilled the eligibility criteria: 20 cross-sectional studies, 6 case–control studies, and 6 cohort studies. The random-effects pooled odds ratios (ORs) showed no increased risk of asthma associated with pet exposure. When stratified by children’s age (0–6 years, >6 years), however, they concluded that pet exposure slightly increased the risk of wheeze in older children (OR 1.1; 95%CI 1.02–1.40), but not in younger children (OR 0.80; 95%CI 0.59–1.08), in whom the effect may even be protective. This conclusion was unusual and may be attributed to differing study designs: The evidence concerning the younger group was derived largely from cohort studies, while that for the older group was based on other study designs.

The role of furry pets in eczema was addressed by a systematic review of studies published between 1950 and 2006 [47]. Only longitudinal and cross-sectional studies were selected (n = 30). Pooled analysis of cat cohort studies (n = 8) demonstrated a protective effect for eczema development (OR 0.76; 95%CI 0.62–0.92). However this effect was not significant in the only study to include cat avoidance behavior as a confounder, and the authors concluded that the protective association observed in their systematic review may have been due to uncontrolled confounding related to selective avoidance behavior. The same group repeated their systematic review, including another 49 articles up to 2010 [10]. They noted that cat exposure increased eczema only in children with filaggrin null mutations, suggesting the possibility of a gene–environment interaction [48].

Another systematic review and meta-analysis examined studies published up to 2007 that investigated the associations between exposure to cats, dogs, and furry pets and asthma and allergic rhinitis [49]. Thirty-two studies met the inclusion criteria: 19 case–control and 13 cohort. Pooling the results of all studies showed that neither asthma nor allergic rhinitis was related to cat exposure. When limited to cohort studies, however, cat exposure was protective against asthma (0.72; 95%CI 0.55–0.93). Again, evidential hierarchy should favor the results from cohort studies.

A recent systematic review [50] investigating the role of cats and dogs in asthma and allergy summarized the literature from 2000 to 2009 according to study design. This extremely comprehensive review included 63 articles, stratified by cat or dog exposure and further grouped by study design. The review focused evidentiary weight on the information provided by the birth cohorts (17 cat and 13 dog), concluding that cat exposure in childhood had no impact on asthma and wheeze up to school age. A major strength of this review is the stratification into broad categories of cat and dog and further stratification by study type, recognizing that the risks within each of these groups may differ. A further strength is the recognition that the information provided by birth cohort studies is likely to give a more accurate picture than is evidence from other study types.

Our recent systematic review [51] aimed to address some of these methodological differences by limiting the included studies to birth cohorts with cat or dog allergen exposure in the perinatal period. We also restricted our review to studies conducted in urban settings, given that in rural and, particularly, in farming environments, exposure to other animals can add another layer of complexity. We retrieved nine studies and concluded that cat exposure may have a protective effect on the risk of allergic disease in low-risk populations but that there was still no clear answer for children at high risk.

What Do the Most Recent Studies Tell Us?

For this article, we systematically reviewed all the studies published from January 2011 to May 2012. The methods and results are outlined in this section, and the most informative studies are annotated in the References section.

We searched Medline using the following strategy in PubMed. Our search strategy included one or more asthma and allergic disease “outcome terms” AND one or more cat related “exposure terms,” published between 1 January 2011 and 8 June 2012. Outcome text words were sensitization OR asthma OR allergy OR hay fever OR wheezing OR rhinitis OR respiratory. Outcome MESH headings were Allergy and Immunology OR Hypersensitivity OR Asthma OR Respiratory Sounds OR Rhinitis OR Eczema OR Dermatitis, Atopic OR Immunoglobulin E OR Bronchial Hyperreactivity OR Food Hypersensitivity. Exposure text words were cat OR cats. Exposure MESH heading was Cats.

Our search identified 19 studies. They included 8 longitudinal studies, 8 cross-sectional studies, and 2 case–control studies. A flow chart of the study selection process is shown in Fig. 1. One author (B.C.) assessed all abstracts for eligibility, then further assessed the full-text articles of eligible abstracts.
https://static-content.springer.com/image/art%3A10.1007%2Fs11882-012-0288-x/MediaObjects/11882_2012_288_Fig1_HTML.gif
Fig. 1

Flowchart outlining the searching process

Tables 1 and 2 present data on study design, study population and sample size, data collection methods, including the sequence of the measurements of the relevant exposures and outcomes, and estimates of the relevant associations of the studies. The studies are grouped according to study design.
Table 1

Longitudinal studies on the association between cat exposure and subsequent allergic diseases (published from January 2011 to May 2012)

Author, Year, Country

Population

Number included in the analysis

Exposure

Outcomes

Exposure and crude and adjusted associations (OR or RR with 95%CI)

Method of assessment

Method of assessment

Herr et al. [52], 2012, France

Population-based birth cohort

1,879

Cat exposure at baseline

Wheeze during first 18 months

Any history of wheeze aOR = 0.65 (0.47–0.89)

Survey

Survey at 3, 6, 9, 12, 18 months

Mild wheeze OR = 0.72 (0.50–1.04)

Severe wheeze aOR = 0.55 (0.35–0.87)

Lampi et al. [53•], 2011, Finland

Population-based birth cohort

5,509

Cat exposure < age 7 years

Allergies at age 31

Atopic sensitization aOR = 0.68 (0.60–0.78)

Survey and skin prick tests at age 31

Asthma ever aOR = 0.90 (0.72–1.12)

Survey

AR in past 12 months aOR = 0.84 (0.74-0.96)

Pekkanen et al. [54], 2012, Finland

AC in past 12 months aOR = 0.85 (0.74–0.97)

AE ever aOR = 0.98 (0.87–1.11)

In atopics: asthma ever RR 1.07, p = .6

In nonatopics: asthma ever RR 1.16, p = .3

Roduit et al. [55•], 2011

Population-based

1,063

Cat exposure during pregnancy

Atopic dermatitis in first 2 years of life

Atopic dermatitis aOR = 0.68 (0.46–1.00)

birth cohort

5 European countries

Survey at age of 2, 12, 18, and 24 months

Survey

Wegienka et al. [56•], 2011, USA

Population based

566

Cat exposure < age 18 years

Cat sensitization at 18 years

Cat sensitization at 18 years

Annual surveys in the first 6-years and follow-up at 18 years of age

IgE levels in blood

1st year aRR = 0.42 (0.21–0.87)

Birth cohort

1–5 years aRR = 1.23 (0.68–2.23)

6–12 years aRR = 0.95 (0.51–1.75)

13 + years aRR = 0.89 (0.49–1.63)

Olivieri et al. [57•], 2012

Population-based adult cohort (20–44 years of age)

6,292

Cat exposure < 16 years

New-onset cat sensitization over a 9- year follow-up

Cat sensitization: aOR = 0.59 (0.47–0.75)

Skin prick test

Baseline survey

European Community Respiratory Health Survey (28 countries)

Epstein et al. [58], 2011, USA

High-risk birth cohort (infants of atopic parents)

636

Cat exposure < 1 year of age

Eczema at 4 years

Eczema: OR = 1.1 (0.7–1.9)

Sensitization in the first 4 years

Interaction between cat ownership sensitization, and eczema (described in text)

Feld1 < age 1 year

Skin prick test and survey

Survey and Feld1 in home dust

Wood et al. [59)] 2011, USA

High-risk birth cohort (children, with at least 1 parent with allergy or asthma)

560

Feld1 (ng/g of dust) at 3 months

Multiple allergic outcomes at 3 or 12 months

Eczema score >1 at 3 or 12 mo aOR = 0.83

p = .12

$Any IgE > 0.35 kUA/L aOR = 0.93 p = .51

$Single wheeze (aOR = 0.83 p = .10

$Multiple wheeze aOR = 1.05 p = .67

Feld1 in home dust

Survey and IgE measurements

Gaffin et al. [60•], 2012, USA

High-risk children (children with atopic dermatitis and a family history of allergy).

299

Cat exposure at baseline (~ 6 months)

Physician diagnosed of asthma at the follow-up

Asthma aOR = 0.16 (0.05–0.53)

Study physician examination after 3 years and 11 months

Survey

OR odds ratio, RR relative risk, AR allergic rhinitis, AC allergic conjunctivitis, AE atopic eczema $ age not reported

Statistically significant findings at p < .05 are highlighted

Table 2

Cross-sectional and case–control studies on the association between cat exposure and allergic diseases (published from January 2011 to May 2012)

https://static-content.springer.com/image/art%3A10.1007%2Fs11882-012-0288-x/MediaObjects/11882_2012_288_Tab2_HTML.gif

OR odds ratio, RR relative risk, NS nonsignificant, IQR interquartile range

% Leung et al. reported multiple symptoms related to asthma, allergic Rhinitis, spirometry, and FENO. Only the significant associations are listed in the table

Statistically significant findings at p < .05 are highlighted

A common methodological challenge that we observed when attempting to compare the results across all these studies is inconsistency in the confounders adjusted for in the final analysis. This is expected given that most of the studies have been designed to investigate a range of risk factors and not specifically cat exposure, which may have led to incomplete collection of all specific potential confounders. Also, the rationale for considering some of the factors as confounders is unclear, given that the potential confounder has to be associated with both pet keeping and allergic outcomes. For example, multiple studies have adjusted for gender of the child, which is unlikely to be associated with pet keeping. As such, gender is unlikely to be a confounder; and while it could potentially modify the association, none of these studies have demonstrated this either.

Overall, the recently published longitudinal studies have suggested a protective effect of early life exposure to cats on subsequent allergies (Table 1) or no impact, while cross-sectional and case–control studies tend to suggest that the current exposure to cats either increases allergies or has no impact (Table 2). A detailed evaluation of the findings is presented below.

Longitudinal Studies (Table 1)

Since the beginning of 2011, nine [52, 53•, 54, 55•, 56•, 57•, 58, 59, 60•] longitudinal studies on cat exposure and subsequent allergies have been published; three of these were conducted in high-risk groups, and one study included a cohort that recruited adults. Strikingly, all the longitudinal studies, including those on high-risk children, demonstrated that early life exposure to cats either protects from developing or subsequently having allergic outcomes or has no impact on these outcomes. Wegienka et al [56•] showed that children who had a cat during the first year of life were significantly less likely to be cat sensitized at 18 years of age. These results are in line with those of Ege et al., who showed that the first year of life is the critical exposure time window in relation to the protective effect related to farm animals [61]. The studies by Lampi et al. [53•] and Olivieri et al. [57•] suggested that the protective effect of early life cat exposure on some allergic diseases lasts well into adulthood.

Interestingly, Epstein et al. [58]showed that although early life cat exposure does not have any impact on subsequent eczema, cat sensitization increases the risk of eczema among those who are exposed to cats, but not among those who are not exposed to cats. This may be related to gene–environment interactions, as shown by the recent finding that exposure to cats increases eczema only among those with filaggrin null mutations that are known to affect skin barrier function [62]. The prevalence of fillagrin null mutations was likely to be high in the study by Epstein et al., since it included children with a strong family history of atopy—that is, children of parents with symptoms of asthma, allergic rhinitis, or eczema and with at least one positive skin prick test (SPT) result to a panel of 15 aeroallergens. Similarly, Roduit et al. [55•] found an interaction between polymorphisms in Toll-Like Receptor (TLR) gene and cat exposure during pregnancy for risk of developing atopic dermatitis. These findings suggest that at least some of the effects of cat exposure may be mediated by microbial diversity, since TLRs are part of innate immunity.

In addition to showing that early life cat exposure reduces the risk of new onset cat sensitization, Olivieri et al. [57•]also showed that acquiring a cat as an adult almost doubles the risk of developing new onset cat sensitization in adulthood. The risk is higher in participants reporting preexisting allergic diseases, already sensitized to other allergens, or with high total IgE levels. Interestingly, all those who developed new sensitization in this group allowed cats in their bedroom. These findings support the last revision of the Allergic Rhinitis and its Impact on Asthma (ARIA) guidelines, which recommend that clinicians and parents consider other sensitized family members when balancing the pros and cons of acquiring a cat. These results also suggest that not allowing cats in the bedroom may help in reducing the risk in such situations. A stratified analysis between acquisition of cats and sensitizations according to early life cat exposure was not performed but may have helped this study to untangle whether the advice on acquiring cats in adulthood could be modified by the person’s childhood cat exposure.

Cross-Sectional and Case–Control Studies

Since the beginning of 2011, eight cross-sectional studies [22, 6369] and two case–control studies [70, 71] that investigated cat exposure and allergies have been published. The majority of these studies were conducted on children, with seven cross-sectional studies on a population-based sample and two on clinic patients. Abbing-Karahagopian et al. [69] measured the cat allergen levels in the second year of life and correlated that with the lung function and allergy outcomes measured during the first year of life in a birth cohort. While acknowledging the lack of temporality between the exposure and the outcome as a limitation, the authors justified this approach by providing references to studies that have shown a stability of allergen levels over a long period. While we agreed with the justification, we felt that the study design could not be labeled as longitudinal and, therefore, listed it with the cross-sectional studies.

Strikingly all the cross-sectional and case–control studies demonstrated that current exposure to cats either increased the risk of having an allergic disease outcome or found that there was no impact. The adverse impact of current pet ownership on allergic outcomes is consistent across various age groups, clinical versus population-based samples, and whether exposure was cat ownership or cat allergen levels. Six out of the eight studies focused on the current exposure to Fel d 1 levels, and therefore, recall bias is unlikely to explain the positive associations observed at least in these studies. One of the major limitations of these cross-sectional and case–control studies is that the lack of information on the timing of introduction of pets and, therefore, how the past cat exposure would modify the current is unknown. It is possible that the current exposure may not have an impact among those who have had cat exposure in the past, since they have developed tolerance.

Interestingly, current cat ownership is related to wheeze even in infancy, while first year of life has been identified as a critical exposure period for development of tolerance [56•]. These findings together suggest that perhaps early life cat exposure may increase allergies initially with development of tolerance subsequently, which is already demonstrated in relation to sensitization [43]. It will be interesting to investigate this hypothesis within the longitudinal studies, especially among high-risk cohorts.

Conclusions

There has been a substantial increase in the number of published longitudinal studies in recent years, and these have clarified the role of early life cat exposure in the development of allergic diseases. Large prospective studies have confirmed that living with a cat or a dog during childhood, especially during the first year of a child’s life, at least does not increase the risk of subsequent sensitization and allergic diseases and that it could be protective. There is adequate evidence on the association between early life exposure and subsequent reduction of allergic disease to consider revising guidelines to indicate that early life cat exposure is likely to protect against allergic disease and definitely does not increase the risk of allergies. However, any advice given to the parents should also incorporate how new acquisition of cats can affect other family members, especially those who are already sensitized [72]. Longitudinal data on exposure to cats in adult life and the subsequent effects are currently scarce and conflicting. Research is urgently needed to determine whether the suggested impact of acquisition of cats in adult life is modified by the person’s childhood pet ownership, to help parents who seek advice on whether or not to get a cat.

Acknowledgments

S.C.D., C.L., M.C.M., and A.J..L are supported by the National Health Medical Research Council. In addition, C.L. is supported by the Sydney Myer Foundation.

Disclosure

No potential conflicts of interest relevant to this article were reported.

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