The prevalence of allergic diseases has increased over recent decades12. As furry pet ownership, especially of cat or dog, increases in Korea, sensitization to pet allergens from domestic exposure is an increasingly important issue. Indirect exposure as well as direct pet ownership can lead to sensitization to pet allergens and allergic disease. The risk of sensitization to pet allergen may be increased in people who do not own pets in areas with a high proportion of pet owners3.

In a survey performed by Seoul Development Institute in 20044, 17.2% of families had pets in their house, and the dog ownership (16.6%) was notably higher than cat ownership (0.8%). Another survey collected by Korea pet food association5, 28.8% of adults replied that they were having pets in their house, and the proportion of dog ownership was 22.7%, which was higher than that of cat ownership (5.6%). In Korea, as the number of pets has increased, the possibility of more indirect exposure to pet allergens might have increased. This could lead to increased numbers of individuals who become sensitized to pet allergens. However, there are few studies about time trends of pet allergy. Thus, the purpose of this study was to investigate the trends of sensitization to pet allergens and characteristics of sensitized patients. For this purpose, we determined the annual sensitization rate to cat or dog allergens in Korea, and explored the associated independent risk factors.

In total, 7,469 patients who underwent allergic profile testing from 2011 to 2015 were enrolled. Of these, 274 (3.67%) were sensitized to dog allergen and 307 (4.11%) were sensitized to cat allergen. We analyzed the annual trends of patients who were positive for cats, dogs, house dust, D1, or D2 among the patients who underwent allergic profile testing from 2011 to 2015 (Fig. 1). The most common clinical manifestation of cat group was atopic dermatitis (30.3%), followed by allergic urticarial (26.4%), prurigo (18.2%), allergic contact dermatitis (3.6%). The most common clinical manifestation of dog group was atopic dermatitis (40%), followed by allergic urticarial (18.7%), prurigo (7%), allergic contact dermatitis (16%).

Annual trends of the number of sensitized patients are shown in Table 1. The number of patients sensitized to cats was 35 (2.44%) in 2011, 49 (3.54%) in 2012, 54 (4.37%) in 2013, 69 (4.21%) in 2014, 100 (5.65%) in 2015. The number of patients sensitized to dogs was 39 (2.71%) in 2011, 40 (2.89%) in 2012, 53 (4.28%) in 2013, 71 (4.33%) in 2014, 71 (4.01%) in 2015. The number of cat-sensitized patients increased steadily until 2015. The number of dog-sensitized patients increased until 2014 and decreased slightly in 2015. There were 1,235 (16.54%) patients sensitized to house dust. Their numbers increased steadily and gradually from 2011 to 2015. Positive results for D1 and D2 were given by 2,318 (31.03%) and 2,150 (28.79%) patients, respectively, with no evident annual trends. By using chi-square test, annual rates of all antigens are showing significant differences between each year (Cat, p=0.04; Dog, p=0.001; House dust, p=0.04; D1, p=0.003; D2, p=0.028).

The clinical characteristics of the patients sensitized to cat or dog allergen are shown in Table 2. The mean age was 24.6±16.5 years in the cat-sensitized group and 25.84±17.4 years in the dog-sensitized group. In the cat- and dog-sensitized group, 127 (41.4%) and 117 (45.5%) patients, respectively, had a family history of allergic disease. Most of the patients were urban residents (97.0% for cat, 95.6% for dog). The most common clinical diagnosis of both groups was atopic dermatitis, followed by allergic urticaria, prurigo, and allergic contact dermatitis.

The proportion of direct exposure to the causative antigen was 48.9% in the dog-sensitized group, whereas 12% in the cat-sensitized group (Table 3). A ‘no exposure’ result to the causative antigen was 78.8% in the cat-sensitized group, whereas 37.6% in the dog-sensitized group. Through the period of study, dogs were pets of 44.5% and 45.5% of patients sensitized to cat and dog respectively.

Independent risk factors for sensitization to each animal allergen were determined (Table 4). Risk factors for sensitization to the cat allergen including sensitization to dog (OR=1.443, p=0.015), but not to house dust (OR=1.076, p=0.61), D1 (OR=1.017, p=0.906), and D2 (OR=1.250, p=0.128). Risk factors for sensitization to the dog allergen included sensitization to cat (OR=1.443, p=0.015), house dust (OR=0.937, p=0.659), D1 (OR=0.928, p=0.611), and D2 (OR=0.869, p=0.347). Spearman correlation analysis conducted to evaluate the association between specific IgE level to each animal allergen and total IgE level did not reveal any significant relationship (rho=−0.019, p=0.705 for cat group and rho=−0.064, p=0.194 for dog group).

The prevalence of allergic disease like atopic dermatitis has been increasing worldwide, including Korea2. Avoidance of the causative allergen is important in allergic diseases. In this regard, the increasing interest in pet ownership in Korea, especially for cats and dogs, is germane. The increased popularity of dogs and cats as domestic pets has led to an increase in animal allergens in society.

The results of this study revealed that the number of patients positive for dog and cat allergens has steadily increased since 2011. The most likely explanation is the increased number of cats and dogs in the community. In one survey performed in Korea5, the number of cat ownership has been sharply increased since 2010 compared to that of dog ownership. The number of cat ownership had been increased consistently since 2000 and showed steep increase since 2010, whereas that of the dog ownership showed plateau between 2000 and 2009, and showed mild increase since 2010 compared to that of cat ownership. Most of people who owned cat replied that they started cat ownership after 2010 (73.5%). That explains the increasing trends of pet sensitization and the result that cat sensitization kept increasing since 2011, whereas dog sensitization showed plateau since 2013 in our study. One study reported that the proportion of sensitization in pet shop workers was not increased compared to that in the control group9. Several studies revealed that sensitization to cats was reduced with low or high exposure to the Fel d 1 cat antigen, whereas the sensitization was highest with moderate exposure to Fel d 11011. In contrast, sensitization was increased in linear correlation in people exposed to house dust, cockroach, and rats11.

Therefore, the overall increase in sensitization to cats or dogs seems to be due to the indirect moderate exposure, rather than direct and frequent exposure. The Fel d 1 cat antigen is considered a ubiquitous allergen. It has been found in many cat-free indoor private/public places, such as offices, hospitals, and schools12. Other studies revealed that cat allergens found in cat-free indoor environments were passively carried, mainly on the clothing of cat owners1314. Consequently, the increased number of cats in society has led to increased indirect antigen transmission from cat owners and the ubiquity of the antigens, resulting in the rise of sensitization to cats12151617. Several studies described that the clothing of cat owners was the main contributor to the dispersal of cat allergen in cat-free places, indicating that a prevention strategy for allergies should include the avoidance of direct cat exposure and also indirect cat exposure from cat owners1218. This indirect transmission concerns mainly the Fel d 1 antigen, because cat allergens are relatively light particles that can be easily carried on clothing131418.

This study was conducted with patients an overwhelming number of whom lived in urban areas (97.0%, 95.6% for cat and dog group, respectively). Pets in the city live in spaces where allergens can be readily present in insulation, carpets, and other household locations. Therefore, it may be easier to increase the ubiquity of antigens, leading to higher sensitization rates. A study conducted in a rural setting could yield different findings.

In this study, the proportion of patients sensitized to cat or dog was 4.11% and 3.67%, respectively, which is lower than the recent sensitization rate of 9.1% for cat and 8.6% for dogs reported in Korea19. The subjects of this study were confined to allergic patients who visited one dermatology clinic. Patients with these allergic diseases tend to avoid pets20, which may explain this relatively lower sensitization rate.

Concerning the exposure modality, exposure to dog allergen was mostly direct, whereas sensitization to cat allergen tended to be indirect, without specific contact. This is consistent with the knowledge that the prevalence of sensitization to cats is relatively high in the Korean population although cat ownership is very low451921. The high number of stray cats and their high reproduction rate could explain the ubiquity of the cat allergen22.

In this study, sensitization to cat was a risk factor for sensitization to dog, and vice versa. Dog and cat allergens share a major epitope, and these major proteins exhibit cross-reactivity2324. This cross-reactivity may explain the association of the clinical and clinical symptoms between dog and cat allergies. This possibility should be explained to patients who are clinically sensitized to dogs or cats. However, house dust, D1, and D2 were not risk factors of cat or dog sensitization. Linneberg et al.25 reported that atopic dermatitis itself could be a risk factor for sensitization to cats. House dust, D1, and D2 are usually positive in atopic dermatitis patients, but no association was uncovered in this study.

The association of total IgE with specific IgE antibody titer to dogs or cats was not statistically significant in both groups. The specific IgE level for dogs or cats was relatively small when compared to total IgE level, so specific IgE level for dogs or cats did not affect total IgE level. Erwin et al.26 investigated the association between total IgE and specific IgE levels, and reported that cats did not show any association with total IgE levels, whereas dust mites showed an association with total IgE levels. The proportion of mite-specific IgE level to total IgE is typically high; the major allergen of dust mites could be a potential enzyme that has a non-specific effect on IgE antibody production. In addition, immunologic desensitization of pet allergen can cause this discordance. High exposure to cat antigen can cause desensitization, therefore the sensitization can be highest with modest exposure to antigen1011.

Our study has several limitations. It is a retrospective study and the modality of pet exposure was only studied in people sensitized to cats or dogs. Nearly all subjects were urban dwellers, which could cause bias. In addition, since a practical survey on pets and an accurate sample survey have not been done in Korea, it is difficult to know the actual number of domestic pets in Korea. Therefore, the relationship between the number of actual pets and the sensitization rate cannot be investigated.

In conclusion, allergies to animal allergens play an increasingly important role in allergic diseases. In industrialized countries, such as Korea, the number of pets is increasing, which will increase the amount of pet allergens in public places and in pet-free settings. Increased exposure to direct and indirect antigens has led to an increase in the number of patients sensitized to dogs or cats annually since 2011. Since dog and cat antigens are cross-reactive, even if a patient is sensitized to one animal, attention should be paid to other species. However, specific IgE to dog and cat allergen is not associated with total IgE.