Cat Felis domesticus (Fel d)

Sensitization to the cat is strongly associated with asthma, especially in environments free of mite and cockroach. Children with cat allergy and problematic severe asthma have higher levels of IgE antibodies towards cats compared with children with controlled asthma (1).

Severe asthma can be caused by multiple sensitizations to lipocalins (Fel d 4) and uteroglobins (Fel d 1) (2) Pork/cat syndrome or allergy to cat dander and pork meat (3) can be mediated by cross-reactive antibodies against pork serum albumin and cat serum albumin (Fel d 2) (4).

It has been documented that Fel d 1 becomes airborne very easily and then is carried by small particles, which in turn transfers it to environments that have not had any exposure to a cat. The concentration in such environments has been at levels that exceed the proposed sensitization threshold of 8µg/gm (5-8).

Furthermore, the low-level cat exposure that occurs in many homes without cats is capable of inducing symptoms in some patients who are sensitive to cats (9).

Taxonomy

The cat was first domesticated 7500 BC, almost 9500 years ago probably from the Felis silvestris or its two other subspecies (10). Reactions to proteins from cat species ocelot, puma, serval, jaguar, lion, snow leopard and the Siberian tiger have been seen in patients having cat allergy and IgE against Fel d 1 (11).

Prevalence

The prevalence of allergy to cats and dogs varies in different geographical regions but is rising across the world. Although, it depends on the time of exposure and allergic predisposition (18) the allergy to cats is a major risk for the development of asthma and rhinitis (29). Cat allergy starts in early infancy progressing up to young adulthood (31). Twenty-six percent of patient clinic visits related to inhalant allergens was because of cat sensitization. Cat sensitization and cat allergy represent a growing public health concern (18).

Sensitization to cat allergens starts early before respiratory symptoms have occurred (2). Nagao et al., showed that one-year-old children with food allergy and/or eczema but no respiratory symptoms were sensitized to major cat allergens already at that early age (32).

A study of almost 13,000 German children reported a sensitization rate of 12.6% to animal dander. The prevalence increased with age from 5.7% in 3–6 years old to 11.5% in 7–10 years old and reached 17.2% in 14–17 years old (33).

A Swedish birth cohort study of over 4,000 children reported an increase in sensitization rates to horse, cat, and dog from 4–16 years, respectively reaching 10.6%, 19.0%, and 22.6% (34) (35).

Increased prevalence of sensitization to common airborne allergens, including from cat and dog, has also been observed in adults (36) although prevalence rates are lower and monosensitization is more frequent (37). In Brazil, sensitization to a cat has increased dramatically from 12% to 30% among allergic children and adolescents between 2004 and 2016, seemingly reflecting a sedentary “indoor lifestyle” (38).

This increase is worrying, because sensitization to a cat is a risk factor for the development of asthma and rhinitis, as reported in a Japanese study showing that sensitization to key pet components was observed in young children before the development of respiratory symptoms (2).

Geographic variation in the prevalence of allergic sensitization to furry animals has been attributed to cultural differences, environmental factors, and rate of pet ownership (39) (40).

 A large multicenter study, the European Community Respiratory Health Survey in adults reported an 8.8% (range, 1.2%–22.4%) sensitization rate to the cat (41). Approximately 26% of European adults coming to the clinic for suspected allergy to inhalant allergens are sensitized to the cat, according to a patient-based study for aeroallergens (Global Asthma and Allergy European Network [GAL2EN]) (40)

Living environment

Every living cat produces Fel d 1 regardless of its sex, living environment, breed, age, etc. which means the highly marketed hypoallergenic cats are non-existent (7). There was an 80-fold difference in levels of salivary Fel d 1 in a group of 64 cats. It was also observed that individual cats were exhibiting up to a 76-fold difference in levels of salivary Fel d 1. The rate of Fel d 1 production gets 3-5 times less after neutering which suggests that Fel d 1 production is influenced by testosterone.

In drier regions like northern Sweden, where mite and cockroach allergens are less existent, Fel d 1 is identified as a major cause of indoor allergy (42). Japanese homes have also reported very high concentrations (160 times) of airborne Fel d 1 when compared to major mite allergens. Upholstered furniture, carpets, and mattresses have stockpiles of Fel d 1 inside homes (43).

Allergic rhinitis

Mild to severe symptoms have been reported as allergic reactions to cats; they include watery, red, swollen, and itchy eyes. Some other manifestations of allergic reactions to cats include itchy nose, nasal congestion, sneezing, coughing, wheezing, itchy throat, and even asthma (7).

Asthma

Exposure to cat and dog not only plays a role in Allergic Rhinitis but also is responsible for the development of asthma (12, 13).  There are several examples of the association between asthma and cat allergy. The number of days with asthma symptoms and the requirement of frequent dosages of β‐agonists and steroids in the second week of school terms in classrooms with moderate to a high prevalence of home cat ownership has been found in cat allergic children in Sweden (14).

Alone in the USA, cat sensitive and exposed asthmatic patients made more than 500,000 emergency visits due to cat allergen-induced asthma (13). A European birth cohort study associates decreased lung function to a combination of sensitization and exposure to pets (15). Another study by Perzanowski and colleagues on a north Swedish population relates to increased asthma severity and persistence with an increase in the level of IgE to cat or dog (16).

Atopic Dermatitis

Hives, rashes or itchy skin are symptoms of an allergic reaction to a cat. Scratches, bites, or even mild exposure to the cat tongue can cause redness or even swell in the affected area (7).

Allergen immunotherapy

Cat immunotherapy is commonly practiced even though limited high-quality evidence in the form of large randomized controlled trials subcutaneous (SCIT) or for sublingual immunotherapy (SLIT) has been recently published (17). No significant proofs of its cost-effectiveness are at the moment available. A consensus document on the cat and dog allergy published in 2017 recommend immunotherapy with cat epithelium for patients with respiratory disease as a symptom of allergy if there is exposure to cat and symptom control is not achieved through environmental control and drug treatments (18). The new immunotherapy treatment with T-cell peptides for cat allergy is promising for the future. This treatment has demonstrated immunologic effects, they have shown efficacy but in some cases also led to non-immediate allergic reactions (19).

Prevention strategies

The removal of cats from home is the most commonly recommended and employed measure for the prevention of cat allergy. As the major cat protein Fel d 1 is a “sticky” allergen, it can take several months after the removal of the cat to alleviate the allergic symptoms particularly in carpeted households (20, 21). It has also been observed that Fel d 1 levels may reduce after up to 20 weeks of removal of a cat. (21). However, Kelly et al., suggested that some months of continuous exposure might lead to a specific IgG 4 response which can induce tolerance to cat in symptomatic patients (22). The duration of tolerance is still unknown, and it has been found that individuals who were exposed to cats in their childhood might develop severe asthma when re-exposed to a cat as an adult (18). Allergy sufferers have been targeted by marketers for sales of hypoallergenic pets, however, their clinical relevance is yet to be demonstrated (23). According to an MIT technology review, several patents related to gene editing have been filed to produce allergen-free cats, but so far has no such cats been produced yet.

Environmental control           

Allergenic Molecules

Clinically significant cat allergens are Fel d 1, Fel d 4, and Fel d 7, they are produces in clinically significant quantities by all cats. Fel d 1 acts as a uteroglobin and is associated with hormone production. Fel d 1 is more likely to reach small bronchioles and cause asthma as it is carried by particles <5 µm in diameter (18).

Reviewer: Dr. Christian Fischer and Dr. Magnus Borres

Last reviewed: November 2020