Management of cat allergy

INTRODUCTION — 

Cat allergy and its treatment are impacted by genetic predisposition, environmental factors, and multiple other human- and cat-related variables. This topic will review the various treatments available for respiratory allergy to cats (ie, allergic rhinoconjunctivitis without or without asthma).

Allergic rhinitis, conjunctivitis, and asthma caused by inhalant allergens, as well as allergen immunotherapy (AIT) for these disorders, are discussed separately:

●(See "Allergic rhinitis: Clinical manifestations, epidemiology, and diagnosis".)

●(See "Allergic conjunctivitis: Clinical manifestations and diagnosis".)

●(See "Pharmacotherapy of allergic rhinitis".)

●(See "Subcutaneous immunotherapy (SCIT) for allergic rhinoconjunctivitis and asthma: Indications and efficacy".)

OVERVIEW OF CAT ALLERGY

Epidemiology — Approximately 12 to 15 percent of the United States population is burdened by allergy to cats [1,2]. Rates are even higher in other countries [3,4]. Most cat-allergic individuals have nasal and ocular symptoms, but a significant portion also have asthma. Symptoms can occur acutely upon exposure to a cat, which is more apparent to patients, or be more chronic in patients who live with cats in the home [5].

Allergens and distribution in the environment — The principal allergen is a protein termed Felis domesticus 1 (Fel d 1) [6]. This protein is produced by the cat's salivary and lacrimal glands and is also secreted by sebaceous glands of the skin. Secondary allergenic proteins have also been identified, including urinary protein (albumin) [6-13]. It is unclear what function Fel d 1 serves in cat physiology, but its importance was demonstrated by the failure of early efforts to "knock" out the Fel d 1 gene. (See 'A hypoallergenic cat' below.)

In homes with cats, allergen load correlates with the number of cats present [14-17]. Owners are exposed through multiple routes: They handle their cats and may be licked, scratched, or bitten. Cat allergen is notoriously sticky, adhering to carpeting, walls, curtains, mattresses, furniture, and other household items. Allergen may also be inhaled because, when cat saliva and dander dries, it becomes easily airborne and is dispersed by the home's heating/air conditioning systems. Approximately 20 percent of cat allergen particles are <10 microns in diameter, and they pass readily into the lower airways and trigger asthma [18]. (See "Allergen avoidance in the treatment of asthma and allergic rhinitis", section on 'Pets'.)

In homes with cats, indoor levels of the cat allergen Fel d 1 can reach >1000 mcg per gram of dust, which is higher than that seen with most other indoor allergens. Even reducing levels by 90 percent may still result in a significant indoor burden for the cat-allergic patient, as symptoms can be provoked with as little as 8 mcg of Fel d 1 per gram of dust [19,20]. (See "Allergen avoidance in the treatment of asthma and allergic rhinitis", section on 'Specific control measures'.)

People who do not live with cats are still regularly exposed, as cat allergens are sufficiently light and sticky that they are carried from homes with cats on clothing, shoes, and accessories. As a result, cat allergens can be measured in significant amounts in schools, workplaces, and houses without cats [2,5,15-17,21].

The degree of exposure also depends upon the duration of exposure (new versus longstanding), whether the cat lives predominantly indoors or outdoors, where the cat sleeps in the home, the sex of the cat, and whether it is neutered or spayed (procedures that reduce allergen load) [14,22].

Tolerance to one's own pet — In some cases, chronic exposure to a cat may actually result in a degree of tolerance to that specific cat. This was borne out in a provocation study, which demonstrated that it was not possible to induce a 20 percent drop in forced expiratory volume in one second (FEV₁; one of the criteria for study enrollment) in patients with cat allergy and asthma who lived with multiple cats for an extended period [23]. In fact, exposure to cats in the home during early life protects against sensitization to cat (ie, the development of allergic antibody [immunoglobulin E (IgE)] to cat) and cat allergy, while new exposure after the first year of life and in adulthood is more likely to result in clinical allergy. The impact of prenatal and early-life exposure on induction of tolerance, introduction of a cat to the household in the first year of an infant's life versus adulthood, and the impact of ongoing exposure to a cat(s) are gaps in our knowledge base and are the subject of ongoing research. (See "Pets in the home: Impact on allergic disease", section on 'Impact of cat exposure on sensitization and asthma'.)

Tolerance to one's own cat does not appear to extend to other cats, for reasons that have not been identified. This tolerance may be disrupted by prolonged absence (eg, going off to college and then returning home in the summer), such that symptoms recur upon exposure to one's previously tolerated pet.

MANAGEMENT

Avoidance — The most effective treatment for cat allergy is avoidance, including finding new homes for pets. However, many cat owners are unwilling to part with their pets, and insistence by the clinician on rehoming may lead to the patient finding a different allergist who is more understanding of their desire to own pets. There are also patients, such as veterinarians, who have ongoing occupational exposures. In addition, keeping a pet is associated with several health benefits, including improvements in cardiovascular health, depression, and loneliness [24,25].

If the patient keeps the cat, we strongly suggest that they exclude the cat from the bedroom. Although sensible, this intervention alone usually does not dramatically reduce symptoms, because of the ease with which cat allergen is carried through the indoor air in homes with central heating, ventilation, and air conditioning (HVAC) systems.

Even after a cat is removed from a home, it can take 6 to 12 months for symptoms to improve to a noticeable degree because allergens remain despite cleaning efforts, and residual allergens take time to lose bioactivity [26].

In some cases, removing the cat from the home may not provide adequate relief, because cat allergen is ubiquitous in the environment, and the exposure obtained in public places may be sufficient to cause symptoms.

Environmental control — Environmental measures such as replacing carpeting with hardwood or other solid-surface flooring, encasing mattresses and pillows with allergen-impermeable coverings, using a vacuum with a high-efficiency particulate air (HEPA) filter, and employing a standalone HEPA filter (or whole-house system) can have a positive benefit with respect to reducing but not eliminating the total indoor allergen load [18,27].

Note that not all HEPA filters meet the rigorous standards set by the United States Department of Energy, which states that a HEPA filter must demonstrate a minimum efficiency that captures at least 99.97 percent of particles as small as 0.3 microns in diameter that pass through it, which encompasses not only airborne allergens but also the majority of bacteria and viruses [28]. Industry has adopted the term "true" HEPA for devices that meet this standard, and these devices are superior to electrostatic filters (although the latter are effective for larger particles, such as pollens). Ozone-generating (ie, ionizing) devices are problematic for people with asthma and should be avoided [29]. However, filtration devices are tested under rigorous and idealized conditions that do not necessarily correlate with real-world use, and patients should understand that the resultant improvement in allergen levels may not be sufficient to noticeably impact symptoms or medication requirements [30]. (See "Allergen avoidance in the treatment of asthma and allergic rhinitis".)

Relevant environmental measures include [31,32]:

●Replacing carpet and upholstered and fabric furnishings that can serve as reservoirs of allergen to the extent possible.

●The placement of tightly woven covers on pillows and mattresses. Because frequent removal of mattress encasings for washing is impractical, a mattress pad or protector can be placed over the mattress encasing and routinely removed and washed with the rest of the bedding.

●Weekly use of a vacuum equipped with a HEPA filter.

●Excluding cats from the bedroom, if possible.

●Use of standalone HEPA air filters in the bedroom (even if the cat is excluded) and family room/den and/or a whole-house HEPA filtration system [33].

Bathing a cat regularly is not practical, since allergen levels returned to baseline in less than a week in one study [34]. In addition, cats are often not amenable to bathing unless trained from an early age.

Allergen-reducing cat food — A specialized cat food is commercially available that contains an egg-based protein that binds Fel d 1 protein produced by the cat. This protein is derived by immunizing chickens with Fel d 1 to produce immunoglobulin Y (IgY) antibodies, the avian equivalent of human immunoglobulin G (IgG) [35]. Eggs enriched with anti-Fel d 1 antibodies are then used to produce a supplemented cat food. Laboratory investigation has shown that regular feeding with this cat food product reduced Fel d 1 levels in cat dander and saliva by approximately 50 percent. In an environmental exposure chamber study, patients experienced some benefit in their nasal symptoms [36]. In an unblinded, uncontrolled pilot study of 40 patients with allergic rhinitis and/or asthma due to cat, exclusive use of this food in the home setting for four months was associated with a reduction in symptoms using a visual analog scale beginning at one month [37]. Controlled studies are needed.

Pharmacotherapy — Cat-allergic patients who have only intermittent exposure (ie, visiting a relative or friend with a cat) may attempt to prevent symptoms with a glucocorticoid nasal spray started five to seven days before and taken throughout the visit (table 1). Cromolyn nasal spray, which is only effective as a prophylactic therapy, can be started two to three days before exposure and continued throughout the visit. Oral antihistamines, decongestants, and antihistamine eye drops can also be taken prophylactically or can be added if symptoms develop (table 2 and table 3). Pharmacotherapy for episodic allergic rhinitis or conjunctivitis is discussed in detail separately. (See "Pharmacotherapy of allergic rhinitis", section on 'Mild or intermittent symptoms' and "Allergic conjunctivitis: Management", section on 'Anticipated exposures'.)

Cat-allergic patients with chronic exposure and persistent nasal symptoms typically require daily usage of nasal glucocorticoids (table 1). Similarly, asthma control may require an inhaled glucocorticoid, the combination of an inhaled glucocorticoid plus a long-acting bronchodilator, or biologics. (See "An overview of asthma management in children and adults".)

Immunotherapy — Cat allergen immunotherapy (AIT) is most often offered to patients who have not improved adequately from environmental control with or without medications, although the evidence base for this therapy is limited and mixed [38-41]. The use of cat AIT in properly selected patients with cat-induced allergic rhinoconjunctivitis, with or without asthma, is supported by multiple guidelines, including those from the British Society of Allergy (BSA), the American Academy of Allergy, Asthma, and Immunology (AAAAI)/American College of Allergy, Asthma and Immunology (ACAAI) Practice Parameters, and other expert groups [38,42,43].

Subcutaneous immunotherapy (SCIT) has been studied most extensively and is the focus of this review. A course of SCIT is generally three to five years in duration.

Referral — Patients who are interested in AIT should be referred to an allergy specialist. AIT is customized for each patient and generally includes all the allergens to which they are sensitized, although, in some countries, AIT is directed only at the patient's most bothersome allergy. The decision to pursue immunotherapy requires shared decision-making. Points of discussion include the costs, the time commitment required, the importance of compliance, and adverse effects or reactions to the immunotherapy itself [44]. (See "Subcutaneous immunotherapy (SCIT) for allergic rhinoconjunctivitis and asthma: Indications and efficacy".)

Patient selection — Based on the available studies of the efficacy of cat AIT, our approach differs based upon whether or not the patient continues to live with cats.

Patients not living with cats — For patients who have removed cats from the home and have only intermittent exposure, we inform them that it may take at least a year for symptoms to improve (the time required for the allergen to dissipate), even after thorough cleaning. If they are interested in waiting this period of time, we suggest they do so to determine their new symptom baseline. If they are still symptomatic on appropriate medications, wish to minimize the use of chronic medications, or do not wish to wait, we suggest AIT, including in the extracts all the other allergens (eg, pollens, dust mite, etc) to which they are sensitized.

Efficacy studies — The efficacy of cat AIT in adult patients who were not living with cats in their homes was studied in small but well-designed challenge studies in the 1970s and 1980s [45-48]. Cat AIT reduced symptoms, delayed symptom onset upon exposure, and attenuated airway hyperresponsiveness during bronchial provocation with cat allergen in adult patients. These early studies were performed with various doses of cat allergen extracts because an optimal dose was not yet known. Two small studies of children with asthma showed improvement in bronchial reactivity to cat allergen [46,49].

In the 1990s, two double-blind, placebo-controlled trials evaluated the efficacy and safety of cat AIT in cat-allergic patients with allergic rhinoconjunctivitis, with or without asthma. These studies served as the basis for the US Food and Drug Administration (FDA) and European regulatory approval of cat extracts for SCIT [50,51]. The maintenance doses used in these two studies are in the effective range that is recommended by various AIT guidelines (ie, 11 to 17 mcg of Fel d 1). (See "SCIT: Preparation of allergen extracts for therapeutic use", section on 'Dosing'.)

●In the first trial, 28 participants with cat-induced allergic rhinoconjunctivitis with or without asthma received one year of SCIT with a cat extract standardized on Fel d 1 (maintenance dose: 13.2 mcg) or placebo [51]. All patients were sensitized only to cat and had complied with measures of cat avoidance, including removing cats from their homes for at least a year, but still had significant symptoms. Cat AIT resulted in greater improvement in the symptom/medication score (81 and 21 percent improvement in active and placebo patients, respectively), 62 percent improvement in symptoms with conjunctival provocation, and a three- to fourfold increase in the dose of allergen that provoked bronchospasm with active treatment compared with placebo. The injections were well tolerated, with only mild and infrequent side effects.

●In the second trial, 28 cat-allergic patients with moderate-to-severe allergic rhinoconjunctivitis and asthma were randomized to three months of SCIT with an extract standardized on Fel d 1 units (target maintenance dose: 15 mcg) or placebo [50]. Patients were matched for age, cat ownership (cat ownership was allowed but not enumerated), and asthma (13 of 28 with clinically diagnosed asthma; all reported cat-induced bronchospasm). Standard validated clinical outcomes included changes in clinical symptom scores, skin tests to cat extract, conjunctival provocation, and peak expiratory flow (PEF) measurements during and after natural exposure to cats in a controlled cat room.

Cat SCIT resulted in improvements in each of the outcomes of efficacy, with a 72 percent reduction in symptoms during cat-room exposure, a meaningful attenuation of conjunctival provocation sensitivity (shifted >100-fold), and a marked attenuation of the drop in PEF upon cat-room exposure (85 L/min drop pretreatment to 29 L/min drop posttreatment: a 66 percent reduction). No serious adverse reactions to treatment were reported.

The authors concluded that a brief three-month course of AIT with a standardized cat extract, administered at an appropriate dose, was a well-tolerated and effective treatment for cat allergy and would be appropriate for patients who are unable to avoid cat exposure or in whom pharmacotherapy has provided insufficient control of symptoms. Limitations of the study included an unspecified number of cat owners and lack of longitudinal assessment of symptoms and medication requirements. The study did not assess the optimal period for maintenance treatment or duration of effectiveness after SCIT was discontinued. However, given the experience with SCIT for other allergens, this short treatment period would be unlikely to lead to prolonged effectiveness after discontinuation.

Patients still living with cats — We focus on optimizing environmental controls and pharmacotherapy for cat-allergic patients who keep their cats because it is unclear if cat AIT is effective in patients who continue living with cats in their home or have ongoing occupational exposure (eg, veterinarians). People who live with cats are exposed to such high levels of allergen on a daily basis that it is uncertain if the doses used in AIT are sufficient to overcome that chronic exposure. There are only a limited number of clinical trials that included a subset of patients living with cats, especially patients with asthma:

●Continued cat ownership was permitted in one of the studies described above that demonstrated efficacy, but the number of patients living with cats was not enumerated, so a subset analysis was not possible [50].

●A large, multicenter, international study of an experimental modified-allergen peptide construct did not demonstrate improvement relative to placebo when administered to cat-allergic individuals living with cats [52]. Note that this was a study of an investigational therapy.

In light of the very limited data available, we inform patients that the effectiveness of cat AIT in this situation has not been conclusively demonstrated, and we do not encourage it in most cases. In particular, patients with cat-induced asthma who live or work with cats and also receive cat AIT may experience worsening of their asthma, such that cat AIT may not be safe for those individuals. However, if a patient allergic only to cat wanted to pursue cat AIT despite the limited efficacy data, we would offer it through informed consent and shared decision-making. In this situation, the patient would be carefully monitored through the first year of cat AIT to determine if continuation of treatment was safe and beneficial. It is the clinical experience of the contributors to this topic that some patients do benefit.

In contrast, if the patient has multiple additional allergies (eg, pollens, dust mites, molds) that could also be addressed with AIT, it is our clinical experience that AIT that includes all the patient's allergens (including cat) often does result in clinical benefit.

Duration of effect — Some information on the duration of effect is available from a series of Swedish studies in which patients received three years of AIT to cat and/or dog and were then evaluated five years after discontinuation of treatment [46,53-57]. Patients were not living with pets in the home. Of the 15 who had completed three years of cat AIT, six reported improved tolerance with continued symptom improvement in their asthma upon exposure to cats, six reported no change after treatment discontinuation, and three reported deterioration in their condition after discontinuation [57]. Thus, 12/15 reported lasting improvement five years after the end of a three-year course of cat AIT.

Thirteen of the study participants who had received cat AIT agreed to undergo bronchial challenge to cat allergen. Objective measures of bronchial sensitivity induced upon provocation with cat allergen had reverted toward pretreatment levels. Further studies are needed to provide clarity to this issue.

INVESTIGATIONAL APPROACHES — 

Novel approaches to treatment are of ongoing interest because of the uncertain efficacy of current therapies for cat-allergic patients who wish to continue living with cats.

A hypoallergenic cat — At present, there is no available hypoallergenic cat breed:

●A naturally occurring "nonallergenic" breed of cat is a myth that continues to be propagated by patients and even medical personnel. The major allergen (Fel d 1) is in the pelt/dander (skin flakes), saliva, and urine [18,58]. Thus, short-haired and "hairless" cats produce as much allergen as other breeds. (See "Allergen avoidance in the treatment of asthma and allergic rhinitis", section on '"Hypoallergenic" animals'.)

●The creation of a genetically modified "hypoallergenic" cat breed is an area of ongoing research. One company is using a gene-editing tool (clustered regularly interspaced short palindromic repeats [CRISPR]) to breed cats that produce little to no Fel d 1 [59,60]. Clinical trials will be needed to establish whether the resultant animals cause fewer symptoms in cat-allergic patients.

Immunization of cats — Immunizing cats against their own Fel d 1 is another approach that is under investigation. It may be possible to vaccinate a cat and induce high-affinity IgG antibodies capable of neutralizing the cat's own Fel d 1, thereby reducing the level of Fel d 1 expressed in the pet's secretions [61]. However, the long-term safety of this approach for the cat, as well as the degree to which it provides clinically meaningful benefit for the patient, remain to be determined.

Monoclonal antibodies

●Monoclonal antibody cocktail specifically against Fel d 1 – One company created two fully human, high-affinity IgG monoclonal antibodies designed to block Fel d 1 binding to IgE more efficiently than the antibodies induced by classic AIT. Phase I/II studies demonstrated improvements in both lung function and tolerance to the amount of cat allergen administered in a cat environmental chamber model (in cat-allergic patients with rhinitis), and this subsequently led to a phase III clinical trial in cat-allergic patients with asthma living with cats in their home [62-64]. However, based on lack of efficacy from its interim analysis, the company subsequently announced that it has terminated its phase III clinical trial due to futility [65].

●Anti-TSLP monoclonal antibody – Tezepelumab is a monoclonal antibody that blocks thymic stromal lymphopoietin (TSLP), a cell-signaling molecule that acts as an "alarmin," a substance capable of rapidly provoking an immune response, such as would occur in a cat-allergic person upon exposure to cat dander. Tezepelumab added to cat subcutaneous immunotherapy (SCIT) was compared with cat SCIT alone in patients with cat-allergic rhinitis who did not live with cats in the home. In a four-arm study (tezepelumab plus cat SCIT, tezepelumab plus placebo, cat SCIT plus placebo, or double placebo), patients were treated for 52 weeks and followed for an additional one year after treatment ended [66]. The patients' worst nasal symptoms during the 52-week nasal allergen challenge were reduced by 36 percent in the tezepelumab plus cat SCIT group as compared with cat SCIT alone. However, at a follow-up nasal provocation challenge one year after treatment, the benefit was reduced to a nonsignificant difference, suggesting a loss of the additional benefit with discontinuation of the drug.

Other forms of immunotherapy — A small number of studies of sublingual immunotherapy and investigational intralymphatic immunotherapy exist, but the data are less robust than those for SCIT [67-69].

SOCIETY GUIDELINE LINKS — 

Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Rhinitis".)

SUMMARY AND RECOMMENDATIONS

●Avoidance – The most effective management of allergic rhinitis and/or asthma due to cat allergy is avoidance of cats. However, patients with pet cats may be unwilling to rehome them. Even if they do so, cat allergen is ubiquitous in the environment, and some patients continue to be symptomatic due to exposures at school or work.

If a patient is willing to rehome their pet cat, they should understand that it can take 6 to 12 months for symptoms to improve to a noticeable degree. Cat allergen is notoriously "sticky" and can remain despite cleaning efforts, and residual allergens take time to lose bioactivity. (See 'Avoidance' above.)

●Environmental measures – Measures such as replacing carpeting with solid-surface flooring, encasing mattresses and pillows with allergen-impermeable coverings, weekly use of a vacuum with a high-efficiency particulate air (HEPA) filter, and employing HEPA filtration for a single room or a whole-house system can have a positive benefit with respect to reducing but not eliminating the total indoor cat allergen load. (See 'Environmental control' above.)

●Pharmacotherapy – Pharmacotherapy for allergic rhinoconjunctivitis and/or asthma due to cat allergy is identical to the general management of these disorders and is discussed separately. (See "Pharmacotherapy of allergic rhinitis" and "Allergic conjunctivitis: Management" and "An overview of asthma management in children and adults".)

●Immunotherapy – If cat-allergic patients are not willing to rehome their pets or if they do so and still have significant symptoms to cat allergen in the environment, a three-to-five-year course of subcutaneous allergen immunotherapy (AIT) is a viable treatment consideration. Patients should be referred to an allergist. (See 'Immunotherapy' above.)

•For cat-allergic patients no longer living with cats who remain symptomatic despite environmental control and a trial of pharmacotherapy, AIT is appropriate. We include cat allergen and all other allergens (eg, pollens, dust mite, etc) to which that individual is sensitized in the immunotherapy treatment solution. (See 'Patients not living with cats' above and "Subcutaneous immunotherapy (SCIT) for allergic rhinoconjunctivitis and asthma: Indications and efficacy", section on 'Summary and recommendations'.)

•For cat-allergic patients who continue living with cats, we explain that the effectiveness of cat immunotherapy in the setting of ongoing high-level exposure has not been demonstrated. (See 'Patients still living with cats' above.)

-For patients allergic only to cat, we discuss the lack of proven efficacy of immunotherapy only to cat with ongoing high-level cat allergen exposure. However, if the patient wanted to try immunotherapy despite the unclear efficacy, it would be reasonable to give a trial of a year, with monitoring for benefit before deciding to continue.

-Patients sensitized to multiple allergens may opt to try immunotherapy (containing all relevant allergens, including cat) if their other allergies are equally bothersome, although we explain that symptoms related to cat exposure may not improve, and shared decision-making is essential.

●Investigational therapies – Effective therapies for cat-allergic patients who wish to continue living with cats are lacking. There are no proven hypoallergenic breeds of cat. Immunization of cats to reduce their allergen levels and novel forms of allergen immunotherapy for cat owners are active areas of investigation. (See 'Investigational approaches' above.)