Key Points for Clinicians and Public Health Professionals

Allergy to Cats and Dogs

A common misconception is that human beings are allergic to cat and dog hair and that short-haired breeds of cats and dogs are less allergenic than long-haired breeds. In fact, human allergy to cat and dogs is related to specific protein allergens, such as Fel d 1 and Can f 1, respectively, rather than hair per se. For this reason, despite advertising claims to the contrary, there are currently no “nonallergenic” breeds of cats and dogs. However, some species appear to provoke allergies in certain individuals and efforts are under way to develop breeds with lower allergenic potential. Cats produce a number of proteins that can act as human allergens. The one considered the most important, Fel d 1, is produced by sebaceous and salivary glands, basal squamous epithelial skin cells, and anal glands of cats.⁶ It can be carried on particles less than 5 microns in diameter that can remain airborne for long periods of time.⁷ A naturally selected line of cats with deficient Fel d 1 is being developed.² Dog breed, gender, and seborrhea appear to influence the levels of Can f 1 allergen detected on fur.⁸ Cat and dog allergens may persist for extended periods in a house even after a pet has left, and both Fel d 1 and Can f 1 have been detected in homes without pets.⁹

Risk Factors for Pet Allergy

Despite the prevalence of pet allergy, it is not clear whether having a pet in the house increases the overall risk of allergy. Host factors play an important role, especially the presence of atopy, defined as “the genetic propensity to develop immunoglobulin E [IgE] antibodies in response to exposure to allergen.”¹⁰ Studies have shown that children born into families that already own dogs (where there is no history of atopy) have a lower risk of allergic disease by 2 years of age compared with families that did not have a dog.¹¹ The genetic basis of allergic predisposition is a subject of active scientific investigation. The timing of allergen exposure also appears to play a role. Although it has been reported that early exposure to cats can increase the risk of early sensitization and development of an immune response to cat allergens, other studies have shown a protective effect for pet ownership early in life, and still others have demonstrated inconsistent results regarding the development of early clinical allergic disease (particularly asthma).^12,13 Farm exposure in early life is also associated with an allergy-protective effect in human beings.¹⁴

Rhinitis and Upper Airway Symptoms

Rhinitis and upper airway symptoms are among the most common complaints in outpatient medical practice. The prevalence in the general population is as high as 20%, but in patients with asthma the prevalence is generally much higher. Symptoms include sneezing, nasal passage obstruction, lacrimation, and itching of the nasopharynx. The diagnosis is generally made by history and physical examination. One of the most important distinctions to make in chronic rhinitis and sinusitis is whether it is allergic or nonallergic. Nonallergic rhinitis and rhinosinusitis can occur in the context of exposure to airborne irritants in the environment. Especially if these irritant exposures continue, this condition is less amenable to treatment with pharmacotherapy.

Diagnostic testing for rhinitis and other allergic conditions related to indoor air exposure can include skin prick or blood testing for IgE antibodies to specific antigens. Skin prick testing involves noting the immediate wheal-and-flare reaction to antigens introduced under the skin. Blood testing using radioallergosorbent testing (RAST) can screen for a wide number of IgE antibodies to specific antigens. Both skin testing and RAST testing are associated with false-positive and false-negative results, and this possibility must be considered in reviewing the results of such testing.

Management of upper airway disorders is generally a combination of exposure reduction and pharmacotherapy. Exposure reduction typically involves multiple measures, such as eliminating irritating chemicals, cleaning more frequently and/or adopting changes in cleaning techniques, and (if relevant) removing or isolating pets from the bedroom and removing fabrics that retain dander and allergens. These measures have been effective in reducing allergen exposure and improving symptoms in the aggregate but not necessarily in the individual patient. Pharmacotherapy includes nasal steroid sprays and antihistamines to control allergic symptoms.

Asthma

Human asthma is characterized by bronchoconstriction and inflammation with airway secretions, leading to obstruction of the airways. Clinically, the hallmark symptoms of asthma are wheeze, cough, and dyspnea. The pathogenesis of asthma involves reversible airway obstruction, and treatment with both short- and long-acting medications can reverse or prevent the symptoms. A variety of allergens can lead to sensitization and new onset of asthma, or exacerbation of underlying asthma in previously sensitized individuals, including animal-related allergens such as dander, cockroach excreta, and dust mites. The role of domestic animals in asthma causation and prevention is complex. Some studies show that possession of pet dogs and cats may be associated with a lower incidence of early childhood asthma, whereas other studies suggest that pet possession may lead to an increased risk of asthma. These differences may be due in part to the nature of the population in question. The effect seems to vary with the age of the children, whether they are in environments with high or low prevalence of pet ownership, and whether there is a family history of allergic disorders.¹⁵ At this time, therefore, it is not possible to offer general recommendations regarding ownership of pets for young families if there is no current asthma or atopy in the family.

In evaluating a patient with suspected asthma, it is important to confirm that reversible airway disease is present. If a patient has spirometry showing obstruction, reversibility can be demonstrated by improvement in spirometry flow rates after bronchodilator administration. If spirometry is normal at baseline, a methacholine challenge test can document abnormal bronchial reactivity.

As with rhinitis, allergic asthma must be distinguished from nonallergic asthma. Nonallergic asthma may be due to exposure to chemical irritants. Diagnostic testing often involves skin prick or RAST testing. A variety of agents are available for asthma treatment, including inhaled corticosteroids, bronchodilators, leukotriene inhibitors, and mast cell stabilizers.

Anaphylaxis

Anaphylaxis is an acute, severe, IgE-mediated reaction to environmental allergens that can occur in sensitized individuals. The clinical presentation of anaphylaxis can involve swelling of the lips and throat with progressive airway edema leading to obstruction and respiratory collapse and death if not treated.

Anaphylaxis can occur as a result of exposure to insect stings (see Chapter 8) but can also occur with exposure to other animal allergens. Anaphylactic reactions can also occur when the immediate precipitating factor is unknown. The diagnosis is typically made by clinical history; laboratory testing is of limited value. Although tryptase or histamine levels may be elevated acutely after an anaphylactic episode, they usually return to normal by the time of a clinical evaluation. The presence of elevated serum IgE to a specific antigen can be of benefit in a patient with a history of anaphylactic episodes, but it is of limited value in predicting whether an asymptomatic patient is at risk of future anaphylaxis because many people with elevated IgE levels do not have anaphylactic symptoms. Although the negative predictive value of IgE is relatively high, occasionally people with normal IgE levels can have anaphylactic reactions to an antigen.¹⁶ The treatment of anaphylactic episodes may include epinephrine, antihistamines, and steroids.