In clinically relevant allergy, individuals suffer reproducible hypersensitivity reactions with exposures to an allergen at a dose tolerated by non-allergic individuals. Diagnosis has routinely involved obtaining a good medical exposure history coupled with detection of sensitization. The presence of allergic sensitization has been identified using serum specific IgE or SPT with extracts. Historically, quantifying symptoms after exposure challenges such as: induction of asthma with measurement of spirometry changes/peak flows; allergic rhinitis with nasal peak flow; and conjunctival provocation with tear formation have not been used in clinical practice, only in allergen research. These valuable tests clinically assess reactivity to specific allergens at the organ where it affects and should correspond well to natural exposure. More recent provocation testing in individuals with negative skin testing and normal serum specific aeroallergen IgE testing have identified IgE antibodies in the nose and therefore these individuals suffer from locally induced allergic rhinitis.24 Because provocation tests are time and resource intensive, they are usually available only in large research settings, but may be needed in individuals who have persistent symptoms with exposure who are skin test and specific IgE negative.

Evaluating for dog sensitization is significantly more complex and challenging than for cats. The correlation between serum IgE to dog and skin testing is poor with only 52.2% agreement and a correlation coefficient r=0.04.25 In vitro assays have sensitivities averaging 70%–75% for most other aeroallergens compared with symptoms induced after either natural or controlled challenges while other suggest specificity ranging from 30%–95% overall.26 Most clinicians use a specific IgE cutoff level greater than 0.35 kU/L in defining sensitization. Data suggests lower levels of detection may improve diagnostic accuracy,27 but those results are rarely reported.

There remains great difficulty in using SPT for detecting dog allergic patients. Commercially available dog extracts used in skin testing is composed of multiple proteins and their potency in extracts can vary considerably, by up to 1,000-fold.27 Dog component allergens have been identified in serum, dander, pelt, hair, saliva, and urine. Extracts prepared from dog liver, serum, salivary glands, and keratinocytes contain fewer IgE binding allergens compared to dander. Initial studies that showed the importance of the dog component allergen Can f 1 suggests that dander is the preferred source for most commercial dog allergen extract preparations,28 but not all companies use that source and there is no idea on the amount of other component allergens present.

Considerable variation in SPT results depending on the extract used in testing.29 The concentrations of clinically relevant individual dog protein allergens may not be at a high enough level and the source material may not contain the appropriate amounts of allergens leading to false negatives. Though patients report considerable symptoms with exposure, commercial crude allergen extracts show slight positive or even negative results.30 Dog crude extracts are not standardized and the amounts of identified allergens in each extract are unclear.

Increasing the confounding factors, crude dog allergen extracts are contaminated with other allergens. Contamination with other allergens that individuals are allergic will cause false positives in both skin testing and in IgE testing when that extract is being used as the target. Allergens such as dust mites have been found in crude dog allergen extracts.31 More effort is required to define what is in each crude extract and improve their purity thereby making skin testing more reliable in identifying sensitized patients. Variations in dog specific component allergens and the contamination with other proteins severely limits the utility of crude dog extracts in SPT to identify sensitized individuals. Unlike allergies to cats, allergies to dogs are significantly less defined and the importance of pure high quality extracts is more critical.

Detecting sensitization to cats has been easier than dogs. Purification and characterization of the cat component protein Fel d 1 occurred in 1973 by Ohman et al.32 This has led to measuring class specific antibodies, production of protein specific antibodies used to measure exposure, crude extract standardization, and generation of T cell specific peptides. In individuals who had a positive exposure challenge, 38 of 41 (92.7%) had positive SPTs.33 Compared to a historical recollection of symptoms to exposure, SPT results equal or >3 mm to cat had a sensitivity of 0.90 sensitivity, specificity of 0.90, with a diagnostic accuracy of 0.90.34 The authors also noted that a tenfold dilutions of cat extract reduced sensitivity without significantly improving specificity.34 Concordance between Phadebas RAST and SPT was 94%35 and Pharmacia CAP System and SPT was 91%.36

In cats, Fel d 1 is the most dominant protein cat allergen where up to 96% of all patients react.37 It does not have significant IgE cross-reactivity with other mammalian proteins and the functional role of Fel d 1 in cats remains unknown. While airborne, it is associated with larger particles (>9 µm) but about 23% is carried on small particles (<4.7 µm in diameter) that stay suspended for several days.38 Though Fel d 1 is the most important component cat allergen, others have been identified (Table 1). Crude extracts for cats, therefore, have been more easily standardized since it can be based on Fel d 1, and therefore advances in cat allergy treatment have been more forthcoming.

Unlike cat allergies where most individuals react against one main protein, the diagnosis of dog allergies is more complex. Slowly, individual dog proteins have been isolated and characterized. Currently, there are 7 named dog component allergens identified as Can f 1–7 by the International Union of Immunological Societies that has standardized allergen nomenclature (Table 2). Though many are classified as a “major” allergen, only >50% of allergic patients need to react,39 and of these, no dog allergen has been identified to have the extent of reactivity compared with cat allergic individuals and Fel d 1. Further complicating studies in dog allergies, allergen concentrations vary between breeds and within.304041 Older dogs produce more dander than younger ones because their skin is drier and some breeds are more prone to eczema and oily seborrhea. Seasonal variation also affects the level of dog IgE antibodies.42

Molecular based diagnosis of allergies is an important advancement in improving sensitivity and specificity43 and will be needed to further define dog sensitization. Assessment of sensitization to allergen components can be done individually (which provides quantitative results on each allergen component), or using an allergen microarray chip (which provides semi-quantitative measurements of IgE antibody binding to numerous components simultaneously). To date, commercially available specific IgE components for dogs (Can f 1, Can f 2, Can f 3, and Can f 5) and for cats (Fel d 1, Fel d 2, and Fel d 4) exists. Studies utilizing component resolved diagnosis in academic studies note that of those who were sensitized, only 64% of adults are Can f 1 IgE positive, with only 32% of those individuals mono-sensitized to Can f 1.44 This suggests that if crude allergen extracts are not sufficiently enrich for each of the other component dog allergen(s), clinicians may be falsely assuming these patients are not dog sensitized.

Diagnosis of allergies based on exposure to specific proteins is known to be critical. Gradually, allergy diagnose is moving away from the crude source of allergens and towards identifying specific causative proteins. For example, the understanding of component allergens in peanuts where Ara h 1, 2, and 3 sensitization is associated with a systemic allergic response in 87% of children with IgE reactivity45 has led to better care. Unfortunately, it will remain unknown what clinical reactions occur with exposure to each dog component allergen without further research. But, individual dog allergen components are slowly being characterized. A greater understanding of the specific proteins that cause specific symptoms based on similar structural features and amino acid sequences may be more important than the exact source of the allergen. In vivo and in vitro testing using the microarray ImmunoCap ISAC technique suggests that allergic sensitization to common pets increases the risk of developing further sensitizations to other furry animals leading to allergies with exposures to them.46 It is clear that sensitization to one allergen predisposes individuals to later sensitization to other aeroallergens, but the mechanism remains unknown. Identification of cross-sensitization with other plants and animals will be important in understanding the development of allergies, how more accumulate, and in clinical management. Advances in linking symptoms caused by specific allergens will improve medical care as tests for a patient's allergen component sensitization profile become more available.

Current recommendations for controlling clinical symptoms caused by dog and cat allergens primarily includes avoidance to exposures. Bathing animals at least twice per week reduces allergens and can eliminate reactions.60 Immediate removal of the pet from the household will not alleviate symptoms particularly if the owner has carpeting in their home. Mammalian allergens are very stable and persist in house dust up to 6 months.

Symptomatic management with the use of anti-histamines and localized steroids are used when avoidance strategies have been utilized but symptoms still continue. Data on dog allergen subcutaneous immunotherapy (SCIT) show evidence of effectiveness, but is clinically less effective than for cats. This slow introduction of allergens to gradually larger doses and consistent exposure to high levels of allergen during immunotherapy for 3–5 years is associated with changes in T cell function that suggests a shift away from T helper (Th) 2 to Th1 phenotype and induction of regulatory T cells.61 The first studies in dog immunotherapy reported in 1963, showed symptom alleviation in a case series of 11 patients treated with dog extract.62 Since then, numerous trials have been conducted to define the true efficacy of dog immunotherapy. Immunologic markers are altered after dog immunotherapy similar to that seen in cats, but, acetone-precipitated extract containing 15.0 µg of Can f 1 per 0.5 mL maintenance dose was required.63 It is unclear if lower levels of Can f 1 in the other commercial extracts will yield similar results since they only contain around 2.5 µg of Can f 1 per 0.5 mL maintenance dose. In contrast, SCIT for cat allergies have been demonstrated to have clear efficacy. Even as early as 5 weeks, nasal symptom and nasal provocation data showed good response (P<0.001).64

The treatment of dog and cat allergies with immunotherapy depends on reliable and consistent allergen extracts. Unlike cat allergen extracts, differences in the manufacturing of dog allergen extracts exist even today. In the United States, allergens are typically manufactured in an aqueous solution. In Europe, products used for SCIT are usually prepared with aluminum hydroxide which forms a complex with the active allergens and is thought to act as a depot thereby releasing the allergens more slowly. Without adequate standardization, the amounts of different specific protein allergens vary enormously, thus making assessment of therapeutic improvement impossible. Safety of SCIT is also an issue, if a patient is highly sensitive, different manufactured lots even from the same manufacturer with different amounts of component allergen proteins can lead to adverse reactions when those individuals are suddenly exposed to high levels.

Component testing measuring patient specific sensitization patterns to dogs and cats will help determine primary sensitization vs. symptoms due to cross-reactivity. These specific components can also be therapeutic targets. Recombinant proteins, peptides, recombinant hypoallergens which mimic natural proteins but are modified to be less allergenic during immunotherapy, and multimeric recombinant allergens are being developed for use in dog and cat allergic individuals with some showing promise. Of these, linear peptide sequences of native allergens have been studied as a novel method to be used in allergy vaccinations for Can f 1.65 These peptides are thought to be less allergenic since they have a reduced ability to cross link IgE epitopes and thus do not initiate cascades of mast cell degranulation. Linear peptide sequences are hypothesized to induce T cell tolerance through the induction of regulatory T cells that can secrete the immunoregulatory interleukin (IL)-10. But, clinical trials suggest that side effects may still exist with peptide treatments. In subjects given intradermal vaccinations of short overlapping peptides of the major cat allergen Fel d 1, 9 out of 40 (22.5%) exhibited late asthma reactions without early or cutaneous responses that may be major histocompatibility complex (MHC) restricted.66 Further refinement of specific peptides and personalization with an individual's MHC will be important. Since dog allergic individuals are not all allergic to only Can f 1, combinations of the appropriate component allergens will be required for optimal therapeutic interventions. To address this, a multimeric recombinant Can f 1, Can f 2, Can f 4, and Can f 6 allergen was created for diagnostic and therapeutic purposes with initial studies showing promise.67 Although the ultimate goal is effective therapeutic intervention with curative treatments, the inability to accurately diagnose dog allergic individuals severely hampers advances. This key factor of having standarize cat allergen extracts amoung others (Table 3), has faciliated advances in treating cat allergies as compaired to dogs.

There has been much debate on the existence of hypoallergenic pets. As noted, of the identified and named specific dog protein allergens, there is none that dominates all others to the extent that Fel d 1 does in cats. Even in cats, there are still 8 specific allergenic proteins that would need to be targeted. The elimination of just Fel d 1 would only help a minority of cat allergic patients who are mono-sensitized to only Fel d 1. Similarly, in dogs, there is concurrently co-sensitization with other dog component allergens most notably Can f 2 with Can f 1. To date, there is no truly hypoallergenic cat nor dog since all of the protein allergens would need to be inactivated.

“Hypoallergenic dog breeds“ though have been used to market dogs that are thought to be less allergenic than other breeds, however, allergen levels vary enormously within breeds and between them.30 The amount of shedding and length of hair are thought to be protective since pet dander and saliva stick to hair and are not released into the environment, but studies find that there was no difference in the air or on the floor.68 In a study measuring the level of allergen Can f 1 via enzyme-linked immunosorbent assay (ELISA) in breeds thought to be “hypoallergenic” (Labradoodle, Poodle, Spanish Water Dog, and Airedale Terrier) found levels in the hair and coat of dogs as well as floor and airborne dust to be similar to non-hypoallergic breeds (Labrador Retriever and a control group).68 As many of these allergens are still being characterized with respect to their allergenicity, their true function in vivo in dogs and cats still remain unclear and inactivating all of them may have a detrimental impact on these animals.

Numerous unmet needs continue to plague the diagnosis and treatment of allergies especially to dogs (Table 4). Of primary importance is the need to create a high quality well characterized dog allergen preparation. Currently, different manufacturers use different processes with different raw materials to create crude extracts of uncharacterized quality and content. These products are not generic, but differ widely in compositions and therefore IgE binding capacity.69 This yields widely variable results in IgE testing as well as SPT. No international standard nor standardization method has been developed to date. All the different commercial products perform differently on patients leading to inconsistent results and data. This results in conflicting data that cannot be extrapolated and interpreted on overall dog allergies and the effectiveness of different therapies. Advanced protein isolation techniques should be used to manufacture pure extracts free of contamination with high component allergen protein concentrations. Data suggests that recombinant allergens may be comparable to natural proteins and should be considered.

The utility of component diagnosis is currently limited to academic studies. Though important, commercial labs detect IgE only to commercially available Can f 1, Can f 2, Can f 3, and Can f 5 and not the entire complement of named dog allergens. Additionally, these components have yet to be characterized in crude extracts used in skin testing for diagnosis and in extract mixtures used in therapeutic immunotherapy. Without adequate quantification of each component, quality studies on the efficacy of immunotherapy (both subcutaneous and sublingual) will be limited. No common structural feature of all allergens has been described due to the limited numbers of clearly defined allergen structures. Increased characterization of protein allergens will lead to better information regarding cross-reactivity thereby helping in the counseling of patients. The definition of an allergen is based solely on the ability of eliciting an IgE response in susceptible individuals. Future directions dictate continued utilization of 2-dimensional (2D) gel electrophoresis coupled with mass spectrometry when screening serum of individuals with clinical symptoms to dog exposure to identify potentially novel allergens. Continued efforts should be committed to fully characterizing these proteins including resolution of their native structure with confirmation not just of their immunogenicity, but also their true clinical allergenic nature with the use of provocation tests. Evidence suggest that each component allergen protein may illicit a unique symptom profile which needs to be defined. Longitudinal studies are required to define which component is protective, at what level, and which induces localized, systemic, or asthmatic symptoms.