A. Introduction

1.  Pruritus is the predominant clinical sign of allergic skin disease. Pruritus is defined as a sensation that elicits the desire to scratch. Pruritus is the most common reason for a dog to be presented to a veterinary hospital. Pruritus may be manifested in animals by licking, chewing, rubbing, hair removal, harassing of owners, irritability, and personality change (lack of tolerance, aggressive behavior). Cats tend to be more secretive about self-trauma.

2.  The concept of a "Threshold Phenomenon" is important in the understanding of pruritus. An individual may tolerate a certain pruritic load without provoking clinical signs but a small increase in that load may push that individual over the threshold initiating clinical signs of pruritus. While some mediators initiate the sensation of pruritus, others simply lower the threshold of pruritus.

3.  "Summation of Effect" occurs whereby additive pruritic stimuli from different coexistent skin diseases may raise an animal above its individual pruritic threshold.

B. Allergic Skin Diseases--Frequency

1.  Globally, the most common allergic canine skin diseases are, in frequency of occurrence, flea allergy dermatitis, atopic dermatitis, and food allergy (adverse reactions to food). These data are more controversial for the cat, but rank order of occurrence probably is the same.

2.  Flea allergy dermatitis is not only the most common skin disease seen in small animal practice in most countries in the world; it is the most common disease of any organ system seen in small animal practice worldwide. Fleas parasitize animals in virtually every area on earth with the exception of locations above 1500 meters elevation and regions such as deserts with very low humidity. However, somewhat surprisingly, flea allergy dermatitis is commonly underdiagnosed all over the world. There are many reasons for this including socio-cultural biases against having ectoparasites.

3.  The reported frequency of occurrence of different allergic skin diseases varies widely from study to study and is highly controversial. Many dermatologists have expressed the opinion that in parts of the world where fleas are common, flea allergy dermatitis comprises between 50% and 80% of allergic skin disease.

4.  Recent data published by Jackson from a referral dermatology clinic at North Carolina State University offers data on the frequency of atopic dermatitis and food allergy in comparison to each other. However, their data on the frequency of flea allergy dermatitis, and flea allergy seen in conjunction with other allergic skin diseases seems surprisingly low. At UC Davis, our dermatology service sees flea allergy dermatitis as the most common allergic skin disease despite being a referral center.

a.  Jackson & others looked at 91 dogs with allergic skin disease:
46%--Atopic Dermatitis only,
23%--Food Allergy only,
20%--Atopic Dermatitis & Food Allergy,
4%--Food Allergy & Flea Allergy Dermatitis
3%--Atopic Dermatitis, Food Allergy, & Flea Allergy Dermatitis
2%--Flea Allergy Dermatitis only (bias of a referral center?!)

b.  One can extrapolate that 66% have atopic dermatitis, 43% have food allergy, 20% of these dogs have both diseases.

C. Differentiating Allergic Skin Diseases by Predilections

1.  Predilections vary between the 3 most common allergic skin diseases.

2.  Prioritize your index of suspicion based on differences in signalment, history, and physical findings (most common affected sites, lesions).

D. Signalment Predilections--Dogs

1.  Breed predilections:

a.  Flea Allergy Dermatitis--Any breed, but more common in 'allergic breeds'.

b.  Atopic Dermatitis--Golden Retriever, Labrador Retriever, Wirehaired Fox Terrier, Dalmatian, West Highland White Terrier, Weimaraner, other small Terriers, Irish Setters, Chinese Shar Pei, Spaniels.

c.  Food Allergy--(Controversial) Labrador Retrievers, Cocker Spaniels, Golden Retrievers, Soft-Coated Wheaten Terriers, Dalmatians, West Highland White Terriers, Collie Dogs, Chinese Shar-Peis, Lhasa Apsos, Springer Spaniels, Miniature Schnauzers, Dachshunds, Boxer Dogs, German Shepherd Dogs.

2.  Age predilections--Age of onset:

a.  Flea Allergy Dermatitis--Any age, more common in dogs >6 months of age.

b.  Atopic Dermatitis--Average 1 1/2-3 years, possible 6 months-6 years.

c.  Food Allergy--Highly variable, can began from 4 months to 14 years, up to 50% may develop the disease at less than 1 year of age.

3.  Sex predilections--Useful or reliable sex predilections not reported.

E. Signalment Predilections--Cats

1.  Breed predilections:

a.  Flea Allergy Dermatitis--Any breed.

b.  Atopic Dermatitis--Any breed.

c.  Food Allergy--Possible breed predilection for the Siamese Cat.

2.  Age predilections--Age of onset:

a.  Flea Allergy Dermatitis--Any age, more common in cats >6 months of age

b.  Atopic Dermatitis--Average 1½-3 years, possible 6 months-6 years?

c.  Food Allergy--Highly variable, reported to occur in cats between 6 months of age and 12 years. Approximately ½ of cats reported have been less than 2 years of age.

3.  Sex predilections--Not reported.

F. History Predilections--Dogs & Cats

1.  Speed of onset:

a.  Flea Allergy Dermatitis--Rapid onset.

b.  Atopic Dermatitis--Insidious, gradual onset.

c.  Food Allergy--Insidious, gradual onset, occasionally more rapid onset.

2.  Seasonality predilections:

a.  Flea Allergy Dermatitis--Allergen availability, warm weather seasonal, contingent on flea availability, severity of winters.

b.  Atopic Dermatitis--Allergen availability, commonly begins seasonally (summer), annually >time spring & fall, finally year around.

c.  Food Allergy--Should not be seasonal.

G. Physical Findings Dogs--Site Predilections

 Flea Allergy Dermatitis--Caudal ½ of dog, bilaterally symmetric dorsal lumbosacral, tailbase, perineum, medial & caudal thighs, umbilicus.

 Atopic Dermatitis--Face, paws, distal extremities, ears, ventrum, caudal carpus.

 Food Allergy--Face, ears, extremities, paws, ventrum.

H. Physical Findings Dogs--Lesions

 Flea Allergy Dermatitis--Crusted papules, erythema, excoriations, chronic changes--lichenification, hyperpigmentation, alopecia, fibropruritic nodules, secondary infection.

 Atopic Dermatitis--Self-trauma leading to erythema, alopecia, excoriation, chronic changes--lichenification & hyperpigmentation, secondary infections, few 'primary' lesions.

 Food Allergy--Self-trauma leading to erythema, alopecia, excoriation, chronic changes--lichenification & hyperpigmentation, secondary infections, few 'primary' lesions, (similar to atopic dermatitis).

I. Physical Findings Cats--Site Predilections

 Flea Allergy Dermatitis--Dorsum from neck to rump, bilaterally symmetric, perineum, medial & caudal thighs.

 Atopic Dermatitis--Head, neck, paws, distal extremities, ears, ventrum.

 Food Allergy--Localization of pruritus to the head and neck is common (42% [White] and 65% [Guaguère]). Pruritus may be generalized as well.

J. Physical Findings Cats--Lesions

 Flea Allergy Dermatitis--Miliary dermatitis (diffuse crusted papules) excoriations, eosinophilic plaques.

 Atopic Dermatitis--Self-traumatic excoriations, Miliary dermatitis (diffuse crusted papules) excoriations, eosinophilic plaques, indolent ulcers.

 Food Allergy--Erythema, excoriations, erosions, ulcerations, and crusts. In general, the magnitude of self-mutilation in cats is greater than that seen in dogs. Miliary dermatitis and symmetric alopecia associated with excessive grooming may be seen. Some eosinophilic plaques and indolent ulcers are reported to respond to elimination diets. Angioedema and urticaria also have been reported as additional rare reaction patterns.

K. Differentiating Allergic Skin Disease by Ruling out Flea Allergy & Food Allergy

1.  There are no laboratory tests available that consistently can rule in or out allergic skin disease in the dog.

2.  Therefore, since flea allergy dermatitis is the most common allergic skin disease, it is logical to rule out flea allergy by several months of strict flea control.

3.  If pruritus remains after strict flea control, initiate a strict elimination diet to rule out food allergy.

L. Ruling Out Flea Allergy Dermatitis

1.  Convince the owner that flea allergy may be the correct diagnosis--Defuse cultural bias, explain why fleas not seen, (flea allergy--most common disease on the planet!).

2.  Show defects in current or past flea control--treating all animals, in & out cats, consistency, substitution of over-the-counter products, environmental issues.

3.  Institute modern flea control for 6-8 weeks--"Prove me wrong!"

M. Ruling Out Food Allergy

1.  Convince the owner that an elimination diet must be incredibly strict.

2.  Show defects in past attempted elimination diets (snacks, food with medicine, flavored heartworm preventatives, IDIs, or other medications, flavored toothpaste.

3.  Institute strict elimination diet for 8-12 weeks. If improvement, continue diet. Do not accept food allergy as the diagnosis without rechallenge with the original diet!.

4.  When rechallenging with the initial food, do not change anything else in the treatment regimen (antibiotics, anti-yeast medications, flea control, antihistamines, topical therapy).

N. 'Ruling in' Atopic Dermatitis

1.  Atopic dermatitis is a diagnosis of exclusion.

2.  'Allergy testing' (either intradermal testing or allergen-specific IgE serology) are performed only in animals that meet the clinical criteria of atopic dermatitis and where other pruritic skin disease that can be more readily diagnosed definitively have been ruled out. Testing is only performed to select antigens in animals where the decision already has been made to embark on the lifetime pathway of allergen specific immunotherapy.

O. Management of Concurrent Skin Diseases

1.  Allergic skin diseases commonly coexist. One must manage ALL allergic skin diseases present to be successful, thus reducing the animal below threshold.

2.  Concurrent confirmed or suspected flea allergy dermatitis must be managed.

3.  Concurrent secondary pyoderma or bacterial overgrowth and Malassezia dermatitis must be managed. Secondary infection is less common in cats than in dogs but probably is underdiagnosed.

References

1.  Rust MK. Advances in the control of Ctenocephalides felis (cat flea) on cats and dogs. Trends Parasitol. 2005; 21:232-236.

2.  Roudebush P. Ingredients associated with adverse food reactions in dogs and cats. Adv Small Anim Med Surg 2002;15(9):1-3.

3.  Jackson HA, Murphy KM, Tater KC, et al. The pattern of allergen hypersensitivity (dietary and environmental) of dogs with non-seasonal atopic dermatitis cannot be differentiated on the basis of historical or clinical information: a prospective evaluation 2003-2004 (abstract). Proceedings North Am Vet Dermatology Forum, 2005:196.

4.  Olivry, T. et al. (2001) The ACVD task force on canine atopic dermatitis. Vet Immunol Immunopath, 81, 143-387.

5.  Gross TL, Ihrke PJ, Walder EJ & Affolter VK. Skin Diseases of the Dog and Cat. Clinical and Histopathologic Diagnosis. Blackwell Scientific, pp 6-9, 142-146, 200-211, 406-410

6.  Reedy LM, Miller WH, Willemse T: Allergic Skin Diseases of Dogs and Cats.2nd Edition. W. B. Saunders Company, Philadelphia, 267 pages, 1997.