Canine Dermatology Series: Secondary Skin Allergies

Hot spots occur as single, intensely pruitic (itchy) lesions resulting from self-inflicted trauma to the skin. These lesions can appear within minutes and predominantly occur when allergic reactions within the skin cause a sensation of itchiness or pain. The dog attempts to alleviate the symptom by licking, chewing, or scratching which leads to trauma and further inflammation to the skin. Labrador retrievers, Golden retrievers, St. Bernards, Collies, and German Shepherds show an increased breed predisposition for developing hot spots most likely because these breeds have a higher incidence for allergic reactions which are the number one underlying cause for hot spots.

Cause: The most common causes for hot spot development in the dog are allergic conditions associated with: flea allergies, atopy, food allergy, parasites (mange), anal gland problems, clipping and grooming (which can cause localized inflammation of the skin), or skin infection. Yeast infections, drug reactions, and autoimmune disorders are less commonly the cause of hot spots.

TABLE 1: Causes of Hot Spots
Common Causes	Uncommon Causes
Allergies--flea, atopy, food	Dermatophytosis
Parasitic--scabies, Demodex	Injection site reactions
Anal gland disease	Drug reactions
Clipping or grooming	Autoimmune disease
Result of deep pyoderma	Panniculitis
	Vasculitis

From: Rosencrantz WS. Pyotraumatic Dermatitis ("Hot Spots"). In Kirk's Current Veterinary Therapy XIII, p. 549.

A role for Staphylococcus infection has been implicated in the development of hot spots. Staphylococcus infection as a cause for hot spots, however, remains controversial since Staphylococcus may inhabit the skin of many normal dogs without producing hot spots. Despite this, dogs with allergies have been found to have an increased number of Staphyloccus inhabiting the skin which suggests that allergies may predispose to both hot spot formation and Staphylococcus infections.

Symptoms: Hot spots appear most commonly on the rump, near the rectum, or on the face just below the ears. They present as intensely reddened, often round patches of moist, sometimes ulcerated areas of skin. The fur over these areas is often matted and coated with a serous discharge, which will eventually crust-over. Intense itching can result in large lesions within a matter of minutes. The lesions are painful and often swollen, and continued self-trauma of licking and chewing may result in thickened skin and scarring.

Diagnosis: Diagnosis of hot spots is easily made by observation of the lesions. The diagnosis of the primary cause for development of a hot spot, however, is frequently made through an elimination process. Skin scrapings are first performed to rule out the presence of parasitic infection such as Demodex, and the dog is usually examined for evidence of "flea dirt" (dried blood excreted by fleas), which would indicate flea infestation. For dogs with recurrent hot spots, additional laboratory work-up is often required. In such cases, cultures to identify bacteria or fungal infections are obtained, and in some instances a skin biopsy may be helpful for identifying some of the less common primary causes for hot spot development. If allergy is suspected, allergy testing or an elimination diet may be utilized. Finally, if all other approaches fail to identify a primary cause, laboratory testing for specific immune or metabolic disorders is often performed.

Treatment: Regardless of the subsequent choices for topical and/or systemic treatment of the hot spot, all the fur covering and immediately surrounding the lesion must be clipped or shaved. In many cases, sedation or a topical anesthetic is required to perform this task because of the sensitivity and pain associated with the lesion. Once the hair is removed, the lesion is gently washed with an antimicrobial shampoo containing either benzoyl peroxide or benzalkonium chloride. Care should be taken to ensure that all residual soap is removed and the lesion should be gently patted dry. Many dogs need to wear a protective device, such as an Elizabethan collar, to prevent incessant chewing, licking or scratching that will produce further trauma to the effected area and delay healing. The following table provides a list of commonly used products for the follow-up treatment of hot spots:

Treatment	Product	Administration	Effect
Astringents	5% tannic acid/aluminum acetate solution (Burow's Solution); Domeboro (1:40) in water, 25% silver nitrate and potassium permanganate (1:1000 - 1:30,000)	Topical	Use within first 24-48 hours; quickly dries out the lesion.
Anti-itch agents	methol (0.12-1%); camphor (0.12-1%); thymol (0.5-1%); cold ice packs	Topical	Substitute cold or heat sensation for itchy sensation
Anesthetic/desensitiz-ing agents	benzocaine; tetracaine; lidocaine; 1% pramoxine ("Relief" or "Dermacool"); benzoyl peroxide ("Oxydex Gel" or "Pyoben Gel"); tars	Topical	Short-acting itch and pain relief.
Anithistamines	2% diphenhydramine ("Histacalm")	Topical	Anecdotal reports of symptom relief
Glucocorticoids	1% Hydrocortisone ("CortiSpray", "Dermacool-HC", "PTD-HC", "Cortisoothe", "CortiCalm")	Topical or systemic	Safe and most effective topical treatment; systemic use should be limited only to short-term treatment since long-term use will cause immunosuppression and increase risk for dermal infections
Combination glucocorticoids and antibiotics	"Tresaderm"; "Gentocin Topical Spray"; "Panalog"; "Otomax"	Topical	Produce best control of symptoms and fastest healing; recommended for short-term use only
Antibiotics against Staphylococcus intermedius	Cephalexin; Primor; Clavamox; Baytril	Systemic	Administered for 14 days then 7 to 10 days beyond clincal cure to control recurrent infection

Prognosis: Dogs that recover from hot spots frequently go on to develop recurring lesions related to the primary underlying cause (i.e. allergy, parasites, etc.). Therefore, identification and elimination of the factors that contribute to hot spot formation are essential for controlling the development of these lesions.

Prevention: Humidity often seasonally coincides with primary conditions that bring on hotspots: fleas thrive in hot, humid weather, and black are more ferocious during the humid months. Additionally, frequent swimming can lead to ear infections that may increase risk for hot spots. When fleas or other biting insects are the primary cause of hot spots, systemic or topical flea preventatives or flying-insect repellants (i.e. "Flys-Off"), respectively, may be applied to the dog. If ear infections are the problem, cleaning the ears several times per week with a veterinary product such as Oti-clens (or similar product), especially following a swim, may help reduce incidence of ear infections and associated hot spot development. Additionally, following a swim, remove wet collars and dry the fur as completely as possible since moist fur in humid weather can lead to itchy skin and thus hotspots. Pollen can exacerbate skin allergies and lead to hotspots because of incessant chewing and scratching. Keeping the dog indoors during times of the day when pollen count peaks (early morning and early evening) helps reduce exposure to pollen.