Web Chapter 42

Hot spots, more appropriately described as pyotraumatic dermatitis, are defined as a circumscribed, moist exudative dermatitis most commonly brought on by self-trauma. The self-trauma results from attempts to alleviate pain or pruritus associated with an underlying disease. Primary bacterial infections in theory could create localized moist exudative lesions with minimal self-trauma, but these are very rare.

The most common causes of hot spots are allergic conditions such as flea allergy, atopic dermatitis, adverse food reaction, scabies, and anal gland problems (Web Box 42-1). Complications from clipping or grooming such as razor burn or trauma also can create localized inflammation that results in pruritus and hot spot formation. Occasionally other infectious conditions (infection with Staphylococcus spp. or Pseudomonas spp., demodicosis, or dermatophytosis) can cause localized multifocal areas of pain and pruritus leading to hot spot–like lesions. Other less common causes of hot spots are listed in Web Box 42-1.

Although dogs of any breed can experience hot spots, certain breeds may be predisposed. These include the golden retriever, Labrador retriever, Saint Bernard, collie, and German shepherd. Importantly, many breeds at risk of hot spots also are predisposed to the common underlying causes of hot spots such as allergic dermatitis. Long hair coat length also has been thought to be associated with a predisposition to hot spots. However, in a report of 40 dogs with hot spots, 50% had short hair and 50% had long hair (Schroeder et al, 1996).

The role of Staphylococcus in the development of hot spots remains controversial. Staphylococcus organisms can be isolated from the skin of healthy dogs. Higher numbers of these organisms are found in allergic dogs, even in those without active skin disease. Therefore allergies may contribute to hot spots not only by inducing pruritus but also by creating a favorable environment for larger numbers of staphylococci to inhabit the skin. One investigator proposes that hot spots are of two types based on histopathologic patterns (Reinke et al, 1987). One is a superficial lesion in which bacteria are considered surface colonizers. The other is a folliculitis that may be a deep lesion. Coagulase-positive Staphylococcus spp., particularly Staphylococcus pseudintermedius, are cultured most commonly from these lesions. This same study showed a strong tendency for young dogs, golden retrievers, and Saint Bernards to be predisposed to the deeper form of hot spots. In another study, lesions in 44 privately owned dogs in a flea-scarce environment were separated histopathologically into four patterns based on the presence or absence of eosinophils or folliculitis (Holm et al, 2004). Eosinophils have not been recorded previously in pyotraumatic dermatitis but were seen in 29 cases. Acute folliculitis was seen in 20 cases. However, no correlation was found between histopathologic type and age, sex, breed, underlying cause, or site of lesion. Samples from 27 cases were cultured for bacteria, of which 25 grew S. pseudintermedius and 2 yielded negative results. In another study Staphylococcus was isolated from all lesions before topical treatment (Schroeder et al, 1996). In control groups treated with a placebo vehicle the condition cleared completely within 7 days without topical antimicrobial treatment. The role of Staphylococcus as a primary cause of hot spots certainly is unclear.