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Pyotraumatic dermatitis (acute moist dermatitis, hot spot)

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01 June 2015, at 12:00am

DAVID GRANT continues the series of dermatology briefs

PYOTRAUMATIC dermatitis is a skin problem induced by selfinflicted trauma as a result of attempts by the patient to alleviate pain or pruritus.

Historically the condition was termed “summer eczema”, as many cases occur at that time of the year. It has also been classified as a surface pyoderma. Although bacteria may be found in lesions, true hot spots are not skin infections, and they do not usually respond to antibacterial therapy alone.

Pyotraumatic dermatitis is very common in dogs (Figures 1, 2 and 3) but rarely seen in cats (Figure 4).

The lesion arises rapidly and is acutely pruritic, enlarging and erythematous, hence the term “hot spot”. They are generally solitary but may spread if not treated promptly. It is more common in long-haired dogs but not exclusively.

The lesion is often painful and care is advisable when examining and touching the dog. Affected areas are often near the underlying cause, for example the posterior dorsal region in fleabite hypersensitivity cases, and the face in dogs suffering from otitis externa.

Underlying causes

  • Fleabite hypersensitivity – the most common.
  • Other parasites particularly if hypersensitivity to the parasite develops.
  • Allergic skin disease – atopy, food hypersensitivity.
  • Anal sac problems such as impaction or infection.
  • Poor coat condition.
  • Otitis externa.
  • Irritant contact dermatitis.
  • Painful musculo-skeletal conditions.
  • Psychoses. Although perhaps less common as an underlying cause, some dogs become intolerant to the pruritus and may demonstrate psychogenic traits such as manic biting or continuing self-trauma after the original stimulus has subsided.

Diagnosis

  • History.
  • Physical examination.
  • Cytology – this is obtained via an impression smear. The presence of bacteria is not marked in many cases. If bacterial involvement is identified by the presence of many bacteria, culture and antibiotic sensitivity testing is advisable.
  • Biopsy – not generally necessary but see under differential diagnosis.

Differential diagnosis

  • Superficial pyotraumatic folliculitis and furunculosis. The differentiation may be quite difficult. There are usually papules and pustules at the periphery of the lesion. Cytological findings demonstrate a more marked inflammatory response with greater numbers of bacteria. Response to treatment is usually rapid in cases of pyotraumatic dermatitis, but not in folliculitis and furunculosis. If doubt exists, histopathological examination will demonstrate involvement of only the surface epithelium in pyotraumatic dermatitis.
  • Demodicosis.
  • Dermatophytosis.

Clinical management

  • Deal with the underlying cause – particularly fleas.
  • Elizabethan collar to prevent further self-trauma.
  • Clip (general anaesthesia or sedation).
  • While under anaesthesia clean with chlorhexidine shampoo (Malaseb, Dechra).
  • Topical therapy such as Hamamelis solution, Dermacool (Virbac) is very useful as it can be sprayed, thus avoiding touching painful lesions. It is also astringent and drying.
  • Glucocorticoids are very effective especially in the more extensive cases such as those depicted in Figures 1 and 3. These may be:
  • Topical, in the forms of creams or as a spray, e.g. hydrocortisone aceponate (Cortavance, Virbac).
  • Systemic, such as dexamethasone (Dexafort, MSD) by injection at a dose of 0.5mg/10kg or prednisolone orally at a dose of 0.5-1mg/kg once daily for 5 to 10 days.
  • Antibacterial therapy is not usually necessary unless secondary bacterial infection is identified by cytology and culture. In these circumstances antibacterial treatment for a minimum of three weeks according to sensitivity testing.
  • Sedation short-term if needed.
  • Recovery is usually rapid and permanent if the underlying cause is recognised and dealt with.

Prevention

  • Good flea control all year round.
  • Clipping the coat in summer.
  • Regular bathing with antibacterial shampoos in susceptible dogs.
  • Regular cleaning of the ears and anal sacs.