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Skin infections, MRSI, perpetuating factors,topicals, antibiotics and more (Proceedings)

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Nov 01, 2010

Skin infections with bacteria (pyoderma) or yeast (Malassezia dermatitis) often are found in dogs secondary to other diseases such as seborrhea, endocrine diseases and allergic diseases. Many of these cases have abnormalities in skin barrier function or desquamation and even when the primary disease is controlled if this defect is not corrected the dog may still be prone to recurrent infections though episodes may be less severe or less frequent. In chronic or recurrent infections other factors may develop which are referred to as perpetuating factors. The most common bacteria to cause skin infections in dogs is Staphylococcus pseudintermedius which used to be called Staph intermedius. However in some cases other bacteria such as Enterococcus, Corynebacteria, E. coli, and Pseudomonas may be pathogenic. The diagnosis of a skin infection is simple if the lesions are recognized and sampled for cytology.

Success treating skin infections requires appropriate antimicrobial therapy and this has been the main emphasis of veterinarians for many years. Topical therapy though considered helpful can actually be essential to successful therapy and in some cases with resistant bacteria such as methicillin resistant Staph (MRS) may become the main or sole method to eliminate the infection. MRSI in dogs presents the same as other methicillin susceptible Staph and is only suspected due to failure to respond to normally effective antimicrobials such as cephalosporins. In general these have not been a zoonotic risk of disease to humans though the same is not true when dogs have MRSA. Successful long term management will require that underlying primary diseases are identified and managed and predisposing factors are eliminated or controlled. Additionally any pathologic changes in the normal anatomy or physiology of the skin that occur because of the inflammation from the infection need to be reversed or controlled. If any part of these components are not addressed then more antimicrobial therapy will be required and success will be limited.

DIAGNOSIS

The diagnosis of pyoderma requires a skin lesion that has neutrophils with bacteria present which are preferably found intracellular within inflammatory cells. If cocci are seen then most commonly the pyoderma is due to Staphylococcus though definitive identification requires a culture. Bacterial overgrowth is diagnosed when no inflammatory cells are present but bacteria are present in abnormally high numbers. The author utilizes greater the one cocci or 0.5 rods per OIF (1,000X) based on work by Dr Colombo. lHistopathology is also helpful in diagnosing pyoderma though bacteria are not often seen. Histopathology is also used to identify primary diseases as well as perpetuating factors. Most suppurative folliculitis and perifolliculitis occur because of pyoderma. The presence of bacteria in a crust or the stratum corneum is also significant. Histopathology is not a good way to diagnose Malassezia though when any yeast are seen the diagnosis should be strongly considered.

The real key to diagnosing pyoderma and Malassezia is to make sure multiple sights and types of lesions are cytologically evaluated. There are many cases where one site may reveal on diagnosis while another site on the same patient and visit may reveal something else. This means it is important to recognize all the lesions that may be seen with these diseases.

THE LESIONS SUGGESTIVE OF -

Pyoderma

The classic primary lesions of pyoderma are: Pustules, furuncles, fistula. Other lesions suggestive or compatible with pyoderma include: Crusts, papules, nodules, lichenification. The spreading ring of scale (epidermal collarette) associated with some erythema, exudate or crusting is also very typical of pyoderma.

The lesions of pyoderma most commonly affect the ventrum, groin and axilla, interdigital and occasional dorsal and lateral thorax. Ventral cervical lesions and anterior elbow are also common sites, especially in atopic dogs.

PERPETUATING FACTORS

Chronic recurrent pyoderma may lead to pathologic changes that become self perpetuating. This helps to explain why some allergic dogs that have their allergies well controlled may still get recurrent pyodermas for a period of time. Infections induce pathology in the skin and this may also contribute to the difficulty in eliminating the infection. Folliculitis can stimulate follicular hyperkeratosis and dilation altering the follicular ostia. Bacteria have greater access to these abnormal follicles and antimicrobial therapy may be required until these pathologic changes can normalize. Folliculitis may result in furunculosis and the released keratin and hair shafts stimulate a foreign body reaction. Though normally this material is broken down and eventually eliminated some cases develop persistent hair shaft sequestrum that appears to be associated with chronic or recurrent cases. In others it may not be hair shafts but remnants of corneocytes are found in the center of microabcesses or scars and the possibility of cocci which may adhere to corneocytes being protected inside a folded or rolled up corneocyte is another possible site for sequestering bacteria and protecting them from tissue levels of antibiotics or the body's immune defenses. Abscess or granuloma formation may alter the ability of some antimicrobials to effectively reach or kill the microorganisms. Another pathologic change that may be less apparent is fibrosis unless it occurs grossly. Fibrosis more often occurs at the microscopic and not the gross level. The fibrosis may be perifollicular or more diffuse throughout the dermis. Certain breeds (Doberman pinscher, bull and Staffordshire terriers, Rottweiler) seem more predisposed to excessive scarring that appears to make resolution of the pyoderma more difficult.