Canine Staphylococcal Pyoderma
Candace A. Sousa, DVM
The term pyoderma refers to any purulent skin disease. In veterinary medicine the term is most commonly used to refer to bacterial colonization or infection of the skin and/or hair follicle. Bacterial pyoderma is second only to flea allergy dermatitis as the most common dermatosis of dogs but this statistic is probably changing with the advent of newer flea-control products.
The skin of normal dogs is populated with small numbers of Micrococcus, alpha-hemolytic streptococci, Propionibacterium acnes, Acinetobacter and Staphylococcus spp. S. intermedius can frequently be isolated from the hair coat of normal dogs. It has been postulated that this might serve as the reservoir for infectious organisms of the skin. S. intermedius is involved in approximately 90% of canine bacterial pyodermas.
Bacterial pyodermas are usually classified based on the depth of involvement from surface to cellulitis. A surface infection or colonization involves the stratum corneum. These include such diseases as intertrigo (skin-fold pyoderma) and pyotraumatic dermatitis (“hot spots”). Superficial pyoderma is the most common canine bacterial skin disease. The infection involves the epidermis below the stratum corneum and/or extends into the hair follicle. Impetigo, superficial folliculitis, and superficial spreading pyoderma are examples of this type of infection. A deep pyoderma occurs when the infection extends through the epidermis or hair follicle and involves a pyogenic inflammation of the dermis or subcutis. Often times there is evidence of rupture of the hair follicle. In addition to Staphylococcus, gram-negative bacteria such as Proteus, Pseudomonas, or Escheria coli can often be cultured. If an antibiotic is chosen that is effective against the S. intermedius, in most cases the other organisms are also eradicated.
Recurrent pyoderma is defined as a bacterial infection of the skin that responds entirely to appropriate systemic and/or topical therapy but recurs within a short period of cessation of therapy, usually within a month.
The presence of bacterial pyoderma is always secondary to an underlying cause. It is the obligation of the veterinarian to try to determine the precipitating cause and to treat or eliminate it in an effort to prevent re-infections. Unfortunately, sometimes we are just not clever enough to determine that cause.
Almost every dermatologic disease of the dog can have bacterial pyoderma as a component. Allergic or pruritic diseases such as flea allergy dermatitis, food allergy, or atopic dermatitis are often complicated by secondary Staphylococcal infections. Diseases of cornification such as congenital or idiopathic "seborrhea", icthyosis, and sebaceous adenitis alter the normal surface microenvironment and allow the overgrowth of bacteria. Endocrine disorders including hypothyroidism, hyperadrenocorticism (either iatrogenic or naturally-occurring), and abnormalities of the sex hormones cause changes in the cornified layer and will often be complicated by secondary bacterial pyoderma. Genodermatoses that cause cutaneous anatomic abnormalities such as color dilution alopecia, black hair follicular dysplasia, and follicular dysplasia often require long term therapy with antibiotics as a part of their management. Parasitic diseases of the dog including demodicosis, scabies, and Cheyletiella infestations and other infectious diseases of the skin such as dermatophytosis, deep fungal infections, and Malassezia dermatitis very often have colonization or infection with S. intermedius as a component.
In some dogs we feel that immunologic incompetence is the reason that there is a pyoderma or recurrent pyoderma. There are only crude tests available to veterinarians to use to access the immune system of the dog. Serum immunoglobulin quantitation and the total lymphocyte count are the only tests that available to private practitioners. Theoretically, an absolute neutrophilia with a lymphocyte count of at least 1000 cells/µl should be seen in immunologically normal dogs with bacterial pyoderma. Identification and quantification of subsets of lymphocytes using markers (CD3+, CD4+, CD8+, etc), neutrophil function tests, and in vitro lymphocyte blastogenesis are tests performed in some universities and in research situations. Very young or very old dogs, animals with a neoplastic disease, or those who are receiving immunosuppressive drug therapy (especially prednisone) are susceptible to bacterial skin infections secondary to immunoincompetence.
Bacterial pyoderma can be diagnosed in several ways. During the examination, characteristic clinical lesions can be seen which are highly suggestive of a diagnosis. Erythema, alopecia, pustules, papules, crusts, and epidermal collarettes (raised borders of detaching stratum corneum present at the margins of circular areas of inflammation) are commonly seen on the skin of dogs that have a Staphylococcal infection. With a deep pyoderma, there are often nodules, erosions, ulcers, and draining tracts. The veterinarian can take a sample of material for a cytologic examination from the surface of the skin, a pustule, or a draining tract. The identification of large cocci, usually in pairs, is highly suggestive of pyoderma caused by S. intermedius. The presence of rods is indicative of a mixed infection (with gram-negative organisms). Many times these bacteria will be seen within the cytoplasm of a neutrophil showing that they are not just contaminants. A skin biopsy is another valuable tool for the diagnosis of pyoderma. Many veterinarians use the dog's response to the use of oral antibiotics as a confirmation of the presence of the infection.
The depth of the infection can have a negative impact on successful drug therapy. Oral, systemic, antibiotics are the first choice for the treatment of canine bacterial pyoderma. The choice of an appropriate antibiotic can be made empirically or based on results of a culture and susceptibility test. A culture and susceptibility test is usually recommended in those cases of bacterial pyoderma that have made no clinical improvement after 2 weeks of treatment with an antibiotic that is usually effective against Staphylococcal infections. A culture and susceptibility is helpful if there is deep pyoderma, for infections caused by gram-negative organisms, or if the animal has been treated previously with several different antibiotics. It is important that the veterinarian prescribe an appropriate dose of an antibiotic for an appropriate period of time (minimum of 3 weeks; may need to extend for up to 8 weeks for deep infections). In every case, oral antibiotic therapy should be continued for at least 1 week after a clinical cure has been seen or 2 weeks after the discontinuation of oral prednisone. (Table 1)
The susceptibility of an organism to a drug is described in terms of the minimum inhibitory concentration (MIC) of drug. Drug efficacy is dependent upon drug concentrations reaching the MIC at the site of infection and will be enhanced if concentrations at the site are several magnitudes above the MIC. However, the concern for drug safety prevents indiscriminate increases in dosage to increase the drug plasma concentration. The breakpoint MIC of a drug is the highest concentration that can be safely attained in blood using the recommended (labeled) dosing regimen. Organisms are considered susceptible to a drug if the MIC is below the breakpoint MIC. Organisms characterized by intermediate susceptibility are inhibited at concentrations that approach breakpoint. The MIC for a resistant organism surpasses the breakpoint MIC of the drug, and for that drug the risk of toxicity outweighs the potential benefits of therapy.
For recurrent skin infections, extended regimens of antibiotic therapy can be tried. These are to be used once the pyoderma has been brought under control. They do raise the potential of inducing or selecting resistant strains of bacteria. There is no "best" regimen to use. Some dermatologists recommend the use of an appropriate antibiotic for one week followed by one week without the drug. Eventually the length of time without antibiotics may be extended. Other veterinarians have recommended using antibiotics for 2 to 4 days per week at the full dose or even every other day. A third protocol involves a maintenance recommendation of once daily dosing. If this is successful then the dose of the antibiotic can be lowered. As a general rule, though, for the use of oral antibiotics to prevent recurrent pyoderma, the dose of drug should not be lowered or the interval without antibiotics should not be extended until you have waited for twice the length of time expected for the pyoderma to recur (i.e. if the animal can go for no more than 2 weeks without antibiotics before recurrence, wait a minimum of 4 weeks using a once daily dose before any further adjustments).
Topical antibacterial therapy is an integral part of the initial treatment of a bacterial pyoderma and is also useful in preventing reoccurrence of the condition. The owner is usually instructed to shampoo the dog as often as the dog needs it and as often as they are able. Shampoos containing benzoyl peroxide, chlorhexidine, triclosan, or ethyl lactate are generally prescribed and owners are advised to let the shampoo contact the dog for a minimum of 10 minutes before rinsing. Use of a final leave-on rinse after bathing will assure that the active ingredient remains in contact with the skin and haircoat. Antibiotic creams or ointments are useful for spot treatment. Mupirocin (Bactoderm®-Pfizer Animal Health) is an excellent topical antibiotic. I try to avoid using products that contain corticosteroids.
Immunomodulatory therapy using products such as Staphage Lysate® (Delmont Laboratories, Inc.), Immunoregulin® (Propionibacterium acnes bacterin; Neogen Corporation), or levamisole may alter lymphocyte and phagocyte immune function by modifying the intracellular cyclic nucleotides of leukocytes. Cimetidine has also been proposed as an immunomodulatory drug because lymphocytes have H2 receptors that theoretically act to modulate cytokine production.
ANTIBIOTICS USEFUL IN DERMATOLOGY
* Cost calculated for a 22.7 kg (50 lb) dog for 3 weeks of therapy, 4/02