Professor, University of Edinburgh, Royal (Dick) School of Veterinary Studies, Summerhall
THE DEVELOPMENT OF PYODERMA
It is believed that staphylococcal pyoderma develops following surface spread from the carrier sites. Factors that ordinarily prevent the development of pyoderma are:
1. The physical barrier of the intact skin. Normal desquamation is extremely important in preventing niches in which pathogenic bacteria could become established.
2. Antibacterial substances present in normal sebum, and secreted onto the skin surface from apocrine secretions. This would include immunoglobulin, and other non-specifically acting substances such as those produced by resident bacteria.
3. The bacterial barrier. If non-pathogenic bacteria occupy all available colonization sites, pathogens will not become established.
There is thus a dynamic balance between the host defenses, the virulence of any potential pathogens and their ability to gain access, and the host defenses that are brought into play in a damage limitation exercise.
FACTORS THAT PREDISPOSE TO RECURRENT PYODERMAS
Pyoderma is likely to prove recurrent in a number of situations
1. Where there is a tendency to surface colonization:
a. Seborrhoea, of whatever cause, is likely to lead to colonization with Staph intermedius.
b. Atopic disease. In this condition it has been shown that Stratum corneum cells of atopics have a greater tendency to adhere to the organism than do cells from normals.
2. Where the integrity of the skin barrier is impaired:This may occur secondarily to any inflammatory skin disease, or one that leads to self-trauma from pruritus. Flea allergy is a rather special case. The secondary infection in such cases is often minor, and does not necessitate antibiotic therapy. However a minority of animals will break with a staphylococcal infection whenever they acquire an infestation. In demodicosis, a recurrence of the infection is often the first sign of recurrence of the mite population.
3. In the immunocompromised animal: This is generally limited to deficiencies in non-specific defenses or in cell-mediated immunity, as antibody response is always evident.
a. Congenital non-specific immune defects. Delayed intracellular killing by neutrophils has been noted in weimaraners and dobermans. Irish setters have been reported with a severe, intracellular killing defect, but this is exceedingly rare. However, absence of the neutrophil complement receptors CD11b and CD18, which aid attachment prior to engulfment, is quite common in Irish setters in the UK and Scandinavia. Such animals would be expected to have difficulties in handling infections of all types.
b. Impaired cell-mediated immunity. It has been documented that atopic dogs have impaired cell-mediated immunity. In animals with impaired cell-mediated immunity from other causes has also been documented. Some animals with recurrent pyoderma have impaired cell-mediated immunity- where their lymphocytes appear to have an inbuilt defect, or where there are serum immunosuppressive factors present. This seems to be an excellent way by which the infection can be potentiated.
4. Food allergy:Typically, affected animals are less pruritic when on antibiotics with their pyoderma controlled, but relapse occurs either immediately on cessation of therapy or within 2-3 weeks. In other cases the pruritus is fully controlled by antibiotic therapy, and the animal appears to be suffering from a food allergy which is sub clinical as far as the pruritus is concerned.
5. In cases of hypersensitivity to staphylococcal antigens:It is well documented in experimental animals that development of hypersensitivity renders the animal much easier to infect. This has been shown also to be true in dogs.
6. Hypothyroidism:This is well recognized as a cause of recurrent staphylococcal infection, but it is not clear whether it results from the accompanying seborrhea, or to impaired defenses that may accompany the condition.
a. Improper use of antibiotics. These must be given in full doses for the full length of time.
b. Use of corticosteroids. Although these may appear to produce an improvement, merely from their anti-inflammatory action, in the long term they are working against our best interests as they (a), tend to dry the skin inducing scaling and thus favoring colonization, and (b), they may further impair cell-mediated immunity.
Suitable antibiotics are directed at the Staph. intermedius. Where there is secondary infection with gram-negatives, these will disappear once the Staph. is controlled unless the animal is severely immunodeficient, or there is an overwhelming infection.
1. Macrolides.Erythromycin and lincomycin are good, first choice bacteriostatic antibiotics. Clindamycin does not appear to offer any major advantages over the other, cheaper, products. The vomiting with erythromycin can be obviated by feeding an anti-emetic 30 minutes prior to each dose for the first two days. There is cross-resistance between the two drugs. The dose for erythromycin is 10-15mg/kg TID, and for lincomycin 20mg/kg BID.
2. Potentiated sulphonamides. Trimethoprim-sulphadiazine (or sulphamethoxazole), ormetoprim-sulphadimethoxine and bacquiloprim-sulphadimethoxine are very useful antibiotics for pyodermas. The dose for the first is 30mg/kg given twice daily, although some clinicians are comfortable with SID dosage. The dose for ormetoprim is 55mg/kg for the first day, and then 27.5mg/kg once daily thereafter. Drug reactions are not uncommon with trimethoprim sulphur, and result from the sulphadiazine, which also has the propensity to induce a transient arthropathy, especially in Dobermans. The tendency to induce keratoconjunctivitis sicca must be watched.
3. Cephalosporins. Cephadroxyl and cephalexin are excellent for the treatment of pyodermas. Although they are broader spectrum than is oxacillin, they do not classify as broad-spectrum drugs, and are perfectly acceptable for the routine treatment of canine pyoderma. Drug reactions are seen, but probably with less frequency than with the sulphonamides. They are bactericidal. Doses are 20-25mg/kg BID.
4. Oxacillin. This is an excellent, narrow spectrum bactericidal antibiotic to which resistance is extremely rare. Unfortunately there are no veterinary preparations. The dose is 20mg/kg TID.
5. Amoxacillin/clavulonic acid. This broad-spectrum bactericidal antibiotic is very useful. Doses used range from 15-20mg/kg BID.
6. Fluoroquinolones. These are very broad spectrum, bactericidal antibiotics. It would be indicated where there is a significant involvement of gram negatives. Recommended doses are 2.5mg/kg BID (or 5mg SID) for enrofloxacin, 2.0mg/kg SID for marbofloxacin. 2.5mg/kg SID for orbofloxacin and 5-10 mg/kg BID for difloxacin. They are, however, not indicated for routine use in dermatology, due to the possibility of inducing resistance in Pseudomonas.
Antibacterial shampoos using products containing benzoyl peroxide (2-3%), clorhexidene (2-4%) or ethyl lactate (10%) form a most valuable supplementary treatment. Chitosanide, which is contained in the new spherulite-based shampoos of Virbac, also has antibacterial properties. Shampoos can be used 1-2 times weekly therapeutically, and are also useful as part of a preventative maintenance strategy.
In the case of deep pyodermas, antibacterial soaks employing povidone iodine are a valuable aid, and can be done twice weekly in the initial stages of treatment. In the case of localised deep pyodermas, benzoyl peroxide gels are of value. Also, where there is evidence of ingrown hairs forming foreign body reactions, these should be gently massaged out in warm water soaks.
THE APPROACH TO THE RECURRENT INFECTION
1. Search for a predisposing cause.
a. Evaluate for seborrhea. If this is evident, look for a cause of the seborrhea.
b. Evaluate for evidence of ectoparasitic disease. Be sure to search repeatedly for demodex in any case of recurrent deep pyoderma, particularly pododermatitis. Biopsies are sometimes necessary to demonstrate mites in such cases.
c. Check for thyroid function, even in animals that are not overtly seborrheic.
d. Skin test, or undertake in vitro tests for atopy, even if animals are not pruritic when their pyoderma is controlled.
e. Do an elimination diet for food allergy, again, even if the animal is not pruritic when the pyoderma is controlled.
f. Unfortunately, work-up for immunological defects is not generally available, but should be pursued if possible if all of the above are negative.
2. Therapy for the recurrent case with no apparent cause.
a. Be sure to use the appropriate antibiotic therapy supported by results of in vitro sensitivity tests. Bactericidal therapy is preferred in recurrent cases.
b. Use good supporting antibacterial shampoos. These may be alternated with antiseborrhoeic shampoos if seborrhoea is evident.
c. Consider using immunotherapy with a staphylococcal product e.g., Staph Phage Lysate (Delmont Labs, Swarthmore, PA, USA).
d. As a last resort some clinicians advocate:
i. Continued antibiotic therapy in normal doses,
ii. Intermittent full courses at full dosage with one month on and one month off.
iii. Intermittent full dose therapy on, say, two consecutive days each week.
iv. Continual low dose therapy, which seems conceptually the least desirable approach.