The CSI approach to pruritic pets–secondary dermatology testing (Proceedings)

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The CSI approach to pruritic pets–secondary dermatology testing (Proceedings)

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Aug 01, 2011

In the first two talks, we discussed the importance of collecting a good history and performing "dermatology-due-diligence on most patients presented with a history of chronic or recurrent pruritus. However, in some cases, that just isn't enough and additional diagnostic tests are needed:

Bacterial culture

In order to understand when and how to culture, it may be useful to first understand a bit about the etiology and pathophysiology regarding pyoderma:

Pyoderma

Bacterial pyoderma is one of the most frequently encountered skin diseases in the dog. In fact, studies have shown that pyoderma is the second most common presenting dermatological complaints when a dog is presented to a veterinary clinic (the first is "not determined"!). Other than abscesses, it is a much less common reason for presentation in the cat. When these patients are presented to the clinic, the underlying question is whether the pruritus is a result of the pyoderma or whether the pyoderma is secondary to the pruritus. Regardless, underlying diseases alter the defense mechanisms of the skin – cutaneous, metabolic or immunologic abnormality are the classic examples. Immunodeficiency syndromes, while chic, are really quite rare. A multicenter study in Europe found that the most common causes were environmental allergies (60%), food allergy (7%), flea allergy (7%), hypothyroidism (7%), hyperestrogenism (4%), demodicosis (4%), and zinc-responsive dermatosis (4%) (Bensignor 2004).

Why is a pyoderma so common in Atopic dogs? (by the way, the new definition for atopic dermatitis includes food allergy). There is an increased adherence by S. pseudintermedius to corneocytes of canines with atopic dermatitis (McEwan 2006, Simou 2005). The defective barrier lipid function found in atopic skin seems to favor colonization and multiplication of staphylococci. This can lead to biofilm production and establishment, and bacterial overgrowth, which promotes adherence and toxin production: the bacteria send signals to one another and once the population of bacteria is high enough ("quorum sensing") a switch is made from proliferation to toxin production, hence further reducing barrier function of the colonized skin (Pin 2006).

Recurrent or poorly responsive pyoderma could be a result of the inability of the antibiotics to keep the bacterial infection under control. Another reason for a relapsing pyoderma is incomplete duration of treatment. It is important that once you start treatment, you continue 1-2 weeks past the point where lesions can be palpated. The surface heals more rapidly that the deeper tissues.

As most of you are aware, the most common organism cultured from bacterial pyoderma is Staphylococcus pseudintermedius. (Most of the organisms formerly called Staphylococcus intermedius are more appropriately named Staphylococcus pseudintermedius based on molecular phenotyping and you are likely to see either this term or "Staphylococcus intermedius group" on antibiograms these days). One of the most comprehensive dermatology textbooks, Small Animal Dermatology, states " Staphylococcus intermedius from dogs is almost uniformly (>95%) sensitive to the various fluoroquinolone antibiotics, amoxicillin clavulanate, oxacillin, and first-generation cephalosporins". This book was printed in 2001 and this fact is no longer true in 2011. Indeed, we are seeing more and more in the way of resistant organisms these days.

Bacterial resistance may not be as new a phenomenon as you may think. Methicillin resistant (MR) strains of gram positive organisms have been reported since the early 70's (Rankin 2007, Devriese 1972). Methicillin resistant organisms are those that are resistant to all Beta lactam antimicrobials. They carry the mecA gene which alters the penicillin binding protein. These organisms can be identified using MecA PCR or Penicillin Binding Protein 2a (PBP2a) latex agglutination. These organisms tend to be resistant to other antimicrobials as well, leaving few available treatment options.

There has certainly been an increase in MR strains found in studies reported over the last 6+ years, although the incidence appears to be regional. In one paper presented by Morris et al, rates of methicillin resistance were: S. aureus 35%, S. intermedius 17%, and S. schleiferi 40%. However, these data were collected from patients where bacterial resistance was suspected. Griffeth et al found that MR coagulase-positive staphylococci were significantly less common in samples that were collected randomly from pet owning households as well as animals that were presented to a dermatologist without regard to clinical history (pyoderma was not a necessary inclusion criteria). These less-biased populations revealed an overall period prevalence of MR staphylococcal carriage in affected dogs as S. aureus: 2%, S. intermedius 7%, S. schleiferi ssp. schleiferi 2%, and S. schleiferi ssp. coagulans 2%. However, the numbers seem to be rising. Hanselman cultured healthy dogs from the community in 2009 and found that 9.8% of the dogs with Staphylococcus pseudintermedius were methicillin resistant!

A study performed at our facility (presented by Dr Karri Beck (my dermatology resident) at the 2010 North American Veterinary Dermatology Forum) also looked at this most challenging organism. In our study, animals presented with a pyoderma were cultured from the affected sites as well as from nasal and anal carriage sites. When the patients reached a clinical cure, these cultures were repeated. Initially, skin cultures yielded 39% (64/165) MRSP as well as smaller numbers of other Staphylococci. Of the dogs with MRSP pyoderma initially, 72% were also positive at carriage sites. Following 'successful" treatment, MRSP was still isolated at follow-up from skin in 37% (11/30), and from carriage sites in 40% (12/30). Perhaps more alarming is the fact that of the dogs that did not have MRSP pyoderma initially, MRSP was isolated from the skin of 31% (17/55) and MRSP from carriage sites increased 5.0% to 27% (15/55) The conclusion was persistence of MRSP on the skin and carriage sites is common after resolution of MRSP pyoderma. Acquisition of MRSP during treatment appears to be common.

Our study underscores the need for appropriate identification and treatment of the bacterial pyoderma. It also underscores the importance of treating aggressively, once the infection is identified. Some of the "resistance" seen in practice may be more managerial (e.g. compliance) than microbiological and only bacterial culture will differentiate. Compliance is a big issue. It is critical that your partner in treatment (the caregiver) is educated on the importance of following the instructions. Newer treatments are available that may help in this regard. Many clients are aware of the issue of bacterial resistance, and their fears may result in premature discontinuation of treatment which could indeed lead to a self fulfilling prophecy