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

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

Key clinical diagnostic points in bacterial infections

     • Always look for an underlying etiology. Separate pruritic pyoderma from pyoderma leading to pruritus

     • Cytology is an inexpensive tool that should regularly be performed

     • Cultures need not necessarily be performed for "first offences" unless the cytology shows the presence of rods (e.g. post grooming folliculitis has been associated with Pseudomonas aeruginosa)

     • When a culture is to be performed, intact pustules that are ruptured are the best, but good samples can be collected from under the raised edge of an epidermal collarette.

          ⇒ Lance the pustule with a sterile needle and swab the site gently with a culturette or culture a collarette

     • Deep infections should be cultured by biopsy. Studies have shown that surface culture predicted deep tissue isolates in only eight of 22 cases; "the majority of cases yielded positive growth of bacteria differing from superficial culture and often resistant to empirical drugs." (Shumaker 2008)

          ⇒ Place a biopsy sample in a container with a small amount of saline and ask the lab to macerate the tissue and culture it. Deep fungal cultures can be performed in the same manner on the same tissue specimen. It is routine for us to collect culture samples in this manner whenever we collect specimens for histopathology. When collection by biopsy is not feasible, one can consider applying digital pressure to the region to collect a specimen by squeezing from the deeper regions of the lesion out through fistulae (when present)

     • Do not be satisfied with a lab report that says Staphylococcus species – ask the lab to speciate

     • Look carefully at the antibiogram in the patients with methicillin resistant organisms.

          ⇒ Clindamycin is often a good choice in Staphylococcus schleiferi cases.

          ⇒ For deep infections, I would go to a higher dose (about 11 Mg/Kg S-BID)

          ⇒ D-test - people with MRSA seem to have an inducible clindamycin resistance. Patients that are resistant to erythromycin seem to rapidly develop clindamycin resistance. Although it is unclear if the same is true for S. pseudintermedius, I might choose an alternate antibiotic, when available.

          ⇒ In a similar vein, some people with MRSA seem to develop a rapid resistance to the quinolones. Once again, choose the antibiotic wisely and don't forget about the benefits of topical therapy

          ⇒ Don't forget about proper infection control in your hospital

     • You can't assume that all drugs in the same class will have the same susceptibility. For example, a bacterium that is resistant to cephalexin may be sensitive to cefovecin; similarly cefpodoxime and cephalothin may have differing susceptibilities

     • Don't forget to look for a concurrent Malassezia dermatitis as one of the interfering factors.......

Fungal Culture and Woods Lamp Examination

It is commonly said that if a cat looks like it has ringworm, it probably doesn't, but if it doesn't look like it, it may very well have it, so be sure to check. A diagnosis of ringworm is an important one due to the possibility of infecting other pets and owners in the household as well as us and other pets in the clinic!

     • Warm the Woods lamp for 5 minutes before attempting to check the pet

     • Only 50 percent of M. canis cases will fluoresce (an apple green color on the hair shaft) in ultraviolet light, and other causes of ringworm will fail to fluoresce. Other products (such as hand creams) can fluoresce, so fluorescence is suggestive but NOT diagnostic of ringworm

     • Fungal cultures should be part of the minimum database for most cats presented with a history of pruritus or chronic dermatitis.

     • The most effective technique to collect samples is the Mackenzie brush technique. Use a new toothbrush (mycologically sterile) to collect specimens. Pluck additional hairs from the periphery of newly developing lesions or hairs that have fluoresced on Woods lamp examination

     • For in-house cultures, I like open plate culture media (as opposed to tubes) such as the Dermatoplate Duo (Distributed By Vetlab supply palmetto Bay FL www.vetlab.com)

     • Check the sample daily. A red color change should occur early in the growth (contaminants will change the color, if given enough time) and is only suggestive of a positive result; a diagnosis should be confirmed microscopically by collecting the specimen with clear tape and pressing it onto a microscope slide with a drop of lactophenol cotton blue stain

     • I have not been impressed by DNA fungal assays, at least those performed in local labs

     • Deep fungal culture is best obtained on surgically harvested material (see above – bacterial culture)

Skin biopsy

A skin biopsy cannot tell you why the pet is pruritic. This test should be reserved for pets where parasites, environmental hypersensitivity, adverse food reactions and microbial dermatitis have been ruled out (with the possible exception of pododemodicosis). Some tumors (CTCL) can be pruritic and are, in fact, more common in atopic individuals

     • Be sure to send the sample to a veterinary dermatohistopathologist

     • Take multiple samples of a variety of lesions in different stages – do not biopsy an ulcer – you will get a diagnosis of "ulcer"

     • Take culture samples concurrently for macerated tissue culture

     • Most importantly, give a detailed history - don't play "test the pathologist"

Blood testing

Is there a benefit in complete hematological and biochemical profiles in the allergic individual? What about thyroid testing? Most of the patients presented to my referral practice for chronic pruritus have had thyroid blood tests. Many of these patients are on thyroid supplementation. Thyroid disease does not cause pruritus. It does predispose patients to a microbial dermatitis and infections are itchy, however. Therefore, if a patient presents with a pruritic pyoderma that is completely antibiotic responsive, I might certainly consider a workup for an underlying disease, including an endocrinopathy. As will be discussed in the next talk, there is the rare patient with a food allergy that is completely antibiotic responsive. If a patient suffers from xerosis (dry skin) and appears unwell, I certainly believe that these patients are good candidates for blood tests. Finally, if one wishes a baseline before administering medications (including steroids) I believe that blood tests have their place. However, if finances are of concern in a young and otherwise healthy pet, one may want to direct funds elsewhere.

Behavioral evaluation

Might behavior play a role? Most veterinarians that have been in practice for some time recognize that we all seem to have patients that are much more "bothered" than others are for similarly appearing lesions. Studies in humans have found that humans suffering from psoriasis and atopic dermatitis will flare under conditions of stress. Interestingly other studies have found that stressed people have an increase in epidermal permeability. Is it too much of a stretch to envision that certain individuals aren't just "uptight" but are actually physiologically predisposed to exacerbation of the clinical signs of atopic dermatitis?