Common mistakes can often cause problems in diagnosing dermatology cases

At one time or another, we have all made the same mistakes when working up a dermatology case. To help us all save time and get the most information with the least amount of work, I thought I would address what in my referral practice appear to be the most commonly made mistakes when working up a dermatology patient. Some mistakes involve medications and the frequency of their use while others can be avoided by just spending an extra second or two that, in the long run, will pay off in reaching the correct diagnosis.

1. Include a history when submitting skin biopsies.

Put yourself in the pathologist's shoes-- that person is only seeing a 6 mm piece of skin whereas you are seeing the complete picture. It is extremely difficult for the pathologist to interpret what he/she is seeing and come to a conclusion when they are not given a brief history of the patient which should include age, breed, sex, clinical appearance, duration of the problem and response to medications. A Polaroid picture accompanying the biopsy sample is submitted by some veterinarians which can also be helpful.

2. Ivermectin when used for demodicosis is used daily not weekly.

I often get referrals of patients with generalized demodicosis that have been treated with a single injection of ivermectin or weekly injections (Photo 1). Weekly ivermectin treats scabies, daily ivermectin treats demodex. And of course, when using ivermectin, dogs must be heartworm negative and not be of a herding breed. I also avoid ivermectin use in elderly patients as I have had patients with neurological side effects in geriatric non-herding breeds. The dose of ivermectin used daily for generalized demodicosis is 200ug/kg/day-800ug/kg/day (off-label use).

Photo 1: Patient with generalized demodicosis.

3.Ear cultures are not the same as ear smears.

An ear smear is a quick three-minute procedure that allows you to readily determine what kind of otitis is present i.e. bacterial vs. yeast (Photo 2). Very simply, an ear smear is performed by obtaining a sample of the ear discharge, rubbing it onto a glass slide, heat fixing, then staining with Dif Quik or Gram's stain. It is then observed under oil and yields either yeast or bacteria. If bacteria are present they are usually cocci (BB shaped) or rods (rod shaped). If rods are present, a culture and sensitivity is then submitted. There are times when a culture and sensitivity is indicated such as when rod bacteria are present or when current therapy is not effective. However most often the added expense of a culture and sensitivity can be avoided by checking a quick ear smear first. If you're not used to performing ear smears -- get started! The more you do, the better you'll get.

Photo 2: Cocker Spaniel with chronic yeast otitis.

4. Perform skin scrapings and combings on all dermatology cases.

This should be automatic. While the owner is giving me the history on the patient, I am combing or scraping the pet (Photo 3). Sometimes when you least expect it, just this little procedure yields surprising results -- such as Demodex gatoi in the cat or Cheyletiella in the dog.

Photo 3: Cat with Demodex cati.

5.If you suspect scabies, treat for it.

We have all missed scabies at one time or another as early scabies presents with little or no lesions, just pruritus (Photo 4). The most common time scabies is missed is in a middle aged or elderly patient that has just become pruritic for the first time in their life. The other main differential, atopy, does not usually start in middle age but much younger i.e. 6 months to 3 years of age. Most patients with scabies are unresponsive to antipruritic doses of steroid. It makes more sense to rule out scabies first with a month's worth of therapy than to perform skin or blood testing for allergy with a resultant life time of immunotherapy. Therapy for scabies includes lime dips every five to seven days or ivermectin 200ug/kg/wk x four to six weeks or milbemycin oxime l mg/kg qod x 16 days or selamectin one dose every 15 days for three doses as well as treatment of the environment. Scabies or Cheyletiella mites should come to mind particularly if the owners are pruritic.

Photo 4: Excessive face rubbing due to canine scabies.

6. The test for food allergy is a hypoallergenic diet trial.

Unfortunately, skin or blood testing for food allergy has not been shown to be accurate (Photo 5, p. 5). The patient must be fed a cooked or commercially prepared diet that does not contain any ingredients to which they have already been exposed i.e. no corn, wheat, egg, beef, chicken, soy, dairy or lamb if they have been on a lamb and rice preparation. It is not enough to change from a current diet to one with "lamb and rice". In food allergy, it is more important to be sure what is not in the food than what is in the food. Unfortunately, a lot of owners read incorrect information about food allergy as I hear daily that dogs are "corn allergic" so they took the patient off of all corn. Corn may be an offending allergen but so can one or more of the seven other ingredients listed above. Most owners do not understand that one "little treat" will cause any problems or they continue to administer medications with cheese or peanut butter -- both are not permitted on a hypoallergenic diet.

Photo 5: Food allergic dogs need to placed on a stringent diet that rules out specfic ingredients such as corn, wheat, egg or beef.

7. Treat a bacterial pyoderma without steroids.

Even a small dose of steroid can serve to cause immunosuppression and not allow the pyoderma to resolve adequately (Photo 6). Pyoderma is pruritic in some patients, but resist the urge to administer steroids to relieve the pruritus while treating the pyoderma with antibiotics. Instead use antibacterial or antipruritic shampoos along with antihistamines to help relieve the itching while treating until at least a week past clearing with antibiotics. It only takes a tiny dose of steroids in some patients to not allow the pyoderma to resolve.

Photo 6: Bacterial pyoderma on the ventral abdomen.

8. Atopy is a disease of rule outs.

Not every pruritic patient is atopic. Remember the diagnosis of atopy is based on age of onset, breed, season affected, response to steroids and year-to-year recurrence of symptoms. Blood testing for atopy does not make the diagnosis, it only serves to provide information as to what to include in the patient's immunotherapy. Once testing for allergy is undertaken be it skin or blood testing, the results must correspond to the time of year the patient is affected...in other words, it has to make sense.

9. A resting cortisol does not make a diagnosis of Cushing's disease.

The proper way to test a patient suspected of having Cushing's disease is either an ACTH stimulation test or low dose dexamethasone suppression test. A single cortisol level in a patient particularly if elevated, just may reflect the patient being stressed or nervous in the office. The true value of cortisol levels is how the patient's body responds when administered Cor-trosyn or dexamethasone.

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10. Listen to the client.

I am asked more often "why didn't my veterinarian refer me sooner?" than hearing "why did my veterinarian refer me to you?" What usually precedes that question is the owner's frustration that the doctor didn't listen to the owner when they tried to tell the doctor what they felt was important information. It's difficult to hear some of the misinformation the clients are gaining from the Internet but when it comes down to it, they live with the patient and as they say "a mother knows her child." It is certainly our role to correct misinformation but sometimes in all of those untrue statements or queries made by clients, there is actually information we need about that pet that is important.