Top 10 dermatology questions technicians get asked (Proceedings)

#1: Why is skin cytology performed and how do we do it?

Skin cytology is defined as the microscopic evaluation of material collected from the skin.  Skin cytology can provide extremely valuable information as to type and degree of infection present, evidence or suggestive features of parasites, a normal or abnormal immune response, or demonstrate the presence of immune mediated or neoplastic diseases.

Samples for skin cytology can be collected via direct impression smear, using clear adhesive tape to lift the sample, using a cotton swab to obtain exudate, or using metal spatulas (or something similar) to obtain material from nail beds.  Different slides should be used for different locations and lesions, and the slides should be labeled accordingly.

Most samples, other than tape, are heat fixed, and then stained with Diff Quik or a comparable stain.  The slides are dried (naturally or with a hair dryer, lighter or bibulous paper-I prefer a hair dryer) and then examined microscopically. It is always recommend to begin at a low magnification to find an area where cellular material appears to be present, and then go to oil immersion for identification.  Important things that are identified on cytology include bacteria, yeast, neutrophils, eosinophils, acantholytic cells (the cells that are present in pemphigus foliaceus or, more rarely, dermatophytosis) and neoplastic cells.

#2: Why and how are skin scrapings performed and why is there a differentiation between superficial and deep scrapings?

The goal of most skin scrapings is to identify parasites, the most common ones being Notoedres cati, Demodex spp., Sarcoptes scabei and Cheyletiella spp.  The scrapings are done differently based on if the parasites are more superficial in the stratum corneum or surface of the skin, versus deeper in the hair follicles.

Superficial scrapings are obtained using a broad sweeping scrape, and surface material is often collected from a larger area.  Because the classic parasite found on superficial skin scrapings is sarcoptic mange, the prime spots to scrape are the ear margins, elbows and hocks where this parasite is most likely to be present.  Notoedres and Cheyletiella can also be found with superficial scrapings.

The main parasite that is being evaluated for with a deep skin scraping is demodex.  A #10 or #15 blade (often dulled) is used to scrape the skin, preferably after mineral oil has been placed, until capillary oozing has occurred.  The skin is typically squeezed before and during the scrape to aid in capillary oozing.  The location of the skin scrapings is at the discretion of the veterinarian, as demodex can be located anywhere.

Regardless of which scraping is performed, the entire slide should be examined with the condenser turned down low to make the mites more refractory, and at a low magnification, such as 10x.

#3: What is the proper way to collect, store and read a dermatophyte culture?

Dermatophytes, also known as ringworm, are a common cause of hair loss, redness, itching, and circular crusty lesions in dogs and cats.  The three kinds of dermatophytes that commonly infect dogs and cats are Microsporum canis, Microsporum gypseum and Trichophyton mentagrophytes.  These organisms are zoonotic, meaning they are contagious to humans.

The method to definitively diagnose ringworm is the fungal culture. Hair, scale and crust is collected and plated on a dermatophyte culture.  These samples can be collected with clean, dry hemostats or the Mackenzie toothbrush technique can be employed to collect hair from all over the body. This technique is commonly employed when screening an inapparent carrier or if the lesions are subtle.  Fungal cultures are typically stored in a dark place at a consistent room temperature and should be protected from ultraviolet light and desiccation.

Dermatophytes utilize protein and produce an alkaline by-product that produces a red color change.  However, after all the carbohydrates are utilized, saprophytic contaminants can utilize the protein and turn the media red as well over time.  For this reason DTM cultures should be inspected daily, and any suspected fungal growth should be lifted with clear plastic tape and stained with lactophenol cotton blue to look for characteristic macroconidia and fungal identification.  Speciation of the dermatophytes should always be performed to determine the source of infection to help prevent future reinfection.

#4: How do I advise owners to clean their environment to decontaminate against dermatophytes?

The environment should be addressed in every case of dermatophytosis that is diagnosed.  This is the most important in cases of M. canis, as the spores can live in the environment for up to 18 months.  Vacuuming, disinfection, steam cleaning, and discarding infected bedding are all important.

All nonporous surfaces should be thoroughly vacuumed and disinfected, including all floors, walls, countertops, windowsills, and transport vehicles.  The cleaner of choice is bleach at a 1:10 dilution (13oz./gallon).  It is advised to clean a test area to make sure the bleach will not damage the surface.  Undiluted bleach is corrosive and irritating and cannot be safely used in homes or catteries.

If possible, all contact bedding, rugs, brushes, combs, toys etc should be destroyed.  Rugs that cannot be destroyed or removed should be washed with an antifungal disinfectant, like bleach or chlorhexidine.  Be sure to tell clients to test the colorfastness of all items before using these agents.

Steam cleaning of carpets is recommended to kill spores and the temperature of water forced into the carpets must be at least 110°F. For this reason a professional steam cleaner should be used, as the rental ones do not reach high enough temperatures.

All heating and cooling vents should be vacuumed and disinfected. Furnaces should be cleaned with high-power suction equipment and furnace filters should be changed weekly.

#5: When should I use a Wood's lamp and how useful is it?

Wood's lamps emit ultraviolet light that is filtered through cobalt or nickel.  It is ideal to let the lamp warm up for 5 minutes before use, and animals should be examined in a dark room.

The Wood's lamp is used to identify the fungal dermatophyte Microsporum canis (i.e. ringworm).  In about 30-50% of cases with this fungal organism, the infected hairs will fluoresce a characteristic candy apple green color under the Wood's lamp.  It is IMPERATIVE to realize that the other two common dermatophytes in small animals, Microsporum gypseum and Trichophyton mentagrophytes DO NOT fluoresce, and that Microsporum canis only glows about 30-50% of the time.  I.E. a negative Wood's lamp test does NOT rule out dermatophytosis.

Furthermore, false positives can occur and are typically caused by topical medications or dead skin cells.  The HAIR should be glowing, not the skin.  Positive Wood's lamp pets should always have the diagnosis of dermatophytosis confirmed with a fungal culture.

Wood's lamps are very useful tools for initially screening patients, selecting the hairs to perform a fungal culture, or in monitoring a patient who is receiving treatment for dermatophytosis.

#6: Why are antibiotics prescribed at a higher dose and longer for animals with skin disease?

The majority of dogs with skin infections have what we call bacterial folliculitis, meaning that the bacteria have colonized the hair follicles. This is in contrast to the classic “hotspot” that a flea allergic dog may create after an hour of chewing on their rump.  This is one of the most important reasons that topical therapy alone is not adequate for dogs with bacterial folliculitis…their infection isn't entirely on the surface of the skin.  Furthermore, the skin does not get as much of the cardiac output of blood as say, the bladder does, which is another reason why dogs with skin infections usually require longer than bladder infections to resolve.  For a dog with a standard bacterial folliculitis at least 3 weeks of antibiotics are needed, but the general rule is to treat until they are 1-week past the resolution of clinical signs.

#7: What is contagious to owners and veterinary personnel that we commonly deal with in dermatology?

Both sarcoptes mites-the canine Sarcoptes scabei and the feline Notoedres cati can cause lesions in humans.  The canine demodex mites are not contagious.  Cheyletiella mites can be contagious to humans.  Dermatophyte infections are also transmissible to humans, with the most common one being Microsporum canis.

#8: Why can't allergies be cured?

Allergies, whether they are due to fleas, food or environmental agents, are caused by the immune system recognizing something normal as abnormal.  They are not caused by a weakened immune system.  Studies have found that animals with flea allergies cannot be desensitized for fleas, so the only treatment is avoidance.  The same is true of animals with food allergies.  Animals with environmental allergies, or atopic dermatitis, can be desensitized to what they are allergic to through the use of allergen specific immunotherapy.  Immunotherapy “retrains” the body's immune system, but animals usually require it for the rest of their life if it works to maintain control over the disease.  In extremely rare cases and usually after years of therapy, immunotherapy can switch the body's immune response and they no longer need to be treated. I've only seen a handful of cases like this out of 1000's of allergic patients.

#9: What do clients need to be told about allergy testing for atopic dermatitis (environmental allergies)?

Allergy testing is currently done for animals with atopic dermatitis (environmental allergies), NOT food allergic dermatitis. It is important to realize that atopy is a diagnosis of exclusion-i.e. allergy testing is performed after the diagnosis is already made, not to MAKE the diagnosis.  After the diagnosis of atopic dermatitis is made, allergy testing is performed to determine what the animal is allergic to in the environment, ideally to start them on allergen specific immunotherapy (allergy vaccine-either as an injection or oral sublingual immunotherapy (SLIT)).  Allergy testing can be performed via intradermal skin testing or serum serological testing.  Intradermal skin testing is considered the gold standard of allergy testing. Because of the expense and labor involved in maintaining the allergens, veterinary dermatologists or practitioners with a large caseload of owners willing to pursue immunotherapy typically only perform it.  Animals should be off injectable steroids for 6-8 weeks, oral steroids for 1 month, and antihistamines, fatty acids and topical steroids for 2 weeks prior to allergy testing.  Serum serological testing can be performed while animals are on these medications.  In my experience, there are many animals that will have a better skin test than blood test, but the blood test can be a valuable tool when skin testing can't be performed.

#10: How does a client perform a proper food trial to evaluate for adverse food reactions?

It is very important to realize that there is currently no accurate blood or skin test to tell if a pet has a food allergy.  The only way to make a diagnosis at this time is to change the pet's food to an appropriate elimination food trial diet for a period of 8-12 weeks.  This is done by feeding an elimination diet containing an unusual protein and carbohydrate source that the pet hasn't been exposed to, or a diet where the proteins are hydrolyzed (made smaller) so the immune system won't recognize it. During this 8-12 week period, the food trial diet must be fed without the addition of table scraps, treats, or chewable supplements (including flavored heartworm prevention).  The only way to successfully perform a food trial is to be strict about what the pet is allowed to eat.

The reason that the trial must be so long is that some pets will not show improvement until up to 3 months off their original food.  It is always recommended to gradually switch to the new diet over a 3-5 day period, water can be given free choice, and the veterinarian in charge of the case is the one to determine if and what treats are allowed.