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VetInfo Digest February 2006

Table of Contents:

Patterns of Skin Disease

Multi-Drug Sensitivity in Herding and Sighthound Breeds

This Month's Note:

Much of this month's VetInfo Digest is devoted to recognizing the patterns of skin disease. This is different than recognizing symptoms or specifically identifying an organism that might be causing the disease through laboratory testing. The various patterns in which skin lesions occur can be a very big hint about the underlying cause. When these patterns are considered along with other signs, especially itchiness, they can give even more information. The biggest problem with this approach to a diagnosis is that skin, more than any other organ, tends to develop secondary infection quite easily and so there is often an original pattern that is overlaid by a secondary pattern and in some cases by more than one secondary pattern. In addition it is not too unusual for dogs to have more than one primary problem and to have secondary infections as well. Even with these complications if the patterns that first show with various skin diseases are recognized it can be helpful in making a diagnosis even in complicated cases.

Just in case I fall into the habit of using veterinary terms somewhere in this newsletter itchiness is generally referred to pruritis in the veterinary literature. Also, this is not meant to be a comprehensive list of skin diseases. I'll just try to include many of the more common skin diseases where patterns of hair loss combined with observing whether itchiness is present, or not, can be helpful in making a diagnosis.

Diseases in which itchiness is the primary problem:

The most common cause of itchiness in both dogs and cats is flea infestation. The pattern of itchiness associated with irritation from flea bites alone is very variable but many dogs with flea infestation chew at their fur very noticeably. Cats are more subtle about attacking fleas, but "overgrooming" is something that we hear mentioned frequently by cat owners when fleas are present. Flea infestation does not have a strong pattern of hair loss or a concentrated itchy region unless flea allergy occurs.

In cats flea infestation is often characterized by a pattern of small scabs around the base of the tail and the neck. The pattern of small scabs is referred to as miliary dermatitis and it is not specific for flea infestation. It is a common sign of skin disease from a number of causes in cats. Fleas are still the number one cause of this condition, though. A sincere effort to eliminate fleas as a contributing cause should be made for any cat with miliary dermatitis, even if fleas are never seen.

Flea allergy is the single most common reason for visits to the veterinarian in areas of the country in which fleas are prevalent. Prior to the development of fipronil ( Frontline Rx), imidocloprid (Advantage Rx), selamectin (Revolution RxA) and lufenuron (Program Rx) flea allergy was estimated to cause 35% of visits to veterinarians overall -- not just for skin disease! Now that these products are available it is still likely that flea allergy causes more visits than any other single disease entity but it is probably less than 35% of the overall visits.

The pattern for flea allergy dermatitis in dogs is hair loss starting at the base of the tail and extending along the spine up to about the level of the last rib. This pattern is often in a "Christmas Tree" pattern with the broad base of the triangle at the base of the tail and the point of the triangle stopping at about the level of the last rib. Sometimes cats exhibit this same pattern but more often flea allergy in cats shows up as multiple tiny scabs on the skin concentrated around the base of the tail and around the neck. When cats get hair loss it tends to be a little more patchy but is still most commonly on the rear half of their body. Flea allergy is more common in older dogs and cats. Flea infestation can occur at any age.

The second most common skin disease in dogs is probably atopy. There is some disagreement over how exactly to define atopy but it is basically allergies to environmental factors such as tree pollens, dust mites, molds, etc. Dogs with atopy tend to have itchiness that affects primarily the front end of their body and more specifically their paws, face and ears. The first signs of atopy tend to be licking at the paws persistently, ear irritation affecting one or both ears and/or face rubbing. Like all of the diseases that affect skin there are exceptions to the rules and some dogs will have other symptoms such as hair loss around the base of the tail or itchiness in the perianal region. The prevalence of secondary infections in dogs with atopy is very high. Bacterial skin infection (pyoderma) and yeast infection (Malassezia) are the most common secondary conditions.

When hair loss occurs with atopy in dogs it most commonly occurs as hair loss around the eyes creating a "raccoon look" or hair loss on the lower legs. It is probably also a major cause of lick granulomas in dogs, which are thick skin swellings that may have a denuded surface and which most commonly occur over the carpus ( wrist joint) in dogs. When atopy affects cats hair loss is also less common than it is dogs. However, in both dogs and cats the secondary illnesses brought on by the allergies can dramatically change the patterns of skin disease. Thinking back to when the disease process first started is sometimes helpful in gaining a mental picture of what the original pattern was.

Food allergies are another cause of itchiness and occasionally hair loss that has to be considered. It is generally agreed that food allergies are much less common than either flea allergy or atopy but they are frequently mentioned as the top three causes of itchiness in dogs. Food allergies also tend to affect the front half of the dog and have a pattern very similar to atopy. Hair loss around the eyes is less common with food allergies. The itchiness associated with food allergies tends to be much worse and may not respond to corticosteroids, which can be an important factor in helping to differentiate these allergies from those induced by atopy. In cats food allergies are thought to be a major contributor to miliary dermatitis, or small scabs usually found around the neck region. In both dogs and cats there is tendency for pets with food allergies to have both skin and gastrointestinal signs, so a cat who vomits frequently and has skin disease would be a little more suspect for food allergies than a cat who has miliary dermatitis but never exhibits signs of gastrointestinal disease.

Sarcoptic mange is caused by a mite that burrows into the skin. It is primarily a disease of dogs but occasionally affects cats. The mite tends to favor burrowing into the skin of the ear margins, elbows and knee regions. There is usually hair loss in these regions when sarcoptic mange is present, but this is not always the case. Most dogs with sarcoptic mange exhibit intense itching and one sign of the disease is a strong scratching reaction with the rear foot on the same side when one of the ear margins is rubbed between an examiner's fingers.

In cats demodecosis, or infection with Demodex cati or Demodex gatoi mites, usually causes itchiness. At times the pruritis is extreme and at other times it seems less worrisome to the cat. Demodex gatoi infection is considered to be a contagious disease in cats, unlike demodecosis in dogs, so it is important to check all the cats in the household for signs, or to treat all cats, if one is found to be infected. The mite seems to prefer being in or around the ears but it is not unusual to see widespread infections involving the head, neck, legs or anywhere on the trunk. Hair loss associated with this mite may be complete or it may be partial giving a sort of scruffy look to affected cats.

Diseases in which itchiness may or may not be a major clinical sign

Malasseziasis is infection with a yeast organism, Malassezia pachydermatitis. This is a very common secondary invader when other skin disease is present. It can cause infection on its own but this is not how infections usually occur.

Malasseziasis can be an extremely itchy disease. It can affect just the feet, leading to chewing on them persistently or it can infect large areas of the body with generalized itchiness. In most cases it causes a more moderate itchiness and some owners don't notice any signs of itchiness at all. When Malassezia infects nails there may be a brown discoloration of the nail near the nail bed as the only visible sign. When it infects skin it tends to produce an oily, thickened, dark colored skin change that can resemble elephant skin, hence the name Malasszia pachyderma. There is a strong predilection for disease in the inquinal region, feet and the underside of the neck but Malasseziasis can cause generalized skin disease affecting almost the entire body in some dogs. West Highland white terriers seem particularly predisposed to generalized malasseziasis.

Dermatophytosis is a fungal infection of the skin. Ringworm is the more common name for these infections. There are several different organisms that can cause ringworm but the most common form in cats is Microsporum canis. Ringworm can cause itchiness but it does not always do so. In cats the most common form of ringworm is probably an inapparent infection, or "carrier" state. Microsporum canis is also common in dogs. Most dogs with the disease become immune for a brief period of time after infection and a carrier state is much less likely in dogs. Dogs also get Trichophyton infections.

In cats the typical patterns for ringworm infection are small scabby lesions around the neck and body which may or may not be accompanied by areas of hair loss. When hair loss occurs it is fairly common for it to occur on the face or front legs but it can occur anywhere in cats. Some cats will lose hair over very wide regions of the body, especially if the ringworm is causing itchiness. In the case of itchiness there is sometimes an obvious difference in hair coat quality in the areas the cat can reach to groom and those that it can not -- so a cat may acquire the appearance of a lion with a mane of hair around its head and neck and very little hair elsewhere. Ringworm can also occur as a very localized infection in area such as nail beds, around the vulva or on the ears.

In dogs most ringworm lesions seem to occur on the face and front legs. One of the reasons for this is that dogs are more prone to Trichophyton infections where are acquired from soil and contaminated objects such as fences. Dogs are more prone than cats to having very small areas of infection in areas such as the lips, ears and vulva. There are dogs who have full blown whole body ringworm infections similar to those that occur in cats but this is unusual in dogs.

While I didn't set out to get into treatment too much in this issue it is important to remember that ringworm treatment in cats almost always requires both topical and systemic (oral) therapy. It is often possible to treat dogs with topical medications alone since they are much less likely to develop inapparent carrier states.

Demodectic mange in dogs is usually not itchy until secondary infection occurs. Infections are commonly associated with demodecosis so this can be a confusing issue. It is important to remember back to the first appearance of hair loss in order to judge how much of a role itchiness played in the beginning. Hair loss tends to occur first along the edges of the lips, around the eyes and on the front legs. Over time patches of hair loss may occur anywhere on the body. Anytime there are patches of hair loss or obviously thinning hair in a dog that is not itchy skin scrapings should be done to check for Demodex mites. Demodecosis can exist as a small local problem or it can affect the whole body. In general, if there are 5 or more patches of hair loss on at least two different regions of the body from which Demodex mites can be identified the infection is considered to be generalized. In most dogs demodecosis occurs between 3 and 12 months of age. When it occurs in older dogs it usually occurs secondary to a disease that is suppressing the immune system such as Cushing's disease or cancer.

Bacterial pyodermas, or bacterial skin infections, are very common in dogs and less common in cats. Pyodermas can be a localized problem and may be very itchy in some instances. The classic really pruritic localized bacterial infection is a "hot spot". These are moist wounds that occur secondary to self trauma. They can be very upsetting to pet owners because the intensity of the itchiness can lead to severe wounds in a very short period of time, often a few hours or less. Golden retrievers and sometimes Labrador retrievers seem to have a special predilection for getting this sores on the side of their face just below the ears.

Pyodermas can also be a more generalized problem. This is particularly true in dog breeds with short to medium length hair, such as Labrador retrievers and dalmatians. Generalized pyodermas can seem to be just a matter of bad dandruff with almost no other signs to truly severe whole body skin infections. In most adult dogs generalized pyodermas occur secondary to other illnesses but paradoxically, treating only the pyoderma is frequently enough to resolve all clinical signs even when an underlying cause such as allergies is present. In these cases it is generally accepted that antibiotic therapy is preferred over other treatments for the symptoms of allergies when they will control clinical signs, by most dermatologists. There is no specific pattern for bacterial pyodermas and they must be considered as a possible primary cause, or secondary complication, in almost all cases of skin disease in dogs, at least during the initial diagnostic process.

Fold pyodermas are infections that occur in skin folds. You might think that this is primarily a problem of breeds with deep skin folds such as shar pei dogs or Himalayan cats but skin fold infections are fairly common in a number of dog breeds. They are less common in cats but do tend to occur in the cat breeds with deep facial folds. We see lip fold infections occasionally in all breeds of cats and often only a very shallow fold is necessary to support an infection. Obese pets of any breed can develop fold pyodermas. This is especially true for vaginal fold infections in female dogs.

There are several things that make skin fold infections a fairly serious problem. Hair from one side of the skin fold grows across the fold and irritates the other side. Moisture is trapped in skin folds and this encourages bacterial growth. The dark moist environment of many skin folds is just idea for bacteria. It can be hard to get skin fold infections under control without surgical removing the offending folds. Obviously this can be an unpopular solution, especially for facial fold infections in breeds with prominent facial folds. Long term antibiotic therapy, either localized or systemic or both, may be necessary to keep the infections under control if surgery is not possible or doesn't resolve the problem entirely. Lip fold infections can have a really severe odor that can be mistaken for dental disease. If the folds are heavy it may be necessary to pull apart any visible folds to see infection in the depths of the fold. In some cases these infections appear to be fairly painful when touched, so be careful if this step is necessary when attempting to determine if a fold infection is present around the mouth.

Hair loss patterns when itchiness is not common

Hormonal diseases in older dogs and less commonly in older cats sometimes have very typical hair loss patterns. This is particularly true for hypothyroidism, although it is very important to point out that hair loss is only one symptom and that it is not present in many dogs who have hypothyroidism. Their is also a typical hair loss pattern for hyperadrenocorticism (Cushing's disease) which is a common disease in older dogs and a much less common disease in older cats. Hyperthyroidism in cats sometimes causes hair loss associated with overgrooming. Although hair loss isn't a major sign of diabetes mellitus in cats it often contributes to a greasy feel to the hair coat. Whenever we see a middle aged cat (especially overweight male cat) who had developed a greasy hair coat we are very suspicious of diabetes.

The most distinct hair loss pattern associated with hypothyroidism in dogs is the development of a "rat tail" or a tail that has no hair on the tip and then along the length of the tail to a varying degree. Sometimes this hair loss pattern will occur with allergic skin disease, especially flea allergy, but in a middle aged or older dog who isn't itchy it is closely associated with hypothyroidism. Less commonly dogs with hypothyroidism will have bilaterally symmetrical hair loss along their sides or other changes in the hair coat quality.

The classic pattern for hyperadrenocorticism is bilaterally symmetrical hair loss along the flanks that occurs in an older dog who often also has a potbellied appearance. There can be skin sores with a very hard consistency caused by calcium deposits in the skin and these can be very itchy, but most of the dogs we see with Cushing's disease are not itchy as a direct result of the disease. On the other hand, secondary bacterial infections of the skin are very common in dogs with Cushing's disease so it can be very important to note whether the hair loss came first or the itchiness came first when trying to decide on the best course of therapy or best diagnostic protocol to follow.

Seasonal alopecia is just what the name implies, hair loss that occurs on a seasonal basis. Hair loss occurs in a bilaterally symmetrical pattern, usually on the sides of the body. This condition is easily confused with the hormonal diseases. Hair loss occurs seasonally, with some dogs losing hair in the spring and getting it back in the fall, some losing it in the fall and getting it back in the spring and much less commonly hair loss in the winter with normal hair growth in the summer. This is most common in boxers, bulldogs and Airedales but it is being seen in other breeds now that the syndrome has been recognized. As far as I know this problem does not occur in cats.

Genetic Testing for Multi-drug Sensitivity

I get a lot of e-mail from folks who read the vetinfo.com web site who believe that I am wrong to suggest that collies and other herding breeds can be given ivermectin for heartworm prevention. This controversy is based on the sensitivity of these breeds to this medication when used at much higher doses, usually twenty times the heartworm prevention dose or more, for treatment of other parasite infections, particularly demodectic mange.

There is still no evidence that I am aware of that indicates that collies or any other breed of dog have adverse reactions to ivermectin, milbemycin or other monthly heartworm prevention medications at the standard doses. I find it kind of amusing that many of the folks who write me believe that I should recommend milbemycin over ivermectin since it is safer. The irony is that milbemycin only requires a ten times overdose to cause symptoms so it is actually more likely to cause reactions in the unlikely event that someone manages to overdose their collie to this degree.

To get back to the point, though, there is good evidence now that the sensitivity to ivermectin is not a simple reaction to a particular medication. Dogs who are susceptible to ivermectin toxicity have a mutation in a specific gene sequence. This mutation affects a drug transport system involving p-glycoprotein. When this molecule is not present in adequate quantities drugs that are absorbed into cells can not be removed by the cell and a toxic exposure results.

The inability of dogs with the specific gene mutation (MDR1d) to process ivermectin also affects their ability to process a number of other drugs, some of which are used fairly frequently in veterinary medicine. These drugs include all of the monthly heartworm prevention medications, narcotics (loperamide, butorphanol), acepromazine, digoxin and dexamethasone. In addition, there are severe implications for dogs undergoing chemotherapy, as several of the most commonly used chemotherapeutic agents as well as the best drug to prevent nausea associated with chemotherapy, ondansetron, can also be toxic to patients with this condition. While the heartworm prevention medications must be overdosed to cause problems the high end of standard dosing schedules for narcotics and acepromazine are within the toxic range for some dogs.

Collies, Australian shepherds, Shetland sheep dogs, Old English sheepdogs, border collies, German shepherds and long haired whippets are widely affected and scattered reports indicate other breeds may be affected to some degree, as well. There probably isn't a major reason to check for this condition unless you have one of the well recognized breeds to be affected with this problem as this time, though.

I think that I would want to know the results of this testing prior to any event that involved anesthesia, treatment for demodectic mange or chemotherapy. Many vets, possibly most vets, use acepromazine prior to anesthesia and use pain relief medications such as butorphanol or possibly morphine that may be affected by the presence of this gene mutation in conjunction with surgical procedures.

Genetic testing for multi-drug sensitivity is available from the Veterinary Clinical Pharmacology Laboratory at the College of Veterinary Medicine of Washington State University. The cost is currently $60 for a single dog or $51 per sample if five samples are submitted at a time. A cheek swab is all that is required and the kits for testing are available from the laboratory. More information is available from the vet school's web site, www.vetmed.wsu.edu/vcpl

Thanks for your Support!

VetInfo Digest

P.O. Box 476

Cobbs Creek VA 23035

All opinions in this newsletter are those of the author, Michael Richards, DVM