The Itchy Dog: An Overview of Diseases

Chris Reeder, DVM, DACVD

Sometimes no matter how hard we try, a diagnosis of pruritic skin disease is frustrating.There are a few key questions and findings that may make life a little easier in dealing with the itchy dog.  The skin as an organ has incredible powers. Not mystical or magical, those are confined to brain along with neurologic pathways to control motion and physiologic/psychologic processes. No, the skin has its own special powers.  It acts as a physical barrier providing innate protection against the evils of the environment.  Skin stretches and can transition from taut to lose, thick to thin, it controls water loss and temperature regulation, it acts as a sensory organ and to many animals, camouflage to help hide from predators or stalk prey.  Skin truly is an incredible organ, though as incredible as it is, an Achilles’ heel does exist…..pruritus. 

Pruritus is derived from the Latin prurire meaning to “itch.”   It is defined as the desire or reflex to scratch.  Itching in the dog is one of the most common presenting complaints from owners to the general practitioner, yet one of the most frustrating and costly as well.  There is help, several key questions and findings may provide a starting point or diagnosis as to why a dog is itching.  Our first step is the question the owner about the dog’s condition:

-          Age of onset of the itching

-          Duration of the itching

-          Other animals in the household affected

-          Housed indoor, outdoor or both

-          Travel history

-          Current and previously fed diets (including human foods)

-          Seasonal history or exacerbations of itching

-          Previous medications, what worked and what didn’t work

-          Current flea/tick control products (other animals treated)

-          Number of bowel movements daily

-          Loose, runny stools or any vomiting

Many animals who have a food allergy will present as young dogs (<1 year of age) with loose or runny stools, >3 bowel movements daily and may be refractory to glucocorticoid use.  Other household animals are not affected and this is typically non-seasonal with itching as a constant feature.  Compare that to the typical dog with atopy (environmental allergies) and we see that most of those dogs present first between 1-4 years of age, have some seasonal pattern, respond to glucocorticoids with a reduction of pruritus and no other animals in the household are affected.  The author’s experience, along with many dermatologists, find that atopy is a much more common presentation than food allergy in roughly 90% to 10%, respectively.  We also see that food allergic dogs often have some compromise to the gastrointestinal barrier function.  This could be a history of intestinal parasites, Parvovirus enteritis, or any other disruption to the gut integrity.  What about a dog whose symptoms appears more atopic but is non-seasonal?  That could be a dog that is suffering from indoor allergens (dust mites, storage mites, human or cat dander, sheep wool, cotton, etc) which allergy testing may be a helpful tool in order to diagnose. 

Scabies can mimic food allergy and atopy very closely.  Other animals or even humans in the household may be affected with pruritus.  The typical approach to dogs with scabies is to identify a pinnal-pedal reflex.  This involves grasping the pinna between the thumb and forefinger of both hands and rubbing the skin together.  A positive response would involve the ipsilateral hind leg start scratching vigorously after a few seconds.  It has been reported that up to 80% plus of dogs with scabies exhibit a positive pinnal-pedal response and these are cases to empirical treat with an acaracidal therapy (e.g. Advantage-multi^® Bayer, Revolution^® Pfizer) which is usually an off-label dosing of one treatment every 2 weeks for 3 total doses.  Common areas affected with scabies infestation include: hocks, elbows, pinnae, groin and scrotum.  All animals in the household should be treated as some may serve as a reservoir even if not itching. 

Flea allergy is truly a hypersensitivity response. This appears as a dose-dependent phenomenon with the higher number of flea bites with salivary protein exposure thus resulting in an allergic response.  The number of bites to induce an allergic response varies with some animals showing no response even with hundreds of bites.  It is important to examine the dog, especially those with long coats, on the rump region.  A severe inflammatory response can be seen in dogs with a strong allergic response to fleas with moist pyoderma and excoriations predominantly over the dorsal lumbar region.  Another common finding in dogs with flea allergy is the discontinuation of flea control over the winter months.  Fleas overwinter indoors and continue to thrive, only at a slower rate than the warm and humid summer months.  Continued flea control for all animals in the household is a very effective means of controlling flea allergies.  Several topical formulations now have both flea prevention and heartworm prevention in a single dose application (e.g. Advantage-multi^® Bayer).  Bathing around the time of application of the topical spot-on formulations of flea preventative is another question to ask the owner.  Typical recommendations are to wait 48-72 hours after application to bathe a dog or use an oral flea preventative (e.g. Comfortis^® or Trifexis^® Elanco). 

   

Ringworm, is neither a ring nor a worm, the saying goes “if it looks like ringworm, it’s probably not” in dogs holds true.  Dermatophytosis in dogs usually presents with pruritus, hair loss, erythema and scaling.  Rarely does ringworm present as focal, circular patches of alopecia in dogs.  Various forms of ringworm can be present in dogs, geophilic, zoophilic, anthrophilic with the geophilic (soil) or zoophilic (animal) forms most common.  Microsporum gypseum is a geophilic dermatophyte whereas Microsporum canis is a zoophilic dermatophyte and the most common species seen in the dog and cat.  Occasionally multiple animals in the household may be affected and about 10% of humans can also have lesions which are typically pruritic.  A dermatophyte test media (DTM) culture is ideal to diagnose and speciate the type of dermatophyte present.  Fluconazole (5-10 mg/kg po daily) is the author’s treatment of choice for dermatophytosis.  Terbinafine (Lamasil^®, 30-40 mg/kg po daily) may also be used for refractory cases. 

Pruritus also damages the cutaneous barrier function both as, what we think is, a genetic dysfunction of the intercellular cement along with direct excoriations of the integument. This barrier defect has led to the development of many good products to help control and restore function.  Wipes, shampoos, lotions, balms and sprays have all been recently developed an are on the maket to help restore or improve skin barrier function in animals.  Shampoos have technology to even prevent bacteria/yeast from adhering to the skin surface (Virbac glycotechnology).  Pro-ceramides have been incorporated into sprays and shampoos to help repair damaged skin (phytosphingosine, Douxo^® (Sogeval)). 

Pruritus may have developed due to an allergic response to food, parasites or the environment and taking the time and understanding what to look for in these cases can make for a much more rewarding treatment outcome.  Many dogs need ongoing management for their atopy, need to be on flea prevention every 3-4 weeks ongoing or must be maintained on certain diets if food allergic.  Scabies is curable, though we do occasional see reservoirs in other household dogs or wildlife making further questioning an important part of the treatment.  Asking the right questions along with a thorough history and physical exam may help increase the correct diagnosis and decrease frustrations levels for the itchy dog.