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Demodex    

        Mr. Shelby got a new Labrador puppy Hershey a few months ago.  On her first exam at 8 weeks he had noted that she seemed to be more itchy than normal and seemed to have quite a bit of dandruff.  In our dry climate it is common for young puppies with their tender skin to be a bit dry and flaky, so we didn’t worry too much about it at that time, but by her 4 month exam she was digging at her skin continually and had started developing bare patches in her coat all over her body.  It was pretty clear that something more than just dry skin was going on.

            There are lots of things that can cause itchy skin in dogs, and finding the cause is the key to fixing the problem.  In Hershey’s case her age was a helpful clue in guiding us to the right course of action.  One of the more common reasons for itchiness and bald spots in the coat in dogs that are under nine months of age is a type of skin mite called Demodex.

            Unlike other species of mites that hop on a host from the outside environment, Demodex mites are transferred from a mother to her puppies within the first few days of life, where they make a home inside the base of the puppy’s hair follicles.  These mites are  normal lifelong inhabitants in all dog’s skin.  (People also have them in some of their hair follicles)  The problems start when a puppy’s immune system hasn’t learned to keep the mite population under control and the mites begin to multiply in the hair follicles to the point where they cause enough irritation to make the skin itchy and make the hair fall out of the follicle.

            We diagnose Demodex by scraping a blade sideways across an affected area until we get deep enough to just start seeing a little blood on the surface.  The debris that is scraped up is then put under a microscope and examined.  The mites look like little caterpillars, and usually in affected dogs lots of them can be seen.  Sometimes the mites are deep enough that they don’t appear on a skin scrape even if they are causing a problem, so if the degree of suspicion is high enough we will occasionally treat for them even when the scrape is negative. 

            There are several ways to treat Demodex.  In very mild cases it may be acceptable to just ignore it.  Almost all dogs eventually mature their immune systems to the point that they can get the mites back into balance on their own.  More affected dogs can be treated with a topical ointment called goodwinol if the problem areas are isolated and small, or orally with a medication called ivermectin  if the problem is more widespread.  The dose of ivermectin used is quite large, and some dogs have a genetic sensitivity to the drug that can cause severe adverse neurological problems, so caution must be used in breeds known to have problems with the drug--especially collies and shelties.  There is a medication called amitraz which can be made up into a rinse and applied topically to dogs as well. Amitraz is quite effective, but bathing a dog with it results in lots of exposure to the bather as well as the recipient of the bath, so I avoid using it when I can. Drugs in the cortisone family, which are commonly used to alleviate itching, will make symptoms of Demodex much worse.

            Occasionally adult dogs will develop problems with Demodex.  When this happens it is often a signal that the immune system is not functioning normally, and the mite problem is just a secondary consequence of something much more serious.

            Cats have their own species of Demodex as well.  It is much harder to find on a skin scrape than dog Demodex, and is implicated as a sneaky reason why some cats may have itchy skin.  Due to the difficulty of diagnosis it is hard to say exactly how common Demodex problems are in cats, but it is currently considered unusual.

            We treated Hershey with oral ivermectin once daily for 6 weeks.  Her itching improved dramatically in a week, but as is the case with most dogs it took quite a long time to settle the problem down completely.  Now she has a full coat and no dandruff and looks and feels like she should.

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