This week we’re talking about mites in dogs.
The most common mite of demodectic mange is Demodex canis. Many dogs raised normally by their mothers possess this mite as mites are transferred from mother to pup via cuddling during the first few days of life. Most dogs live in harmony with their mites, never suffering any consequences from being parasitized. If, however, conditions change to upset the natural equilibrium (such as some kind of suppression of the dog's immune system), the Demodex mites may gain the upper hand. The mites proliferate and can cause serious skin disease.
Unlike sarcoptic mange, demodectic mange is not considered a contagious disease and isolation of affected dogs is generally not considered necessary. Current thinking is that mites can be transferred from one dog to another but as long as the dog is healthy, the mites simply add into the dog's natural mite population and no skin disease results. In rare circumstances spread of disease is possible if a severe infection is involved. While there are still assorted theories about dog-to-dog transmission of Demodex mites, there is no question that mites cannot be transmitted to humans or to cats.
Localized demodicosis occurs as isolated scaly bald patches, usually on the dog's face, creating a polka-dot appearance. Localized demodicosis is considered a common puppyhood ailment and approximately 90% of cases resolve with no treatment of any kind. This is quite a contrast to generalized demodicosis as described below so it is important to be able to distinguish localized from generalized disease. It seems like this would be a simple task since localized demodicosis classically involves several round facial bald spots and generalized demodicosis involves a bald scaly entire dog; still, reality does not always fit into neat categories in this way. Some guidelines used to distinguish localized demodicosis include:
Treatment is not necessary or recommended for localized demodicosis. Resolution of a localized demodicosis lesion should be at least partially apparent after one month though total resolution can take up to three months. Approximately 10% of localized demodicosis cases will progress to generalized demodicosis. Enlarged lymph nodes are a bad sign -- often foretelling generalized mange.
Generalized demodicosis entails much more extensive involvement of the skin. Large patches of skin are affected and, if allowed to progress untreated, the entire surface of the dog may be impacted. Sometimes a polka dot appearance results but if there are more than four spots present, the patient is no longer classified as having the "localized" form and treatment rules for generalized disease may be applied. The secondary bacterial infections make this a very itchy and often smelly skin disease. These infections may require antibiotics, but it is important to realize that infection will probably not resolve fully until the mites are gone.
The biggest change in treating this condition came with the release of the isoxazoline flea products (Bravecto®, Nexgard®, Credelio® and Simparica®). The isoxazoline products are oral medications labeled to kill fleas and ticks in dogs. It turns out they have substantial activity against Demodex mites and are successful enough that they have become the treatment of choice. These products are typically given orally every two to four weeks, depending on the protocol. Most dogs are simply cured by this protocol
Young dogs that develop demodicosis are believed to have a genetic immunological defect that allows the mite to gain the upper hand. As the puppy grows up and his or her immune system matures, the immune system tend to naturally regain control of their mite infestation; in fact, 30-50% of dogs under age one year recover spontaneously from generalized demodicosis without any form of treatment. Usually treatment is recommended, though, to facilitate recovery.
Most demodicosis occurs in young dogs, under age one and a half. An older dog should not get demodicosis unless there is an underlying problem with the immune system. In such cases, demodicosis is considered an indication to seek a more serious hidden condition such as cancer, liver or kidney disease, or an immune-suppressive hormone imbalance.
Occasionally, a dog fails this treatment and is relegated to the more traditional methods such as: high dose oral ivermectin, a protocol that is not safe for many herding breeds; topical moxidectin (Advantage multi®); daily oral milbemycin (Interceptor®); or even labor intensive dipping with amitraz (Mitaban®).
Treatment should be accompanied by skin scrapes every two to four weeks. In this way the effectiveness of treatment is assessed, and dosing can be changed. There are several protocols describing how to determine if treatment can be stopped. The idea is to eradicate every single mite from the dog's body so that the condition cannot recur. This typically entails either continuing treatment for a significant time after the patient appears recovered and/or rechecking skin scrapes a significant time after treatment is finished.
Dr. Lamb is the Veterinarian at the Manchester Animal Hospital.
