If you think it's scabies, treat it

Always keep scabies in mind as a differential in your pruritic canine patient, and, remember, very few cases are typical.

Canine scabies, or Sarcoptes scabiei var. canis is a transmissible infestation of the skin of dogs that is transferable to other species. Scabies have a 17- to-21-day life cycle. The female mite excavates a burrow in the skin and lays her eggs behind her (Photo 1).

Photo 1: Scabies eggs from a skin scraping in oil viewed under low power. (PHOTO COURTESY OF DR. ALICE JEROMIN)

Mites prefer hairless skin. Canine scabies mites have a terminal anus vs. dorsal anus found in feline Notoedres cati (Photo 2). Preferred hosts include members of the canis species such as foxes, wolves, coyotes and dogs, but can cause disease in other species such as cats and humans. Lesions on people consist of papules within 24 hours of exposure accompanied by pruritus. Canine scabies mites on a person are difficult to isolate. Although they burrow, they will only remain on the unpreferred host for a few hours to a few days. Pruritus can persist for several weeks following eradication of the mite.

Photo 2: Canine scabies mite viewed under low power. (PHOTO COURTESY OF DR. ALICE JEROMIN)

Canine scabies have been found living on cats affected with mites. The off-host survival time is temperature and humidity dependent. At room temperature, all mite stages can survive two to six days. Female mites at low temperature and high humidity can survive off-host for four to 21 days. The incubation period is unknown but severe pruritus is evident at 21-30 days.

Clinical signs and symptoms

Pruritus is usually nonresponsive to antipruritic or even immunosuppressive doses of steroids. If the dog is itchy despite a high dose of steroids, be sure to consider scabies. Since the mite prefers nonhaired areas, affected areas include one or more of the following: elbows, hocks, ventral abdomen and/or ear edges. Lesions range from none with just pruritus to crusting (Photo 3) (be aware that owners sometimes "pick off" the crusts), excoriations, alopecia, lymphadenopathy, corneal ulceration or aural hematomas.

Photo 3: Crusty ear edge of a Poodle with scabies mites. (PHOTO COURTESY OF DR. ALICE JEROMIN)

Differential diagnoses

Differentials can include other ectoparasitic diseases such as Cheyletiella, atopy, Malassezia dermatitis, and food allergy.

Suspect scabies if the dog has been exposed to other dogs via kennel or grooming; exposed to foxes, wolves, or coyotes; if several dogs in the household are pruritic; if a new puppy comes into the household and pruritus starts; and in patients on immunosuppressive doses of steroids for other internal medicine problems. Hypothyroid patients may have large numbers of mites and not seem to be all that pruritic particularly if they are profoundly hypothyroid (Photo 4).

Photo 4: Hypothyroid dog with scabies mites. (PHOTO COURTESY OF DR. ALICE JEROMIN)

Diagnosis

Skin scrapings can yield mites, fecal pellets or eggs. (Keep in mind that scrapings may be negative up to 70 percent of the time).

Other diagnostic techniques include:

» Skin biopsies (alluding to allergic dermatitis sometimes found with eosinophils)

» Positive ear-edge/rear-leg reflex (pinnal-pedal reflex) in atopic patients.

» Positive skin test reaction to house dust mites

» Serological test (available in Europe)

» A response to therapy ("Ivermectin response test")

Treatment

Bathing should be performed to remove seborrheic crusts, antibiotics for secondary infection, steroids at anti-inflammatory doses, and treatment of the environment (e.g., pet bedding/toys/carpeting) with a pesticide. Acaricidal therapy should be undertaken for the dogs and cats in the household with one of the following:

» Lime sulfur 2-3 percent dips every five to seven days.

» Amitraz (Mitaban) dip once weekly x 2 dips (only if >16 weeks of age). Not for use in cats.

» Ivermectin (Ivomec) orally or subcutaneously at 200ug/kg every seven to 10 days for a total of three to six doses (not FDA approved and not commonly used in cats) after first testing heartworm negative. Contraindicated in Collies, Shetland Sheepdogs, Old English Sheepdogs, Australian Shepherds, other herding breeds and crosses.

Before using, you may want to check a p-glycoprotein deficiency test from Washington University's Medical School (St. Louis). I do not use Ivermectin in geriatric patients because they tend to experience more neurologic problems even at a low dose.

» Milbemycin (Interceptor) orally at 1 mg/kg body weight every other day for 8 doses or 1 mg/lb body weight once weekly for four to six weeks after first testing negative for heartworm. (Plumb says 2 mg/kg po q 7d x 3 doses or 0.75 mg/kg sid x 30d)

» Metaflumizone/amitraz (ProMeris for Dogs) used once monthly for two months or biweekly for four doses (cure rates at day 56 were 75 percent for monthly use and 83 percent for biweekly use in dogs only).

» Selamectin (Revolution) one tube topically every 15 days for three doses on cats and dogs. (Not approved for scabies in cats? Label says q 30 days, some animals may need every 15 days.)

» Fipronil (Frontline) Spray Treatment used for dogs and cats at 1 spritz/lb body weight every 21 days for 2 doses or Frontline Plus for dogs used monthly for multiple treatments.

Dr. Jeromin is a pharmacist and veterinary dermatologist in private practice in Cleveland, Ohio. She is a 1989 graduate of The Ohio State University College of Veterinary Medicine and an adjunct professor at Case Western Reserve University's College of Medicine in Cleveland.