Demodectic Mange in Dogs

Causes

There are three species of Demodex mites that have been found in dogs. 

Demodex canis is the most common Demodex mite, and is transmitted from a mother to her puppies in the first few days of nursing. It lives in the hair follicles and sebaceous glands, is considered normal flora of the skin, and is usually present in small numbers.

Demodex injai is a large, cigar-shaped  Demodex mite found in the pilosebaceous unit. It is unknown how it is transmitted. It is only associated with adult-onset disease along the dorsal topline of a dog, mostly in West Highland White Terriers, Wirehaired Fox Terriers, and Shih Tzus.

The third Demodex mite is Demodex cornei, which lives in the stratum corneum of the epidermis. Its mode of transmission is also unknown. It is thought to be a variant of D. canis.

This is a microscopic view of Demodex, the cigar-shaped mite causing demodicosis in dogs.

Risk Factors for Demodicosis

Demodicosis is a strange and interesting disease. It is an overgrowth of an otherwise “normal” interaction of a parasite with its host. This overgrowth appears to have an immunopathologic basis, which is not understood.

• Dogs with generalized demodicosis may have abnormal or depressed T-cell function.
• Development has been associated with oclacitinib (Apoquel®) treatment.
• Genetic factors (especially with juvenile onset demodicosis), metabolic diseases, and other causes of immunosuppression may predispose to demodicosis.
• Exposure to dogs with demodicosis–there is some argument as to whether Demodex can be spread from an affected dog to another dog. So far it appears that it is not contagious except to immunocompromised dogs.

Clinical Signs

Demodicosis is a disease that has definite age and breed tendencies, or predilections.

D. canis is seen more frequently in the following breeds:

• American Staffordshire Terrier
• Shar-Pei
• Boston Terrier
• English Bulldog
• West Highland White Terrier

Demodex injai is noted more often in these breeds (as mentioned above):

• West Highland White Terrier
• Wirehaired Fox Terrier
• Shih Tzu

Demodicosis is usually seen in dogs under one year of age, typically showing just a few lesions. Generalized demodicosis, which is fortunately much less common, can be seen in both young and old dogs.

Signs of demodicosis are as follows, depending on the severity and species of Demodex mite:

• Patchy hair loss–most commonly on the face, especially around the mouth, the eyes, and the front legs–this is the most common presentation, especially in young dogs, and is usually from D. canis infestation
• Hair loss may also be on the trunk and feet
• Pododemodicosis–lesions are localized to the feet
• Demodicosis is usually not itchy unless a secondary bacterial infection develops
• Mites can cause follicles to distend, get infected, and rupture (furunculosis)
• Skin can become severely inflamed, exudative, and granulomatous
• D. injai may cause seborrheic dermatitis of the dorsal trunk, comedones, alopecia, erythema, and hyperpigmentation 
• Demodicosis can begin as localized and sometimes worsen to generalized disease

Differential Diagnoses

It is important to rule out other causes of bacterial folliculitis and furunculosis. Demodicosis can also be mistaken for ringworm (dermatophytosis). Any other causes of inflammatory hair loss can look like demodicosis so they must be ruled out, especially if the patient does not respond to appropriate treatment.

Diagnostic Tests

• Skin scrapings are diagnostic in most cases (the veterinarian gently scrapes a scalpel blade across the skin in several places, to attempt to scrape up Demodex mites from the hair follicles, then examines the slide under the microscope)
• Hair plucking can be used in the eyelid area (similarly, hair is plucked, then examined on a slide under the microscope, looking for mites at the hair roots)
• Skin biopsies may be needed in chronic, granulomatous areas or in Shar Peis
• Blood tests will usually be normal unless there is underlying disease causing immunosuppression
• Fecal samples will usually be negative, although mites can be a rare finding

Chronic patients should be thoroughly evaluated for their overall health and husbandry. Immunosuppression can be an underlying factor in the spread of Demodex, so diagnostic testing for diseases such as hyperadrenocorticism (elevated blood cortisol production), hypothyroidism, other systemic diseases, diabetes, and cancer, should be done.

Dogs with a history of chemotherapy, other immunosuppressive medications, and steroid administration are at increased risk for generalized demodicosis.

This English Bulldog has generalized demodicosis. This can become a chronic condition where mites cause the follicles to distend, get infected, and rupture (furunculosis). Once infected, the condition can become very itchy and even more miserable.

Treatment

Localized D. canis lesions usually spontaneously resolve (over 90%). Most of the time dogs under 1 year do not need to be treated, and are monitored for remission. Usually two negative skin scrapings 1 month apart are sufficient to diagnose remission in young dogs. 

The goal for treatment of generalized demodicosis is remission, which is achieved in about 70% of dogs. 

Generalized demodicosis is often a management problem. Diagnostic testing as listed above may reveal issues that require treatment. If there is no apparent illness, and no history of immunosuppressive medications, consider stress as a possible factor in immune suppression. Stress can take many forms: 

• Nutritional–not enough food or the wrong kind of food, carbohydrates, proteins, or nutrients
• Exercise–not enough or too much, too much crating 
• Temperature–too hot, too cold, too humid
• Emotional–illness or pain, loss in the family

Talk to your veterinarian about your dog’s nutritional, immunity, and exercise needs and the possibility for supplementation.

Juvenile onset of generalized demodicosis is thought to be heritable, and these animals should not be bred.

Effective medications include:

• Isoxazoline parasiticides
  • These are the treatments of choice at the label doses for flea control, with excellent efficacy against demodicosis, and are safe for avermectin-sensitive breeds.
  • They include fluralaner (Bravecto®), lotilaner (Credelio®), afoxolaner (Nexgard®), and sarolaner (Simparica®).
  • Side effects such as vomiting, diarrhea, anorexia, or neurologic signs including seizures, are not common.

• Ivermectin
  • Ivermectin is very effective.
  • Your veterinarian may want to start therapy with a low dose to watch for any signs of sensitivity to ivermectin.
  • Treat for 30-60 days beyond negative skin scrapings, with an average of 3-8 months of total treatment.
  • This is non-FDA approved usage. Do not use in avermectin-sensitive breeds with ABCB-1 mutation.

• Milbemycin Oxime
  • Milbemycin oxime cures 50-85% of cases, depending on the dose.
  • ABCB-1 mutation dogs may have neurologic signs with higher doses, but tolerate milbemycin better than other drugs in this class, such as ivermectin.
  • This is non-FDA approved usage.

• Moxidectin
  • Moxidectin can be given orally once daily or topically once a week.
  • This is non-FDA approved usage.
  • Do not use this in avermectin-sensitive (ABCB-1 mutation) breeds.

• Amitraz
  • Amitraz is a dip concentrate that must be diluted and handled carefully–see below for precautions. It is used every 7-14 days until the dog is cured.
  • Clipping your pet’s coat and bathing with a benzoyl peroxide shampoo before dipping have been shown to improve the response to treatment.
  • Do not use amitraz collars or spot-ons as they are ineffective.
  • Best results occur with regularly scheduled dips of correct concentration.
  • Up to 30% of chronic cases are not completely cured.
  • You may need to do maintenance rinses every few weeks if your dog is not cured.
  • Possible interactions: Remind your veterinarian that your dog is getting amitraz dips regularly, so they will keep this in mind when prescribing medications. Amitraz can interact with some anesthetics and sedatives such as xylazine and acepromazine.
    • Amitraz precautions: Up to 30% of patients can show drowsiness, lethargy, anorexia or depression for 12-36 hours after treatment. Rarely, more serious side effects can occur. Yohimbine is an injectable antidote that can be administered. You should apply amitraz in a well-ventilated area with an apron and gloves to avoid contact, and avoid its use if you are taking a monoamine oxidase inhibitor (MAO). Amitraz can also alter blood sugar in some diabetic people.

Amitraz Dipping

Apply a ¼ inch strip of artificial tear ointment to both eyes before bathing or dipping your dog. This will help protect their eyes from soap and dip. If you think any soap or dip has gotten into your dog’s eyes, rinse well with an eye rinse solution or water for 15  minutes, and contact your veterinarian or emergency clinic immediately.

Use as small amount of reconstituted dip as possible in a tub or plugged sink, keeping in mind that amitraz dip is toxic. Gently re-sponge the dip over your dog for the allotted time, avoiding splashing. Be sure to wear protective gear for your eyes, skin, and clothes. Consider using slightly loose gloves, so you can take them off easily to give treats to your dog during the dipping process. Alternatively, you can wash one or both hands and give the treats with your glove(s) on, or have another person give your dog treats to help them be more confident and comfortable with their dips. Or you can choose to comfort your dog with soothing words and give them treats when you are finished.

Let your dog’s coat dry naturally without blow-drying or toweling off, if possible. After you bathe and dip your dog, cover them with a hooded bathrobe towel or dog pajamas. This will help prevent your dog from getting chilled and will protect your furniture and home.

Potential Outcomes

The prognosis for your dog depends greatly on a few things–your compliance being the most important! You can greatly increase the odds of your dog having a cure by following the course of treatment as recommended by your veterinarian. Don’t miss doses of oral medications and don’t miss dips as scheduled (mites will be reproducing in your dog’s hair follicles if you miss doses). 

If your dog has genetic, immunological, or other underlying disease issues that are contributing to his demodicosis, they may need isoxazoline treatments vs. other medications.

Especially for dogs with generalized demodicosis, it is important to follow-up to be sure your dog is cured. Skin scrapings are usually done every 3-4 weeks until resolution, then continue to treat your dog for 1-2 months beyond when skin scrapings are negative.

Since up to 30% of dogs with generalized demodicosis never achieve a full cure, it may be necessary to continue to treat your dog to control their signs of Demodex. Talk to your veterinarian about a longer-term plan for your dog, which might include ongoing isoxazoline antiparasitics, dips, and antibiotics, or some rotation, pulse therapy (on again-off again), or combination of the three.

Use training treats to get your dog accustomed to the bathtub and the dipping experience. Wash your hands of any dip, or remove your glove(s) before giving treats to your dog. Be sure to reward them with treats during and after the dip, and try to make the whole experience as fun as possible.