CAUSES AND TREATMENT OF SCABES IN DOGS (PART 2: DEMODEX)

Scabies in dogs is caused by two different species of mites: Sarcoptes and Demodex. Although the common symptom is itching, the manifestations of each disease are somewhat different. The disease is caused by scabies and is spread from sick animals. The scabies burrow and lay eggs under the skin, causing intense itching as soon as they are infected. Meanwhile, hair coat mites are parasitic mites in hair follicles. The disease only occurs when hair coat mites multiply too much to overcome resistance, causing skin irritation and hair loss (Ravera et al. 2013). Although scabies is not dangerous to your dog's life, your pet's health can decline quickly if there is a serious infection or loss of appetite or sleep.

DISEASE CAUSED BY DEMODEX
Pathogen characteristics
Form
Demodex canis is a species of tick belonging to the genus Demodex, family Demodecidae. Each Demodex species is specific to its host, for example, the main parasite in dogs is Demodex canis, the main parasite in cats is Demodex cati. Therefore, the disease is not transmitted between different animal species.
Adult Demodex canis is 150-285 micromet long, females are shorter and rounder than males. Demodex canis has a long transparent body with eight short legs, attached to the first body segment. 
Demodex moves at a speed of 8-16 mm/hour mainly at night, causing the mites to burrow into the hair follicles. The body is covered with scales to attach itself to the hair follicles, and Demodex has a pinhead-like mouthpart that feeds on skin cells, hormones, and oils (sebum) that accumulate in the hair follicles.

Life cycle
Mating takes place in the follicle and the egg is laid inside the follicle or sebaceous gland. After 3-4 days, the eggs hatch into six-legged larvae and the larvae develop into adults in about 7 days. The total lifespan of Demodex is several weeks, and the Demodex corpse will decompose inside the hair follicles or sebaceous glands.

Epidemiology
Demodex parasitic in dogs is mainly Demodex canis, sometimes the parasitic species in dogs is Demodex injai or Demodex cornei but it is very rare (Plant et al., 2011). Demodex parasitizes in the dragon follicles and sebaceous glands, usually found on the cheeks, nose, chin, forehead, temples, eyelids, eyebrows and also on the scalp, neck, and ears. They can also be found on the penis, nasolabial folds, buttocks, and in the sebaceous glands in the inguinal mucosa.
Demodex is transferred between hosts through contact of hair, eyebrows, and sebaceous glands on the nose. Many puppies receive initial contact from their mothers during the first days of their nursing life. However, the immune system of healthy animals helps control the number of Denodex canis, so it does not increase the chance of developing the disease (Ravera et al., 2013).

The likelihood of progression of clinical disease is influenced by many factors including; genetic defects, structural and biochemical changes of the skin, immune disorders, hormonal status, breed, age, nutritional status, stress, hair length, stages of the estrus cycle, reproduction, endometritis and debilitating diseases. Of these, the immune status is said to be the most significant. Systemic disease requires an ecologically favorable skin environment and is immune to the extreme infestation of Demodex canis. In large quantities, Demodex can increase the rate of cellular self-destruction or immunodepletion of TCD4 cells. An increase in cell destruction of peripheral leukocytes leads to the progression of clinical manifestations.
The disease can occur in dogs 18 months of age and younger due to immunodeficiency associated with visceritis, malnutrition, or debilitating health. Puppies can also develop the disease due to an immature immune system or immunodeficiency specific to ticks.  Increased prevalence in certain breeds suggests a genetic basis for immature dog-onset disease. In dogs over 18 months of age, the disease can occur due to drug-induced immunosuppression or systemic disease, so dogs with an adult-onset illness should undergo a detailed physical examination to identify underlying diseases.

Clinical pathology
Symptoms of celiac disease include: Dogs often rub themselves against their face or head, shedding hair in patches, dermatitis, excess oil on the skin, scabs on the skin, and swollen paws. The disease can occur with different pathologies such as:
Local lesions: Classified as local when there are no more than 4 skin lesions, the diameter of the lesion site <= 2.5cm. Lesions are often seen in the puppy's facial area. Most local cases will resolve on their own after 6-8 weeks without treatment when the immune system is complete.
Diffuse lesion: Classified as a diffuse lesion when there are more than 4 lesions on the skin and the diameter of the lesion is >2.5cm. This case is usually prone to secondary bacterial infections, when the dog is severely itchy and has a bad smell.
Foot dermatitis: The lesion is only located on the foot. It causes infection caused by secondary bacteria located between the pads and the toes. This is the most difficult type to cure.

Diagnose
Demodex canis lives mainly in the sebaceous glands and hair follicles. They can persist in the skin of animals without manifestations of pathology. Shedding and skin lesions develop when the tick population overgrows. Although both skin surface and deep surface scraping methods can detect the presence of ticks on the skin, deep scraping can detect Demodex ticks in some cases when the scratch is superficially negative.
The deep skin shaving technique aimed at a small area of skin (< 2 cm2) can be helpful if you press lightly on the skin or actually squeeze the skin of interest between your thumb and finger to try to squeeze the mite deeper to the surface of the skin more superficially. In some breeds such as Old English Sheepdogs and Shar-Peis, it can be especially difficult to spot ticks on skin scratches. In such cases, when the Demodex infection is negative but the results of multiple shaves are negative, a skin biopsy is appropriate.

Technique: Apply a drop of mineral oil to the circular cutting blade (#10 or #20). Pinch the skin at the area of interest and shave vigorously to remove the superficial cuticles. This area will turn red (no blood) if the shaving is done correctly. Put the oil/residue mixture on a microscope and test at 10x magnification. The tick and its eggs will be clearly visible at low magnification, but accurately identifying the tick will require 40 times magnification. It may be necessary to use a needle to puncture the debris on the microscope blade, especially when there is significant hyperkeratosis.
In sensitive areas where it is difficult to scrape the skin (e.g. feet, middle skin, around the eye socket), it can be done using the "trichoGram" technique. How to do: Pluck out a small amount of dragon in the affected area using tongs, clamping in the direction of dragon growth; between clamping close to the surface of the skin and pulling out all the dragon fibers that have grown. Put a drop of mineral oil on the glass plate, place the dragon fibers in parallel order on the mineral oil, separate them to fully evaluate the roots and tops of the mycelium. Examining the hair fibers under a microscope can find Demodex mites clinging to the hair fibers or sometimes hiding behind the hair. It is also possible to find chimpanzees, Cheyletiella mites and their ticks.

In rare cases, no ticks are observed by shaving or trichoGram, skin biopsy may demonstrate the presence of Demodex mites in the ciliary follicle or in the sebaceous glands/ducts. This technique is necessary in cases of Demodex involving the feet.
The method of fecal flotation should be carried out in conjunction with a physical examination when cyst palpation is suspected. Due to the dog's licking behavior, the fur pouch can be combed from the fur skin and will move through the digestive system to be found in the feces. This diagnostic method can be especially useful for aggressive dogs, which have difficulty taking skin samples.

Disease prevention and treatment
The general prognosis for the resolution of skin lesions is good, but the prognosis depends on the state of health and the underlying conditions or the presence of immunosuppression. Comprehensive treatment should include (i) Effective use of medications, (ii) Evaluation of any underlying disorders and appropriate treatment when detected, (iii) Antibiotic treatment in the presence of purulent dermatitis (iv) Several months of treatment may be required to remove the tick.
It is recommended to continue distributing the selected treatment for 1-2 months after the tick is no longer detected on the scrape on the skin.
Treatment methods include the use of products containing the Isoxazoline group (Afoxolaner, Lotilaner, Sarolaner, Fluralaner), or products containing the macrocyclic Lactone group (Milbemycin oxime, Moxidectin/ Imidacloprid, Ivermecin, Doramectin). Note that some dogs, especially herding breeds such as Collies, Shetland Sheepdogs, Border Collies, Australian Shepherds, and Old English Sheepdogs, may have mutations in their MDR1 gene and thus increase the risk of toxicity to macrocyclic lactones. Signs of intoxication include dilated pupils, excessive salivation, coma, coma, and death.

Some of the procedures that can be used:
+ Rotenone ointment: apply 1 time a day and 2 times a day for hairy areas. Warn owners that areas of hair loss may become larger at the beginning of treatment, from mechanical hair loss during the application of the drug and the outward spread of ticks.
+ Benzoyl peroxide gel: Apply once a day to areas of hair loss. It does not kill or remove ticks from hair follicles but helps to limit secondary infections and withdraw hair follicles. 
Many cases of localized disease can go away on their own, but secondary infections must be resolved because they can progress to a systemic form, so attention should be paid to new lesions or clinical signs of deterioration.

For immature dogs:
+ Treatment of all secondary bacterial and parasitic infections simultaneously. Corrects nutritional imbalances. Castrate dogs with systemic diseases.
+ Topical treatment with Amitraz (diluted solution 0.025 -0.06%) for dogs >= 4 months old. Bathe the dog with a shampoo containing benzoyl peroxide and dry it, then dip the diluted amitraz on the dog, making sure to wet all skin surfaces and let it air dry. Take the drug once a week until the skin scraping test is negative when the two times are 2 weeks apart; followed by another Amitraz dip.

For adult dogs:
+ Treatment of all secondary infections, with the choice of antibiotics based on the bacterial substrate for chronic cases. Severe bacterial infections need to be treated for remission before starting Amitraz drops. Dogs with foot disease, should be allowed to stand in amitraz solution. Severe cases can be soaked once a week with Amitraz with a 2-fold increase in concentration.
+ Ivermectin: The initial dose is 0.1 mg/kgP, orally or subcutaneously 1 dose for 7 days, then increase to 0.2 mg/kgP, once a week for the next 3 weeks. If live ticks are seen and the young stage is seen, increase to 0.3mg/kgP, once a week for another 1 month and re-evaluate the animal. Do not give ivermectin to dogs < 12 weeks of age. Not concurrently tolerated by Amitraz.
+ Doramectin 0.6 mg/kgP, orally or subcutaneously 1-2 times/week.
+ Milbemycin: 1-2 mg/kg orally once a day, used for dog breeds where Ivermectin is contraindicated. If live ticks and/or juvenile ticks are found after 1 month of treatment, the dose can be doubled. Treatment should be continued for 2 months after two negative skin scrapes 2 weeks apart. Milbemycin should not be used concomitantly with Ivermectin or Amitraz.
One of the most common reasons for treatment failure is the premature end of treatment. Therefore, the end of treatment should be based on the duration of treatment, rather than clinical manifestations, since dogs that have improved clinically can still contain fur capsules. Therefore, it is necessary to:
+ Perform a re-examination and shave the skin every 4 weeks to monitor the reaction.
+ Continue treatment until 2 consecutive negative scrapes are obtained. A minimum of 4 to 6 negative skin shaving positions. Usually, treatment is needed for at least 3 to 4 months.
+If the number of groping does not decrease after a few skin shaves, especially if groping is found to be reproducing (eggs, larvae, and pupae), re-investigate to find the underlying cause or consider alternative treatments.

+Monitor the sick dog for 12 months after discontinuation of treatment, with re-examination and skin scraping every 3-4 months to monitor for recurrences. Remember, during monitoring, the presence of any alive, dead Demodex mites and/or Demodex tick fragments on skin scrapes is considered positive, indicating the need for further treatment.

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Source: Nguyen Duc Hien, Nguyen Ngoc Phu Vinh, Huynh Minh Tri, Pham Minh Thu, Tran Khanh Long (2023). The disease is common in dogs. Can Tho University Press..