CAUSES AND TREATMENT OF SCABES IN DOGS (PART 1: SARCOPTES)

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 it has a serious infection or has lost appetite or sleep.

SARCOPTES

Pathogen characteristics
Form

       Scabies is caused by Sarcoptes scabiei species belonging to the genus Sarcoptes, family Sarcoptoidae Sarcoptes scabiei species is very small in size, specializes in burrowing to parasitize human and animal skin, causing scabies. The strain that causes scabies in dogs is Sarcoptes scabiei ver.canis.
       Sarcoptes scabiei is an obligate parasite of mammals and birds. Adults are oval white, slightly yellowish, with brown legs and mouth. Females are about 330 - 450 Micrometer long. Adults and nymphs have eight legs, larvae have six legs. Eggs are oval, transparent, thin shell, about 100-150 Micrometer long. In females, the ends of the two front pairs of legs are bell-shaped suction cups, and the two pairs of hind legs are long bristles. In males, only the third pair of legs ends in long bristles, the remaining pairs of legs all have suction cups at the end. The scab's abdomen is flat, scattered with a few fluff and bristles. The dorsal surface is slightly convex, with many longitudinal grooves and horizontal grooves that are almost parallel to each other, except for a few places where finger-shaped spines appear. The number, position and shape of the spine structures on the dorsal surface help identify the scabies. 

       The number of scabies living on an infected dog usually does not exceed 100, most commonly 10-15. However, dogs with psoriatic mange can have thousands on the surface of the skin and scabies can be found in objects that have come into contact with infected dogs, from bed sheets and floors.

Life cycle
       The scabies burrow into the keratin and surface layers of the epidermis. Scabies burrow by secreting saliva to break down keratin and by moving their heads and mouth parts. It uses its two front pairs of legs to widen the tunnel.
       The scabies eat some of the skin material that burrowing creates, but is mostly compacted against the walls of the tunnel. The scabies penetrate into the granular layer and membrane of the epidermis, only a new part of the tunnel consists of eggs, larvae and feces of the scabies embedded in the keratin layer. Older parts of the burrow are sloughed off the skin's surface along with the keratin.
       Male mites are more mobile than females, they are found on the skin and in burrows dug by the female in search of a mate. After copulation, the male scabies dies, the female continues to dig tunnels in the skin at night and lay eggs. After penetrating the host's skin, the scabies destroy the subcutaneous tissue for food and dig tunnels for shelter... Favorable conditions for them to dig tunnels are at night, about 3 times a day. - 5mm, the tunnel segments end when they reach the stratum corneum of the skin. The tunnel is hard to see and is the most characteristic clinical manifestation of scabies.
       Females live for 4-6 weeks, digging up to 5mm of new tunnels every day. About 2-3 days, the scabies lay 1 time, each time 2-4 eggs. After 2-3 days the eggs hatch into six-legged larvae, significantly smaller in size than adults. The larvae try to escape from the main tunnel by digging another tunnel to penetrate the skin surface. From the surface of the skin, it digs a sheltered place in the keratin layer to molt. The larvae undergo two molts. The first time after 3-4 days, the larvae molt to create 8-legged protonymph pupae. The next molt produces a tritonymph (also called deutonymph) and after 4-7 days the pupa develops into an adult. A typical life cycle lasts 2-3 weeks.

Resistance
     Scabies can be separated from their host animal. Their chances of survival depend on the ambient temperature and humidity. Scabies can survive in human dwelling (21°C, humidity 40-80%) for 24-36 hours. In cool and humid conditions, they can last longer. At 10°C and 97% humidity, scabies can survive up to 18 days. High temperatures and dryness will kill the scabies for several hours.

Transmission
        The disease is easily transmitted between dogs, almost always through direct contact. Since the scabies only exists for a short time outside the host, movement can also occur indirectly through cages, cage fillers, or grooming equipment. Wild foxes and panda dogs can also serve as sources of bacterial infections. The infection does not require direct contact with wildlife. Instead, visiting caves, sewers, or any place where mangrove wildlife has been can lead to infection.
       Although scabies is easily transmitted, some exposed dogs do not manifest scabies clinically. In households with many dogs, it is common for only a few dogs to have symptoms.

Clinical pathology
       The incubation period before symptoms develop can range from a few days to several months. For first-time scabies infections, it usually takes 2-6 weeks for itching and skin damage to occur. In contrast, subsequent infections often show symptoms for only 24 – 48 hours.
        The main clinical signs can vary from asymptomatic to severe itching. Scabies causes papules that eventually coalesce to form large erythematous areas and crusts, complicated by secondary bacterial infection. Dogs commonly affect the edges of the ears with hair loss, red skin, scaling and keratosis of the ear edges. Other affected areas are the caudal edge, lateral edge of the legs, face, thorax and abdomen. Dermatitis may also be seen. Long-term illness can be serious with irritability, weight loss and lymphadenopathy.
        Systemic sepsis due to secondary bacterial infection and acute glomerulonephritis due to Streptococcus pyogenes infection are complications of scabies that can be fatal. Streptococcus spp. and Staphylococcus spp. have been isolated from scabies fecal pellets and from burrows in the skin, as scabies can facilitate the spread of the bacteria. The metabolic burden of scabies can contribute to malnutrition, especially in cases of severe scabies and crusting.
Dog scabies is transmitted to humans, causing rashes and itching. However, Sarcoptes scabiei ver.canis does not live on human skin for more than three weeks. If the itching does not disappear within three weeks, it may be due to another agent.

Diagnose
The history and clinical signs will usually allow the expected diagnosis of scabies. Shaving the skin inflamed by scabies under the microscope can reveal their scabies, feces, or eggs. Some ELISA test kits are available for Sarcoptes-specific IgG, however their use is limited due to false-positive and false-negative results.

Disease prevention and treatment
Treat
       Long-haired dogs need to pinch the fur away from scab skin areas and bathe the whole body with a shampoo with Benzoyl peroxide. The shampoo loosens the scales and causes the scabies killer in the shampoo to penetrate deep into the pores. The stepdaughter has developed resistance to certain Organophosphate-dipped drugs. Two bath medications that still work against them are Amitraz and sulfur. However, sulfur has an unpleasant odor, stains the dog's coat with a bright color, and can irritate the skin. Dip your dog medication once a week for six consecutive weeks (or until symptoms go away). Continue treatment for another two weeks after the dogs seem to be cured. It is important to treat all dogs that come into contact with the affected individual.

       Ivermectin 0.2 -0.3 mg/kg orally or injected subcutaneously 2-4 times, each time 7-10 days apart, is effective against scabies. However, at doses used to treat scabies, the drug caused central nervous system problems and mortality in Collies, Shetland Sheepdogs, Old English Sheepdogs, Australian Shepherds and other cattle breeds (and their hybrids). Check your dog's heartworm status before giving ivermectin, as it can cause a reaction in dogs that are positive for heartworm larvae.
       Milbemycin oxime may be used in place of ivermectin in breeds where ivermectin is contraindicated. Selamectim, amitraz used topically also helps prevent scabies in dogs.
       Corticosteroids relieve severe itching and may be required for the first two to three days of treatment. Ulcers on the infected skin require oral antibiotics and topicals. Dogs often need systemic antibiotics, therapy for secondary infections. Oclacitinib 0.6mg/kg within 12 hours for 14 days. Prednisolone 1 mg/kg per day is tapered for 2 weeks, and antihistamines may be helpful.

Prevention
      Adult scabies can live for 21 days outside the host, so indoor environmental treatment is recommended to prevent recurrence. All exposed dogs, cats, and rabbits must be treated, even if there are no symptoms. Cleaning the environment includes washing all bedding weekly and disinfecting all holes.
Isolate and treat sick dogs for 4 weeks until all animals are asymptomatic to avoid infection.

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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.