Definition
Sarcoptic mange (fox mange or scabies) is a highly pruritic and contagious skin disease caused by the mite Sarcoptes scabiei. Transmission is usually through direct contact with infected animals. It is possible for dogs to act as asymptomatic carriers. The mite has the potential to cause mild disease in humans.
Cause
The Sarcoptes mite burrows into the top layers of the skin creating an extremely pruritic dermatitis. The adult female lays her eggs in a skin tunnel. The eggs develop into larvae, then 2 stages of nymphs before becoming adults. The entire life cycle is completed in the skin and takes 17–21 days.
The canine Sarcoptes mite is relatively host-specific, but can occasionally cause disease in other species, including humans. Owners can potentially develop a severely pruritic, papular rash if they are in contact with infected dogs.
The clinical features are associated with a hypersensitivity to the mites. This means it is possible for dogs to act as asymptomatic carriers.
Signalment and history
Sarcoptic mange may affect dogs of any age or type although it seems to be more common in young dogs, which may reflect a higher risk of exposure to the mites. There may be a history of scabies diagnosis or clinical signs in animals the dog has contact with as the mite is very contagious.
Some areas are known to have an increased risk of sarcoptic mange due to infection from local wildlife (for example urban foxes, coyotes).
Dogs that are immunosuppressed may be more likely to become infected. Immunosuppression may result from:
- Disease (for example neoplasia, hyperadrenocorticism (Cushing's), hypothyroidism)
- Medication (for example glucocorticoids, chemotherapy)
Particularly high mite counts can be seen in these patients.
Clinical signs
The degree of pruritus is often severe (rated >8 out of 10). Dogs may stop other activities (such as eating, playing, exercising) in order to scratch.
- The most commonly affected areas are the ear margins, axillae, lateral elbows and lateral hocks. Spread from these areas generally progresses to the head and ventral trunk before becoming generalised
- Initial lesions include erythema, papules, alopecia and small haemorrhagic crusts
- The intense pruritus leads to self-trauma and excoriations
- Secondary bacterial infection is common
- Chronic disease results in alopecia, lichenification and hyperpigmentation
- Many dogs also have a generalised lymphadenopathy
On rare occasions, dogs may show severe pruritus with no other visible signs.
Diagnosis
Differentials
Allergic skin disease (atopic dermatitis, food allergy), Malassezia (yeast) dermatitis, Otodectes (ear mites), other parasitic skin disease (e.g. Cheyletiella).
Pinnal-pedal reflex
- A positive pinnal-pedal reflex is suggestive (i.e. the dog attempts to scratch with a hind leg if the edge of the pinna is rubbed or scratched)
- This test is estimated to be positive in 80% of dogs with ear lesions caused by sarcoptic mange; however, it may also be positive with allergic diseases, therefore the result should be confirmed with skin scraping, serology and/or trial therapy
Superficial skin scrape
- A definitive diagnosis is achieved by observing adult mites, nymphs or eggs on microscopy
- Mites are difficult to find: multiple scrapings are required. Elbows and margins of ear pinnae should be sampled. The ideal sample area has red raised papules and yellowish crusts, but is not excoriated (i.e. near the edge of lesional areas)
- Identification of just one mite on skin scraping is diagnostic
- A negative skin scraping does not rule out sarcoptic manage (typically scrapes have <50% success)
Serology
- An ELISA test that detects Sarcoptes IgG is available
- Can be useful in dogs where sarcoptic mange is suspected and skin scrapings are negative
- Note: dogs may not produce this antibody until up to 5 weeks after infection and it is unknown how long after infection this test remains positive. Potential cross-reactivity with house dust mite antigen (i.e. atopic dermatitis) is also possible
Trial treatment
- In many cases it is justified to trial treat patients with an appropriate acaricide when scabies is suspected
Treatment and management
In some dogs affected areas may resolve spontaneously as the dog develops immunity to the mite; however, treatment should be started as soon as diagnosis is achieved.
Anti-pruritic therapy
- Consider short-term therapy if required on welfare grounds
- Note: immunosuppressive therapy such as corticosteroids could reduce the immune system’s ability to naturally eliminate the parasites
Antimicrobial therapy
- Required if secondary yeast/bacterial infections are present
- Antimicrobial shampoos or systemic therapy
- Considerations include appropriate spectrum of activity and sufficient treatment duration
Parasiticide (acaricidal) therapy
- The disease is highly contagious and all dogs in the household should be treated
- Treatment duration is at least 4 to 6 weeks
- Consider environmental treatment (cleaning +/- insecticide) as mites can survive up to 21 days in environment
- Topical treatments:(i) thick-coated dogs may need clipping prior to treatment; (ii) pre-treat with antiseborrhoeic shampoo to remove skin crusts; (iii) treatments should be applied all over body (low success with focal treatments); (iv) regular swimming may limit treatment efficacy; (v) compliance issues (difficulty of application) may limit this option
- Systemic drugs (for example macrocyclic lactones).
