E.I. Papadogiannakis, DVM, Cert.SAD, PhD
Dept. of Veterinary Public Health, National School of Public Health
As in any dermatological case, the diagnostic approach for ectoparasitic skin diseases should be done in a systematic way.
A detailed history should always be obtained. The presence of pruritus as well as its characteristics should be recorded. Although pruritus constitutes a common manifestation of ectoparasitic skin diseases, its absence does not rule them out For example, localized demodicosis, some cases of cheyletiellosis and pediculosis are not pruritic or exhibit only slight pruritus.
Another important aspect of the history is the determination of the source of the suspected infestation. Most ectoparasites are transmitted by direct contact with infested animals or even humans. Demodectic mites could be considered an exception, although these mites are transmitted only early in life when pups are suckling and ingest them.
Most ectoparasites are contagious, and history could provide evidence for that when humans or other animals are affected in the household. However, there might be an ectoparasitic skin disease in the dog without contagion to occur.
After history has been obtained, a thorough clinical and dermatological examination has to be undertaken. Primary and secondary lesions along with their distribution pattern should be recorded. There are no pathognomonic skin lesions for ectoparasitic skin diseases, since a variety of both primary and secondary ones are usually present depending on the duration of the infestation, the parasite species involved and the specific immune response of the dog.
DIAGNOSTIC TECHNIQUES FOR LOCATING ECTOPARASITES:
Skin scraping is the most frequently used test in small animal dermatology, aiming to find and identify small and microscopic ectoparasites. Skin scrapings should be obtained in any case of alopecia, pyoderma, keratinization disorder and pruritic dermatosis, although some veterinary dermatologists suggest all dogs with skin disorders should have a skin scraping.
There are two types of skin scrapings: superficial and deep. The species of suspected ectoparasite(es) determines the type of skin scraping to be performed. Some ectoparasites (e.g., Cheyletiela mites) live on the skin, subsisting on the debris and exudates that are produced on its surface, therefore superficial skin scrapings enhance the success in finding parasites. On the other hand, other ectoparasites (e.g., Demodexa nd Sarcoptes mites) live within the skin, for at least part of their life cycle; in these cases deep skin scrapings enhance the success in finding parasites in these cases.
The equipment needed to perform a skin scraping is mineral oil, a scalpel blade, microscope slides, coverslips and a microscope.
For a superficial skin scraping to be performed, sometimes clipping the hair coat may be necessary. Scissors rather than electric clippers should be used, because superficial mites can be lost if electric clippers are used. The lesion to be scraped should be moistened with mineral oil to aid the collection of skin debris. The skin is scraped using a dull surgical blade perpendicular to the skin surface to avoid the collected material to smear over surrounding hair and tissue. Debris is placed on a microscope slide containing one drop of mineral oil and slightly emulsified by using the blade. A coverslip is added and the slide is examined under low microscopic power (x10).
Deep skin scrapings collect debris from the intrafollicular and superficial dermal areas.
Before the skin scraping is performed, the affected skin should be squeezed between the thumb and the forefinger to extrude the mites from the hair follicles. Blood must be visible grossly to confirm that a sufficiently deep skin scraping has been performed. After debris collection the rest of the procedure is similar to that described for superficial skin scrapings.
Regardless of the type of skin scraping performed, a minimum of three sites should be scraped. The periphery of a typical unexcoriated lesion should be chosen for scraping, as most parasites move away from sites of inflammation. The microscope slides should be examined very carefully for mites, ova, and fecal pellets.
Acetate Tape Impression
This test is indicated when Cheyletiella mites, poultry mites, and lice are suspected. It is more reliable in dogs with heavy infestations. Clear, pressure-sensitive acetate tape is pressed to the hair surface and to the skin adjacent to parted hairs or in shaved areas. The tape is then stuck with pressure on a microscopic slide and examined. Attention must be paid to collect scale from the skin and not from the distal part of the hair. Superficial parasites are found very close to the skin surface.
Coat Brushings and Flotation
This technique is indicated in any pruritic or scaling dermatosis. Brushing debris may contain adult fleas, flea excreta, ticks, lice or Cheyletiella mites. In this test, large areas of the dog's body are brushed and the collected scale and debris are put in a fecal flotation solution. A coverslip is applied to the surface of the flotation solution, allowed to stand for 10 minutes and then transferred to a microscope slide and examined. Hairs should be carefully examined for the presence of eggs glued to the shafts.
The examination of a hair plucking (a trichogram) under the microscope can provide diagnostic information regarding ectoparasite infestation. The bulb section must be examined for Demodex mites, since these are the only ectoparasites intimately associated with the proximal section of the hair, owing to their intrafollicular habitat. The hair shaft should also be examined for ectoparasites since lice and Cheyletiella lay their eggs along the proximal to mid-section of the hair shaft and attach their ova using either a cement-like substance or thread.
This test is used primarily to find Otodectes cynotis mites. Many species of ear mites migrate to the ear margins to deposit eggs; therefore, if swabs from the external ear canal are negative, skin scrapings from ear margins or the periaural skin should be performed.
The animal should be restrained and the external ear canal gently swabbed with a cotton-tipped swab. The collected debris should be transferred to a microscope slide by gently rolling the swab in a drop of mineral oil. A coverslip is then added and the slide examined under low microscopic power.
This test is particularly useful for diagnosing Sarcoptes mange when mites evade detection.
The test is an ELISA assay for determination of circulating anti-Sarcoptes IgG in the blood serum of suspected dog.
The monoamine oxidase inhibitor Amitraz can be used as a topical acaricide applied as a 0.025%-0.05% sponge-on solution, for either weekly use or fortnightly treatment interval, for the treatment of sarcoptic mange, cheyletiellosis and generalized demodicosis. Half a milliliter of undiluted amitraz solution can be mixed with 30 ml of propylene glycol or mineral oil to treat demodectic otitis externa or localized lesions such as pododemodicosis.
Amitraz should not be used in Chihuahuas, in pregnant or nursing bitches or puppies less than 3 months of age. Care should be taken if the drug is being handled by, or applied to, a diabetic owner or patient respectively, as individuals exposed to it and its vapors can develop transient hyperglycaemia. It should also be noticed that the α2 adrenoreceptor agonistic properties of amitraz may induce central nervous system (CNS) depression, bradycardia and sedation.
Fibronil is a GABA receptor inhibitor and can be used either as 0.25% solution or as 10% concentrated solution at 30 days intervals, or every 15 days, for the treatment of flea and tick infestations, sarcoptic mange, cheyletiellosis and otoacariasis. For the treatment of sarcoptic mange it is mainly indicated for early infestations and for those individuals in which the use of alternative products is contraindicated (e.g., very young puppies and pregnant or nursing animals). A single otic application of two drops of 10% solution was effective in controlling Otodectes cynotis infestation in one study, with no adverse effects. Fibronil has the advantage, over other acaricidal drugs, that it can be used in puppies over one week of age.
Ivermectin has been used (off-label use) in the treatment of sarcoptic and otodectic mange and cheyletiellosis as s.c injections every 14 days or per os every 7 days, at the dosage of 0.3-0.6 mg/Kg for 30 days. Because the drug is not licensed for this purpose the owner consent should be obtained prior to its off-label use. Because idiosyncratic reactions can occur in Collies, Shetland sheepdogs, Australian shepherds, Bobtails and Border collies, this drug should not be used in these breeds or their crosses, as it can affect the CNS, causing ataxia, tremors, mydriasis, salivation, depression and even coma and death. Injectable ivermectin has been given daily per os at dosages of 0.6 mg/Kg for several months for treatment of canine generalized demodicosis (off-label use).
Milbemycin oxime is well tolerated by Collies and other breeds sensitive to ivermectin. It has been successfully used in sarcoptic mange and cheyletiellosis at the dosage of 1-2 mg/Kg per os once or twice weekly for 3 weeks (off-label use). For canine generalized demodicosis, a daily dosage of 0.5-2.0 mg/Kg has been described (off-label use). The drug can also be used in canine nasal mite infestation at the dosage of 0.5-1 mg/Kg.
Injectable moxidectin, given per os at the dosage of 0.4 mg/Kg every 15 days for two occasions, was useful to treat canine sarcoptic mange (off-label use). Administered per os daily at 0.2-0.4 mg/Kg/day it gave good results against canine generalized demodicosis. The drug can also be used for treatment of canine otoacariasis at the dosage of 0.2 mg/Kg per os or s.c twice, every 10 days. Contraindications for its use are the same as for ivermectin.
Selamectin is a novel avermectin and its spot-on formulation is the only systemic treatment licensed for the control of canine sarcoptic mange, cheyletiellosis, some ticks and fleas (adults and eggs), although it can be effective in canine otoacariasis in spite of being not registered for this use. The drug was effective against canine nasal mites when given 4 times at two weeks apart.
Imidacloprid blocks the post-synaptic nicotinic acetylcholine receptors and is safe and effective adulticide drug for flea control with residual activity. Imidacloprid 10% (w/v) in combination with permethrin 50% (w/v) in a spot-on formulation has been found to be effective against fleas and ticks.
Insect growth and development inhibitors (IGIs) do not kill adult fleas but will decrease the viability of eggs and/or interfere with the development of larvae and pupae. They are available as a spot-on or as orally administered formulations. IGIs are Lufenuron, Cyromazine, Methoprene and Pyriproxifen.
Permethrins and Pyrethrins are effective flea adulticides whose mechanism of action is achieved by disruption of Na+ channels that lead to paralysis and death of the parasite. They are also effective against lice infestations in dogs.
For flea control there are many additional products in the market available containing organophosphates, carbamates and other pyrethroids (apart from permethrin) and botanicals (apart from pyrethrins).