comparative dermatology: parasitic disorders

Comparative Dermatology: Parasitic Disorders DONNA WALTON ANGARANO, DVM LAWRENCE CHARLES PARISH, MD

E ctoparasites commonly parasitize domestic animals. In some instances they are simply a nui- sance; in others, they cause significant disease. In severe cases, ectoparasites may cause debilitat-

ing or life-threatening disease. Some ectoparasites transmit disease to animals, humans, or both.

In many regions, the presence of external parasites is one of the most common reasons animals are presented to veterinarians, far more so than humans consult their physicians. The population of parasites varies in different parts of the country. Likewise, some parasites are species specific. Of special concern is the possibility that human beings may be affected by the same or similar parasites.le3

This article reviews some of the common dermatologic disorders caused by parasites in animals and humans. Particular emphasis is given to those parasites that are zoonotic.

Fleas

Pet owners in most regions of the country have experi- enced the frustration of the presence of fleas on their dog or cat. These small, wingless insects feed on the blood of warm-blooded animals. They may inhabit many species, including dogs, cats, ferrets, and humans.4 Many dogs and cats have fleas with no visible clinical signs. Others are allergic to flea saliva, and the bite from a single flea may result in prolonged pruritus. Flea allergy in the dog or cat may develop at any age. The major sign of flea

From the Department of Small Animal Surgery and Medicine, College of Veterinary Medicine, Auburn University, Auburn, Alabama, and the De- partment of Dermatology, Jefferson Medical College of Thomas Jefferson University, Philadelphia, Pennsylvania.

Address correspondence to Donna W. Angarano, DVM, Department of Small Animal Surgery and Medicine, College of Veterinary Medicine, Au- burn University, Auburn, AL 36849-5523.

0 1994 by Elsevier Science Inc. l 0738-081x/94/$7.00

allergy is pruritus at the rump, tail head, hind legs, and abdomen. Papules, erosions, excoriations, alopecia, erythema, and hyperpigmentation may develop (Fig 1). Cats may also have lesions around the neck. Unfortunately, signs tend to exacerbate with age.

Although anti-inflammatory and antipruritic medications may decrease the symptoms of flea allergy, the major treatment goal is avoidance of fleas. Because fleas do best in a warm (65 - 80°F) and humid (75 - 85%) environment,5 flea control is obviously easier in some parts of the United States than others.

The cat flea, Ctenocephalides felis, causes most of the flea problems of animals and humans in the United States.6 The human flea, Pwlex irritans, may also inhabit animals.’ Recently, scientists have developed a more ac- curate understanding of the life cycle of the flea. It is hoped this knowledge can be used to better control fleas on pets and in the environment.8

The adult cat flea spends most of its life on the pet.8 In fact, when removed from the pet, the flea does not survive long. The adult female has a life span of 6 months to 1 year. During this time, she may lay as many as 20 to 25 eggs a day. These eggs fall off the animal and develop into larvae in a few days. There are three larval stages which may last several days to several months. A cocoon devel- ops. The pupa stage within the cocoon is the most resist- ant to current flea control measures.6 Finally, the unfed adult emerges and seeks a host. Fleas encountered in the environment are usually recently hatched adults search- ing for their first blood meal.

Ctenocephalides felis prefers to feed on dogs and cats; however, if these hosts are not available, recently hatched adults will feed on humans. This frequently occurs fol- lowing family vacations during which humans and animals have vacated the house or when a pet dies or moves with the previous homeowner. The first warm-blooded host entering the environment is usually attacked. Flea bites in humans are often arranged as clumps of ery-

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Figure 1. Alopecia, hyperpigmentation, and lichenification of the rump and hind legs of a Welsh corgi withflea allergy dermatitis.

thematous papules (Fig 2). Mimicking papular urticaria, these lesions are frequently on the legs and affect only a few members of the family. These intensely pruritic lesions may become excoriated or result in furunculosis.9

In 1991, American pet owners spent $871.6 million in an effort to control fleas.s Unfortunately, there is no easy solution. Many insecticides in various forms of application are effective against fleas; however, a single application does not afford control. Consistency and persistency are necessary for an effective flea control program. Em-

Figure 2. Eythematous papules resulting from flea bites.

Figure 3. Microscopic view of Sarcoptes scabiei var cams. Xl 00.

phasis must be given to the animal and the indoor and outdoor environment.‘0-13

Although many successful options exist, a combina- tion of dips (total body rinses) and sprays is a common treatment on animals.6J1 Care must be taken to follow label instructions for frequency of use, dilution, and pre- cautions for both animals and humans.

In the indoor environment, it is important to use insecticides effective against adult and immature fleas.“J* Adulticides are frequently combined with insect growth regulators. Methoprene and fenoxycarb are insect growth regulators commonly used in the United States.6 Recent studies have demonstrated the efficacy of boric acid for indoor environmental control.6

Methods of controlling fleas will undoubtedly change during the next several years, as new and better products evolve that kill the developing flea. It is hoped these methods will also decrease the time and effort needed to control fleas.

Scabies

Scabies is an intensely pruritic, nonseasonal, contagious dermatopathy caused by a rather species-specific, sar- coptiform mite. i4,15 Canine scabies, Sarcoptes scabiei var canis (Fig 3), is common in most regions of the United States. Feline scabies, Nofoedres cafi, is less common and limited to a few metropolitan areas.5 Porcine scabies, Sar- copfes scabiei var suis, has decreased in incidence as a result of aggressive therapeutic and prophylactic treatment programs. l6 Human scabies, Sarcopfes scabiei var

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hominis, is common, particularly in people in confined environments.‘7-20

Scabies mites are generally considered to be species specific 21; however, most scabies mites are capable of causing dermatitis in other species, including humans. In the majority of cases, mites on an unusual host are not capable of reproduction.22

Clinical disease caused by scabies is characterized by intense pruritus. The mites spend their entire lives in or on the skin of the host. The female mite secretes a substance that dissolves the stratum comeum to form a depression. The mite moves forward, digesting and consuming the damaged tissue. 23 When she finds a favorable site, she lays her eggs which hatch in 3 to 10 days.’ The life cycle is completed in 2 to 3 weeks, and the adult lives for 4 to 5 weeks.

There is evidence to suggest that scabies is not simply a parasitic but also a hypersensitivity disorder.24*25 Pruritus varies widely among affected individuals and is not pro- portional to the number of mites present. Initial infections have a latent period between the inhabitance of burrow- ing mites and the onset of clinical signs. Previously in- fected patients have an immediate reaction. Clinical signs of papules and pruritus may persist after removal of the mite. Affected people and animals may show positive reactions to intradermal skin tests using fleas, house dust mites, and other ectoparasites.26

In the dog, clinical signs consist of extremely pruritic papules, crusts, excoriations, and alopecia. Lesions are most prominent at the ear margins and on the elbows. The ventral abdomen and thorax may also be affected. Occasionally, lesions may affect the entire body. Periph- eral lymph nodes are enlarged. Despite intense pruritus, secondary bacterial pyoderma is uncommon.

The canine scabies mite is often difficult to demonstrate with skin scrapings. Mites are most easily found on young or immunosuppressed individuals. Multiple scrapings are performed, focusing on areas of crust for- mation. Despite negative skin scrapings, dogs with a his- tory and clinical signs consistent with scabies should be treated.

Treatment is aimed at eradicating the mite on the animal. Parasiticidal efficacy varies in different regions of the country; however, the treatments most often used include 2 to 4% lime sulfur applied as a weekly total body dip for 6 to 8 weeks or ivermectin given orally or subcuta- neously at 200 to 300 pg/kg.27 A second ivermectin treatment is administered in 14 days. Lime sulfur and ivermectin are effective treatments. Ivermectin is the easiest to administer and is commonly used, but it is not approved for this use in the dog. It should be used with caution and should be avoided in the collie28.29 and Old English sheepdog.30

Scabies is transmitted from host to host by direct contact. Under experimental conditions with an ideal tem- perature (10 o C) and humidity (97%), mites remain capable of infesting for 36 hours after removal from the host.23 In veterinary medicine, it is a common practice to treat all in-contact dogs, as asymptomatic carriers have been reported.5 Because the mite is usually species specific, it is not necessary to treat other species of pets (ie, if a dog is determined to have scabies, in-contact dogs are treated but not in-contact cats).

Mites may be transmitted to other species but generally do not reproduce on the unusual host. It has been esti- mated that 33% of affected dogs transmit the disease to a human in the household.’ Humans contracting canine scabies typically demonstrate pruritic papules and excoriations on the wrists, arms, waist, or other areas of restric- tive clothing contact. Canine mites are not found in scrapings from humans. In most instances, treatment of the animal results in disappearance of signs in humans.

In rare cases, canine mites have been shown to be capable of infesting and reproducing on humans. In these experimental situations, canine mites were demonstrated on skin scrapings31

Details of human scabies infection with the char- acteristic papules and burrows are found in several sources.32-34 There have been recent additions to treatment options for humans.35

Cheyletiellosis

Cheyletiellosis is a moderately pruritic, contagious dermatopathy of animals that is zoonotic to humans. Three species of mites which may infect the dog, cat, and rabbit are Cheylefiella yasguri, Cheylefiella blakei, and Cheyle- fiella parasifavorax, respectively. The mite has weak host specificity.5

Cheyletiellosis is less common than demodicosis or scabies, but is common in some regions of the country. The mite is large (250 X 400 pm) and unique in appearance due to its claw-shaped accessory mouthparts (Figs 4 and 5). The nonburrowing mite lives in the keratin in pseudotunnels and feeds on surface debris and tissue fluids. Cheylefiellu are generally considered to be obligate parasites, although mites have been reported to survive off of the host for up to 10 days under experimental conditions.36 The mites are transmitted by direct contact or fomites. Their life cycle is complete in 35 days. Animals of any age may be affected, but the disease is most common in young animals, particularly those in pet stores, shelters, or other places where numerous animals contact each other.

Cheyletiellosis is frequently called “walking dandruff” because of the clinical sign of seborrhea sicca. Le-


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Figure 4. Microscopic view of Cheyletiella yasguri. The claw- shaped accessory mouthparts give it an unique appearance. X200.

sions of scale, crusts, and erythema are most prominent over the dorsal trunk. Pruritus is moderate in severity. An unusual finding has been the observation of mites mov- ing in and out of the nostrils of cats.37

Because Cheyletiella mites are large, they may be visible with the naked eye (hence the term walking dandruff’).

They may also be identified from superficial skin scrapings or by collection using transparent acetate tape. Alter-

Figure 5. Cheyletiella. Closer view of accessory mouthparts. X320.

natively, a flea comb may be used to collect scales for microscopic examination. In some cases, mites may be difficult to demonstrate and asymptomatic carriers have been identified.

Many insecticides, including ivermectin, are effective in destroying the mite. 38 Although cheyletiellosis is not considered a common dermatopathy in veterinary medicine, its prevalence may be underestimated because of false-negative samples and because it is responsive to treatment with flea products. The affected animal, all in-contact animals, and the environment should be treated. Successful treatment may require 6 to 8 weeks of therapy.39

Cheyletiella mites are contagious to humans by direct contact with affected dogs,‘O cats,41 or rabbits. Lesions in humans consist of pruritic macules, vesicles, pustules, and papules with a necrotic center. Lesions are located on the arms, trunk, or thighs where the affected animal has been held (Fig 6). The lesions caused by Cheyletiellu are difficult to distinguish from those caused by scabies or canine scabies. Cheyletiellu mites are not usually found because they do not burrow or reproduce on humans. This has been described as the “bite and run” behavior of Cheylefiellu. Lesions in humans may be treated sympto- matically and resolve when the pet is treated.41

Pediculosis

Lice are species-specific ectoparasites that spend their entire life on the host. They are transmitted by direct contact or by fomites. Under ideal conditions, they may live for several days off of the host.

Lice are classified as Anopluru, or sucking lice, and Mullophugiu, or biting lice. Although similar in size, sucking lice have a narrow head, whereas the head of biting louse is approximately as thick as the body. Sucking lice feed on blood and tissue fluid; biting lice feed on keratin and surface debris. Thus, sucking lice are more important clinically, as they may cause anemia in a debilitated host.

Lice are uncommon on dogs and cats in most parts of the United States. When present, they may cause pruritus, excoriations, and secondary bacterial infections. More severe problems may be seen in the young or debilitated dog or cat. Flea products are generally an effective treatment5

Pediculosis is more common in large animals. Cattle, goats, and horses are often affected, and swine and sheep less frequently. Clinical signs vary depending on the species of lice; however, scales and pruritus are often present over the dorsal trunk. More significant problems are weight loss, decreased milk production, and damage to the hide or wool

Lice appear to be more prevalent during cool weather.

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Figure 6. Eyfhemafous papules caused by Cheyletiella.

This may be the result of their inability to survive high temperatures or of the crowding of animals or increase in length of hair coat during winter months.‘*

One author (DWA) is aware of several instances when lice from large animals were transmitted to people by direct contact. Treatment of the animals or cessation of contact resulted in eradication of the lice.

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Details about pediculosis corporis, pediculosis capitis, and pediculosis pubis can be found elsewhere. The key characteristics of these infestations are scratch marks and excoriations.N,43*”

Demodicosis

Demodex are species-specific, follicular mites that are found as a normal resident on most species of animals and in humans (Fig 7). Demodicosis is rare in the horse, uncommon in cats, goats, and cattle,‘* and common in dogs. It is second to flea infestation in the frequency of canine ectoparasites in most parts of the United States. Although canine demodicosis is not contagious to humans, the disease is discussed because of its frequency of occurrence. In addition, demodicosis is a disease of immunosuppression. 45 It is possible that human demodicosis may become a significant dermatopathy as it is rec- ognized in patients with acquired immunodeficiency syndrome or in patients compromised by chemotherapy.

Canine demodicosis refers to two drastically different dermatologic conditions caused by Demodex canis. Local- ized demodicosis is a common, readily diagnosed, and

Figure 7. Microscopic view of Demodex folliculorum. X400.

548 ANGARANO AND PARISH Clinics in Dermatology 2994;12:543-550

Figure 8. Alopecia, crusts, and excoriations on the face of a Doberman pinscher with generalized demodicosis.

easily treated syndrome. Most cases occur in young dogs, with the face and front legs most often affected. The majority of cases of localized demodicosis undergo spon- taneous remission. If treatment is desired, a variety of topical medications are effective. Benzoyl peroxide is one of the most popular because of its antimicrobial and folli- cle flushing activity.

Generalized demodicosis is a disease that frustrates most veterinarians.5*45 Large portions of the body may be affected by alopecia, erythema, scaling, crusting, and hyperpigmentation. The face and interdigital areas are often affected (Fig 8). Signs may be generalized. In addition to skin lesions, generalized lymphadenopathy is usually present. Secondary deep pyoderma may be severe, with Staphylococcus intermedius, Pseudomonas sp, or both in- volved. Demodicosis is not usually considered to be pruritic; however, many dogs with generalized lesions will itch and rub, mostly because of secondary infections. Al- though genetic studies have not been completed, generalized demodicosis is usually considered to result from an inherited immunodeficiency specific for the Demodex mite. Dogs with generalized demodicosis should not be used for breeding.5

Generalized canine demodicosis usually begins at 8 to 14 months of age. In older dogs, generalized demodicosis is most often associated with allergic skin disease and chronic use of glucocorticoid therapy. Neoplasia and metabolic disease may also cause immunosuppression resulting in adult-onset demodicosis.

Amitraz is the only approved, effective treatment for generalized demodicosis.46 Bathing prior to the dip is helpful. Amitraz dips should be repeated once weekly or every other week depending on the severity of disease.

Unfortunately, many cases of generalized demodicosis relapse after cessation of the dips. In some animals, this may be due to concurrent disease, treatment of which

results in improvement. Some cases are controlled with total-body maintenance dips given every 1 to 2 months.

In preliminary reports, 50 to 80% efficacy has been shown with milbemycin oxime, administered orally for 60 to 90 days. 47*4e Ivermectin has also been shown to be effective in the treatment of canine demodicosis. Dosages range from 200 to 600 ,ug/kg given orally once a day. Treatment may be necessary for 30 to 90 days.49 Ivermec- tin should not be used at this dosage in the collie or related breeds.

Whether Demodex is responsible for such conditions as rosacea or seborrheic dermatitis in humans is unclear. The mite can be found on the nose of most adults.50

Conclusions

Ectoparasites are common causes of dermatologic problems in veterinary medicine. Some ectoparasites are strongly species specific, whereas others readily affect multiple species. An understanding of ectoparasites of animals is important to physicians because of the poten- tial for transmission to humans. In addition, knowledge will increase as comparisons are made between human and veterinary diseases that share a common etiology.

Drug Names

lime sulfur: LymDyp@ ivermectin: IvomeP benzoyl peroxide: Pyoben@, OxyDex@ amitraz: Mitaban@ milbemycin oxide: Interceptor@

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