Chapter 6 Diseases of the Integumentary System

The skin makes up the largest organ system in the body. It comprises approximately 24% of the total body weight of a newborn puppy and about 12% of the body weight of an adult animal. It consists of three distinct layers: the epidermis, the dermis, and the hypodermis, or subcuticular layer (Fig. 6-1) (refer to an anatomy and physiology textbook for the exact function of each layer). The skin serves as a barrier between the animal’s body and the environment. It not only protects the animal from physical, chemical, and microbiological injury, but the sensory organs found in the skin allow the animal to feel pain, heat, cold, touch, and pressure. The skin is also a storage depot for electrolytes, water, proteins, fats, and carbohydrates, and it assists in the activation of vitamin D by solar energy.

Figure 6-1 A, Canine skin and underlying subcutaneous tissue. Notice that epidermis of canine skin includes folds from which compound hairs arise. B, Normal canine skin. C, Normal feline skin. Note the thin epidermis and compound hair follicle arrangement of both species.

(A, From Colville T, Bassert JM: Clinical anatomy and physiology for veterinary technicians, St Louis, 2002, Mosby, by permission; B and C, From Scott DW, Miller WH Jr, Griffin CE: Muller and Kirk’s small animal dermatology, ed 6, Philadelphia, 2001, WB Saunders, by permission.)

The hypodermis stores fat for insulation and energy reserves. The animal’s skin has many functions:

• Enclosing barrier: Protects the internal environment of the body from water and electrolyte loss.

• Environmental protection: Protects the internal environment from the external environment.

• Temperature regulation: Maintains the animal’s coat and regulates the blood supply to the cutaneous tissues, which regulate heat dissipation and retention.

• Sensory perception: Contains sense organs for touch, temperature, and pain.

• Motion and shape: Allows for motion and provides a definition to the body.

• Antimicrobial: Contains antimicrobial and antifungal properties.

• Blood pressure control: The peripheral vascular beds within the skin help to control blood pressure.

• Secretion: Contains both apocrine and sebaceous glands.

• Adnexa: Produces hair, nails, hooves, and horny layers of the epidermis.

• Storage: Stores electrolytes, water, vitamins, fat, proteins, carbohydrates, and other substances.

• Pigmentation: Processes within the skin (e.g., melanin formation) help to determine coat and skin color and provide solar protection.

• Excretion: The animal’s skin has a limited excretory function.

• Vitamin D production: The skin is essential for solar energy activation of vitamin D, which is necessary for normal calcium absorption.

TECH ALERT

The skin is an important indicator of internal disease.

Any physical condition that disrupts the normal functions of this barrier can result in disease. Increased moisture, chemical exposure, increased temperature, hormonal change, and physical damage can produce a breach in the barrier, allowing the invasion of disease-producing microorganisms.

Problems relating to the skin are the most frequent complaints presented in smallanimal medicine. Technicians deal with these complaints daily as clients ask questions and seek help for the treatment and prevention of skin diseases afflicting their pets. This chapter focuses on the most commonly seen skin problems of companion animals.

ECTOPARASITES

External parasites are responsible for many skin problems seen in small-animal medicine (see a parasitology text for detailed information on the life cycles of the individual parasites discussed in this chapter). The most commonly diagnosed ectoparasites are as follows:

• Ear mites (Otodectes cynotis)

• Fleas (Ctenocephalides spp.)

• Ticks (Ixodes spp., and others)

• Mange (Demodex canis, Sarcoptes scabiei, Notoedres cati)

• Warbles (Cutebrae spp.)

• Myiasis (fly maggots)

• Lice (Linognathus setosus)

Some of these parasites live on the skin; some live within or under the skin; and some pierce the skin, sucking blood meals that produce severe cutaneous reactions. These reactions include inflammation, edema, and itching. In many cases, the animal itself is responsible for increased damage to the skin through licking, chewing, and scratching.

Ear Mites (Otodectes cynotis)

Ear mites live on the surface of the skin in the external ear canal, feeding on epidermal debris (Fig. 6-2).

Figure 6-2 Adult male ear mite, Otodectes cynotis.

(From Hendrix CM, Robinson E: Diagnostic parasitology for veterinary technicians, ed 3, St Louis, 2006, Mosby, by permission.)

DIAGNOSIS

• Under an otoscope, the large, white, adult mites can be seen crawling on the surface of the crusty exudate.

• Adult mites and eggs can be seen on microscopic examination of smears taken of the exudate.

Fleas (Ctenocephalides spp.)

Fleas are blood-sucking ectoparasites that feed sporadically on mammals and birds (Fig. 6-3). Fleas produce severe skin irritation as a result of their frequent bites. Flea saliva is highly antigenic in some animals and will produce an allergic dermatitis. Fleas can act as vectors for diseases and as intermediary hosts for the dog tapeworm (Dipylidium canium). Pets, as well as their environment, can become infested with massive numbers of fleas.

Figure 6-3 Adult male and female Ctenocephalides felis, cat fleas, the most common flea found on dogs and cats.

(From Hendrix CM, Robinson E: Diagnostic parasitology for veterinary technicians, ed 3, St Louis, 2006, Mosby, by permission.)

Ticks (Ixodes spp. and Argasid spp.)

Ticks are seen commonly on outdoor dogs and cats, especially during the summer months. These blood-sucking, arthropod parasites are not host specific and will infest all warm-blooded animals in the area (including humans). Heavy infestation may produce anemia in the host. Ticks also can transmit many bacterial, viral, rickettsial, and protozoan diseases. Lyme disease is one high-profile example of tick-borne disease. Ticks are divided into two main families: Ixodidae (hard ticks) and Argasidae (soft ticks). Most of the commonly found ticks belong to the Ixodidae family. Some of the best known members of this family include Rhipicephalus sanguineus (the brown dog tick (Fig. 6-4), Dermacentor variabilis (the American dog tick), and Amblyomma spp. All but Rhipicephalus spp. gain access to the host outdoors. Rhipicephalus spp. typically inhabit buildings and kennels. One soft tick, the spinose ear tick (Otobius megnini) can be found in the ear canals of dogs and cats in the southwestern United States.

Figure 6-4 Rhipicephalus sanguineus (brown dog tick) invades both kennel and household environments.

(From Hendrix CM, Robinson E: Diagnostic parasitology for veterinary technicians, ed 3, St Louis, 2006, Mosby, by permission.)

Ticks injure animals by several means: irritation of the actual bite, as vectors of disease, and through a neurotoxin found in the saliva of 12 different Ixodes species. This neurotoxin causes tick paralysis, an ascending, flaccid paralysis of dogs.

DIAGNOSIS

• Finding a tick on the animal is the definitive diagnosis.

• A history of exposure to wooded, grassy areas known to have tick infestations suggests the diagnosis.

TREATMENT

• Manual removal of all ticks. Soak the tick in alcohol; firmly grasp the head parts using a curved mosquito hemostat and pull to remove the tick. Destroy the tick by crushing or soaking in alcohol. One should never use bare hands to remove a tick or crush a tick, because the blood contained within the tick may contain infectious microorganisms. Never use a lighted cigarette, gasoline, or kerosene to remove a tick, because their use will result in serious damage to the animal’s skin.

• Collars: Preventic collar (Virbac, Inc., Fort Worth, Tex.) is the only collar that is effective against ticks (even though others claim to be effective). These collars are effective for about 3 months if they do not get wet.

• Topical systemic treatments: Frontline is a spot-on product that is effective against fleas.

• Advantix

• Revolution

• Environmental treatments include the following:

Adams Home and Kennel Spray

Vet-Fog Fogger

Siphotrol Plus Fogger

Permectrin Pet, Yard, and Kennel Spray (Bio-Ceutic, St Joseph, Mo.)

Some yard sprays and garden sprays will also kill ticks

• The tick population in the environment can be decreased by removing brush, limiting rodent populations, and keeping grassy areas cut.

Mange

Three primary diseases called “mange” are seen in the dog and cat: demodectic mange, sarcoptic mange, and notoedric mange. These diseases are the result of tiny mites living on or in the skin, where they produce irritation and inflammation. The symptoms for each of the diseases are distinct, and diagnosis must include identification of the mite through skin scrapings or biopsy.

Demodectic Mange

Demodex canis, a cigar-shaped mite, lives within the hair follicles of most dogs and some cats (Fig. 6-5). These mites spend their entire life cycle on the host. In most dogs, the immune system holds the number of mites in check; however, in dogs with compromised immune systems (such as puppies with poor nutrition or other parasites, or dogs with chronic disease), the number of mites becomes excessive, causing disease. There appears to be a hereditary predisposition to demodectic mange, and certain breeds seem to be at greater risk.

Figure 6-5 Adult Demodex canis. These mites resemble eight-legged alligators.

(From Hendrix CM, Robinson E: Diagnostic parasitology for veterinary technicians, ed 3, St Louis, 2006, Mosby, by permission.)

Demodectic mange occurs in two forms: localized, the more commonly seen form, and generalized, the more severe but less common form.

CLINICAL SIGNS (GENERALIZED DEMODEX)

• Animals usually will be febrile.

• The entire body surface will be involved.

• Secondary bacterial skin infections with pustules will be seen.

DIAGNOSIS

• Diagnosis is by mite identification, usually through skin scrapings. Place a drop of mineral oil on the lesion and firmly scrape while squeezing the skin. Transfer the material to a clean glass slide and examine under a microscope. This mite is found easily.

• Culture and sensitivity of the skin lesions may be necessary if a secondary bacterial infection is present.

TREATMENT

• Treatment of Demodex depends on the age of the patient, the extent of the lesions, and veterinarian preference.

Localized

• Rotenone (Goodwinol ointment): apply to lesions daily

• Rotenone (Canex): apply to lesions daily

Sarcoptic Mange (Scabies)

Scabies is an intensely pruritic, contagious disease of animals. The mite, Sarcoptes scabiei var. canis, has a rounded body with four pairs of legs (Fig. 6-6). The female mite burrows into the epidermis and lays eggs. This burrowing produces intense itching and inflammation within the skin. Scabies can occur in dogs of any age, sex, or breed. Humans may experience development of visible lesions after exposure to infected animals; however, the mites do not survive off the animal host for longer than a few days. If the owner experiences development of small, red papules on his or her skin, a medical doctor should be consulted.

Figure 6-6 Adult Sarcoptes scabiei mite that causes scabies, an intensely pruritic disease.

(From Hendrix CM, Robinson E: Diagnostic parasitology for veterinary technicians, ed 3, St Louis, 2006, Mosby, by permission.)

DIAGNOSIS

• Identification of the mite through skin scrapings. These mites are difficult to find in many cases because they are located deep in the epidermis. Scraping should be deep to maximize the chance of finding the mite. Multiple scrapings of the same animal may be necessary to locate the mite. Mites can also be seen on skin biopsies.

Cuterebra “Warbles”

The Cuterebra fly lays eggs in the soil. These eggs mature into a larval stage similar to a grub that directly penetrates the host’s skin (Fig. 6-7, see also Color Plate 5). Here, in a subcutaneous pocket, the larva continues to mature, finally leaving the wound to become an adult fly. A fistula or opening in the swelling allows the larva to breathe while maturing; the larva can be seen moving up and down in the opening to the fistula.