The hair tends to cycle in a specific manner. Initially, hairs are in the growing or anagen phase, progressing through an intermediate or catagen phase into a resting or telogen phase (Figure 13-1). The time period that hairs are in the catagen phase is short, and finding hairs in this stage of the cycle is difficult using a trichogram (plucking hairs and looking at them under the microscope). A variety of conditions including temperature, photoperiod, genetics, and hormones can affect the hair cycle.^1,⁶

FIGURE 13-1 Phases in the life cycle of a hair. A, Anagen begins with the renewal of the intimate relationship between the papilla and the undifferentiated cells that partially enclose it. B, As anagen proceeds, matrix cells generate a new hair that pushes upward toward the surface and in the process dislodges the old club hair. C, Mature anagen hair follicle consists of infundibular, isthmus, and inferior segments. D, During catagen, the entire inferior segment of the follicle shrivels upward as a thin cord of epithelial cells and is followed upward by the papilla. E, During telogen, the club hair rests in its cornified sac at the level of the hair erector muscle.

(From Moschella SL, Herley HJ: Dermatology, ed 3, Philadelphia, 1992, WB Saunders.)

The hair follicles are located within the dermis and are associated with sebaceous and apocrine glands. Hairs are important in thermoregulation, protection of the skin from external factors, and as an ornament to attract other animals. The types of hairs found in equine skin are simple primary hairs. Horse hairs tend not to shed at once. Instead their hair sheds in patches or in a mosaic pattern.¹

Ambient temperature affects the texture of the hairs. In warm temperatures the hair coat comprises thick, medullated hairs. Piloerection of hairs occurs to aid in cooling the body. In contrast, in cold temperatures the coat comprises longer, finer, and poorly medullated fibers. This type of hair coat provides more insulation from cold. The hairs of the fetlock, mane, and tail do not shed as do the other hairs on the horse.¹

Additional Skin-Related Structures

The horse and other Equidae have skin structures unique to these species, including the ergot, chestnut, and hoof. The ergot is a small mass of cornified tissue located in a tuft of hair on the flexor surface of the fetlock. The ergot is a vestige of the second and fourth digits of extinct Equidae. The chestnut refers to a mass of horny tissue on the medial surface of the radius and is believed to be a vestige of the first digit. The hoof is the horny covering over the distal end of the third digit. The hoof includes the frog, important for shock absorption and stimulation of blood flow to the foot, and other components that provide a firm structure to facilitate motion.

HISTORY

A patient history of the horse can provide the clinician with useful clues to help determine the underlying cause of a dermatologic problem. Figure 13-2 is an example of a form with different types of dermatologic questions. For example, most allergies produce the clinical sign of pruritus. The owner may notice the horse rubbing, scratching, licking, or chewing, or physical examination may reveal evidence of pruritus (i.e., broken hairs and excoriations). Pruritus does not confirm a diagnosis of allergic disease. Other conditions such as parasitic infestation or secondary bacterial or fungal skin infection may be pruritic. These differential diagnoses may be ruled out by performing the diagnostic tests discussed later in this chapter.

FIGURE 13-2 Sample equine history form.

Insect allergies and atopy frequently recur seasonally. However, in the warm climates of the United States, some horses may have year-round or nonseasonal pruritus with insect allergies or atopy. The practitioner should also include food allergy on the list of differential diagnoses for horses with nonseasonal pruritus.

Questions concerning whether other horses in the stabling or pasture facility have dermatologic lesions may be important. Similarly, knowing if grooming aids (brushes, combs, etc.) are used on more than one horse is important. Pruritus that affects multiple animals may indicate the possibility of a parasitic or infectious process.

Response to previous therapy is another useful historical clue. The practitioner should determine the dose and frequency of medication administration. If the appropriate drug or drug dose is not used, then the patient may not respond.

Other factors to consider include feed, stabling environment, and supplements or other medications (e.g., wormers, sprays, shampoos) used regularly. If any of these factors changed before the development of the dermatologic lesions, then the change may be related to the problem in the horse.

PHYSICAL EXAMINATION

The examiner should consider the type of skin lesions and lesion distribution when performing a physical examination on the equine patient with dermatologic disease. Pustules and vesicles are fragile skin lesions that are not found easily and are most common in bacterial or autoimmune skin diseases. Pustules and vesicles are considered primary lesions. Other examples of primary lesions include papules, wheals or hives, erythema, and nodules. Secondary lesions commonly observed in horses may include scale, crusts, excoriations, fistulae, ulcers, necrosis, hyperpigmentation, hypopigmentation, lichenification, hyperhidrosis, and scars.

The location of lesions is important because certain dermatologic conditions tend to occur on certain areas of the body. This tendency is especially true with parasitic hypersensitivity reactions because certain parasites have a predilection for certain areas of the body.⁷ For example, pinworms tend to cause intense pruritus of the tailhead, and black flies tend to have a predilection for the head, ears, and ventral abdomen. More detailed information concerning insect feeding preferences is covered in the insect hypersensitivity section. The horse should be examined for evidence of external parasites such as ticks or flies.

The clinician should always perform a thorough physical examination, including determining the temperature, heart rate, and respiratory rate. If abnormalities in any of these parameters exist, then the horse may have a systemic disease or more than one medical problem. For the health of the animal, exploring and treating all possible medical problems is important.

COMMON CAUSES OF PRURITIC DERMATOSES

Pruritus is the sensation to rub, lick, scratch, or chew.⁸ Horses most commonly rub the skin and hair coat when they are pruritic (Figure 13-3). Clinical evidence that a horse is pruritic may include the presence of broken hairs, excoriations, hemorrhagic crust, alopecia, lichenification (thickening of the skin), and hyperpigmentation. Lichenification and hyperpigmentation occur most commonly after chronic inflammation and pruritus.

FIGURE 13-3 Regional pruritus: differential diagnosis of pruritic skin diseases.

Conditions that tend to be pruritic include parasite infestations such as lice, mites, ticks, Onchocerca spp., Habronema spp., and pinworms or allergic reactions such as insect hypersensitivity, food allergy, contact allergy, and atopy. Occasionally, pruritus occurs with bacterial or fungal infections.

Parasitic Skin Diseases

LICE (PEDICULOSIS)

Two types of lice may be found on horses. Biting lice feed on epidermal debris, whereas sucking lice feed on blood and lymph. Haematopinus asini, the sucking louse of horses, most commonly infests the mane and tail and the hairs behind the fetlock and pastern. The biting louse, Damalinia equi, tends to feed on the dorsolateral trunk.

Lice are host-specific and complete their entire life cycle on the host. Female lice attach their eggs to hairs. In cool, moist environments, lice can live for 2 to 3 weeks; they live only a few days off of the host. Lice infestations are transmitted by direct or indirect contact. Brushes, combs, and tack serve as fomites for transmission of lice. Adult horses may act as asymptomatic reservoirs for infection.

Lice infestations can occur year round, but they are more common in the winter in northern climates. Cooler skin and hair coat temperatures are more favorable for development of eggs within the female, oviposition, and egg development.⁹ Skin temperatures on the legs of horses, with the possible exception of Draft-breed horses, are too cold for louse development.

Lice infestation has no age, breed, or sex predilection in horses. Affected horses tend to be restless and have a poor appetite. They have a dry, dull hair coat with patchy areas of hair loss and excoriation. The coat may appear moist and have a sour or mousy odor. Skin fasciculation may be the only evidence of early infestations in foals. Some horses may be asymptomatic carriers that are identified only when foals or other susceptible animals become infested. Heavy infestations of sucking lice may cause anemia or severe debilitation.

The diagnosis of lice infestations is made by demonstration of adults or eggs or both on the hairs. Sometimes, lice cannot be visualized easily in direct light. In these circumstances the veterinarian may need to comb the hair coat with a fine-toothed comb to find the lice. Skin biopsies may reveal a nonspecific superficial perivascular eosinophilic dermatitis with or without intraepidermal microabscesses.¹

When managing a lice infestation, the veterinarian should treat all animals on the premises. Ivermectin at 200 μg/kg orally every 2 weeks for three treatments is effective for treating sucking lice. Biting lice do not respond to ivermectin treatment, and thus one should treat affected horses with other medications, such as dips. Common types of dip may be used to treat sucking and biting lice infestations include lime sulfur, pyrethrins, methoxychlor, malathion, coumaphos, crotoxyphos, pyrethroids, lindane, permethrin, selenium sulfide, imidacloprid, phoxim, and fipronil.¹⁰^–¹² Lime sulfur and pyrethrin dips have fewer associated adverse reactions in horses than do other dip treatments. All products should be used according to label directions. None of these treatments affect lice eggs, and two to three repeat applications of the dip are recommended at 2-week intervals. For optimal efficacy, one should treat the whole body of the horse when applying the dips. Other precautions include cleaning and disinfecting all tack and grooming supplies (brushes and combs), decontaminating the stabling area using a commercial premise spray used to kill fleas because lice may survive for days to weeks in the environment, and, although reinfestation is difficult to prevent, treating animals returning from shows, breeding farms, or training facilities prophylactically.

MITES (MANGE)

A variety of mites may infest horses. Sarcoptes scabiei var. equi (scabies, head mange), Chorioptes equi (leg mange), Psoroptes equi (body mange), Pyemotes tritici (straw itch mite), and Trombicula and Eutrombicula species (chiggers) are species associated with equine pruritic skin disease.⁷ The pruritus accompanying a mite infestation results from a combination of mechanical irritation and hypersensitivity to the mite and by-products of the mite (i.e., feces).¹³ The primary skin lesion from a mite infestation is a maculopapular eruption. These skin lesions are often difficult to find with chronic infestations.

Sarcoptes scabiei

In the United States, S. scabiei infestations in horses are a reportable disease.⁷ This mite is highly contagious and infests horses and transiently infests human beings (self-limiting disease) in contact with parasitized horses. This parasite is capable of surviving off the host for up to 3 weeks, and transmission of scabies by fomites or in the environment is possible. Scabies mites burrow in the superficial epidermis and lay their eggs. In early infestations, mites are found in highest concentration around the head and neck. These mites seem to prefer the ears on horses. As disease progresses, mites usually spread over the entire body. A scabies infestation in a horse results in intense pruritus and generalized scaling and crusting with excoriations and lichenification. A secondary skin infection may result from the pruritus. One may elicit the itch-scratch reflex by scratching the horse over the withers, causing the horse to tuck its nose close to its chest and make smacking noises and exaggerated lip movements. This reflex suggests scabies but is not diagnostic, and one may elicit the reflex in some normal horses.¹⁴ The scabies mite is circular with short legs, a terminal anus, and long unjointed pedicles. Unfortunately, as in other species, the scabies mites are difficult to find on skin scrapings. Therefore a negative skin scraping does not ensure that the horse does not have scabies.

Ivermectin at a dose of 200 μg/kg orally is effective for treating horses with scabies. As in other species, the practitioner should repeat ivermectin treatment at 2-week intervals for a total of two to three treatments. Within 2 weeks the eggs hatch and new adult mites are present. Topical treatments such as lime sulfur dip, lindane, coumaphos, diazinon, malathion, or toxaphene also have been used. The practitioner should dip horses every 7 to 10 days for three to six treatments, treat all horses in contact with the infested horse, and decontaminate fomites (e.g., tack, brushes) and the environment.

Chorioptes equi

Chorioptes equi mites are host-specific and do not parasitize human beings (Figure 13-4). Unlike the scabies mite, infestation with C. equi is not a reportable condition. C. equi mites spend their entire life cycle on the host feeding on epidermal debris. Preferred feeding sites include the distal limbs and perineum. Infested horses exhibit intense pruritus with stomping of feet and rubbing of the perineal area. Skin lesions include alopecia, erythema, and crusting of the pasterns, fetlocks, or perineum. Draft-breed horses or other horses with feathered fetlocks are predisposed to C. equi. The mite tends to be more common in the winter months. C. equi should be included in the list of differential diagnoses for horses with pastern dermatitis or greasy heel. Treatment of this mite is the same as for S. scabiei. However, ivermectin-resistant C. equi mites have been reported, and topical dip therapy may be indicated if the horse does not respond to oral therapy. If a topical medication is used, clipping the hair around the fetlocks and pasterns to ensure optimal contact of the dip with the skin and mites is advisable.

FIGURE 13-4 Chorioptes mite.

Psoroptee equi

Psoroptes equi mite infestations are a reportable condition in horses in the United States. P. equi mites are highly contagious to other horses but do not infest human beings. These mites do not burrow but live on the skin surface. They are biting mites that feed on serum and cellular components. Transmission of P. equi mites is by direct contact and exposure to fomites. Mite infestations tend to begin on the forelock, mane, and tail and spread to the trunk. These mites have been known to infest the ear canal and cause otitis externa. Affected horses typically show head shaking or rubbing. As with many other mites, pruritus results in alopecia, papule, oozing crusts, excoriations, and ulcers on the skin. The intensity of pruritus varies. History and clinical signs suggest the diagnosis of P. equi mange. Skin scrapings are required to diagnose this condition definitively. P. equi has an oval body with segmented pedicles. Treatment for P. equi infection is similar to that described for sarcoptic mange.

Trombiculidiasis

Trombiculidiasis (chiggers, red bugs, harvest mites) is caused by an infestation with larvae of free-living adult mites (genus Eutrombicula or Neotrombicula). Larvae are most prevalent in grasses, forests, or swamps in late summer and fall. Small rodents are the natural host. Pathognomonic skin lesions are papules or wheals with a small orange or red dot (trombiculid larvae) in the center. Trombiculidiasis is seasonal, but the seasonality varies with genus and species. Eutrombicula alfreddugesi is active in the late spring and peaks midsummer, whereas Neotrombicula automnalis is most active from the late summer to midautumn.¹⁰ Larvae feed on the host in the late afternoon and early evening. Areas of the body most commonly infested are the face, muzzle, distal limbs, ventral thorax, and abdomen. The diagnosis is based on observation of the distinctive lesion with the larvae approximately 0.2 to 0.4 mm in size, oval in shape with six legs, in the center of the skin lesion. Trombiculidiasis is a self-limiting disease. Some horses are uncomfortable with this mite infestation and should be treated. Recommended treatments include lime sulfur dip, permethrin, pyrethrin, cypermethrin, and phoxim sprays or dips as a one-time treatment in association with prednisolone at a dose of 0.5 mg/kg orally for 3 to 5 days.

Pyemotes tritici

Pyemotes tritici (straw itch mite) normally parasitizes the larvae of grain insects. Occasionally this mite parasitizes human beings and horses.¹⁵ Infestation in horses occurs from contaminated hay fed in overhead racks. The mite produces a maculopapular crusted eruption on the head, neck, and trunk that is only occasionally pruritic. The diagnosis is based on history and clinical examination. This disease is self-limiting, and the owner should remove contaminated forage or feed the horse from the ground until all contaminated forage has been consumed. Infestations from fomites or hay fed on the ground have not been reported.

Ticks

Ticks are an important ectoparasite in the horse because of their potential role in disease transmission, including a variety of viral, protozoal, rickettsial, and bacterial infections. Tick infestations are most common in the spring and summer.

Argasid ticks lay eggs in cracks and crevices in the environment, and immature ticks infect hosts after hatching. Larvae and nymphs suck blood and lymph and then drop off to develop into adults. These ticks infest barns, sheds, and other areas where animals are found. In horses, Otobius megnini (spinose ear tick) tends to infest the ears and ear canal. Clinical signs of infestation include otitis externa, head tilt, head shaking, ear rubbing, and occasionally aural hematomas.

Dermacentor, Ixodes, and Amblyomma species are the most common ixodid ticks of horses. These ticks are found outdoors and have complicated life cycles in which all stages of the life cycle are parasitic. The severity of clinical signs depends on the density of the infestation and whether the horse develops a hypersensitivity reaction to the bites. Infestations occur most commonly on the ears, face, neck, groin, distal limbs, and tail. Early lesions consist of papular to pustular eruptions that rapidly develop into crusts, erosions, ulcers, and hair loss. Hypersensitivity reactions may be local or general. Local responses appear as nodules that develop at the site of the tick bite. Although the pathogenesis is unknown, cutaneous basophil hypersensitivity is believed to be involved.¹⁶ Systemic reactions are characterized by whole-body urticaria or multifocal urticarial plaques. In Australia, a hypersensitivity reaction to Boophilus microplus has been observed in sensitized horses.¹⁷ Intense pruritus, papules, and wheals develop as soon as 30 minutes after ticks begin to feed.

A definitive diagnosis is made by observation of ticks attached to the horse or in the ear canal. Treatment aims at killing ticks on the horse. The practitioner should apply pyrethrin or pyrethroid sponge-on dips to the body of the horse, taking extra care to soak skinfold areas. Usually, only one treatment is necessary unless reinfestation occurs. Resistance to insecticides can occur rapidly, and knowledge of local resistance patterns is important. Infestations of O. megnini require mechanical removal of as many ticks as possible. One part rotenone and three parts mineral oil applied twice weekly is an effective otic parasiticide for horses. The author has used a commercially available pyrethrin otic preparation (Otomite Plus, Vibac, Fort Worth, Tex.) for small animals in horse ears with success. Ivermectin also has been shown to be effective in treating ticks at a dose of 200 μg/kg orally.

Onchocerciasis

Onchocerciasis is a nonseasonal skin disease of the horse caused by the parasite Onchocerca cervicalis. This nematode lives in the ligamentum nuchae and produces microfilariae that migrate to the skin and are ingested by the intermediate host, Culicoides species. Microfilarial populations in the skin vary, and the highest concentrations occur in the dermis of the face, neck, and ventral midline, especially the umbilicus (Figure 13-5).¹⁸ Microfilarial populations vary seasonally and are highest in the spring, which interestingly is the peak season for the Culicoides vector.¹⁹ Microfilariae are more superficial in the dermis during the spring and summer months. Clinical signs of onchocerciasis are believed to be caused by an idiosyncratic hypersensitivity reaction to one or more microfilarial antigens because many horses that have circulating microfilariae do not have any gross skin or ocular lesions.²⁰ Whether the reaction is directed only at dying or dead microfilariae is unknown.

FIGURE 13-5 Ventral midline dermatitis caused by onchocerciasis.

Onchocerciasis has no breed or sex predilection and usually affects horses 4 years of age and older. Clinical signs are nonseasonal but may be worse in the spring and summer, most likely because of the added irritation from the vector. Lesions may occur on the face and neck, on the ventral chest and abdomen, or in all these areas.^21,²² Early lesions begin as a thinning hair coat. As the disease progresses, lesions may vary from focal to generalized areas of alopecia, scaling, crusting, and plaques. Affected areas may be severely excoriated, ulcerated, oozing, and lichenified. Annular lesions in the center of the forehead suggest the disease (Figure 13-6). Leukoderma usually develops at the site of lesions and is irreversible. Ocular lesions include sclerosing keratitis, vitiligo of the bulbar conjunctiva, white nodules in the pigmented conjunctiva, uveitis, and a crescent-shaped patch of depigmentation bordering the optic disk.²³

FIGURE 13-6 Equine onchocercosis.

The practitioner may make a presumptive diagnosis of onchocerciasis by demonstrating the microfilariae in the skin of animals with compatible historical and clinical findings. Skin scrapings and direct blood smears are often negative. One can demonstrate microfilariae most reliably with a mince preparation or via histologic examination of skin from a biopsy specimen.²² Mince preparations require a 4- or 6-mm punch specimen of tissue obtained from the ventral abdomen. One places the tissue specimen in a Petri dish with a small amount of physiologic saline, minces it with a scalpel blade or razor, and incubates it at room temperature for 30 to 60 minutes. The specimen can then be examined microscopically for evidence of the rapid motion of the microfilariae. Skin biopsies reveal a superficial perivascular eosinophilic dermatitis. Often, microfilariae are visible in the superficial dermis.⁷

Ivermectin at a dose of 200 μg/kg orally is the treatment of choice.^7,^21,²⁴ A single dose often produces remission of clinical signs within 2 to 3 weeks. However, some horses require two to three monthly treatments before clinical signs resolve. Approximately 25% of horses have an adverse reaction, such as ventral midline edema or pruritus, which occurs 1 to 10 days after treatment. In rare cases, severe umbilical and eyelid edema and fever may occur. In confirmed cases of onchocerciasis, one should perform a thorough ocular examination to look for any signs of eye disease associated with onchocerciasis. Anecdotal reports suggest treatment may precipitate an episode of uveitis.

Prednisolone at a dose of 0.5 mg/kg orally may be necessary in the first week of treatment to prevent exacerbation of skin and ocular lesions as a result of massive destruction of microfilariae but is not routinely recommended. Indications include the development of ventral midline edema, pruritus, and fever. None of the currently available anthelmintics kill adult parasites in the ligamentaum nuchae, and affected individuals require periodic retreatment with ivermectin when clinical signs recur. The prevalence of cutaneous onchocerciasis has decreased significantly with the advent of routine dewormings with ivermectin.