TICKS

Etiology

Horses are susceptible to tick infestations by many species of ticks that feed on wild or domestic ungulates in different regions. Soft ticks (Argasidae) and hard ticks (Ixodidae) may be involved. Ixodes spp., Dermacentor spp., Boophilus spp., Amblyomma spp., and Otobius megnini (spinose ear tick) are most often involved. Exotic ticks, such as Anocentor nitens (tropical horse tick), if found on horses in the United States, are reportable.

Life cycles among tick species may require one, two, or three hosts and a few weeks to 2 years to complete, depending on environmental conditions. Details of life cycles and hosts can be found elsewhere.¹ With multihost ticks, the larvae and nymphs usually require small vertebrate hosts, while the adults feed on larger animals. Tick infestations occur in spring or summer months (the exception being the winter tick, Dermacentor albipictus) or nonseasonally in the tropics. In warm climates, if rainy and dry seasons are distinct, ticks will be more active during the rainy season. Most ticks live in forests, grasslands, and scrub, infesting passing animals. Some live in the burrows or nesting areas of the host, enabling them to reach the host easily whenever conditions are favorable.

Pathogenesis

Clinical manifestations of tick infestation result from tick feeding. Local reactions result in papules, nodules, wheals, and sometimes pustules around the feeding site. Highly pruritic reactions with papular to urticarial lesions have been attributed to hypersensitivity to Boophilus spp. Other species of ticks, including Otobius megnini nymphs, Anocentor nitens (an important vector of equine babesiosis), and Amblyomma maculatum, feed in the ear canals of horses. The resulting irritation results in head shaking, ear rubbing, head tilt, or a lop-eared appearance and creates conditions conducive to the development of secondary bacterial otitis externa.

Infectious agents are readily transmitted by ticks for several reasons: (1) ticks feed multiple times throughout their life cycle; (2) many infectious agents are transmitted during tick maturation to the next stage in the life cycle; (3) ticks take relatively large blood meals, which are concentrated by secreting the host’s own fluids back into the host directly or as coxal secretions; and (4) immunosuppressive substances are present in the tick saliva that reduce host defenses, allowing the microbe to become established.¹ Heavy tick infestations can result in poor nutritional and immunologic condition and even anemia from blood loss. Tick paralysis results from salivary proteins found in females of some tick species, which secrete the neurotoxic proteins during feeding. Just one tick may cause partial paralysis, but a more severe or extensive paralysis may result from larger numbers of ticks feeding. Foals and ponies are more likely to be affected.^1,²

Clinical Findings

Clinical syndromes resulting from tick infestation include (1) mild to severe papulonodular dermatitis, with or without pruritus; (2) otitis externa; (3) systemic infection with tick-borne agents such as Babesia caballi or B. equi (Chapter 59), Anaplasma phagocytophilum (previously E. equi, Chapter 42), Borrelia burgdorferi (Chapter 35), Francisella tularensis, and Theileria annulata; (4) anemia and poor condition; and (5) flaccid, ascending motor paralysis, particularly in foals or ponies.^1,^3,⁴

Diagnosis

Finding ticks on the horse is diagnostic. Identification of ticks is useful in determining which control measures will be most effective. Keys to aid identification can be found in parasitology texts and elsewhere.^5,^6,⁷ When exotic ticks are suspected or tick management problems arise, consultation with governmental authorities or a parasitologist for proper identification is strongly recommended.

Pathologic Findings

Histopathologic findings for tick-induced papules and nodules range from eosinophilic and neutrophilic perivascular dermatitis with dermal edema to intraepidermal vesiculopustular dermatitis. Chronic, nodular lesions often contain large numbers of lymphohistiocytic cells forming lymphoid nodule or follicles.^8,⁹

Therapy and Prevention

Control measures for tick infestations depend on the species involved. Topical or systemic acaricidal treatments are normally combined with management changes such as clearing brush, keeping pastures mowed, and avoiding tick habitat during the season of greatest tick activity. With multihost ticks, control of other tick hosts in the environment (rodents, mice, deer) may prove helpful when possible. Acaricides normally used include synthetic pyrethroids, organophosphates, and avermectins. Chlorinated hydrocarbons and formamidines (amitraz) are contraindicated because of environmental danger and toxicity to horses, respectively. Products are usually applied as sprays or dips, or administered systemically. Resistance to acaricides varies regionally.

Public Health Considerations

Infestation of horses with ticks should alert their human owners that they too are at risk of tick exposure and thus potential exposure to tick-borne diseases. Humans in contact with tick-infested animals should inspect themselves for similar infestation and remove ticks as soon as possible before engorgement occurs. The longer a tick remains attached, the greater the likelihood for transmission of infectious agents to the host.

MITES

Sarcoptic Mange

Psoroptic Mange

Etiology

Psoroptic mange in horses has been attributed to several species of psoroptic mites that may or may not be truly distinct species. Psoroptes equi, P. ovis, P. natalensis, and P. cuniculi have been implicated in equine infestations. Because P. ovis is reportable in cattle and sheep and because these mites are very similar to one another (possibly one species), appropriate authorities should be contacted for advice, especially if infested horses have had exposure to sheep or cattle or have been acquired recently.

Epidemiology

Mites are readily transmitted by direct contact or contact with fomites. Psoroptic mange mites live on the skin surface and feed by puncturing the epidermis, causing serous exudation, erythema, crusting, and pruritus. The female mite lays eggs in the surface debris. Eggs hatch within 2 to 3 days and develop to maturity within 11 to 14 days. Off-host survival of adult mites is shortest during periods of high temperature and low humidity. On average, survival on premises is 2 to 3 weeks but may be longer depending on conditions.¹⁰

Pathogenesis and Clinical Findings

As with sarcoptic mange, pruritus from psoroptic mange is thought to be caused by irritant and hypersensitivity reactions to the presence and secretions of the mites. Papules, crusts, scaling, alopecia, and lichenification are the initial lesions. Lesions tend to be distributed around the outer ear, forelock, mane, tail, ear canal, and inguinal and axillary areas, with less involvement of the trunk and legs. Secondary bacterial infections may result.

Diagnosis

Diagnosis requires microscopic identification of Psoroptes spp. mites from multiple superficial scrapings and crusts obtained from lesions. Unlike sarcoptic mange, mites are found more readily with psoroptic mange.

Therapy

Prevention

Thorough physical examination and isolation of newly introduced horses should prevent introduction of Psoroptes mites into the herd. Fomites that could potentially transmit the infestation indirectly (e.g., buckets, tack, blankets, grooming equipment) should not be shared between new horses and other horses on the farm.

Chorioptic Mange

Etiology

The most frequently diagnosed mange in horses, chorioptic mange is caused by the surface-living mange mite Chorioptes bovis (Fig. 64-1). As with the previously discussed mange mites, there is likely only one species of Chorioptes mites that infests several hosts.

Fig. 64-1 Female Chorioptes bovis mite.

(Courtesy Ellis Greiner.)

Epidemiology

Transmission occurs by direct and indirect contact. The mite feeds on cutaneous scale and debris without burrowing or puncturing the epidermis. Mite eggs are laid and subsequently develop in skin debris, completing the life cycle in 2 to 3 weeks. Mites and mite eggs can survive on debris within buildings, bedding, and on grooming equipment for extended periods, depending on temperature, humidity, and conditions of hygiene. Asymptomatic carriage of mites is common in sheep and cattle and may occur in horses. Mite populations and subsequently clinical signs appear to increase in the winter and decrease in the drier summer months.

Pathogenesis and Clinical Findings

Because pruritus can range from severe to nonexistent in chorioptic mange, it is presumed that, as with other mite infestations, hypersensitivity plays a role in the pathogenesis of lesions. Chorioptic mites have a predilection to infest the skin of the legs from below the carpus and the hock. Chronic lesions may spread to the ventral thorax and abdomen and occasionally extend up to the tail base. Initial lesions include erythema, papules, and crusts arising on the lower limbs. With time these may progress to more extensive crusting, ulceration, lichenification, and secondary infection. A difficult-to-treat, chronic, pastern dermatitis may result. Draft breeds with feathered (long-haired) fetlocks seem especially suited to maintaining infestations. Foot stamping, biting at the lower limbs, and rubbing of the lower limbs are symptoms usually noted by the owner.

Diagnosis

Diagnosis, by demonstrating the mite in superficial skin scrapings taken from multiple sites, can be difficult. Recommended techniques for catching these fast-moving mites include (1) using clear cellophane tape to pick up mites and crusts from the skin surface (although mites can be difficult to remove or position for identification, and parasitologists find such preparations frustrating); (2) adding insecticide to the oil used for the scrapings; and (3) vigorous brushing into a pan of soapy water. With the third technique, allow the mixture to settle out in a deep, narrow container. Decant and then centrifuge the settled material for microscopic examination of the pellet, or process the settled material as for fecal flotation.^9,¹¹ Field collections of crusts and debris can be placed in a blood sample tube for transportation to the laboratory for examination. In warmer weather the mites may only be obtained from scrapings of the coronary band.⁹

Therapy

Chorioptic mange can be difficult to treat because of its superficial location on the skin and mites’ ability to survive in the environment. Oral ivermectin is effective at reducing mite populations but cannot eliminate them.¹² Effective topical treatments include a series of three whole-body baths, 5 days apart, using 1% selenium sulfide shampoo, and two treatments, 2 to 3 weeks apart, with 0.25% fipronil spray.^13,¹⁴ Fipronil spray should be applied to all affected areas, including both the forelimbs and the hindlimbs from above the carpus and stifles, respectively, distally to the hoof and in sufficient quantity to dampen the hair and wet the skin. Other topical products that should be effective include 2% lime sulfur, organophosphate products, and synthetic pyrethroids. Ideally, the legs of heavily feathered animals should be clipped (owners often resist this for cosmetic reasons) and the legs thoroughly washed and skin debris removed before each parasiticidal application. Dips or washes may need to be repeated at 5- to 7-day intervals for 3 to 4 weeks. Care should be taken to apply these products to all areas vulnerable to infestation, including rear quarters, tail base, ventral abdomen and thorax, and even the head, if the horse is biting at or rubbing infected forelimbs with its head. One author suggests a combined approach using ivermectin systemically in combination with topical treatment.¹⁵ A key component to treatment is isolation of infested horses and treatment of the environment and fomites. All bedding should be removed from the stables, followed by thorough cleaning and treatment of premises with insecticide. If stalls or paddocks can be vacated for 3 or more weeks, there is less likelihood mites will survive in the environment. All bedding for infested horses should be replaced with fresh product daily, if possible, or on treatment days, at a minimum. Grooming and feeding equipment should be segregated, cleaned, and treated.