Ticks, Mange, Pediculosis (Lice), Chiggers / Harvest Mites (Trombiculidiasis) and Straw Itch Mites (Forage Mites)

Table of Contents

Ticks         
Ticks Affecting Horses
Tick Paralysis
Mange
Burrowing Mites
Pediculosis (Lice)
Chiggers / Harvest Mites (Trombiculidiasis)
Straw Itch Mites (Forage Mites)

Ticks

These are obligate ectoparasites of many terrestial vertebrates worldwide. They are actually large mites and belong to the subclass Acari. Two of the three families of ticks, Ixodidae (hard ticks) and Argasidae (soft ticks) parasitize livestock. Although the two families share some basic features they differ in many others including morphologic, physiologic, feeding habits, and reproductive patterns.

Members of the family Ixodidae are by far the most important and numerous parasites of livestock. The genera and species affecting horses are listed in Table 4.1 along with their hosts, geographic distribution, hosts, location on the animal, and season of occurrence.

Some important features of hard ticks are:

• They can be readily seen with the naked eye. The unfed larvae are about 1 mm and just visible.
• The life cycle consists of: engorged female > eggs > larva > nymph > adult male/female > engorged female > ...
• There are one, two and three host ticks.

• One host tick: the entire development from larva to adult takes place on one host.
• Two host tick: larva and nymph occur on one host, and the adult on another.
• Three host tick: each stage of development, larva, nymph, and adult takes place on a different host. The life cycle can take as long as three years to complete.

• "Ornate" ticks have light colored spots or lines on the scutum
• Among the diseases Ixodes spp. can transmit to horses are:

• Lyme disease, babesiosis, ehrlichiosis, tick paralysis, EIA, sleeping sickness

• A number of species of ticks are capable of causing tick paralysis which is referred to below.
• Ticks can be vectors of many diseases including piroplasmosis, EIA, Equine granulocytic ehrlichiosis (EGE) caused by Anaplasma equi ( formerly Ehrlichia equi)
• Horses heavily affed by ticks may be anemic

Ticks Affecting Horses

Amblyomma spp. (americanum, cajennense maculatum)

The Lone-star (silvery spot on female) and Gulf Coast ticks occur in the South Eastern to South Central USA and in tropical Latin America. The range of these ticks is expanding in North America and they are found on horses.

Boophilus spp. (annulatus, microplus)

The Cattle Fever Tick, B. annulatus, is a one host tick that has been eradicated from the United States. Eradication was successful because it is a one host tick and prefers to feed on cattle. B. microplus has a broader host range including horses, goats, sheep and deer and is an important vector of Babesia equi.

Dermacentor spp.
D. albipictus

The winter tick is an unusual member of this genus. This inornate, one host tick, occurs widely on horses in North America is also found on cattle, sheep, and goats. It is mainly present in groin and pectoral regions during the fall and winter months. Heavy infestations may cause loss of appetite, depression and debilitation.

D. andersoni

The Rocky Mountain wood tick is found in the Western US and Canada. It can produce tick paralysis in foals, dogs, cats and some other domestic species.

D. nitens

The Tropical horse tick, prefers the external ear canal of horses but may be found in other areas as well as on cattle, sheep, goats and deer. Vector of Babesia caballi.

D. occidentalis

The Pacific coast tick is mainly found in coastal areas of western USA. It is a three host tick that may be found anywhere on the horse. In regions where these ticks occur, horses should be examined for ticks after exposure.

D. variabilis

This tick, referred to as the America dog tick, occurs in the Eastern and Central USA, Eastern Canada, and Mexico. It causes tick paralysis in dogs most commonly, cats, some domestic animals including foals.

Ixodes spp (pacificus, scapularis, ricinus, holocyclus)

These ticks are well known for disease transmission through out the world. Ixoses holocyclus in Australia is the most virulent tick paralysis producer known. Species of Ixodes transmit Lyme disease to man and dogs and parasitize horses in North America. Although Ixodes pacificus prefers to feed on lizards and small rodents it is the vector of Anaplasma equi to horses.

Otobius megnini

This argasid, soft tick, is found in the ears of horses, dogs, cats, cattle, and occasionally humans. Only the larvae and nymph forms are found in the horse's ears. The adults can survive on the ground for as long as two years.

Diagnosis

Ticks and particularly the larger engorged females can be readily seen on the skin. A number of morphologic characteristics are used by parasitologists to identify the various species of ticks.

Treatment

Spot-on, spraying, showering or dipping with acaricides.

Control

Horses that have been in areas where ticks are prevalent should be carefully examined daily and the ticks removed.

Tick Paralysis

More than a dozen species of ticks with varying geographic distribution have the capacity to produce tick paralysis. This condition mainly affects humans (children especially), dogs (most commonly), cats, cattle, sheep, goats, and foals infrequently. It is assumed to be due to a protein neurotoxin produced by ticks. The ticks mainly involved in North America are D. variabilis, D. andersoni, and D. occidentalis. Young animals are most susceptible.

The circulating neurotoxin interfers with acetylcholine release at neuromuscular junctions leading to an ascending paralysis. Toxicity is mainly associated with female ixodid ticks and incidence of the disease is related to the latter's activity in the spring and early summer.

The incubation period is 5 - 7 days.

Clinical signs: Temperature is normal. Abnormal fore- and hindlimb reflexes with incoordination and weakness with ascending paralysis. In a few hours the animal is unable to move, breathing is difficult and death may ensue in several hours.

If ticks are removed in time recovery takes place within 1 - 3 days.

Diagnosis is based on the sudden paralysis and presence of ticks. Rapid recovery after tick removal is confirmatory.

There are no observable lesions and blood and fluid values are normal.

There is no treatment other than the prompt removal of ticks.

Measures should be directed toward reducing exposure to ticks and to their removal when present.

Mange

Mange is caused by a small number of parasitic mites. The parasitic mites are small and just visible to the naked eye. When engorged with blood they may exceed 0.5 mm in length. Based on their behavior they are divided into two major categories, burrowing mites and non-burrowing mites.

Some important features of mites are:

• They are taxonomically complex comprising a least eight families.
• Most parasitic mites are in prolonged contact with the skin and can cause a skin disease called mange.

Unlike ticks, most species complete their life cycle, from egg to adult, on the host, and consequently transmission is mainly by contact.

Mange is highly contagious.

Burrowing Mites

Sarcoptic Mange

This form of mange is caused by largely host specific strains (not absolute) of the burrowing mite, Sarcoptes scabei. Many animals and humans worldwide are infected with this species. However, this form of mange is rare in North America. The last reported case in the horse was in 1958.

It is the most severe form of mange encountered in horses in North America. Lesions are found mainly on the head, neck, shoulders, thorax, and flanks. There is intense pruritis with the formation of small papules followed by vesicles and crust formation.

Psoroptic Mange

It is caused by the nonburrowing mite, Psoroptes equi which mainly infects the poll, mane and tail regions. There is itching, alopecia, formation of thick crusts, and thickening of the skin.

Chorioptic Mange

It is caused by the nonburrowing mite, Chorioptes equi which mainly infects the lower limbs (foot mange). The lesions are similar to those of sarcoptic mange. There is intense itching and horses will paw, lick and bite their lower limbs.

Demodectic Mange

This form of mange, caused by Demodex equi and D. caballi, is rare in horses. These mites live in hair follicles and sebaceous glands. D. equi is found on the body while D. caballi occurs on the muzzle and around the eyes.

Lesions occur on the forelimbs, neck, face, and shoulders. They consist of areas of alopecia, nodular formation and scaling; itching is absent.

Diagnosis

The mites of psoroptic and chorioptic mange can be seen in skin scrapings. Deeper scrapings or biopsies may be necessary to demonstrate the mites of sarcoptic mange which is now extremely rare in the horse.

Material can be conveniently collected after applying mineral oil to the skin prior to scraping. Mounts under coverslips are then examined under the low power of the microscope.

Deep scrapings are required to demonstrate demodectic mites, eggs and larval forms.

Treatment

Organophosphate insecticides or a lime-sulphur solution is applied by dipping, spraying or sponging.

Ivermectin is effective except for demodex. Several treatments are necessary at 3 - 4 week intervals.

The above treatments should be accompanied by clipping of the hair, removal of scabs and crusts.

It is important that all horses in a stable or in close proximity be treated.

Some adult mites can live as long as three weeks off the host.

Pediculosis (Lice)

Lice are wingless, flattened insects that are ectoparasites of mammals worldwide. They constitute two orders: Anoplura (sucking lice, blood feeders) and Mallophaga (biting lice).

Two species of lice affect horses: Damalina equi a biting louse that feeds on skin and hair, and Haematopinus asini a sucking louse that pierces skin and sucks blood and tissue fluids. Infestations are common.

The life cycle is completed on the host and transmission is mainly by direct contact. Infections are more severe in the late winter and early spring.

The skin irritation results in itching manifested by rubbing, biting and kicking. The coat may be roughened with loss of hair and there may be secondary bacterial infection. In heavy infestations individuals become anemic and lose weight and condition.

Diagnosis

This is based upon finding eggs or lice. The latter are 2 - 4 mm in size and thus can be readily seen when hair is parted.

Biting lice are more active than sucking lice.

Treatment

Dipping or spraying with insecticide solutions; two treatments two weeks apart are recommended. Ivermectin is effective against sucking lice but not biting lice.

Frequent and thorough grooming is helpful in controlling pediculosis.

Chiggers / Harvest Mites (Trombiculidiasis)

Chiggers are the larval stage of harvest mites (Trombicula), the adults of which live on plants and some invertebrates. The larvae ordinarily feed on small rodents. During the late summer and fall they may also feed on horses. The feet and muzzle are most often affected. They are most active after sunset.

Lesions begin as papules leading to scales, scabs, hair loss and intense itching. There is pawing and stamping with rubbing and biting affected areas.

They transmit the threadworm Onchocerca cervicalis.

The infestation usually lasts only a few days. Insecticide washes may be applied and the pruritis can be controlled with glucocorticoids.

Straw Itch Mites (Forage Mites)

These mites feed on organic matter associated with grain and straw. They may infest the skin of the head and neck while horses feed from a rack and on the limbs when they feed from the ground.

The skin reaction resembles that seen with chiggers. The condition is self-limiting. The itching and irritation can be ameliorated by insecticide washes and glucocorticoids.