of an inch) (A). Both live in the large intestine and cause damage which may result in colic, diarrhea, and poor condition. Their eggs are passed in the feces and hatch into what become infective larvae. Infective larvae crawl up pasture forage, shown in a drop of moisture on a blade of grass (B), where they are ingested. Large strongyle larvae migrate in the walls of arteries supplying the intestinal tract. This may cause damage and thrombi or blood clots (BC) in the blood vessels (BV) to the intestine (I), which passes across the width of the bottom of Figure C. This is a predisposing cause of most cases of colic in the horse.

Fig. 9-2. Adult ascarids (Parascaris equorum) that became so numerous that they blocked and caused the rupture of the small intestine, resulting in the death of the horse.

Bots

Hot flies (Gastrophilus spp.) lay eggs on the horse’s hair—primarily around the forelegs, shoulders, chest, neck, throat, jaws, and lips—during the summer and late fall (Fig. 9-3A, B). The hot flies are killed by the first hard freeze in the fall. After ingestion of the bot eggs, or migration of larvae into the mouth, larvae develop in the oral and pharyngeal tissue. Later they emerge and are swallowed or migrate to the stomach and upper small intestine where they attach (Fig. 9-3C). The following spring, after living for 9 to 10 months in the horse, they are fully grown and are about 0.8 to 1 inch (2 to 2.5 cm) long and reddish orange (Fig. 9-3D). These detach and are passed in the feces. Some, as they are passed, may reattach around the anus for a few days. Once excreted, they burrow into the manure or soil and, after 2 to 4 weeks, pupate to the hot fly, thus completing their 1-year-long life cycle. Without treatment, they are present in most horses.

Fig. 9-3. Bot flies (Gastrophilus intestinalis) (A) emerge from pupae in the summer. They are nearly the size of bees and have no mouth, so they do not bite, as is commonly believed. During their 3- to 5-week life, they lay eggs or nits on the horse’s hair (B) around the lips, jaws, neck, head, forelegs, and shoulders. This activity may cause horses to bunch, seek protection in dense shrubbery or in water, or run erratically. Larvae from these eggs migrate to the oral cavity and to the stomach, where they develop into adults over the following 9 to 10 months attached to the lining of the stomach (C). The following spring, they detach, are passed in the feces (D), and after 2 to 4 weeks, pupate to the bot fly (A), completing their 1-year-long life cycle.

The immature larvae may damage the oral and pharyngeal tissue. However, the greatest damage is to the stomach and duodenum, where they cause inflammation, which may result in colic, ulcers, or even perforations.

Pinworms

Adult pinworms (Oxyuris equi) occur primarily in the horse’s colon and rectum. Their sticky yellow eggs are deposited around the anus, causing irritation. As a result, the horse may scratch its rump against walls and posts, wearing away the hair at the base of the tail and rump. This is thought to be the pinworm’s only harmful effect. Similar symptoms occur as a result of tail mange mites (Chorioptes equi), the biting louse (Damalinia equi) and allergic dermatitis due to hypersensitivity to biting midges (Culicoides spp.). Diagnosis of oxyuriasis is made by clinical signs and by finding the eggs. Clear tape applied to the anal and perineal region will remove the eggs. The tape should be placed in mineral oil on a glass slide and examined for the characteristic eggs (Fig. 9-6C). Larvae hatched from pinworm eggs are ingested and pass to the colon, where they develop into the egg-laying adult. The life cycle takes about 5 months.

Stomachworms

Larvae of stomachworms or spirurids (Habronema and Draschia spp.) develop inside the maggots of the common horsefly or stablefly found in manure. These flies carry the larvae and deposit them around the horse’s lips, nostrils and eyes as well as in wounds. Swallowed larvae develop into adults in the lining of the stomach, where they lay eggs which are passed in the feces. They are not commonly present in horses in the United States. In one study, they were present in only 3 to 13% of Thoroughbreds in Kentucky. Their presence in the stomach rarely causes problems, although a heavy infestation may cause stomach inflammation. Ivermectin administration is an effective treatment.

The major problem caused by stomachworms comes from larvae deposited around the eyes, causing persistent inflammation, or in skin wounds, causing skin sores sometimes referred to as “summer sores.” The larvae cannot penetrate normal intact skin, but when deposited in the wound they migrate, extending the wound, and prevent healing. Yellow granular type lesions develop and can enlarge rapidly. Diagnosis is generally made from the history and appearance of the lesions, but if necessary can be confirmed by finding larvae in a skin biopsy.

Treatment of stomachworm larvae is to administer ivermectin twice 3 to 6 weeks apart. This kills the larvae and leads to a rapid resolution of lesions. Large or secondarily infected lesions may require additional therapy, including removing excess granulation tissue and applying antimicrobial ointment and corticosteroids to decrease inflammation. Prevention of reinfection requires fly control and immediate wound care. Lesions may be covered and fly repellents applied to the covering. Topical pastes containing organophosphates and anti-inflammatory agents may be used instead. For inflammation around the eyes caused by stomachworm larvae, application of antibacterial and corticosteroid ointments to the eye several times daily is usually an effective treatment.

Less Prevalent Equine Internal Parasites

Internal parasites not sufficiently prevalent to warrant routine control measures, although they may occur in sufficient number to cause problems in some specific areas or situations, include hairworms, tapeworms, threadworms, lungworms, and Onchocerca cervicalis.

Hairworms

Hairworms (Trichostrongylus axei) occur in the stomach of horses and other species. They usually occur only in horses grazed or housed with ruminants. Adults, which live deep in the stomach lining lay eggs which are passed in the feces. Infective larvae develop from these eggs and are ingested. Usually hairworms cause few problems. However, heavy infestations can cause poor condition and anemia. Treatment is to administer an avermectin.

Threadworms

Threadworm (Strongyloides westeri) adults can live both in the ground and in the small intestine. Infective larvae from their eggs can be excreted in the mare’s milk. The larvae infect the foal by ingestion or by penetrating the foal’s skin. In the infected foal, larvae migrate into the blood, which carries them to the lungs. They are coughed up, swallowed, and become adults in the small intestine. Because they stimulate a sufficient immune response, they rarely occur in other than nursing foals. Adult worms in the small intestine cause inflammation and, in sufficiently large numbers, can cause diarrhea and unthriftiness in 2-to 3-week-old foals. Treatment is to administer ivermectin or oxibendazole to the foal. Threadworm infestation is prevented by administering ivermectin to the mare the day of foaling and to the foal at 2 to 3 weeks of age.

Tapeworms

Tapeworms (Anoplocephala spp.) are flatworms present in the intestine of 10 to 67% of horses in North America. Although they have been associated with severe gastrointestinal disease and death, and utilize ingested nutrients, they usually do not cause clinical signs. However, they may cause chronic unthriftiness, intermittent colic, diarrhea, progressive emaciation, and anemia. Occasionally, heavy infestations may cause acute problems due to (1) clusters of the worms obstructing the intestine, or (2) cecal perforations resulting in colic and death. Pyrantel pamoate administered at 2 to 3 times the normal dose is effective for the treatment and prevention of tapeworms. Benefit is optimized by treating 2 weeks prior to and at the conclusion of the grazing season.

Lungworms

Lungworm (Dictyocaulus arnfieldi) infected horses generally but not always have been exposed to donkeys, or pastures used by donkeys, within the past several years. Donkeys serve as a reservoir whose infective larvae are passed in the feces. Infection in donkeys is generally relatively harmless to them. In horses, however infection may cause airway inflammation and an accumulation of pus in the airways. Diagnosis is based on finding infective larvae in the feces or the parasite in the lungs. Affected horses should be separated from donkeys, removed from contaminated pastures, and administered ivermectin.

Onchocerca cervicales

O. cervicales is transmitted to horses by the bite of its intermediate host, the biting midge (Culicoides spp.), whose control is given in Tables 9-3 and 9-4. O. cervicales was found in the crest of the neck of 59% of Thoroughbreds in Kentucky, with the incidence being higher the older the horse. None were found in the ligaments or tendons of the legs. Although no clinical signs were present, some older horses had massive mineralization of caseous material around the parasite.

INTERNAL PARASITE CONTROL

To minimize parasite infestation and, therefore, reduce the likelihood of harm due to parasitic diseases, an internal-parasite control program must decrease the potential for the transmission of parasites between horses and their environment. To accomplish this objective requires the selection and utilization of effective dewormers along with sound management practices that enhance environmental control. Control programs should be developed based on climate, management practices, and economics for each specific farm. The program should be aimed at controlling (1) large and small strongyles for all horses, plus ascarids for yearlings and younger horses; (2) bots in all horses; and (3) other intestinal parasites that may be a problem in that farm, stable, or individual.

With a good internal-parasite control program, colic, anemia, diarrhea, poor growth or weight loss, reduced stamina, and reduced performance due to intestinal parasites are prevented. A good internal-parasite control program is particularly important for young horses. Young horses are generally more heavily infested than older horses, and horses affected early in life may have their entire future health and performance impaired.

There are four aspects of importance for the control of internal parasites in horses: (1) choice of dewormer (anthelmintic), (2) dewormer administration schedule, (3) nondewormer management practices, and (4) monitoring the results so the program can be modified as indicated.

Dewormer Drug Selection

A multitude of dewormers or anthelmintics and combinations of them are available in different forms for treating and preventing internal parasite infestation in horses. The many different dewormers currently available and recommended can be divided into five major chemically and pharmacologically similar classes: (1) avermectins, which currently include only ivermectin: and moxidectin, (2) organophosphates, (3) tetrahydropyrimidines, which currently include only pyrantel, (4) benzimidazoles and probenzimidazoles, and (5) piperazine, which is often used in combination with phenothiazine and benzimidazoles (Table 9-1). The spectrum of efficiency and resistance is similar for all the drugs in each of these five classes. Dewormers containing the same class of drug are called by different names by each manufacturer marketing them. Therefore rely on the name of the active drug listed on the label and not on the marketer’s name for the product.

A dewormer effective against the parasites you want to treat should be selected. The routine recommendation has been that a dewormer from a different class should be used on a rotational basis: i.e., a dewormer from a different class should be used than was used the previous time the horses on that farm were treated. It was believed that this procedure assisted in preventing the development of parasites resistant to the dewormers used. Recent studies in both horses and sheep, however, have demonstrated that alternating dewormer classes neither delays nor enhances the development of resistance to them. Consequently, as suggested, deworming treatment programs that alternate between dewormer classes, or that use the same dewormer for 1 year or as long as it is effective, appear to be equally acceptable procedures for the control of equine internal parasites.

Small strongyles resistant to piperazine, phenothiazinc, and all of the benzimidazole drugs have been reported. No other parasites have been reported to have developed a resistance to any of the other dewormers. Once resistance to a drug develops, it may be 3 years or more before that drug will again be effective against those parasites on that farm. Benzimidazole-resistant small strongyles are susceptible to benzimidazole + piperazine combination products and ivermectin, pyrantel, and the organophosphate dichlorvos.

Deworming medications may be added to the feed, given by stomach tube or put in the back of the horse’s mouth as a drench or in a paste form with a syringe. One method of administration is no more or less effective than another if all of the correct amount of the medication reaches the animal’s stomach within a fairly short period of time. Thus, dewormers added to the feed are effective only if all of the dewormer is consumed within a few hours or less.

Avermectins, one of the newest and most broad-spectrum class of parasite medications, have a wide margin of safety, including for pregnant and reproducing horses and for foals. They have a greater than 95% efficacy against parasites of the skin, respiratory tract, and blood, as well as the gastrointestinal tract. They are also effective against blood-sucking lice, flies, and mites as well as both the adult and migrating larvae of nearly all of the major internal parasites of the horse. In addition to its broad spectrum of activity, ivermectin suppresses fecal intestinal parasite egg excretion by the horse for 8 to 10 weeks following its administration, as compared to 4 to 5 weeks for all other nonavermectin dewormers currently available. As a result, it gives equal or better protection when administered one-half as often as other dewormers. However, avermectins are not effective against tapeworms (cestodes), trematodes (flukes), or eyeworms (Thelazia lacrymalis). Although avermectins don’t cause the death or detachment of ticks, they do prevent their molting and egg production.

TABLE 9-1 Deworming Drugs for Horses

It has been suggested from field observation that degradation of feces from ivermectintreated animals is slowed because its excretion in the feces may kill dung-degrading organisms. However, studies indicate this doesn’t occur. No difference was found in the rate of decomposition of feces from horses treated bimonthly for 2 years with ivermectin and those either untreated or treated with other dewormers. There was no difference in the amount of fecal-fouled pasture where horses were treated only with ivermectin as compared to pasture used by horses on a rotational deworming program.

Although most deworming medications, except cambendazole and organophosphates, are safe for use throughout pregnancy, it is best not to give any medication not required for the life of the horse during the last few weeks of pregnancy. If any problems occur at foaling, any medication recently administered may be blamed. Cambendazole, carbon disulfides, and organophosphates should not be given to the mare during the first 3 months of pregnancy because of possible, but unproven, inducement of teratogenicity. Organophosphate dewormers are also not recommended for foals.

Dewormer Administration Schedules

Three different types of dewormer administration schedules or programs have been recommended and used: interval, seasonal, and continual. Interval programs, if done properly, are effective for all horses. Seasonal programs should be used only for mature horses not on a good interval program. Continual dewormer administration is a good alternative for individual horses where all horses on the premises are not on the same or on an effective internal parasite control program.

Interval Deworming Programs

In the mid-1960s, year-round bimonthly administration of dewormers for all horses was proposed and has been widely recommended and used since that time. Less frequent dewormer administration has been recommended and used by some for nonbreeding farms with lower concentrations of less valuable horses. Following these recommendations maintains adequate internal-parasite control at some farms and stables, whereas at some it hasn’t provided satisfactory strongyle control or prevented environmental contamination with strongyle ova. At such farms and stables, satisfactory control was obtained by administering non-avermectin dewormers monthly or ivermectin every 2 months. This, as well as the effect of inadequate parasite control on colic incidence has been well demonstrated. In one study, a moderate incidence of colic was occurring even though a non-avermectin dewormer was being administered every 2 months. In horses given a non-avermectin dewormer monthly, and in those given ivermectin every 2 months, the incidence of colic decreased 61 to 93%, whereas there was no change in colic incidence in the other horses, who continued to be given a non-avermectin dewormer even 2 months.

Seasonal Deworming Programs

In addition to interval dewormer programs for horses of all ages, as described above, seasonal dewormer programs for mature horses have been developed to try to improve or maintain good internal-parasite control with less frequent administration of dewormers. The object of seasonal control programs is to administer dewormers at the right time to minimize fecal strongyle egg excretion and, therefore, pasture contamination, thus prolonging subsequent reinfestation of the horse. In cooler climates (e.g., the northern United States, the United Kingdom, and northern Europe), peak egg excretion occurs in the spring and summer. In warmer climates (e.g., the southern United States to Buenos Aires), pasture infectivity decreases during the hot summer months.

To prevent peak egg excretion in cooler climates, avermectin dewormers should be given just before grazing new spring pasture and again in the middle of the summer (e.g., in the northern hemisphere in May and again in July). Alternatively, a non-avermectin dewormer should be given twice. 1 month apart, beginning at each of the two times or monthly from spring through the middle of summer. In the fall following a freeze, a dewormer effective against bots should be given (e.g., an avermectin or an organophosphate).

In warmer climates, an avermectin dewormer should be given in late summer and early winter (e.g., in the southern United States, in August and October); instead, a non-avermectin dewormer should be given twice, 1 month apart, beginning at each of these two times, or monthly from late summer through early winter. Studies in Ohio and England found that this program resulted in a sixfold decrease in pasture infectivity, thus allowing the horse to graze in relative safety long after dewormers were administered.

Seasonal control programs should not be used for horses less than 1 year of age. Since adult and immature ascarids, as well as strongyle and bot control, are necessary in young horses, they should be given a dewormer every 2 months beginning at 6 to 8 weeks of age until 1 year of age. If the treatment interval is lengthened to even 10 weeks, there is a sharp increase in their fecal ascarid and strongyle egg excretion. To avoid treatment complications, the dewormer administered to young horses should not cause a sudden parasite kill, as occurs with organophosphates and piperazine. Slow-kill dewormers, such as ivermectin, pyrantel, or benzimidazoles, are less likely to cause complications. After foals are 1 year old, at which age they have developed an immunity to ascarids, they may be switched to a seasonal control program.

For bot control, the most important treatment is in the fall. In areas where it freezes, a dewormer effective against bots (e.g., avermectins or an organophosphate) should be given to all horses right after the first hard freeze, which will kill bot flies. It has been recommended that this fall bot treatment be delayed until 1 month after a hard freeze so that all ingested larvae will have reached the stomach. However, if a dewormer effective against migrating bot larvae is administered (e.g., avermectins or organophosphates), this 1-month wait isn’t necessary or recommended. In addition to giving a dewormer effective against bots, anytime bot eggs or nits (Fig. 9-3B) are present, they should be removed from the hair or killed by topical application of insecticides such as 0.06% coumaphos, 0.12% malathion, or 0.03% lindane. Ideally, or if bots are considered a major problem on that farm, additional bot treatments should be given bimonthly beginning 1 month after bot eggs first occur on the horse’s hair.

Continual Deworming Programs

A third intestinal parasite control program, besides interval and seasonal programs, is the continual feeding of low amounts of a deworming medication. However, many small strongyles developed a resistance against the dewormers used in this manner in the past, making them ineffective. One in which no parasite resistance has been demonstrated, and the only dewormer currently on the market recommended and found to be effective when fed continually, is pyrantel tartrate. It is available in alfalfa-flavored pellets (Strongid C, Pfizer). When fed continually at its recommended dosage, it is effective against both adult and larval stages of large and small strongyles, ascarids, and pinworms, and may prevent tapeworm infestation. Unlike dewormers used in interval or seasonal parasite-control programs, a continuously administered dewormer need not be given to all horses on the premises to maximize control in an individual. Thus, this is a beneficial alternative in boarding facilities where deworming is left up to individual owners rather than all horses being treated at the same time. Regardless of the control programs used by others at a facility, internal parasites, except bots, can be prevented in horses fed pyrantel tartrate daily. Before beginning this control program, a deworming dose of ivermectin should be administered. In addition, bot treatment as described previously for seasonal deworming programs is necessary since pyrantel tartrate is not effective against bots. One study suggested that feeding pyrantel tartrate daily decreased the risk of colic as compared to horses dewormed interchangeably with benzimidazoles and ivermectin an average of 5 times annually. Although in 1993 it cost about $ 15/month to feed the 1000-lb (454-kg) horse pyrantel tartrate daily, the company selling it claims, but without presenting supportive data, that pyrantel tartrate increases feed utilization and offsets its cost. Like all claims lacking valid supportive data, especially when a reason for a bias exists, skepticism is warranted.

Environmental Control of Internal Parasites

There are a number of management practices besides the administration of deworming medications that are beneficial in minimizing internal parasites in horses. One of the most important is to minimize ingestion of fecally contaminated feed and water. Almost all infected larvae are near a fecal mass. The concentration of infective larvae is 15 times higher in fecally contaminated areas of a pasture than in noncontaminated areas. Thus, decreasing fecal contamination of feed and water, and fecal proximity to grazing, is quite beneficial in minimizing the horse’s ingestion of infective larvae. Procedures to do this include the following.

Fig. 9-4. On arrival to a new farm, all horses should be vaccinated (as described in Table 9-5), receive a non-benzimidazole dewormer, and be kept isolated for a minimum of the first 7 and preferably 14 days. Most infectious diseases are contagious for 7 to 14 days before the appearance of clinical signs of that disease, and viable strongyle eggs are excreted for up to 4 days after an effective dewormer is administered. If a horse is from a recent disease outbreak area, 30 days of isolation is recommended. The isolation pen should be well away from the stables and other horses.

Monitoring Internal Parasite Control

To monitor the efficiency of an internal-parasite control program, fecal samples can be examined for the presence and number of intestinal worm eggs or oocysts that they contain. This is done as described in Figures 9-5 and 9-6. It should be done at least twice yearly, once before and once 7 to 14 days after a dewormer is administered. Fecal samples should be checked from at least 10%, or 6, of the horses in each age and housing or pasture group. If the control program is effective, fecal egg counts on all samples should be less than 50 eggs per gram (epg) or fewer than 10% of the fecal samples should contain parasite eggs. If this is not found, the control program should be modified accordingly. Over 80% of the post-treatment samples should be free of all parasite eggs, and there should be greater than a 90% reduction in pre-treatment to post-treatment egg counts. If this does not occur, it indicates ineffective treatment, either because the proper amount of deworming medication wasn’t received by the horse or because the dewormer used was ineffective, which may occur because of the development of parasite resistance to that dewormer.

Fig. 9-5. Fecal flotation to determine the presence of intestinal parasites. A small amount of feces (½ to 1 tsp or 2 to 5 g) is mixed in a salt or sugar solution in a test tube. A microscope slide cover slip is placed on top of the tube, in contact with the solution (A). Oocytes, or eggs from intestinal worms, if present in the feces, float to the top of the solution and adhere to the cover slip. The cover slip is placed on a glass slide and viewed under a microscope (B). If the amount of feces used is determined, the number of eggs per gram can be determined as an indication of the adequacy of that farm’s control program.

Internal-Parasite Control Program

The best internal-parasite control program for each farm or stable may be determined by the following procedures:

Fig. 9-6. Oocysts, or eggs of intestinal parasites, as viewed under the microscope at 100 × magnification. Shown here are oocysts from strongyles (A), ascarids (B), and pinworms (C).

EXTERNAL-PARASITE PROBLEMS AND CONTROL

In addition to internal parasites, there are numerous external parasites, or ectoparasites, that annoy farm horses and transmit numerous diseases. These diseases include encephalomyelitis (sleeping sickness), Rocky Mountain spotted fever, Colorado tick fever, tuleramia, Powassan encephalitis virus, anaplasmosis, piroplasmosis or babesiosis, equine infectious anemia or swamp fever, tick paralysis, and Lyme disease, whose incidence is increasing. Although less commonly affected by Lyme disease than dogs and people, horses can be affected, resulting in a reluctance to move and/or a transitory lameness with joint swelling sometimes apparent. Lyme disease is caused by a bacterium, spread by Ixodes ticks (e.g., deer or bear tick or western black-legged tick) when they are attached for 24 hours or longer. In addition to spreading these diseases, a horse may develop a localized reaction or infection, or an allergy to ticks or other external parasites, which can result in hair loss or hives (Fig. 1-5). Most external parasites are a problem for horses only during warm weather. Lice, mites, and some ticks, in contrast, are a problem primarily during the winter or early spring. This is because their life cycles occur entirely on the animal, whereas all or a portion of the life cycle of the other external parasites of the horse occur partially or totally off the animal. Factors helpful in the identification of external parasites, their effects on the horse, and nonpesticide control procedures are summarized in Table 9-2. Although nonpesticide control procedures are quite helpful, and for some external parasites necessary for their control, the proper use of pesticides is more important. The type of pesticides recommended for control of each external parasite are given in Table 9-3, and specific pesticides and fungicides recommended for use on horses are given in Table 9-4.

Pesticides have maximum efficiency only when applied to the horse and surrounding area at the right time. Knowing the right time requires knowing which insects are present. If an external parasite causing a problem can’t be identified, a specimen should be collected and shown to someone capable of identifying it, such as a veterinarian or diagnostic laboratory, university, or agriculture extension personnel. These individuals may also have the latest information on the seasonal occurrence of external parasites, species found in a given location, the best or approved methods and materials available for their control, recommended times of treatment, and other useful information on local external parasites.

Two approaches should be taken in controlling external parasites: 1) controlling them in the area where horses are kept, and 2) controlling them on the horse. By doing a thorough job of spraying the area and facilities, many external parasite problems can be minimized or eliminated. A product containing both an insecticide and an insect growth regulator (Table 9-4) not only to kill adults but also to inhibit egg hatching and larval development may be quite helpful or necessary to eliminate heavy infestations. Controlling external parasites on the horse often is more difficult than eliminating them on the premises. The control of many external parasites on the horse frequently requires at least daily application of a pesticide during peak insect season. However, too frequent an application of some pesticides induces a systemic toxic effect. Because of this, pyrethrin insecticides are frequently used for horses. Pyrethrins and other pesticides for control of external parasites are available in many different forms, including dips, sprays, dusts, wipes or smears, injections, rubbers, pourons, and feed additives. Horses have a very sensitive skin and may have a skin reaction to the solvents and other ingredients in a pesticide formulation. This occurs most commonly when they are treated with formulations not approved for application to horses, but may occur in some horses treated with approved products. Some formulations may cause burning, blistering, or cracking of the skin. Because of these possible effects, a product not previously used on a particular horse should be applied to only a small area of the horse and used thereafter only if there is no adverse reaction to it in the following 24 hours.

Fig. 9-7(A,B). Hair loss due to itching caused by lice infestation (pediculosis). Lice may also cause blood loss and poor condition. They occur particularly on debilitated horses and, like mites, whose life cycle (like that of fleas) occurs on the animal, but in contrast to other external parasites in which much of the life cycle occurs off the animal, lice are a problem primarily during the winter. See Table 9-3 for treatment and control.

Pesticides can also be toxic to the horse and the people who apply them, and can be destructive to the environment if they are not used and handled in a safe and correct manner. A few precautions to follow when choosing and applying pesticides for the control of external parasites of horses are the following:

INFECTIOUS-DISEASE PROBLEMS AND VACCINATION PROGRAMS

Preventing infectious diseases in horses requires in addition to a good vaccination program isolating all incoming horses as previously described (Fig. 9-4). On broodmare farms, new horses should not be introduced into the resident population, but instead kept separately. If an outbreak occurs, healthy horses in contact with sick horses should be considered potential incubators and sources of the disease. Because of this, in order to minimize the spread of the disease, no horses should be moved to other barns or sites on the farm. In addition, people working with diseased or exposed horses should either not handle other horses on the farm or should disinfect themselves and change clothes before doing so.

Fig. 9-8(A,B,C). Ringworm (dermatophytosis or girth-itch). Three different-appearing skin lesions, all of which were due to ringworm. Be careful not to use brushes, combs, or tack from an affected animal on another animal, as infection is readily spread in this manner. For diagnosis, treatment, and control see Table 9-2.

A good vaccination program for the control of bacterial and viral diseases, like a good parasite control and feeding program, is an essential aspect in the care of the horse. A good vaccination program will vary depending on numerous factors, including: (1) disease prevalence in that area and farm, (2) degree of confinement, (3) number of horses, (4) what the horses are used for, and (5) frequency of contact with other horses. Because of these and other variables, and the continual development of new vaccines, a vaccination program should be set up that is adapted as needed for each farm, stable, or situation. Regardless of the vaccination program used, all horses on a farm should be on the same program and schedule when possible. This will maximize herd immunity and thus minimize infectious-disease challenge, thus protecting those that may respond poorly to vaccination. Manufacturers’ recommendations for vaccine storage, handling, and administration should be followed to maximize the vaccine’s efficacy.

Diseases for which vaccination should be considered include tetanus, encephalomyelitis (sleeping sickness), influenza, rhinopneumonitis (equine herpes virus or EHV), strangles or distemper (strep equi), rabies, anthrax, botulism, equine viral arteritis, salmonellosis, Potomac horse fever, (monocytic ehrlichiosis) and, in broodmares on farms with a high incidence of foal septicemia, Clostridium perfringens type C and D toxoids. A minimum vaccination program for all horses includes tetanus toxoid and encephalomyelitis every spring (Table 9-5). More frequent vaccination and vaccination for influenza, rhinopneumonitis, and strangles are indicated in many situations (Table 9-5).

Acute allergic reactions to vaccines are uncommon but are a life-threatening emergency requiring prompt administration of epinephrine. Local irritant tissue reactions to vaccines are much more common. These reactions usually resolve without treatment, but oral administration of anti-inflammatory drugs, topical application of warm compresses or drawing agents, and gentle exercise may be helpful. If the neck is affected, the horse may be reluctant to lower its head to eat or drink, and feed and water should be positioned accordingly. Horses that repeatedly react to a vaccine may benefit by prior administration of non-steroidal anti-inflammatory drugs, or the use of a different injection site or a different brand of vaccine.

TABLE 9-2 External Parasites of Horses Identification, Effects and Nonpesticide Control^a

Tetanus

Tetanus (lockjaw) is caused by neurotoxins produced by the bacteria Clostridium tetani, particularly in deep or closed wounds such as puncture wounds, but may occur, due to intestinal infections. This highly fatal disease is a constant threat for all horses as the bacteria are widely distributed in soil and manure. The disease affects all animals, with the horse being particularly susceptible.

Vaccination of all horses for tetanus is recommended, as given in Table 9-5. For horses not previously vaccinated or with an unknown vaccination history, tetanus toxoid should be given twice, 1 to 2 months apart. Protection is attained within 2 weeks after the second injection and may persist, but not necessarily at protective levels, for as long as 5 years. For all horses, yearly revaccination with the toxoid is recommended, as it is following an injury. The broodmare should be given the toxoid prior to breeding and again 1 to 2 months before the expected foaling date. The foal should receive an adequate amount of the vaccinated mare’s colostrum and be vaccinated with the toxoid at 2, 4, and 6 months of age.

Tetanus antitoxin is indicated for the injured horse that is not known to have been vaccinated with the toxoid in the past year. It provides immediate protection which lasts for about 2 weeks. Following injury, both antitoxin and toxoid should be administered to unvaccinated horses or those with an unknown vaccination history. Following foaling, both antitoxin and toxoid should be administered to unvaccinated mares and the antitoxin administered to her foal. After receiving the antitoxin, the horse should be revaccinated with the toxoid as described in the previous paragraph.

Administration of tetanus antitoxin, but not toxoid, carries the risk of inducing serum hepatitis 3 to 9 weeks following its administration. Most cases of serum hepatitis, even with treatment, are fatal. A proper tetanus vaccination program eliminates this risk, as well as greatly decreasing the risk of tetanus, because antitoxin administration then is not needed or recommended.

TABLE 9-3 External Parasites and Type of Pesticides Recommended for Their Control on Horses^a

^a See Table 9-2 for Nonpesticide control procedures and Table 9-4 for specific pesticides recommended for horses.

^b Corticosteroids are helpful in decreasing itching. Tar and sulfur shampoos or sprays formulated for animals are also helpful. Lotions or oily preparations or a sheet covering affected areas to provide mechanical barriers to gnats may be helpful if used before onset of severe itching. Densensitization or hyposensitization is not helpful.

Clinical signs of tetanus in affected horses begin following an incubation period of 3 days to several weeks, but usually 1 to 2 weeks. Muscle spasms and a stiff paralysis progressing from the head to the neck, front, and then hind legs occurs. The jaw muscle is the most commonly affected muscle, giving rise to the name lockjaw for the disease. Prolapse of the third eyelid, as well as eyelid retraction, erect ear carriage, flared nostrils, and hyperesthesia, are the most consistent clinical signs. These are usually followed by stiffness, resulting in a sawhorse stance, and reluctance to feed from the ground. The horse may be unable to swallow, resulting in saliva dripping from the mouth and regurgitation of ingested food and water through the nose. Convulsions, muscle rigidity, sweating, and fever may follow. Colic, urine retention, and difficult respiration may occur. About 50 to 75% of affected horses die within 2 to 10 days after the onset of symptoms, usually as a result of an inability to breathe. For those surviving, clinical signs may be apparent for up to 6 weeks, and founder, muscle damages aspiration pneumonia, and decubitus ulcers may occur.

Treatment of tetanus includes early administration of tetanus antitoxin and an antibiotic, and cleaning the infected wound. Tranquilizers may help alleviate muscle spasms, paralysis and pain. Supportive care is very important and should include housing in a dark, quiet stall with padding and thick bedding to minimize injuries. Slings or bales of hay may be necessary to help keep the horse standing or lying on its sternum. Urinary catheterization and frequent manual removal of feces may be necessary.

TABLE 9-4 Pesticides Recommended for External Parasite Control on Horses^a

^a Categories may overlap, e.g., some pyrethrins are insecticidal, repellent, and botanical. For all, follow product label directions closely. Many sprays require 4 to 8 oz (120 to 240 ml) per horse to give good residual control.

^b These inhibit egg hatching and larval development which may be necessary to eliminate heavy animal and premise infestations. They are usually combined with an insecticide such as pyrethrin for use on animals and chlorpyrifos for premise treatment (e.g., Siphotrol Plus II House Treatment, Vet-Kern, Zoecon Corp, Dallas, Texas).

^c 1 oz of 50% wettable powder in 1 gallon of water (10 g/l.) of Orthocide (Ortho Products, San Francisco).

Equine Encephalomyelitis (Sleeping Sickness)

Equine encephalomyelitis or encephalitis is caused by three antigenically different arbovirus strains: Eastern (EEE), Western (WEE), and Venezuelan (VEE). Wild animals and birds, which are not affected by the viruses, serve as their reservoirs. The viruses are transmitted to horses and people by blood-sucking arthropods, mainly mosquitoes. Disease therefore occurs primarily during the season in which mosquitoes occur. VEE is the only virus of the three that is transmitted from one horse to another. It can also be transmitted by insects to people and other animals. Although it’s possible for EEE, neither it nor WEE is transmitted from horses or people to other horses or people; i.e., a horse or person cannot get EEE or WEE, but can get VEE from other horses or people.

Prevention of equine encephalomyelitis is to vaccinate twice, 2 to 3 weeks apart, with annual revaccination 2 weeks to 2 months before blood-sucking-insect season begins, as summarized in Table 9-5. Vaccines are available that contain all three viral strains, just EEE and WEE, or only VEE. There is increased specific antibody production to all viruses when bivalent or trivalent vaccines are administered. The response to VEE vaccination alone is lower in horses previously vaccinated against EEE and WEE. Vaccination is recommended against all strains to which a horse may be exposed, whether that exposure is from being in an area where that strain has occurred or being exposed to horses (for VEE), or other animals or birds that have been. In the United States and South America, clinical disease due to EEE occurs primarily, but not exclusively, in the east. WEE in the west, and both in the center; whereas VEE, along with both EEE and WEE, occur primarily in Mexico, Central America and northern South America. In 1962 a severe epidemic of VEE occurred in Venezuela and Colombia infecting and killing countless horses, and infecting 300,000 people, over 300 of whom died. The last occurrence of VEE in the United States was in 1971, but an outbreak occurred in Chiapas, Mexico in July 1993.

TABLE 9-5 Equine Vaccination Program^a

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