Health-management Interaction: Horses: Introduction

Proper management can reduce the incidence of many disease conditions in horses. Informed management of the environment and diet, routine foot and dental care, and adherence to an appropriate deworming and vaccination program form the basis of a preventive health program. Client education is important for compliance. Owners are more likely to follow recommended changes in husbandry programs once they appreciate the advantages: diet manipulation reduces the incidence of certain types of colic and exercise-induced myopathies, good dental care improves feed utilization, minimizing exposure to barn dusts and molds reduces the risk of chronic obstructive pulmonary disease, and individually designed deworming and vaccination programs reduce morbidity and mortality rates due to parasitism and infectious disease.

Housing:

Stabled horses are exposed to numerous respiratory and GI pathogens, including viruses, bacteria, mold spores, dust mites, and parasites. Stable environments affect disease transmission in terms of air quality and ventilation, population density, and general cleanliness. Barns should be constructed to optimize ventilation and light, minimize exposure to dust and molds, provide temperature regulation, facilitate cleaning and disinfection, and provide ample space for each horse. Windows and skylights provide sunlight and natural ventilation. Sunlight is a potent killer of many bacteria and viruses; it also promotes coat shedding and regular estrous cycles. Eight air changes per hour is considered adequate ventilation for temperate climates and average humidity. (See also ventilation, Ventilation: Introduction.) Ceiling or wall-mounted fans can be used to increase air circulation on hot humid days. Stall doors that are open at the top or made of heavy mesh screening provide better ventilation. Stalls should have nonslip flooring and walls or partitions that prevent direct contact between horses in adjacent stalls. Suggested stall dimensions for adult horses and mares with foals are 3.6 × 3.6 m and 5.0 × 5.0 m, respectively. Doorways should be at least 2.4 m high × 1.2 m wide.

Recurrent airway obstruction (chronic obstructive pulmonary disease, Recurrent Airway Obstruction) is associated with airway hypersensitivity to environmental allergens and irritants. The most commonly incriminated allergens are fungal spores and pollens. The population of spores increases with the dustiness of the barn, bedding, and forage. Management changes that help prevent this condition include substituting wood chips, peat moss, or shredded paper for dusty straw bedding; avoiding dusty concentrates; using shallow rather than deep feed containers; and soaking hay before feeding at ground level. Air quality can be improved further if bedding and feed are not stored above the stalls. Riding areas and the dust they generate should be situated away from stalls to reduce exposure to dust.

Regular disinfection of stables and feed and water buckets helps reduce persistence of infectious agents in the environment. Organic debris inactivates most chemical disinfectants; therefore, disinfection should begin with physical cleaning (ie, hosing, scrubbing) of all surfaces followed by chemical disinfection. Phenols, quaternary ammonium compounds, and chlorine are the most commonly used disinfectants. To further reduce spread of infectious disease, stalls should have walls or partitions to prevent direct contact between horses in adjacent stalls. Pregnant mares, mares with foals, and weanlings should be kept separate from yearlings and adult horses. Ideally, new arrivals should be isolated from the resident horse population for 30 days to reduce introduction of contagious diseases, including respiratory viruses and bacterial infections (eg, influenza, rhinopneumonitis, and Streptococcus equi ).

Feeds should be stored in dry containers to reduce contamination with molds and animal excreta. Moldy hay and silage feeding have been associated with cases of equine botulism. Opossum feces can transmit infective sporocysts of Sarcocystis neurona , the causative agent of equine protozoal myeloencephalitis ( Equine Protozoal Myeloencephalitis: Introduction). Contamination of feeds by deer urine has been incriminated in the spread of certain strains of Leptospira ( Leptospirosis: Introduction).

Pasture:

Ensuring that horses have ample time on good quality pasture provides optimal ventilation, a source of good forage, and the opportunity to graze and exercise. Exercise improves condition, prevents boredom-related abnormal behaviors (eg, cribbing and weaving), and reduces the risk of large-intestinal impactions. Grazing also helps reduce the incidence of gastric ulcers. Reducing the time spent in poorly ventilated barns reduces exposure to many inhalant allergens incriminated in the development of recurrent airway obstruction. Access to good forage provides a natural source of vitamins and fiber. If horses are fed in groups, sufficient space should be allowed to minimize competition and to ensure even the most submissive horse has access to an adequate diet. Feeding hay and grain in elevated feeders off the ground reduces ingestion of sand, infective parasite eggs, and animal excreta.

Safe, durable fencing should be used for pastures and paddocks to reduce the risk of self-trauma. Double fencing between paddocks minimizes transmission of contagious disease between horses. Overcrowding should be avoided. Overgrazed pastures, which result from overstocking, lead to extremes in ground conditions (eg, dust or mud), contribute to increased parasite burdens, and favor overgrowth of potentially toxic plants. Excessive dust increases the risk of respiratory infections among young horses by inhalation of soil saprophytes, such as Rhodococcus equi ( Rhodococcus equi pneumonia). The risk of this potentially fatal bacterial pneumonia on farms where the disease is enzootic can be reduced by minimizing exposure of young (<4 mo old), susceptible foals to aerosolized R equi using environmental control strategies such as decreasing dust formation on pastures and paddocks, housing foals in well-ventilated areas, rotating pastures, reducing the size of mare-foal bands, irrigating and planting dirt areas with grass, and removing feces frequently from stalls, paddocks, indoor arenas, and pastures. Breeding mares earlier in the season to ensure foaling during colder weather may reduce the number of susceptible foals exposed to dry, dusty summer conditions. On farms where R equi is endemic and foal morbidity and mortality rates are high despite attempts at pasture management, the incidence of disease can be reduced with preventive administration of one dose of hyperimmune plasma containing high concentrations of antibodies against R equi to newborn foals within the first week of life followed by a second dose 25 days later.

Overstocking in barns and pastures favors outbreaks of other contagious respiratory infections caused by viral and bacterial pathogens spread between horses via aerosolization of respiratory tract secretions. Enteric infections with Clostridium difficile and C perfringens can become endemic on some farms. An increased incidence of Clostridium diarrhea in newborn foals has been associated with foaling on dirt, gravel, or sand surfaces, and stall confinement or limited turnout on dry lots during the first 3 days of life.

Whenever possible, horses should not be pastured on sandy soils because sand ingestion during grazing predisposes to colonic and rectal impaction, chronic diarrhea, and weight loss. If sandy pastures are unavoidable, the risk of sand colic can be reduced by feeding psyllium and by providing trace mineralized salt with equal parts of bone meal.

Horses grazing on pastures near water may be at increased risk of contracting certain diseases such as Potomac horse fever, caused by Ehrlichia risticii and disseminated by aquatic insects and snails ( Potomac Horse Fever). Pastures should be kept free of standing or stagnant water to reduce the breeding grounds for mosquitos carrying West Nile virus, another equine pathogen ( West Nile Encephalomyelitis: Introduction).

Nutrition:

Diet plays a role in the health of the horse from birth through old age and is an often overlooked method of disease control. (See also nutrition: horses, Nutrition: Horses: Introduction.) In young horses, developmental orthopedic disease ( Developmental Orthopedic Disease: Overview) is the result of rapid growth, trauma to articular cartilage or growth plates, genetic predisposition, and nutritional imbalances. Dietary management involves regulating energy intake to avoid excessive rates of growth and weight gain. The proper balance of protein, calcium, phosphorus, zinc, and copper is important in supporting healthy endochondral ossification and in stabilizing bone collagen and elastin synthesis. The amount of nutrients required in the diet for normal bone development are dictated by rate of growth. Excessive energy intake contributes to osteochondrosis by decreasing bone density and cortical thickness.

Deficiency of protein must be severe to interfere with endochondral ossification. Rapidly increasing protein intake may produce faster bone growth; however, if the diet lacks adequate minerals to support this increased growth, altered endochondral ossification can be seen. Calcium and phosphorus balance affect bone density, rate of growth, and cartilage thickness. Inadequate amounts of copper and zinc have been associated with an increased incidence of osteochondrosis and osteodysgenesis.

Some of the most common mistakes made when feeding young horses include feeding excessive grain and leafy legumes (eg, alfalfa, which results in too high an energy intake), feeding a diet with too little zinc or copper to support rate of growth, and feeding a diet with an improper calcium:phosphorus ratio. Cereal grains and grass forages are low in calcium, phosphorus, protein, and lysine. Excess energy from cereal grains may be more detrimental than excess energy from grass forages; one reason may be that energy from grain is derived from starch, whereas energy from grass forage comes from microbial production of volatile fatty acids. Starch, but not volatile fatty acids, stimulates insulin secretion, which has been implicated in stimulating hormone changes that contribute to ostechondrosis. Older horses often have dental problems that compromise feed intake and mastication. Extruded or soft pelleted feeds are ideal. Hay should be good quality, leafy, and easy to chew.

Diet manipulation can help treat, control, and prevent other disease conditions. Horses with recurrent airway obstruction should be fed as dust-free a feed as possible. Adding water or oil to grains decreases dust. Hay should be thoroughly soaked and fed close to the ground. If complete pelleted feeds are fed, hay can be removed completely from the diet. On sandy soils, hay should be fed off the ground to reduce sand ingestion. Dietary management can be used to reduce the risk of gastric ulcers. Alfalfa hay, with its high calcium and protein concentration, acts as a buffering antacid and has a protective effect on the nonglandular squamous mucosa. Small hay meals fed frequently or access to pasture also reduces the risk of gastric ulceration.

Nutritional management for Quarter Horses with hyperkalemic periodic paralysis is focused on decreasing dietary intake of potassium and increasing renal potassium losses. Dietary manipulation includes avoiding high potassium feeds such as alfalfa hay, brome grass, canola oil, soybean meal or oil, and sugar or beet molasses and replacing them with timothy or bermuda grass, beet pulp, and grains such as oats, corn, wheat, or barley. Affected horses should be exercised regularly and have access to pasture.

Heavily muscled breeds of horses including Quarter Horses, draft horses, and warmbloods are prone to myopathies associated with elevated muscle glycogen stores and polysaccharide storage inclusions in type II muscle fibers. Successful management of this condition, known as polysaccharide storage myopathy, focuses on increasing the fat content of the diet and on eliminating or reducing grain intake.

Stall confinement, poor-quality or high-fiber feed, inadequate water intake, and ingestion of foreign material (eg, rubber fencing) predispose to intestinal impaction. Management practices to reduce the risk of impaction include ad lib access to fresh water (warm water may be preferred during cold weather), adequate exercise, good quality feed, and good dental care. If impaction has been a problem, poorly digestible feeds (eg, mature forages) should be placed with low-fiber, highly digestible forages (eg, growing grass or legume hays). A complete pelleted or extruded feed helps maintain soft feces.

Foot Care:

Proper foot care is essential. Corrective trimming and shoeing maintains proper foot balance and helps reduce the risk of laminitis, thrush, white line disease, and hoof cracks. In young foals and weanlings, feet should be examined and trimmed regularly to ensure a proper weight-bearing axis. A balanced foot in a young horse minimizes the risk of angular limb deformities. (See also lameness in horses, Lameness in Horses: Introduction.)

Dental Care:

Routine dental care reduces the incidence of dental disease. Sharp teeth and uneven dental wear predispose to tongue and cheek lacerations, poorly masticated feed, decreased feed intake, choke, and weight loss, and may contribute to mouth pain when the horse is bridled. Frequent dental examination allows early detection of tooth root abcessation before more serious complications (eg, sinusitis) develop. (See also dentistry in large animals, Large Animals: Overview .)

Parasite Control:

Parasites and the damage they cause to the intestinal vasculature and mucosa contribute to GI disturbances, including impactions, motility disorders, diarrhea, and peritonitis. Migrating ascarids contribute to pulmonary inflammation in foals. The principal internal parasites of horses are nematodes. With the advent of more effective anthelmintics, the nematode species of importance have shifted from large strongyles to cyathostomes. There is no single parasite control program that is ideal for all horses. Age of the horse, population density, and pasture size and quality can affect choice of programs. As horses age, they develop resistance to reinfection with certain parasites, such as Strongyloides westeri and Parascaris equorum . Resistance to other strongyles is incomplete.

The 3 major classes of anthelmintics commonly used are avermectins and milbemycins, benzimidazoles, and pyrimidines. Avermectins have a broad range of activity, are effective at low dosages, suppress fecal egg counts for long periods of time, and are active against adult and migrating larval nematodes and various ectoparasites including lice, mites, ticks, and bots (eg, Gasterophilus spp ). Treatment of mares with ivermectin within 24 hr of foaling greatly reduces transmission of S westeri to foals via the dam’s milk. Prompt deworming of the mare soon after delivery minimizes the newborn foal’s potential exposure to parasite ova in the mare’s manure as a result of coprophagy. Due to their efficacy against adult and migrating roundworms, avermectins should be included in the foal’s anthelmintic schedule. Avermectins are not effective against tapeworms ( Anaplocephala ), Onchocerca adults, flukes, or encysted cyathostome larvae. Milbemycins are effective against encysted cyathostome larvae. Benzimidazoles are effective against most nematodes; however, cyathostomes have developed resistance to this class of anthelmintic. Increased dosages of this drug class are required to kill migrating immature large strongyles and encysted cyathostomes. The pyrimidines have reasonable efficacy against nematodes and good activity against tapeworms when used at increased dosages. A new combination anthelmintic, praziquantel/ivermectin, has demonstrated excellent efficacy against field strains of equine cestodes (ie, tapeworms) in addition to most nematodes. An effective deworming protocol for horses should incorporate ivermectin/praziquantel or a double dose of pyrantel pamoate during the fall to remove tapeworms and a larvicidal dose of anthelmintic (eg, 10 mg/kg fenbendazole, PO, sid for 5 days, or a single dose of moxidectin) during the winter months to remove encysted cyathostome larvae. (See also gastrointestinal parasites of horses, Gastrointestinal Parasites of Horses.)

Vaccination Program:

The goal of vaccination is to develop and maintain both individual and herd immunity against infectious diseases. Commercial vaccines are available for rabies, encephalomyelitis (Eastern, Western, and Venezuelan), tetanus, influenza, equine herpesviruses 1 and 4, botulism, equine ehrlichiosis (Potomac horse fever), equine viral arteritis, rotavirus, West Nile virus, equine protozoal myelitis, and Streptococcus equi (strangles). Vaccination programs are formulated based on the animal’s age, use, and level of exposure. Broodmare vaccination is important to provide active immunity for the mare and passive immunity for the foal via transfer of colostral antibodies. Vaccination guidelines for foals have been modified due to the interference of maternal antibodies with the initial vaccination response. In most instances, the primary vaccination series includes 3 doses.

The following vaccination recommendations assume that foals are born to vaccinated mares and have absorbed adequate colostral antibodies with IgG levels >800 mg/dL .

Tetanus:

Recommended for all foals and horses. Initial vaccination at 6 mo of age, with a 3-dose series at 4- to 6-wk intervals, followed by annual boosters. Broodmares should receive a booster 4-6 wk before foaling. A horse with an unknown vaccination status that sustains an injury should receive a dose of tetanus antitoxin along with a dose of tetanus toxoid. A second dose of toxoid should be given 4 wk later.

Rhinopneumonitis (Equine Herpesvirus 1 and 4):

Recommended for all foals and horses. Initial vaccination begins at 5 mo of age followed by 2 more doses at 4- to 6- wk intervals. Young horses are most susceptible and should be vaccinated at 3- to 4-mo intervals. Pregnant mares are vaccinated against EHV-1 during months 3, 5, 7, and 9 of gestation and 4-6 wk before foaling.

Encephalomyelitis (Eastern and Western):

Recommended for all foals and horses. Initial vaccination between 4 and 5 mo of age (3-4 mo of age in highly endemic areas), with a 3-dose series at 4- to 6-wk intervals, followed by semiannual boosters. Broodmares should receive a booster 4-6 wk before foaling.

Influenza:

Recommended for all foals and horses. Due to the persistence of maternally derived antibodies, initial vaccination using the IM vaccine should begin at 9-10 mo of age followed by 2 additional doses given at 4-wk intervals. If the intranasal vaccine is used, a single dose can be administered at 9 mo of age. Pregnant mares should receive an annual IM booster 4-6 wk before foaling. If the mare was not vaccinated during the last trimester of her pregnancy, then the 3-dose vaccination series for her foal can begin as early as 5 mo of age, with subsequent doses given at 4- to 6-wk intervals. Young performance horses should be vaccinated every 3-4 mo. Adult horses are usually vaccinated annually.

Rabies:

Recommended for foals and horses in areas where rabies is prevalent. Vaccination of all horses should be encouraged. Initial vaccination should begin at 6 mo of age, followed by a second dose at 7 mo and a booster at 1 yr of age, followed by annual boosters. Broodmares can receive a booster before breeding or 4-6 wk before foaling.

Potomac Horse Fever:

Vaccination is suggested in areas where the disease is endemic. Initial vaccination can begin at 5-6 mo of age, followed by a second dose in 3-4 wk and a booster at 1 yr of age. Annual boosters in the spring are recommended. Pregnant mares should receive a booster before foaling.

West Nile Virus:

Vaccination of all foals and horses in the continental USA is recommended. Initial vaccination can begin at 5 mo of age followed by a second dose in 4 wk. A booster should be administered in late summer for foals born early in the year. Pregnant mares should receive a booster 4-6 wk before foaling.

Botulism:

Vaccination is recommended for horses in the mid-Atlantic states and other regions of the USA where the disease is common. Initial vaccination involves a series of 3 doses administered at 4-wk intervals followed by annual boosters. Foals from vaccinated mares can begin their primary vaccination series at 5 mo of age. Broodmares that have never been vaccinated should receive an initial series of 3 doses administered at 4-wk intervals during the last trimester, followed by annual boosters administered 4-6 wk before foaling.

Strangles:

Use of this vaccine is restricted to farms where strangles is endemic. Initial immunization with the IM vaccine involves a 3-dose series administered 4 wk apart beginning at 5 mo of age. If the intranasal vaccine is used, vaccination can begin at 11 mo of age with a second dose given at 12 mo and annual boosters thereafter. Broodmares on endemic farms should receive an annual booster 4-6 wk before foaling.

Rotavirus:

On farms where foal rotaviral diarrhea is a problem, pregnant mares should be given a 3-dose series at 3- to 4-wk intervals, during the last trimester of pregnancy. Foals obtain passive immunity through absorption of colostral antibodies.

Foals with failure of passive antibody transfer (ie, IgG levels <200 mg/dL) and/or foals born to unvaccinated mares can receive their initial vaccination for equine herpesvirus 1 and 4, tetanus, and Eastern and Western equine encephalomyelitis in a 3-dose series beginning at 3-4 mo of age. These foals can receive their first dose of rabies vaccine at 3 mo of age, followed by a booster at 12 mo. Influenza vaccination can be started at 9 mo of age. Foals born to mares that have never been exposed to or vaccinated against West Nile virus can receive their first vaccination at 3 mo of age.

Perinatal Mare and Foal Care:

During the last trimester, pregnant mares should be on a slowly rising plane of nutrition that includes a concentrate containing at least 12-14% protein. The diet should be balanced in calcium, phosphorus, copper, and zinc. During the immediate prepartum period, bran or psyllium can be added to the diet as a laxative to keep feces soft and to reduce the risk of postpartum impactions.

In addition to routine deworming throughout pregnancy, mares should be dewormed within 24-48 hr of parturition to reduce the foal’s exposure to parasite ova. Mares should receive annual booster vaccinations 4-6 wk prepartum to improve colostral quality. In addition to producing antibodies in response to vaccination, mares produce antibodies to pathogens indigenous to their environment. Because colostrum is produced during the last 3-5 wk of pregnancy, mares should not be moved to new premises during the immediate prepartum period, and mares and foals should remain in the foaling environment during the early postnatal period.

Prior to parturition, the mare’s udder and perineum should be cleaned and fresh bedding added to the stall to optimize the hygiene of the foaling environment and reduce the neonate’s exposure to bacterial pathogens. Ideally, the flooring in the foaling stall should be easy to disinfect. Immediately after birth, the foal’s umbilicus should be dipped with either 2% iodine or chlorhexidine disinfectant to decrease the risk of ascending umbilical remnant infection and septicemia. A gravity enema is often administered prophylactically to stimulate meconium passage and reduce the likelihood of meconium impaction. Early colostrum ingestion is essential to ensure adequate absorption of colostral antibodies and to stimulate early gut closure and reduce uptake of potentially pathogenic bacteria via translocation across the intestinal mucosa. (See also management of the neonate, Management of the Neonate.)

Foals are born virtually without gamma globulins and rely on passive immunity through absorption of colostral antibodies for protection against common equine infectious diseases during the first 4-8 wk of life. Intestinal absorption of colostral antibodies is optimal during the first 10 hr of life and ceases after 24 hr. A newborn foal’s serum IgG concentration should be > 800 mg/dL after colostrum ingestion. Foals with <400 mg/dL of serum IgG should be supplemented with additional colostrum if <18-20 hr old, or given a plasma transfusion if >20 hr old or if compromised GI function is suspected.

In general, foaling mares should be kept in an environment where they can be monitored frequently both before and after foaling. Attended foalings are mandatory to help avoid foal loss due to correctable periparturient problems (eg, dystocia, failure to rupture fetal membranes normally, and maternal rejection or aggression).

See also feeding the orphan foal, Feeding the Aged Horse and the Orphan Foal .