It is widely recognized that internal parasitism adversely affects the health and performance of horses. Parasite control is considered an essential feature of any equine management program, but too often critical assessment of parasite control efforts is replaced by mechanical implementation of a generic parasite control program. The result is complacency. This article will discuss the impact of parasitism, benefits of control, and major features of an effective parasite control program for horses.

Biology of Common Parasites
Before discussing parasite control, it is essential to understand some biological characteristics of internal parasites and how they interact with horses. Parasites are different than other infectious organisms because, unlike bacteria and viruses, they cannot multiply within the body of the animal they infect (i.e., the “host”). Instead, each generation of parasites must leave the host as reproductive products (usually eggs) and pass into the environment, where they develop into an infective form. Because developing parasites are exposed to prevalent climatic conditions during this phase of their development, parasite transmission often follows distinct seasonal patterns within geographic regions.

Over five dozen species of internal parasites are known to infect horses. Most of these are nematodes (worms) which are specific to horses and cannot be transmitted to other animals or to humans. Certain nematode parasites, such as thread-worms (Strongyloides westeri) and roundworms (Parascaris equorum), are ultimately eliminated by acquired immunity (immunity that develops from having an infection) and are rarely seen in horses over six and 15 months of age, respectively. Another group, known as the large strongyles (Strongylus spp.), was historically important because its members can cause the greatest damage to horses on a “per worm” basis. The prevalence of large strongyles in North America has decreased markedly in recent years. Frequent treatment has eradicated large strongyles from most well-managed horse herds.

Currently, the major internal parasites of mature horses are a homogeneous group of worms known as the cyathostomes (or small strongyles). Adult cyathostomes live in the large bowel. Females lay eggs which are carried into the environment via the horse’s manure. Under conditions of favor-able temperatures (45°F to 85°F), moisture, and oxygen, cyathostome eggs hatch and eventually develop into infective, third stage larvae (L3s). Horses become infected with cyathostomes by grazing forage contaminated with infective larvae.

Once inside the horse, L3s are carried to the large bowel where they invade tissues and begin a process of gradual maturation. After intervals ranging from five weeks to 2.5 years, mature cyathostomes emerge from the gut tissues and begin to lay eggs. The transmission cycle repeats itself once or twice annually with seasonal fluctuations. Because cyathostomes are the major parasites of concern in well-managed, mature horses, the remainder of this article will focus on their impact and the potential benefits of controlling transmission of cyathostomes. Nevertheless, many of the subsequent comments are applicable to other types of equine parasites as well.

Consequences of Parasitism
Internal Internal parasitism adversely affects horses and the horse industry in various ways, but the negative consequences can be described in two general categories: health and economics.

HEALTH
Cyathostomes compromise the health of the host differently as each new life cycle stage develops within the horse. For instance, recently ingested L3s cause inflammation when they penetrate the gut lining. After further development within the host, the same worms (now fourth stage larvae or juveniles) can cause severe, focal damage when they emerge from tissue cysts (protective sacs the larvae live in for weeks to several months) and release inflammatory waste products into the host’s tissues. Finally, mature cyathostomes fasten their mouths to the gut lining and ingest tissue fluids containing protein. More damage is probably inflicted when cyathostome numbers are high. The health consequences of internal parasitism in horses can be classified as clinical or subclinical disease: Clinical Disease—One definition of a clinical disease is an obvious abnormality that can be diagnosed by observation. Common clinical conditions caused by equine parasitism include weight loss or poor growth, rough hair coat, lethargy (sluggish), diarrhea, anorexia (off feed), and colic (abdominal pain).

Clinical diseases - also include physiologic abnormalities that can be measured by a single procedure, such as fever, anemia (a deficiency of the oxygen-carrying component of blood), hypoproteinemia (a deficiency of protein in the blood), and changes in blood cell components that accompany inflammation.

The mechanisms of clinical cyathostome disease are complex and largely unknown, but some common signs can be explained simplistically. Diarrhea, for example, results if the secretory activity of the gut is stimulated or if mechanisms for resorption of water are sufficiently compromised. Another common consequence of parasitism is colic, which can be defined as abdominal pain that usually originates in the digestive tract. Parasitism can cause gut movements to increase or decrease, resulting in intestinal cramping or abnormal accumulations of gas or fluid, respectively. These conditions are manifested clinically as colic. The loss of appetite frequently observed with parasitism may be a side effect of various inflammatory processes. Certain parasites of cattle and sheep have been shown to affect feed intake by altering the hormonal status of the host.

Nevertheless, the central event in many parasitic diseases is loss of protein from the gut. Because cyathostomes inhabit the large bowel, plasma leaking into this portion of the bowel is subsequently lost in fecal material. The inflammation that accompanies parasitism also signals protein resources to manufacture immunoglobulins (proteins that have antibody activity) and inflammatory cells, which effectively divert protein away from processes like growth and weight gain.

Subclinical Disease— Subclinical disease is the most common consequence of parasitism in well-managed horses, and may be responsible for greater economic losses than clinical conditions. Horses in apparent good health may have subclinical parasitic disease that is potentially measurable. Examples of subclinical conditions include decreased feed efficiency and suboptimal performance.

Without a doubt, demonstration of subclinical disease in horses remains difficult because standard techniques for measuring equine performance have not been adopted. Though great inherent variability exists among breeds of horses and even among usage classes within a breed, the major roadblock to studying subclinical equine disease is that western culture traditionally doe not view horses as production animals. The measurement of equine feed efficiency is a pertinent example. Anecdotes and subjective impressions abound claiming improved performance in horses maintained on optimal, preventive parasite control programs, but there are few experimental data to support these contentions.

Suboptimal performance may be even more difficult to measure because it obviously has different meanings for horses in various use categories. In brood mares, for instance, impaired performance may be manifested as irregular estrus cycles, failure to conceive, giving birth to a small or weak foal, or inadequate milk production. Other examples of suboptimal performance include slower times for equine athletes or failure of a halter horse to achieve peak condition.

Immunity—One health consequence of internal parasitism that should be regarded as beneficial is the development of acquired immunity by the host. As mentioned previously, immune horses can eliminate certain parasitic infections (e.g., threadworms and round-worms), but immunity to cyathostomes and other parasites is usually incomplete and may be manifested as resistance to disease rather than to infection. Accordingly, an “immune” horse harboring a significant number of cyathostomes probably experiences less clinical and subclinical damage than foals, weanlings, or yearlings with comparable infections.

The development of immunity or resistance to cyathostomes is highly desirable because parasitic challenge abounds and is inevitable. Excessively rigorous parasite control efforts can deny sufficient exposure to worms and, ultimately, interfere with the development of adequate immunity. It may seem contradictory, but optimal parasite control programs should not suppress parasitism totally. Rather, a control program for cyathostomes should permit sufficient leaks to allow the development of acquired immunity. Admittedly, such delicate balance would be difficult to achieve because the thresholds between resistance and subclinical disease have not been identified, and are likely to vary among individual horses.

ECONOMICS
Economic losses attributable to parasitism arise from several sources and vary in magnitude. The most dramatic expenses result from replacing an animal that dies of parasitic disease. Returning a clinically diseased horse to normal health can incur similar expenses when one includes the costs of therapeutic drugs, veterinary bills, additional labor, and dietary, housing, or other management alterations. The costs associated with subclinical disease are more insidious but potentially high, and include decreased feed efficiency, reduced average daily gain, and those related to suboptimal performance. The horse industry in North America also incurs huge annual losses through expenditures for inefficient or inappropriate control efforts, including the use of ineffective drugs, incorrect timing of deworming treatments, and failure to monitor the continued efficacy of control regimens.

Benefits of Parasite Control
Parasite control is universally practiced, but the perceived benefits remain largely subjective and poorly documented. The scientific literature contains numerous reports of the alleviation of clinical disease in individual horses through therapeutic deworming, but salvage treatment to remediate a health crisis can hardly be considered efficient parasite control. In one of the few population studies of the benefits of parasite control, the incidence of colic in a herd decreased 80% after measure were implemented to improve control of cyathostomes. Until better data are available, the potential benefits of effective parasite control essentially are the opposites of the clinical and subclinical effects discussed previously.

In summary, parasite control is important to the health and productivity of horses. Effective control of internal parasites may decrease expenses for therapeutic drugs and veterinary bills. Feed efficiency can also be impacted when internal parasites are controlled. Horses may be maintained with less feed expense in comparison to horses which are not subjected to periodic dewormings. Better performance may be difficult to document, but enhanced reproduction, athletic performance, or appearance and behavior may accompany improvements in parasite control. The final potential benefit is that appropriately-designed control programs are likely to be more effective than current practices at most farms, and in many cases, can be implemented at lower costs.

Effective Equine Parasite Control

It is not feasible to recommend specific control programs for every management situation. Horse owners should work closely with their veterinarians to design an appropriate deworming program. The following principles should be considered when designing parasite control programs for mature horses. Parasite control must be preventive in nature. Delaying treatment until clinical signs appear is not prevention. The objective of control is not only removal of worms. The goal is to also prevent future transmission by disrupting parasite reproduction. Fewer eggs on pasture decreases the incidence of infection. Parasite reproduction can be blocked by treating at intervals determined by the duration of efficacy of the drug used for the prior deworming. Control efforts should be intensified during seasons when environmental transmission is greatest. Control programs should not be perennial, nor should they be so rigorous as to suppress all transmission. Such programs may block the acquisition of immunity and, ultimately, leave a horse highly susceptible to parasitism. Anthelmintic resistance by some parasites is common and widespread. Control programs should utilize only dewormers that have been proven effective in the herd.  The continued efficacy of drugs in the herd should be monitored at least annually.  A good nutrition program and stress-free management complement parasite control programs. Appropriate stocking density is one of the few pasture management techniques that aid in parasite control. In most locations and climates, pastures cannot be left vacant long enough for pasture rotation to contribute significantly to control programs.  Stall hygiene has almost no impact on the transmission of cyathostomes. When the preceding factors are included in a parasite control strategy and meticulously implemented, the horse owner can expect decreased clinical and subclinical impact on animals, improved health and performance, and ultimately lower expenditures for health maintenance.

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