Investigations into the Incidence and Control of Selected Parasites and Pathogens which Infect Arkansas Horses

Investigations into the Incidence and Control of Selected Parasites and

Pathogens which Infect Arkansas Horses

I. Literature Review

Parasite control, particularly of gastrointestinal parasites, is an important aspect of the overall

health management of horses. An understanding of the prevalence and pathogenicity of the parasites

offers insight towards effective management programs utilizing both drugs and husbandry techniques. Six

groupings of internal parasites that are of main concern, due to either pathogenicity or prevalence,

include; small strongyles, large strongyles, tapeworms, bots, pinworms, and ascarids.

a. IMPORTANT EQUINE PARASITES

Small Strongyles

Cyathostomes (small strongyles) are the most prevalent intestinal parasites in horses around the

world. Currently, there have been 83 different species of nematodes identified that infect horses. Of these,

50 are cyathostomes and are commonly referred to as small strongyles. A typical small strongyle infection

includes thousands of adult and larval stage nematodes, and is comprised of 5 to 10 of the most

prevalent species (Lichtenfels et al., 2008).

Cyathostomes have a typical “trichostrongyle” life cycle, with adult females depositing eggs in the

cecum and large intestine, which are passed with the feces into the environment. Favorable

environmental conditions allow eggs to hatch within one week, although this can take up to four weeks

(Reinemeyer, 1986). Once the first stage larvae (L1) have emerged from the egg, they live on dissolved

nutrients, undergo a molt and develop to second stage larvae (L2). These larvae in live on a nutrient-rich

feces and molt to the third stage, infective larvae (L3). The L3 migrate from the feces and can survive on

pasture for up to 11 weeks in the winter, but only 2-3 weeks in the dry summer months (Reinemeyer,

1986). Once L3 have been ingested by the equine host, prepatency usually lasts for 5-6 weeks, although

a prepatent period of up 8 weeks has been observed (Klei and French, 1998). Inhibition of the parasitic

L3, which occurs in the cysts in the mucosa or submucosa of the large intestine, can be influenced by

season, infection levels, and acquired immunity by the host. These cysts can endure for up to 3 years in

older horses (Klei and French, 1998). During non-inhibited development, the L3 develop to fourth stage

larvae (L4) within 6-12 days in the cysts and move into the lumen, where they further develop into adults,

with 50-55 percent residing in the large colon. A large portion of the pathogenicity of cyathostomes is due

to larval cyathostomiasis, a condition in which a large number of L4 emerge from cysts in the large

intestine and cecum, causing severe colitis, diarrhea, and possibly death, especially in younger animals

(Klei and French, 1998).

Large Strongyles

Large strongyles are the most pathogenic nematodes that infect horses, and are arguably the

most pathogenic of all parasites in horses. There are three species of the genus Strongylus that are the

most important large strongyles in horses. These species are Strongylus vulgaris, S. equinus, and S.

edentatus (Lichtenfels et al., 2008). The large strongyle life cycle is dissimilar from that of small

strongyles, with a prepatent period that is normally six months to one year, depending on the species

present. Adult large strongyles are found in the cecum and colon; however, they are attached to the wall

of the intestine and suck blood, damaging mucosa in the process; thereby giving these parasites the

common name of blood worms. The other prominent difference between small and large strongyles is the

migration of the larval stages of each of the large strongyles. Each of the three species of large strongyles

has a unique migratory path going from the gut, to various organs and then back to the gut. Small

strongyle larvae simply migrate in the mucosa of the cecum and large intestine.

The pathogenicity of large strongyles is primarily due to the migration of the larvae, usually the L4

stage, before development into adult worms. S. vulgaris is the most pathogenic species of large

strongyles, with migration occurring primarily into the cranial mesenteric artery. After ingestion of the L3

larvae by the equine host, development of L4 larvae occurs, followed by migration through the wall of the

small intestine, cecum, or ventral colon, into the arterioles, then into small arteries, upstream to the larger

arteries, and eventually to the cranial mesenteric artery. Larvae are passed back to the cecum or large

colon where they form nodules in the walls of the intestine. Adults are only sexually mature upon leaving

these nodules; a process that takes approximately 6 months after ingestion of L3 larvae (Drudge, 1978).

S. edentatus does not possess the high pathogenicity of S. vulgaris because the migration of L4 larvae

occurs primarily in the liver rather than arteries. Ingested larvae move through the cecum, through portal

veins to the liver, through the peritoneal lining of the abdominal cavity, and then back through the

intestinal wall to the mucosa. This migration period, from ingestion to development of adults moving into

the mucosa of the ventral colon, requires approximately 11 months (Drudge, 1978). The migration of S.

equinus is very similar to that of S.edentatus; however, once the larvae leave the liver they also travel to

the pancreas before returning to the mucosa of the cecum. The development of this less common large

strongyle takes approximately nine months from ingestion of larvae to development of adults in the cecum

(Drudge, 1978).

Tapeworms

Cestodes (tapeworms) are increasingly thought to be an important gastrointestinal parasite in

horses, with research into the correlation of infections with colic, or intestinal disturbances (Proudman,

2003). There are three species of tapeworms with importance in horses, Anoplocephala perfoliata, A.

magna, and Paranoplocephala mamillana. Each species resides in a distinct location of the intestinal

tract, and pathogenicity of these parasites is dependent on both their location and infection rate in horses

(Lyons et al., 2006). The most common species, A. perfoliata, is thought to be the most pathogenic

because it resides near the ileocecal junction, leading to incidences of spasmodic colic and cecal

ulcerations, with the potential of death of animals with heavy infections. A. magna is the largest of the

three species, but is relatively uncommon and resides in the posterior small intestine. The smallest

species is P. mamillana, which is found in the anterior small intestine or stomach, and is also relatively

uncommon (Lyons et al., 2006).

The life cycle of tapeworms is indirect, with orbatid mites serving as intermediate hosts for the

infective stages. The entire life cycle requires approximately four to six months, with a two to four month

period of development in the intermediate host and two months for development in the definitive host

(Drudge, 1978). The mite ingests embryonated eggs from the environment and the cysticercoid (larval

stage) develops in its body cavity. Horses ingest infected mites on pasture, and the larvae develop into

adults in the intestinal tract. The scolex of the adult attaches to the horse’s intestinal wall and maturation

occurs through the growth of the strobila from the “neck” of the tapeworm towards the posterior intestine

of the host. The proglottids that make up the strobila each contain male and female reproductive systems

resulting in proglottids full of eggs (gravid). These gravid proglottids pass with the feces into the

environment, releasing eggs for mites to ingest (Lyons et al.,2006).

Bots

“Bot” is the common name for the maggot stage of the bot fly that infects horses. There are

several species of the genus Gastrophilis that infect horses; with each “colonizing” a different location on

the stomach mucosa. The two most common species are G. nasalis and G. intestinalis. Pathogenicity is

due to the pits formed in stomach tissue, as well as occasional perforation and peritonitis. Adult flies in the

environment mate and the females cement eggs (“nits”) containing first stage larvae on the hairs covering

the horse’s body, concentrating on the legs, shoulders, and neck. Dependent on the species, eggs either

hatch spontaneously after one week or are stimulated to hatch by the horse licking or chewing on the

area containing the eggs. First-instar larvae migrate through oral tissue and develop into secondinstar

larvae in three weeks. The second-instar larvae migrate to the back of the throat and are swallowed,

passing to the stomach where development into third-instar larvae occurs after three to four weeks. The

third-instar larvae create pits in the lining of the stomach, where they can remain for up to 10 months

before detaching and passing into the environment with the feces. Upon entering the environment, the

larvae burrow into the ground to pupate for approximately one to two months. Adult flies emerge, mate,

and females lay eggs for approximately two months prior to their demise (Drudge, 1978).

Pinworms

Pinworm infections are found in all ages of horses, and are important because of the irritating

effect they have on the host (“indirect”) pathogenicity. The common pinworm is the species Oxyuris equi,

which is found in the large intestine. Females migrate to the anus, where they rupture and deposit eggs

around the perianal region of the horse. The development of infective larvae in the eggs requires three to

five days. Upon ingestion by the host, larvae develop into fourth stage larvae within three to 10 days.

Fourth stage larvae develop into sexually mature worms over five months as they are attached to the

mucosa of the large intestine. Irritation to the host is due to the migration of the females out of the anus

and their subsequent rupture. Egg deposits dry on the horse’s skin, which causes severe pruritis around

the tail head and can cause secondary bacterial infections from horses rubbing their tail against any

available surface. Horses can sustain an infection of over 20,000 pinworms with no obvious, specific

clinical signs other than tail rubbing (Drudge, 1978).

Ascarids

Parascaris equorum (ascarids) commonly infect young horses, particularly those under one year

of age. The pathogenicity of ascarids is due to the possible rupture of the small intestine, and possible

damage in the liver and lungs from large numbers of migrating larvae. Adult ascarids reside in the small

intestine and are the largest nematode parasites of horses. Individual females can lay up to 200,000 eggs

per day, which pass with the feces into the environment and become infective in two weeks. Infective

eggs remain in the environment for many years in a resistant shell, and hatch upon ingestion by the

equine host. Larvae released from the eggs migrate through the intestinal wall, through portal veins to the

liver, and into the lungs. Immature larvae are coughed up and swallowed, move to the small intestine, and

develop into mature adults. The entire life cycle requires four months, with migration and development in

the host requiring three months (Drudge, 1978; Lyons et al., 2006).

b. CONTROL OF IMPORTANT PARASITES

Chemical control of parasites is an important part of the overall health management program for

horses. Anthelmintic use should be primarily based upon the helminth incidence and the drug’s spectrum

of activity. There are currently three classes of anthelmintic compounds in use for the treatment of equine

gastrointestinal nematodes; macrocyclic lactones, tetrahydropyrimidines, and benzimidazoles.

Praziquantel (quinoline class) is also used for the control of tapeworms; however, it is only marketed in

combination with macrocyclic lactones.

Macrocyclic Lactones

The macrocyclic lactone class of anthelmintics includes two subclasses of compounds,

milbemycins (including moxidectin) and avermectins (including ivermectin), both of which cause flaccid

paralysis of the nematode by interfering with neurotransmission and muscle cell function (Wescott, 1986).

Moxidectin and ivermectin are nearly identical in chemical structure, but moxidectin lacks a sugar group

that is contained on the ivermectin compound. This alteration gives moxidectin exceptional lipophilic

properties, enabling it to target encysted cyathostomes (late L3/L4 mucosal cyathostome larvae). Both

moxidectin and ivermectin are labeled for the control of bots, adult large-mouth stomach worms,

pinworms, ascarids, adult and L4 small strongyles, large strongyles, and adult hairworms

(Trichostrongylus axei) (Brady and Nichols, 2009).

Tetrahydropyrimidines

Tetrahydropyrimidines (pyrantel salts) include pyrantel tartrate and pyrantel pamoate. Both of

these compounds are approved for the control of mature infections of large strongyles, small strongyles,

pinworms, and ascarids. The pyrantel salts cause nematode paralysis by stimulated release and

maintenance of acetylcholine at neuron synapses (Brady and Nichols, 2009). Pyrantel tartrate usage is

recommended after horse treatment with a larvacide, such as moxidectin, and is administered at a low

daily dosage. Daily pyrantel has also been shown to control tapeworm infections (Kivipelto et al., 1998).

Pyrantel pamoate at a triple dose has also been shown effective against tapeworms (Kivipelto et al.,

1998) and has been approved and labeled for double dosage use for the control of cestodes (Phoenix,

2005).

Benzimidazoles

Benzimidazoles have been on the market longer than the other two classes of anthelmintics,

spanning over fifty years of use by way of multiple formulations. Currently in the horse industry, the two

compounds used most often are oxibendazole and fenbendazole. Fenbendazole is labeled against

ascarids, pinworms, small strongyles, and large strongyles, as well as encysted small strongyle larvae

when given at a double dose for five consecutive days (Brady and Nichols, 2009). Benzimidazoles act on

nematodes through interference of metabolism by microtubule inhibition (Roberson, 1977; Rew and

Fetterer, 1986).

c. RESISTANCE TO EQUINE ANTHELMINTICS

Anthelmintic resistance is a cause for concern and is the result of frequent use of anthelmintics in

the horse industry. The most common ways to measure efficacy of deworming products are the use of

fecal egg counts (FEC), egg reappearance periods (ERP), and fecal egg count reductions (FECR). There

is a lack of consistency with the measurement of resistance, leading to conflicting reports of its

prevalence in the equine industry. The World Association for the Advancement of Veterinary Parasitology

(WAAVP) defines resistance as a FECR percentage that is less than 95% (Coles et al., 1992). Analysis

methods of FECR tests differ among researchers and the accuracy of some methods has been

questioned, although no consensus has been achieved (Denwood et al., 2010).

The development of resistance to all of the major classes of anthelmintics has been associated

with several factors. The primary factor contributing to resistance has been the high frequency of

treatment, particularly with only one compound or class of anthelmintic (van Wyk, 2001; Brady and

Nichols, 2009). Underdosing is also a factor in resistance development (Brady et al., 2008), along with a

low presence of refugia maintained on farms. Refugia is defined as the population of nematodes that

remain unexposed to chemical compounds, i.e., free-living populations on pasture, animals not treated

with the compound, or encysted larvae not exposed to the anthelmintic (van Wyk, 2001; Brady and

Nichols, 2009).

Benzimidazoles are the most common anthelmintics with documented resistance, particularly

with cyathostome populations. Although resistance has been documented over several decades (Little et

al., 2003) and in multiple countries (Kaplan, 2002), fenbendazole remains one of the most popular

anthelmintics in use today (Brady and Nichols, 2009). Multiple studies have shown the efficacy of

fenbendazole to be extremely low, with FECR percentages of 84.4% (Varady et al., 2004), 65.1% (Varady

et al., 2000), -36% (Rossano et al., 2010), and no significant reduction in FEC (Martin-Downum et al.,

2001; Chandler and Love, 2002). According to Kaplan (2002), benzimidazole-resistant cyathostome

populations have greatly overwhelmed the populations of susceptible cyathostomes, leaving the majority

of farms with only resistant strains.

Tetrahydropyrimidines have been shown to be resisted by cyathostomes in many countries. The

prevalence of resistance to this compound has not been as widespread as resistance to benzimidazoles,

perhaps due to the fact that it has not been on the market as long (Brady and Nichols, 2009). Pyrantel

tartrate, given as a daily top-dressing, may be responsible for the development of resistance in this class

(Kaplan, 2002). No published data reveals this direct correlation; however farms with documented

pyrimidine resistance have also had a history of daily pyrantel tartrate use (Tarigo-Martinie et al., 2001;

Kaplan, 2002). Research conducted on pyrimidine resistance is generally correlated with resistance to

benzimidazoles, which could indicate cross-resistance (Lyons et al., 2001; Brady and Nichols, 2009).

Although resistance has been documented with cyathostomes, pyrantel pamoate has been shown to

remain effective against Oxyuris equi infections (Reinemeyer et al., 2010a).

Macrocyclic lactone resistance has been documented for ascarids, but has not yet been shown in

cyathostomes. In a 2002 review article, Kaplan reported no findings of ivermectin resistance; however

researchers did report ascarid resistance to ivermectin (Boersema et al., 2002; Brady and Nichols, 2009;

Reinemeyer, 2010b). In the United States, there is currently no evidence of cyathostome resistance to

milbemycins, and recent studies show efficacies of moxidectin to be 99.9-100% (Chandler and Love,

2002), 99.1% (Martin-Downum et al., 2001), and 100% (Rossano et al., 2010); however, the ERP for

moxidectin was shorter than previously reported according to Rossano and colleagues (2010). Other

research has demonstrated ivermectin to be effective against cyathostomes, with a reported efficacy of

>99% (Klei et al., 2001).Efficacy against Oxyuris equi has also been shown at 96% for adults and >99%

for fourth stage larvae (Reinemeyer et al., 2010a).

d. PARASITE MANAGEMENT

Deworming Protocols

Research has been conducted in various EU countries to document how horse owners and

veterinarians are controlling parasites. In the UK in the late 90’s, horse owners said they used rotational

deworming practices and were influenced in their decision to do so by advertisements and magazine

articles and occasionally their veterinary surgeon (Lloyd et al., 2000). This process was confirmed by

Allison et al (2011), who found that 50% of horse owners receive their deworming advice from a

veterinary surgeon, and about 30% used professional advice to develop a selective deworming protocol.

In Ireland, only 54% of horse owners devised their deworming protocols based on veterinary advice, and

none of them used selective deworming (O’Meara and Mulcahy, 2002). In Denmark, where anthelmintics

have been available only by prescription since 1999, veterinarians are responsible for determining when a

horse needs treatment. Most veterinarians (97%) reported using fecal egg counts to guide their treatment

decisions, but in cases of foals or horses with “suspicion of clinical parasitic disease” fecals were not

performed prior to treatment (Nielsen et al., 2006). This same group of veterinarians also reported low

utilization (11% of practitioners) of fecal egg count reduction tests to determine anthelmintic efficacy and

resistance (Nielsen et al., 2006).

The use of proper deworming protocols is an important aspect of internal parasite control. In the

past, a practice known as ‘interval deworming’ was recommended by parasitologists (Drudge and Lyons,

1986). This practice called for the treatment of horses every 6-8 weeks, primarily targeting the removal of

Strongylus vulgaris in order to prevent verminous colic (Drudge and Lyons, 1986; Kaplan, 2002). This

strategy has been widely implemented, and strictly followed, since its introduction (Kaplan, 2002). Interval

dosing has been extremely successful in controlling Strongylus spp. but it has led to resistance by

cyathostomes, which are now considered the most important internal parasite in horses (Duncan and

Love, 1991; Larsen et al., 2011).

In order to address the resistance of cyathostomes, parasitologists have begun to implement new

treatment strategies. Selective treatment and rotational deworming have been examined in many studies

around the world, and both have been shown to be effective on horse farms that harbor resistant

parasites (Duncan and Love, 1991; Gomez and Gorgi, 1991; Brady et al., 2008; Becher et al., 2010;

Larsen et al., 2011).

Rotational deworming has been studied as a strategy to regain effectiveness where resistance by

certain parasites has been documented (Brady et al., 2008). Following a fast rotation between different

classes of anthelmintics, fenbendazole (10 mg/kg for five days) was shown to have an efficacy of 98.7%

in mature horses on a farm with documented benzimidazole resistance (Brady et al., 2008). Reinemeyer

et al (2010b) found that foals infected with ML-resistant strains of P. equorum could be treated with

pyrantel pamoate and have a significant reduction in adult worms. Although researchers have suggested

it, additional research on fast vs. slow rotation has not been published (Kaplan, 2002; Brady et al., 2008).

Recommendations of rotation between drug classes are numerous, with an agreement that only effective

anthelmintics be used (Nielson et al., 2010, Reinemeyer et al., 2010b). Additionally, a recommendation of

rotation based on parasite prevalence by season has been forwarded (Nielson et al., 2010), but there

have not been studies published to confirm or refute these suggestions.

The most novel approach to equine deworming is selective treatment, which is a program based

upon diagnosing internal parasites in horses, and then treating individual horses based upon that

diagnosis (Kaplan, 2010). Usually, this diagnosis is made by performing fecal egg counts on all horses

and then treating only those over a certain threshold (Gomez and Georgi, 1991). The selective treatment

protocol has been used in small ruminant production with some success, with treatment criteria based on

the use of FAMACHA or production characteristics such as weight gain, milk yield, or wool yield (Kenyon

et al., 2009; Gaba et al., 2010). In horses, the use of selective treatment has been implemented in the

European Union through regulation of deworming products, which are only available with a prescription

from a veterinarian (Anderson et al., 2012).

While various studies have confirmed that selective treatment helps maintain efficacy of current

drugs (Duncan and Love, 1991; Gomez and Georgi, 1991; Becher et al., 2010; Larson et al., 2011), there

have not been any definitive studies on when fecal samples should be taken, or any that prove that

selective deworming aids in actually reducing resistance. In 2010, Becher and colleagues found that out

of 129 horses sampled each month for 10 months, only 29.5% needed treatment (FEC >250 EPG). This

study demonstrated that a significant decrease in the number of treatments can be obtained, thereby

maintaining refugia and potentially decreasing the selection pressure for development of resistant

parasites (Becher et al., 2010). In the United States, there have not been recent studies to confirm the

selective treatment data coming from the EU; however, this could be due to the fact that the American

Association of Equine Practitioners (AAEP) has only recently recognized selective treatment as a

protocol. The new (2013) AAEP recommendations for deworming programs include different guidelines

for horses under 3 years of age versus horses over 3 years of age, with more traditional guidelines for

treating young horses (every 3 months) to control P. equorum and prevent disease associated with large

strongyles. The guidelines for older horses recommend the use of fecal egg counts and fecal egg count

reduction tests to ensure that only indicated horses receive treatments and the drugs in use maintain their

effectiveness (Nielsen et al., 2013).

II. Introduction

The presence of gastrointestinal parasites can reduce animal health and body condition. In

horses, this is indicated by a poor hair coat, diarrhea, poor body condition and in some cases, colitis

(Drudge and Lyons, 1986). As clinical signs are not definitive for parasitism, fecal flotations are performed

to confirm parasite burdens in poorly performing animals. Quantitative flotations give fecal egg counts

(FEC), measured in eggs per gram (EPG), and constitute the most effective tool for parasitological

interpretations in live animals. The flotations show the eggs shed in the feces by mature helminths

residing in the digestive tract, which are in turn used as an indication of the population in the horse. Fecal

egg counts are generally performed only when there is already suspicion of infection and treatment has

already been recommended. Commonly, treatment is given preemptively to healthy animals in order to

prevent the development of clinical signs (Kaplan, 2002).

Anthelmintic drugs are used to control parasite infections and several are currently on the market

for use in horses. Historically, fenbendazole has been one of the most commonly used anthelmintics in

the United States, but ivermectin is probably the most popular anthelmintic today (Chandler and Love,

2002). The newest drug on the market, moxidectin, is also commonly used although it is contraindicated

for use in foals younger than 6 months of age or severely debilitated horses due to its lipophilic properties.

Moxidectin can be used therapeutically in conjunction with pyrantel tartrate, which is given at a daily

larvacidal dose in the feed. The drugs used for anthelmintic treatment should be dependent upon the

efficacy and farm-specific protocol.

Several protocols of anthelmintic intervention have been utilized by equine caretakers, with

interval treatment the most common. Interval treatment calls for the use of anthelmintics every 6-8 weeks

in horses sharing a pasture, regardless of parasite burden. This protocol has led to the development of

resistance, particularly by small strongyles (Larsen et al., 2011). Exposure of entire populations of

helminths to particular chemicals results in establishing a parasitic gene pool of only those resistant to the

drug. In requiring the treatment of all animals, interval dosing exposes all parasites to the drugs used on

that particular farm. Resistance to the most commonly used drugs in the equine industry has been

thoroughly documented in multiple countries and is often correlated with interval dosing protocols

(Kaplan, 2002). In order to prevent the extreme resistance currently found in small ruminants, equine

veterinarians have begun to recommend different protocols (Kenyon et. al, 2009; Nielsen et al., 2013).

Selective treatment has been gaining ground in veterinary parasitology; however, its use in the

field has not been thoroughly documented or evaluated. Various methods have been implemented in

selective treatment, with the use of fecal flotations to distinguish two groups of horses on each farm as

the basis of this protocol. One group of animals, the high-shedding horses, is treated with an anthelmintic

while the others, the low-shedding horses, are left untreated. Determination of treatment is based on a

pre-selected threshold, generally between 200-250 EPG. The untreated animals help to maintain refugia

(a population of the parasites not exposed to the drugs); a biological means of diluting the gene pool of

those helminths resistant to chemicals (van Wyk et al., 2001). This can help reduce the rate/degree of

resistance, which in turn can improve anthelmintic efficacy.

Efficacy of commonly used drugs has been severely depressed by resistance in small strongyles.

Small strongyles are the most common gastrointestinal parasites found in horses and are responsible for

the majority of eggs found in a fecal egg count; and proportionally greatly determine the treatment threshold

(Love et. al, 1999). Large strongyles are also found in the FEC but their eggs are similar in size and shape

to the small strongyles and are therefore not differentiated in flotations. However, identification can be made

through the use of coprocultures, larval harvest, and larvae identifications (Ivens, 1978). The FEC is used

to estimate efficacy of anthelmintics by performing flotations at the time of treatment and again 14-21 days

post-treatment, comparing the egg counts. This is known as a fecal egg count reduction test (FECRT) and

is presented as a percentage of efficacy. Drugs are considered to be efficacious with

≥95% FECR. Selective treatment could help maintain efficacy by reducing resistant populations of

parasites through monitoring parasite burdens with FEC and the FECRT (Larsen et al., 2011).

The objectives of this study were; (1) to determine the prevalence of helminths in our area by egg

and L3 determinations, (2) to determine if certain horses maintained low FEC, therefore eliminating the

need to treat them on a year-round basis and (3) to determine the effectiveness of four common

treatments (moxidectin, ivermectin (pioneer and generic), fenbendazole and pyrantel tartrate) via a

standardized fecal egg count reduction test.

III. Materials and Methods

Timeline

This study was conducted from February 2011 through October 2011.

Horses

Fecal samples were collected from 226 horses housed on 14 farms in Northwest Arkansas,

Central Arkansas, and the University of Missouri in Columbia. Selected farms had to maintain a herd of at

least 10 horses for the duration of the trial. At the beginning of the study, horses ranged in age from 8

months to 35 years and included 99 mares, 126 geldings, and one stallion. There were 39 breeds

represented at the farms. On-going farm management procedures, with the exception of anthelmintic

treatments, were kept in force at each farm for the study.

Fecal Samples

Fecal samples were collected from each animal at pre-treatment (PRT) (day -7 to day 0) and at

post-treatment (PT) (3-5 weeks following treatment). Re-treatment and re-sampling was separated by

approximately 3 months. Eighty-nine horses were sampled/treated once, 116 horses were

sampled/treated twice, and 21 horses were sampled/treated three times. Samples from the horses were

taken either rectally or collected from individual paddocks or stalls and refrigerated at 5°C until

examination within 2-5 days following collection. Fecal samples were quantitatively examined using single

centrifugation of 1 g feces in saturated MgSO4 (Martin-Downum et al, 2001). Coprocultures were also

conducted for samples with a FEC ≥ 20 EPG for the first 6 months and ≥ 50 EPG for the remainder of the

study, using standard techniques (Ivens et al., 1978). A total of 933 fecal samples and 259 coprocultures

were evaluated during the study.

Treatments

Several anthelmintics were used for treatment in the study; moxidectin (MOX; Quest , Pfizer),

ivermectin (IVER; Zimectrin , Merial), generic ivermectin (GIVER; IverCare Farnam), ivermectin with

praziquantel (IVER-PRA; Zimectrin Gold , Merial), fenbendazole (FEN; Safeguard , Intervet), daily

pyrantel tartrate (MOX-PYR; Strongid C 2X , Pfizer), which was preceded by moxidectin according to

manufacturer instructions, and pyrantel pamoate (PYR PAM; Strongid , Pfizer). All dosages were given

according to label dose rates and horse weight as determined with calibrated equine weight tape

measurement at the heart girth. Treatments were given only to horses with a FEC >200 EPG. Owners

and/or farm managers chose the anthelmintic at each treatment and were given the option to change

treatments should their choice be ineffective (<90% FECR) at any point in the study.

Statistical analysis

Statistical analysis was performed using SAS for repeated measures (PROC MIXED, SAS Inst.

Inc., Cary, NC) as described by Littell et al., 1996. Egg counts were transformed to the log 10(x + 1) prior

to analysis and significant differences were determined when the model F-test proved significant (p <

0.05).

IV. Results

Fecal Egg Counts

Of the 933 fecals examined during the study, 303 had EPG of zero, 407 were <200 EPG, and 223

were >200 EPG. In the group of horses sampled for all three phases, 126 samples were analyzed, with

37 samples with an EPG of zero, 58 samples <200 EPG, and 31 samples >200 EPG. In the group of

horses sampled for only two phases, there were 550 samples analyzed, with 187 with an EPG of zero,

234 samples <200 EPG, and 129 samples >200 EPG. For horses sampled for only one phase, 256

samples were analyzed, with 79 that had an EPG value of zero, 115 samples <200 EPG, and 61 samples

>200 EPG. There were 37 samples that contained eggs other than Strongyle-type eggs, including five

with Oxyuris equi, two with Parascaris equorum, and 35 with cestode eggs (Figure 1).

Figure 1. In 933 fecal samples collected from 227 horses over 8 months, 630 samples had Strongyle-type

eggs, 5 samples had Oxyuris equi eggs, 2 samples had Parascaris equorum eggs, and 35 samples had

cestode eggs. Data presented on a logarithmic scale.

Coprocultures

All three major large strongyle species were found in samples from horses in Northwest

Arkansas. Seven of the 259 coprocultures had large strongyles, with one of the samples containing S.

vulgaris, five with S. equinus, and two with S. edentatus. One sample had both S. vulgaris and S.

equinus. The other 252 coprocultures contained only cyathastome larvae.

Treatments

A total of 156 treatments were given during the study. Of these, 107 treatments were MOX, 23

were ivermectin (10 IVER, 8 GIVER, and 5 IVER-PRA), 13 were FEN, and 13 were pyrantel (MOX-PYR,

PYR, PYR-PAM) (Figure 2). Over the entire study, 101 horses did not require treatments, correlating to

44.5% of the animals used in the study.

100

1000

Egg Types Found in Fecals

Strongyle-type eggs

Oxyuris equi eggs

Parascaris equorum eggs

Cestode eggs

Figure 2. Out of 156 treatments given during an eight month selective deworming study, 107 were MOX,

23 were ivermectin (10 IVER, 8 GIVER, and 5 IVER-PRA), 13 were FEN, and 13 were pyrantel (2

MOXPYR, 6 PYR-PAM).

In the horses sampled for three phases, six of the horses were considered CON (untreated

group) animals (0 EPG or <200 EPG) throughout the study, five horses were treated one time (FEC >200

EPG), 10 horses were treated twice, and there were no horses that needed to be treated at all three

phases. Two of the horses treated once were given MOX during the first phase and were in the CON

group for the other two phases. Two other horses were in the CON group for the first two phases, then

treated with MOX in the third phase, while the last horse was in the CON group for the first and third

phase, and treated with GIVER during the second phase. Of the 10 horses requiring two treatments, 8

were treated initially with MOX, then had EPG lower than the threshold in the second phase, then

required treatment again in the third phase. One of these horses was treated with GIVER and the others

were all treated with MOX. In two other horses that required two treatments, the initial fecal sample put

them in the CON group but they were treated the remaining two times with MOX.

100

1000

Anthelmintics

MOX

IVER

GIVER

IVER-PRA

FEN

MOX-PYR

PYR-PAM

The horses sampled for two phases consisted of 50 CON animals, 44 that were treated once, and

22 that were treated twice. Of the 44 horses treated once, 32 were treated with MOX, six were treated

with IVER, three were treated with FEN, two were treated with MOX-PYR, and one was treated with

IVER-PRA. Of the 22 horses treated twice, eight were treated with MOX both times, four were treated

initially with GIVER and then MOX, four were treated initially with FEN and then IVER, two were treated

with FEN and then PYR-PAM, one was initially treated with FEN and then MOX, one was treated initially

with FEN and then IVER-PRA. Horses sampled for one phase included 45 CON animals and 44 treated

horses. Of the treated horses, 21 were treated with MOX, five were treated with IVER, two were treated

with FEN, and two were given PYR-PAM.

Drug Efficacy

Efficacies were determined for MOX during the first phase of treatments, and MOX and IVER

during the second phase of treatments. During the first phase of treatments, 134 CON animals had a PRT

FEC average of 43.9 EPG (arithmetic mean-AM) and a PT average FEC of 96.9 EPG (AM), resulting in a

FECR of +177.7% (based on AM). The MOX treated animals (N = 56) had a PRT average FEC of 768.8

EPG (AM) and a PT average FEC of 13.8 EPG (AM), resulting in a FECR of 98.2% (based on AM). The

PRT average and PT average were statistically different (P<0.05) for CON and MOX animals in the first

phase of the study (Figure 3).

Figure 3. First phase treatment groups with pre-treatment (PRT) and post-treatment (PT) fecal egg counts

(FEC) for each group. All FEC are averages for all animals in each treatment group. CON animals

(N=137) had PRT FEC of 54.6 EPG and PT FEC of 98.7 EPG; MOX animals (N=53) had PRT FEC of

759.4 EPG and PT FEC of 4.5 EPG; IVER animals (N=11) had PRT FEC of 304.3 EPG and PT FEC of 0

EPG; FEN animals (N=13) had PRT FEC of 994.6 EPG and PT FEC of 420 EPG; MOX-PYR animals

(N=4) had PRT FEC of 321.3 EPG and PT FEC of 0 EPG; PYR-PAM animals (N=2) had PRT FEC of

838.5 EPG and PT FEC of 139 EPG. CON and MOX were statistically different (P<0.05) for PRT FEC

and PT FEC, as indicated by a,b subscripts. Data presented on logarithmic scale.

During the second phase of treatments, 103 CON animals had a PRT average FEC of 32.3 EPG

(AM) and a PT average FEC of 110.2 EPG (AM), resulting in a FECR of +241.2 % (based on AM). The

MOX treated animals (N = 30) had a PRT FEC average of 434.1 (AM) and a PT average FEC of 10.7

EPG (AM), resulting in a FECR of 97.5% (based on AM). The IVER treated animals (N = 12) had a PRT

FEC average of 334.6 EPG (AM) and a PT average FEC of 2.1 EPG (AM), resulting in a FECR of 99.3%

(based on AM). The PRT and PT average FEC of the MOX and IVER treated horses were not different

than each other, but both were statistically different than the CON animals (P<0.05) (Figure 4). During the

third phase of treatments, there were 34 CON horses with an average PRT FEC of 31 EPG and a PT

average FEC of 142 EPG, resulting in a FECR of +358%. All treated horses in the third phase were

100

1000

CONa

MOXb

IVER

FEN

MOX-PYR

PYR-PM

Anthelmintics from First Phase

PRT FEC

PT FEC

treated with MOX (N = 27) with an average PRT FEC of 955 EPG and an average PT FEC of 0.1 EPG,

with a FECR of 99.9% (Figure 5).

Figure 4. Second phase treatment groups with pre-treatment (PRT) and post-treatment (PT) fecal egg

counts (FEC) for each group. All FEC are averages for all animals in each treatment group. CON animals

(N=103) had PRT FEC of 32.3 EPG and PT FEC of 110.2 EPG; MOX animals (N=30) had PRT FEC of

434.1 EPG and PT FEC of 10.7 EPG; IVER animals (N=12) had PRT FEC of 471.7 EPG and PT FEC of

3.2 EPG; MOX-PYR animals (N=2) had PRT FEC of 405 EPG and PT FEC of 0 EPG; PYR-PAM animals

(N=2) had PRT FEC of 1510 EPG and PT FEC of 14.5 EPG. No horses were treated with FEN in the

second phase. MOX and IVER were statistically different from CON (P<0.05) for both PRT FEC and PT

FEC, as indicated by a,b subscripts. Data presented on logarithmic scale.

100

1000

10000

CONa

MOXb

IVERb

MOX-PYR

PYR-PM

Anthelmintics from Second Phase

PRT FEC

PT FEC

Figure 5. Third phase treatment groups with pre-treatment (PRT) and post-treatment (PT) fecal egg

counts (FEC) for each group. All FEC are averages for all animals in each treatment group. CON animals

(N=9) had PRT FEC of 66.7 EPG and PT FEC of 99.2 EPG; MOX animals (N=12) had PRT FEC of 542.2

EPG and PT FEC of 0.1 EPG. No horses were treated with IVER, FEN, MOX-PYR, or PYR-PAM during

the third phase. Data presented on logarithmic scale.

A total of 19 horses were treated with drugs other than MOX and IVER during the study. Thirteen

horses were given FEN, with an average PRT FEC of 995 EPG and an average PT FEC of 420 EPG.

Based on these averages, the FECR of FEN was 57.7 % during the study (Figure 3). There were four

horses given pyrantel pamoate, with an average PRT FEC of 1174 and a PT average FEC of 77 EPG,

resulting in a FECR of 93.4% (Figure 3; Figure 4). Seven horses were treated with MOX-PYR, with an

average PRT FEC of 299 EPG and an average PT FEC of 0 EPG, resulting in a FECR of 100% (Figure 3;

Figure 4).

V. Discussion

The fecal egg count data presented here is consistent with similar studies. A major problem with

relying on fecal egg counts for a selective treatment protocol is that the diagnostic test primarily targets

only strongyle parasites, primarily cyathostomins. Although these are the most abundant gastrointestinal

parasites in adult horses, other parasites can be implicated in disease and should be monitored (Kaplan

100

1000

CON

MOX

Anthelmintics from Third Phase

PRT FEC

PT FEC

and Nielsen, 2010; Nielsen et al, 2014). In this study, only 37/933 samples (3.9%) contained eggs from

parasites other than strongyles with the most common being cestode eggs. This suggests that the

prevalence of gastrointestinal parasites other than small strongyles is low enough to not warrant frequent

treatment, which has been previously suggested (Kaplan and Nielsen, 2010). The number of samples

with P. equorum eggs, another important parasite, was very low (N=2) but the majority of horses sampled

in this study were at least one year of age; a “cut-off” age for when clinical infections with this parasite

become quite rare (Nielsen et al, 2013).

Although indirect monitoring of large strongyle infections does occur with fecal egg counts, their

eggs cannot be differentiated from those of cyathostomins and coprological techniques must be used; a

technique that requires a certain amount of proficiency (Nielsen et al, 2014). A survey of Danish

veterinarians reported 41% of respondents used coprocultures in their practice and most perceived that

large strongyles rarely caused problems (Nielsen et al, 2006). In the current study, coprocultures were

only done on samples that had fecal egg counts greater than 20-50 EPG. The prevalence of all three

major species of large strongyles was very low, with only 7/259 cultures positive for at least one species

(2.7%). The major pathogenic nematode, S. vulgaris, was only identified in one sample. This low

prevalence has been found in another study using PCR-screenings (Nielsen et al, 2012b) and as few as

one to two yearly larvicidal treatments are expected to adequately control S. vulgaris infections in most

adult horses (Nielsen et al, 2012a).

The treatment frequency results from this study are consistent with other selective deworming

studies, wherein not all horses need to be dewormed regularly, based on fecal egg counts (Becher et al,

2010; Kaplan and Nielsen, 2010, Larsen et al, 2011). During this study, 44.5% of the horses did not

exceed the threshold for treatment (>200 EPG); leaving about one-half of the horses untreated. Since it

is common for all horses to be treated on a regular basis, such as every three months with interval

deworming, using fecal egg counts to determine treatment could reduce the number of treatments given

on a specific farm, as well as to individual horses; which is desirable in terms of economics and expanded

refugia.

Efficacy data in this study is consistent with current studies, although there is little published data

concerning anthelmintic efficacy with selective treatment protocols. Fenbendazole in this study had a low

sample size (N=13) and a FECR of 57.7%. Resistance of fenbendazole is well documented in the U.S.

and Europe (Varady et al., 2000; Chandler and Love, 2002; Varady et al., 2004; Rossano et al., 2010),

which could account for the owner’s decisions to use other products in this study. The efficacy of pyrantel

products in this study is difficult to elucidate since there were very few horses on these products, and the

manufacturer recommendations for pyrantel tartrate include pre-treatment with moxidectin. It is unclear if

low egg counts were due to the pyrantel products or moxidectin. Ivermectin efficacy in this study was very

high, with an average FECR of 99.7%. This is consistent with recent studies evaluating ivermectin efficacy

in the face of concerns with macrocyclic lactone resistance (Klei et al, 2001; Larsen et al., 2011). The

efficacy of moxidectin was also high, with an average FECR of 98.5%, and potential contribution to the

low FEC in the pyrantel treated horses. Repeated studies have shown consistently high efficacies of

moxidectin, although many legitimate concerns have been raised regarding its prolonged residues in

tissue leading to possible resistance to this drug in the future (Kaplan, 2002; Rossano et al., 2010).

VI. Conclusion

The results of this study, combined with information from other published studies and the AAEP,

offer some guidance for anthelmintic treatment of horses using selective treatment protocols. Fecal egg

counts should be integrated into regular herd health protocols, particularly those with adult horses, in

order to reduce egg shedding and treatment rates, which could reduce anthelmintic resistance (Kaplan

and Nielsen, 2010). For veterinarians, fecal egg counts should be a part of regular practice and a strong

recommendation to clients for proper management of equine health. While current research is making

strides in managing equine gastrointestinal parasites, more research is needed in the field. Extensive field

surveys of treatment rates, egg shedding rates, and anthelmintic efficacy have not been reported, and

would greatly impact reception and implementation of the current AAEP recommendations (Nielsen,

2012c).