STR1210008 Pfizer Animal Health Technical Bulletin January 2011 Evidence-Based Equine Internal Parasite Control Utilizing Fecal Egg Counts and Environmental Risk Assessment to Effectively Control Equine Internal Parasites Nathan Voris, DVM, MBA John M. Donecker, VMD, MS, Dipl ABVP (equine) Pfizer Animal Health Five Giralda Farms Madison, NJ 07940 Robert E. Holland, Jr., DVM, PhD George A. Conder, PhD Pfizer Animal Health 5300 N. 28th Street Kalamazoo, MI 49007 Key Points The quantitative fecal egg count (FEC) is the gold standard diagnostic for identifying internal parasitism and the cornerstone of an evidence-based parasite control plan. The baseline FEC should be obtained early in the spring (April) in northern climates or early in the fall (September) in southern climates. Identifying individual horses that have increased vulnerability to internal parasitics is essential to the success of a well-managed parasite control program. Anthelmintic treatment will always be compromised if it is not supported by management practices that minimize parasite exposure. By recognizing environmental risk factors and implementing management modifications that reduce parasitic exposure, the residual capacity of anthelmintics utilized in the program will be maximized and the overall parasite control program will be more effective. Individual and herd FEC information combined with identification of environmental risk factors are integral to the development of an evidence-based parasite control plan. Equally essential are realistic goals and the continual assessment of individual program components. Although there is no universally accepted standard, commonly used thresholds for initiating treatment are 200-500 eggs per gram (EPG) for individual horses. When a FEC is obtained before and after antiparasitic treatment, it becomes a FEC reduction test (FECRT), a determination of the degree of parasite resistance to a specific anthelmintic. All brands are the property of Pfizer Inc, its affiliates, and/or its licensors. 2010 Pfizer Inc. All rights reserved.
Prior to the introduction of the modern classes of anthelmintics, parasite control strategies were primarily focused on eliminating the highly pathogenic Strongylus vulgaris (large strongyle), which was recognized as the leading cause of equine colic. 1 Veterinarians and horse owners were acutely aware of the danger of Strongylus vulgaris and elimination of that threat required frequent treatment with the products available at that time. Traditional methods of rotational deworming developed as more anthelmintic product classes were made readily available until, for the most part, the veterinarian s knowledge and skills were replaced by the convenience and affordability of paste anthelmintics. Due to the development of convenient and effective treatment options, large strongyle infections are now unusual in well-managed herds. In fact, virtual elimination of large strongyles in treated herds revealed the true prevalence and importance of the small strongyle. Small strongyles (cyathostomins) now account for nearly 100 percent of the equine parasite burden and are considered the most significant internal parasite of adult horses. 2 Parasite control strategies that successfully reduced the threat borne by the large strongyle, in many cases, are no longer appropriate for the control of today s primary parasite burden. Complete elimination of the small strongyle is not practical, nor necessary, for the adult horse to maintain good health. 2,3 In fact, one of the unintended consequences of effective, readily available equine anthelmintics is widespread overuse, which has led to selection of anthelmintic resistance in the small strongyle. Veterinary literature has heavily reported the emergence and expansion of anthelmintic resistance and this information is beginning to spill over into horse owner publications. Client expectations, responsible use of the currently available anthelmintic products, and the pursuit of evidencebased parasite control programs are all reasons for veterinarians to again assume a prominent role in the management of this aspect of equine health. This report will lay the framework for such a program by exploring the diagnostic value of quantitative fecal egg counts (FECs) and how that information complements the evaluation of parasite exposure risk factors in developing an individualized deworming program consistent with the unique needs of a given equine operation. Step 1: Establish Baseline Egg Counts The quantitative FEC is the gold standard diagnostic for identifying internal parasitism 2,4 and the cornerstone of an evidence-based parasite control plan. The FEC should be incorporated into the annual wellness evaluation of every horse as a cost-effective means of monitoring the intestinal parasite status of individual horses. Horses with an elevated risk of parasite exposure or those found to carry heavy parasite burdens should have FECs performed more frequently. Because the small strongyle is currently the principal equine internal parasite, the present day FEC is primarily a measure of the number of small strongyle eggs shed by adult cyathostomins. However, should a horse be infected with large strongyles or ascarids, ova from egg-laying adult worms of these species will also be identified in fecal material. It should not go unmentioned that cyathostomins present a set of unique diagnostic challenges that require additional consideration when interpreting FEC information due to the hypobiotic stage of larval maturation. A horse with substantial hypobiotic infection can have a negative or low FEC. Successful anthelmintic elimination of egg-laying adult cyathostomins can signal hypobiotic (encysted) larvae to emerge from the intestinal wall, mature and subsequently begin shedding cyathostomin eggs. 5 This event could easily be misinterpreted as a sign of anthelmintic failure or resistance. The number of egg-laying adults typically represents only a small percentage of the total (encysted and lumenal) cyathostomin population, so that a FEC often underrepresents the intensity of the total cyathostomin burden. 2
In light of the above, the baseline FEC should be obtained early in the spring (April) in northern climates or early in the fall (September) in southern climates. Egg shedding by cyathostomins has a distinct seasonal pattern in North America. In northern temperate climates, FEC increases in the spring, peaks in the late summer and autumn, and declines in the winter when hypobiotic infection predominates. In southern temperate climates, FEC rises in the autumn, peaks in the winter and early spring, and declines in the summer when hypobiosis predominates. Timing the FEC at the beginning of the optimal small strongyle shedding season will additionally correspond to the time of year when reinfection is suppressed due to the relatively low rate of egg shedding and the preceding period of natural pasture decontamination in southern climates or winter husbandry practices that limit infective parasite exposure in northern climates. 6 Finally, the egg reappearance period (ERP) of any anthelmintic product administered prior to the baseline FEC should be considered (see Figure 1). A FEC should be obtained no sooner than four weeks after the ERP of the previously administered anthelmintic. Product Class Benzimidazole Pyrantel Pamoate Ivermectin Moxidectin Figure 1 Egg Reappearance Period 4 weeks 4-6 weeks 8 weeks 12 weeks In a well-managed parasite control program, FEC results obtained under these guidelines will reflect the horse s natural ability to resist parasite infection. A horse with a high FEC (> 500 eggs/gram) following nonideal parasite environmental conditions and/or the exhaustion of the residual capacity (egg reappearance period) of an anthelmintic, should be considered immunologically vulnerable to internal parasite infection. A real world illustration of sustained individual vulnerability to parasite infection can be observed in a European study where horses (n = 424) on 10 farms were given periodic anthelmintic treatment and tested for FEC over a three-year period. 7 Individual horses shed cyathostomin eggs in fairly consistent patterns despite regular anthelmintic treatment. For example, horses that had a low or negative pretreatment FEC tended to have similarly low or negative parasite burdens when retested. However, horses with a history of a FEC > 200 eggs/gram had a high probability of having a similarly high FEC at future test intervals. The authors concluded that deworming was generally effective on a shortterm basis, but high-shedding horses had a natural tendency to incur reinfection and resume fecal shedding of parasite eggs at pretreatment levels. Step 2: Measure Shedding Levels As described above, identifying individual horses that are particularly vulnerable to internal parasitic infection is an important consideration when interpreting FEC data. Identification of these individuals within a herd dynamic is essential to the success of a well-managed parasite control program due to the phenomenon of overdispersion. Overdispersion is a commonly recognized characteristic of parasitism where a small minority of hosts harbor the majority of parasites on a continuing basis. Quantitatively, overdispersion can be likened to the 80-20 rule (where it is estimated that 80 percent of the total parasite burden originates from 20 percent of the horses). Identifying horses that are consistently low egg shedders (< 200 eggs/gram) and those that are consistently moderate or high egg shedders (200-500 eggs/gram or > 500 eggs/gram, respectively) will enable farm management to make evidence-based husbandry modifications to complement appropriate anthelmintic usage in protecting horses from natural sources of parasite reinfection. High-shedding horses should be the focus of a targeted parasite control program, including anthelmintic treatment and separation from lowshedding horses. This approach will likely reduce the treatment frequency for other horses in the herd, minimizing costs and reducing selection pressure for anthelmintic resistance development. 3
Step 3: Know the Variables In addition to individual and herd FEC results, evaluation of environmental and management risk factors is integral to creating a successful parasite control strategy. Various factors can contribute to a higher incidence of infective parasite exposure and positive FECs. These factors include: Age of Horse. Young horses are more susceptible to parasite infections (especially Parascaris equorum) than adult horses due to their immature immune system. Adolescent horses should be grazed on clean pastures separately from older horses. Intensive parasite control measures should be specifically tailored for this high-risk age group due to the invasive nature of larval migration associated with large roundworm infections. Horses of advanced age can also have an increased risk of parasite infection and should be handled accordingly. Local Climate Cycle. Exposure to infective parasites is seasonally cyclic and should be taken into consideration when building a parasite control program. Egg shedding by cyathostomins in northern temperate climates, increases in the spring, peaks in the late summer and autumn, and declines in the winter when hypobiotic infection predominates. In the southern temperate climates, shedding increases in the autumn, peaks in the winter and early spring, and declines in the summer when hypobiosis predominates. Poor Pasture Management. Infrequent mowing, sporadic collection or disposal of manure (less than twice weekly), and continuous grazing without pasture rotation increase likelihood of infective parasite exposure during grazing. Inadequate Sanitation. Although exposure occurs predominantly in pastures, stabled horses with intermittent access to pasture can incur substantial cyathostomin burdens, as well. 8,9 The ability of the cyathostomin L2 stage to develop within two days into infective L3 underscores the importance of routine sanitation of living areas. 5 Stalls and feeding areas should be cleaned daily and corrals and paddocks at least twice weekly. Transportation. Also to be considered are the effects of commingling horses that occurs due to frequent interherd movement. When horses that have received inadequate deworming care are introduced to a resident herd, the new arrivals may become a major source of fecal egg shedding. Incoming horses of unknown parasite status should be quarantined from the resident population until they have received FEC testing and anthelmintic treatment as needed. Overstocking. Risk of parasite exposure increases when the stocking density is greater than one horse (or any combination of Equidae) per two acres. 4,8 Anthelmintic treatment will always be compromised if it is not supported by management practices that minimize parasite exposure. By recognizing environmental risk factors and implementing management modifications that reduce parasitic exposure, the residual capacity of anthelmintics utilized in the program will be maximized and the overall parasite control program will be more effective. To further underscore the point, many parasitologists believe that careful herd and environmental management is at least as effective as anthelmintic treatment in preventing clinical disease and controlling the emergence of resistant parasites. Step 4: Determine the Horse s ID Individual and herd FEC information combined with identification of environmental risk factors are integral to the development of an evidence-based parasite control plan. Equally essential are realistic goals and the continual assessment of individual program components. Total eradication of fecal egg-shedding in horses on pasture is neither sustainable nor a suitable goal for managing gastrointestinal parasitism. One of the paradoxes of parasite control is that maintaining a FEC-negative herd creates an immunologically susceptible population and occurs only as a result of anthelmintic overtreatment, which increases the rate of selection for resistance. Additionally, there is little evidence to indicate that a continuous FEC-negative status is necessary for normal development of foals 4
or well-being of adult horses. 2 Finally, parasitologists generally agree that the proper objective of parasite control is to maintain the parasite burden at a low level rather than to eliminate parasites entirely. 5 This reasonable middle ground avoids overtreatment that leads to resistance, keeps parasite control programs within cost-effective boundaries, and helps the host maintain partial immunity to overwhelming infection. What is an appropriate FEC target for an effective parasite control program? There is limited data that correlates FEC levels with clinical effects. One recent study demonstrated improvements in body condition score and live weight gains based on treatment of horses with a herd FEC mean of 94-125 eggs per gram (EPG). 10 Although there is no universally accepted standard, commonly used thresholds for initiating treatment are 200-500 EPG for individual horses and 100-300 EPG for the herd mean. 2,11 In addition to being an indicator of intestinal parasite status, the FEC can be utilized as a tool to evaluate the effectiveness of individual anthelmintic products. When a FEC is obtained before and after antiparasitic treatment, it becomes a FEC reduction test (FECRT), a determination of the degree of parasite resistance to a specific treatment. The response to anthelmintic treatment can be determined by using paired fecal samples obtained before treatment and 10-14 days afterward. The FECRT is calculated using the following formula: % FEC reduction= (pretreatment EPG-posttreatment EPG) pretreatment EPG X 100 Cutoff FEC reduction values for determining resistance vary depending on the class of anthelmintic used. For macrocyclic lactones (ivermectin or moxidectin), < 98% reduction in FEC indicates resistance. For benzimidazoles or pyrantel salts (pyrantel pamoate or pyrantel tartrate), values between 80% and 90% are suspicious of resistance, and < 80% reduction in FEC indicates resistance. 2,12 While the FECRT provides information concerning resistance to anthelmintics, it does not present a complete clinical picture. When utilizing the FECRT to evaluate an anthelmintic product, results should be considered in light of the horse s ongoing parasite exposure circumstance and individual immunologic vulnerability to reinfection. Overdispersion tendencies, seasonality of the fecal shedding, and the generally reduced efficacy of all three equine anthelmintic classes in younger horses, 13,14 all play a roll in affecting rates of parasitic reinfection. Low or sporadic levels of apparent anthelmintic resistance as determined by FECRTs should not be cause for entirely discounting the value of any of the three antiparasitic drug classes. Eliminating any single anthelmintic class from a parasite control program places greater reliance on the remaining anthelmintics, increasing the velocity of selection for resistance. Conclusion Over the last 40 years, the role of the veterinarian in many equine parasite control programs has been reduced to advisory, primarily due to the decline of the large strongyle and the safety, efficacy, convenience, and affordability of currently available paste anthelmintics. Unfortunately, this same ready availability of anthelmintics along with the one-size fits all rotational deworming strategy that virtually eliminated the parasite responsible for most episodes of colic decades ago has led to the overuse of anthelmintics and placed heavy selection pressure for resistance on the primary parasite of concern today, the small strongyle. As a result, the basis for which veterinarians derive their advisory information and strategies employed to combat equine internal parasitism must be modified to fit the challenges presented by the current parasitic population. By implementing an individualized parasite control approach that relies on diagnostic evaluation to identify a horse s intestinal parasite status, appropriately utilizes anthelmintic products based on the horse s clinical needs, and effectively recognizes and mitigates the risk factors that contribute to reinfection, veterinarians will once again assert their knowledge of parasitic epidemiology into this vital aspect of equine health, modernize and enhance parasite control in their patient populations, and reduce the selection pressure that is creating anthelmintic resistance. 5
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