Reducing parasite resistance on equine operations using a comprehensive, whole-farm approach

Key Concepts that Provided a Basis for the Development of the Project Methods and Protocol

1. Important changes have occurred in equine parasite populations due to the introduction of new classes of dewormers. Large strongyles are now rare and small strongyles are the parasites of concern in adult horses, while ascarids remain the most important parasite in foals and weanlings. 
2. Traditional parasite control programs feature rotational use of dewormers at regular intervals. This strategy is 40 years old and was designed to eliminate highly pathogenic large strongyles. This strategy was very successful and disease from large strongyles is now very rare. 
3. Small strongyles are present in all horses but are relatively mild pathogens and only produce disease when the parasites are present at very high levels. Frequent deworming treatments are not needed to keep most adult horses healthy. 
4. The strategy promoted today is to use products with proven efficacy that are administered at the appropriate time of the year based on the parasite burdens of individual horses. 
5. In northern climates, egg shedding tends to be low in winter, whether or not the horses have been dewormed, and egg shedding increases in early spring through the summer months. 
6. Adult horses vary greatly in immunity to parasites and shedding of small strongyle eggs. Most adult horses tend to have good immunity against small strongyles; 40-60% of adult horses tend to be low shedders; 20 to 30% are moderate shedders; and 10 to 30% are high; 80% of eggs come from 20% of the horses on a farm. 
7. Adult horses tend to shed roughly the same number of eggs throughout their life time; low shedders will often remain low and high shedders have a tendency to remain high. 
8. Horses less than 3 years of age require special attention and more frequent deworming since they are more susceptible to infection and developing disease. 
9. Anthelmintic resistance is highly prevalent in small strongyles and ascarids. Resistance is the ability of parasites in a population to survive treatment with a dewormer. Resistance is an inherited trait. The rate of development of resistance is determined by the degree of selection pressure from repeated exposure to a dewormer and the extent to which surviving parasites pass their genes to the next generation. Resistance occurs over time. With continued reproduction of resistant worms, eventually the resistant population is high enough that the dewormer fails.
10. To reduce the rate at which resistance occurs, it is critical to maintain parasites that are not exposed to the dewormer. This is known as the refugia. Parasites that are not exposed to the dewormer will not develop genes that are resistant to that dewormer. 
11. Egg reappearance time is defined as the time interval between the last effective deworming treatment and the time it takes for significant egg shedding to be detected. Egg reappearance periods that are shorter than anticipated are an indication that resistance is developing to that product. The table below shows the egg reappearance period for the products when they were first developed and when the drug is fully effective. To evaluate egg shedding in adult horses, the sample should be collected 4 weeks after the egg reappearance period for the dewormer used. 

Key Concepts Utilized in the Development of Project Deworming Protocol

1. Fecal egg counts should be conducted on all farm horses and deworming efforts should be focused on horses with high small strongyle egg contamination potential. 
2. A basic foundation of anthelmintic treatments should be considered for all horses. This generally consists of one or two yearly treatments. 
3. A product that provides control of strongyles, bots and tapeworms should be used in the fall, usually after the grazing season. 
4. Efficacy of dewormers on the farm should be determined using a fecal egg count reduction test. 
5. Deworming treatments should be used during times of peak transmission – usually spring through fall when the number of non-resistant parasites will be highest in the pasture.
6. A product labeled for encysted larvae should be used in horses that have a history of parasite infection and disease. 
7. Yearlings and two year old horses have a greater risk of parasite infection and disease due to reduced levels of immunity. More frequent deworming treatments (often 4 or more per year) may be necessary to control parasites in young horses. Conducting fecal egg counts on a regular basis will help with deworming decisions and conducting fecal egg count reduction tests will ensure that the products are working.
8. Foals and weanlings are very susceptible to parasite infection and great care should be taken to ensure that the foals are dewormed with the correct product at the appropriate time for the parasites that are present. Targeted deworming based on fecal egg counts is not recommended for this age group. Very specific deworming protocol has been established for foals and weanlings. 
9. Fecal egg counts provide a measure of the number of eggs being shed into the environment. A fecal egg count of zero does not necessarily mean that the horse is parasite free. The horse may have small strongyle larvae that are encysted or adults that are not shedding eggs and may harbor parasites other than small strongyles. 
10. FECs from an individual horse will vary from sample to sample by as much as 50%. A horse with a FEC of 1000 may actually be shedding 500 or 1500 eggs per gram. 
11. Low shedders are those horses that are considered to be below the threshold for deworming (generally between 100 to 500 eggs per gram; high shedders are generally over 1000 EPG. 
12. Just deworming the high shedders significantly reduces the eggs that are put into the environment.In order to reduce the selection pressure that can lead to the development of resistant parasites, horses with low to moderate fecal egg counts should not be dewormed.

Project Methods  

1. Data was collected about the farm operation including number of horses, stability of the horse population, acres of pasture, turnout schedule, pasture conditions, BMPs adopted on the farm, and previous deworming practices. 
2. FEC data included name and weight of horses, amount of product used, FECs prior and post deworming. Horse profiles were generated and included age, sex and breed of the horse, horse activity and stress levels, and any health issues that the horse experienced. 
3. The compiled data for all farm operations was used to determine overall parasite burdens on PA farms, the % of horses on the farm that are “shedders” and to evaluate efficacy of the products used on the farm. The data will also be used to determine if resistance to a product is found on all farms in PA or if resistance is farm specificProject Methods - Conducting and Interpreting Small Strongyle Fecal Egg Counts in Horses.
4. Although specific protocol has been developed for this project based on AAEP guidelines, the farm manager and veterinarian were encouraged to provide input into the deworming program that was developed for each farm.
5. In order to empower horse owners to make changes in their deworming program, it is important that clientele have the knowledge and skills necessary to be confident they are making good management decisions. To ensure sufficient knowledge about equine parasites, resistance development and deworming management practices, farm owners participating in the research portion of the project were required to complete the short course, Reducing Parasite Resistance on Equine Operations Using a Whole Farm Approach.
6. All farm owners that completed the short course and wanted to be part of the project were accepted. The farm partners met at predetermined sites and conducted fecal egg counts on all horses on the farm in order to monitor egg shedding and identify low and high “shedders”. Since lack of monitoring equipment was an obstacle to conducting egg counts, farm partners participated in previously scheduled monitoring sessions and utilized microscopes and supplies strategically placed in Extension offices. Trained Extension staff provided assistance and ensured that protocol was followed.
7. Fecal egg counts were conducted approximately every 8 to 12 weeks beginning in April or early May. The start time for the project varied based on geographic location of the farm and climate conditions.
8. The Paracount – EPG system, consisting of a volumetric vial and a McMaster’s slide of specific volume, was used to conduct FECs. Anthelmintic efficacy was determined by conducting pre and post de-worming egg counts for the products that were provided.
9. Although no farms were prevented from participating in the project, only those farms with a minimum of three horses that were moderate to high strongyle shedders pre-treatment were included in the treatment efficacy assessment
10. For the project, low shedders were defined as horses with a FEC of 1-200 EPG, moderate shedders have a FEC of 200-500 EPG, and high shedders have a FEC of 500 EPG. Generally horses were targeted for deworming if the egg count exceeded 300 EPG. Since there is quite a bit of variability in manure samples obtained from the same horse, this number was only a recommended guideline for this project.
11. The deworming protocol was developed using AAEP guidelines for young and adult horses. If foals and yearlings were present on the farm, the farm managers was encouraged to monitor egg shedding in these young horses, but the data was not included in the determination of high and low shedders and product resistance.     

Determining Resistance - Fecal Egg Count Reduction Tests

1. Since a major project goals was to determine which dewormers are effective on PA farms, it was important to conduct pre and post deworming fecal egg counts of horses in the project.
2. The detection limit for the Paracount EPG System system is 25 eggs per gram. Based on this detection limit, horses with small strongyle fecal egg counts of 500 to >1000 are excellent candidates for conducting fecal egg count reduction tests. Horses with egg counts of 200 to 500 are considered to be suitable for the test but the results need to be carefully evaluated. For this project, farm owners were supplied with dewormers for horses that have FECs of >300 EPG and were asked to recheck egg counts in 14 days post deworming. Monitoring sites with trained staff and equipment and supplies were available for all scheduled FECs and post deworming sessions.
3.  To ensure product compatibility and consistency in post deworming checks, participants were provided with the deworming products for all moderate to high shedders on the farm.
4. To calculate the % fecal egg count reduction he following formula was used:% FEC = (FEC before deworming – FEC post deworming) / FEC before deworming X 100 
5. Since resistance occurs at the farm level (not the horse level), fecal egg count reduction tests should be preferably be performed on 5 to 10 horses on each farm that have moderate to high egg counts (over 300 EPG for this project). Only farms that had at least 3 horses with egg counts over 300 were included in the project data. The % fecal egg count reduction data for all the horses tested on the farm was added together and an average taken. Resistance was determined based on the average of the fecal egg count reductions for each dewormer tested.
6. Egg shedding reduction can vary based on the dewormer used. Benzimidazole (Anthelcide, Safeguard, Panacure) and pyrantel (Strongid, Exodus) should reduce egg shedding by at least 90%. Ivermectin and Moxidectin (Quest) should reduce egg shedding by 95% or more. If egg shedding is not reduced to these levels, then resistance should be suspected.
7.  Farm partners were asked to evaluate efficacy of pyrantel, fenbendazole, and ivermectin on their farms. Pyrantel was chosen as the first product evaluated because of varying reports of resistance levels.
8.  If fewer than 5 horses were used for egg count reduction tests, the results needed to be carefully evaluated unless there was very high efficacy of the product (over 98% reduction in egg shedding) or very low efficacy (less than 80% reduction in egg shedding is consistently seen for all horses). In addition, since egg shedding can vary by as much as 50% from sample to sample, if the % reduction fell in a grey zone -   defined as 5% less than the 90% target for benzimidazoles and pyrantel and 95% for ivermectin and moxidectin, then resistance needed to be determined carefully. It was sometimes necessary to repeat the test.