HOW-TO SESSION: LIFE STAGE MANAGEMENT Table 3. Anthelmintic Efficacy: Interpretation of FECR Values Used to Determine Anthelmintic Efficacy Against Small Strongyles Anthelmintic


Benzimidazoles Pyrantel


Ivermectin/moxidectin


FECR When Drugs Were First Introduced to Market


99%


94–99% 99.9%


Adapted from AAEP Parasite Control Guidelines.27 FECR Suggesting


no Evidence of Resistance 95%


90% 98%


FECR Suggesting Suspected Resistance


90–95% 85–90% 95–98%


FECR Indicative of Resistance


90% 85% 95%


shorter after treatment with both compounds in this drug class. When first marketed, ivermectin and moxidectin suppressed strongyle egg shedding for 8 weeks and 12–16 weeks, respectively. Recent re- ports have documented reduced ERPs of less than 5 weeks forMLanthelmintics.7,19 Additional studies have demonstrated that L4 small strongyle larvae residing in the intestinal lumen are able to survive treatment with MLs.15 The two most popular fecal egg counting tech-


niques are the modified Wisconsin method, that involves centrifugation and flotation using a Sheath- er’s sugar solution and can detect egg counts as low as 1–5 EPG; and the McMasters technique, which relies on flotation using one of several solutions (e.g.,


saturated salt, saturated sugar-salt, ZnSO4,or Sheather’s sugar) and the use of a calibrated count- ing chamber, and has a lower limit of detection of  25–50 EPG. The modified Wisconsin method offers an increased likelihood of observing cestode eggs if present and is the preferred method to use when performing a FECRT due to its low egg detection limit. Fecal samples collected for either method should be as fresh as possible and stored in airtight bags or containers to reduce egg hatching. If refrig- erated, fecal samples can be stored for at least 5–7 days. The reader is referred to other references for details regarding sample handling and storage as well as detailed testing procedures.26 The FECRT is the only method currently avail-


able to determine whether anthelmintic resistance is developing. Originally designed to evaluate re- sistance among strongyle species, the FECRT has also been applied to ascarids, although results should be interpreted cautiously. To perform a FECRT, a fecal sample is collected prior to deworming and 10–14 days following treatment. The numbers of EPGs in the pre- and post-treatment samples are used to cal- culate the percent reduction in FEC using the follow- ing equation:


EPG pre-treat.  EPG 14 days post-treat. EPG pre-treat.


 100FECR (%)


The FECRT can be used for each horse individually, but in the interest of economics and labor, it is often sufficient to test groups of individuals previously identified as moderate to high shedders. The FECR values obtained from those groups are used to infer drug efficacy for the farm. Horses should not


Anthelmintic


Benzimidazoles Pyrantel


Ivermectin Moxidectin


have received anthelmintic treatments for at least 8–10 weeks prior to performing the FECRT. If moxidectin was the last drug administered it is pref- erable to wait at least 12–14 weeks before collecting a pretreatment sample. The FECRT can also be used to screen newcomers arriving on the farm. Only foals old enough to be shedding parasite eggs should be included in the screening. Shedding of (small) strongyle eggs typically does not commence until foals are 6 weeks of age or older and ascarid eggs usually do not develop in fecals until foals are at least 8–12 weeks of age. Egg-counting tech- niques with greater sensitivity should be used to calculate FECR and only animals with high-enough pretreatment egg counts (e.g.,100 EPG) should be included. Given that variability among individual animals can be quite extreme, FECR should be de- termined for at least five to 10 animals within a farm population and then the average FECR calcu- lated for the group tested. Guidelines for interpret- ing FECRT results are presented in Table 3. It is important to recognize that efficacies differ among the various anthelmintics. The values presented in Table 3 are suggested cutoff values to use to decide whether parasites are still susceptible to a given drug if there is evidence of developing resistance or if widespread resistance exists. ERP is defined as the time interval between the


last effective deworming treatment and the resump- tion of significant egg shedding. Table 4 lists the ERP for commonly used equine dewormers. Occa- sional monitoring of the ERP for a given drug class on a farm is a reasonable way to determine whether resistance is beginning to develop against drugs pre- viously considered effective. A shortening of the ERP is considered a precursor to the development of


Table 4. Cyathostomin Egg Reappearance Periods (ERP) for Commonly Used Equine Anthelmintics


Expected ERP When Drug Is Still Effective


4–5 wk 4–5 wk 6–8 wk


10–12 wk


ERP When Drug Was First Introduced


6 wk


5–6 wk 9–13 wk 16–22 wk


Reprinted from AAEP Parasite Control Guidelines.27 AAEP PROCEEDINGS  Vol. 62  2016 475


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