Why guess when you can test! A guide to evaluating anthelminthic efficacy in small ruminants

Small ruminants are particularly susceptible to infections by gastro-intestinal nematodes (GIN). Since the 1950’s1, these infections have mainly been managed with anthelminthic (AH) drugs. The efficacy of these molecules was assumed to be flawless and was indeed seldom checked. Nowadays, the ever-increasing list of herds facing resistant or multi-resistant GIN is bringing a change in parasite management worldwide2,3. One aspect of this change is the increasing need to evaluate efficacy of dewormers when they are used in small ruminants. This article aims to clarify the circumstances that lead to testing and gives indications on how to proceed.

When to test for anthelmintic efficacy

One obvious situation that dictates testing the efficacy of dewormers is a suspicion of reduced efficacy by the farmer or the veterinarian. Farmers are usually the first ones to detect a case of reduced efficacy of the molecule(s) they are using. Frequently, they report a persistence of clinical signs such as poor body condition score, anemia, diarrhea, poor fleece quality, lower milk production, and mortality despite the animals being treated. For the practitioner, a case of reduced efficacy is suspected when treatments on the same flock are more frequent than usual4. It is important to note that these listed signals come quite late and can trigger an emergency treatment of a whole flock. These situations have a high chance of leading to economic losses, hence the importance of anticipating diagnostic tests. Reduced efficacy can signal the presence of resistance in the farm, but it can also be the consequence of underdosing, maladjusted nutrition or poor drug bioavailability.

Testing efficacy of AH molecules in a herd doesn’t, and in the best-case scenario, shouldn’t, be done only when efficacy is dwindling. Efficacy of AH families should be monitored after each use when efficacy is unknown, and then regularly over the years. Resistant strongyle can be brought into a farm with animal trade: AH efficacy should be evaluated before any herd mixing takes place.

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How to check that a dewormer is still effective against GIN

Checking if an anthelmintic is working can be done by conducting a post-treatment pooled fecal egg count (FEC). To perform a pooled FEC, at least ten animals should be sampled individually after treatment, with each animal contributing equally to the mix (1 to 3 gram of feces5,6). Parasitological examinations can be conducted using the modified McMaster technique (figure 1)5–7, or by using the Mini-FLOTAC® device5.

 fecal egg countFigure 1. Pooled FEC as described by 6 using the modified McMaster technique8

Pooled FEC are not officially supported by the WAAVP (World Association for the Advancement of Veterinary Parasitology), but this strategy has the merit of being simpler and more economical to perform. In addition, studies have shown good correlations between pooled FEC and the average of individual FEC 5. The main drawback when calculating fecal egg count reduction (FECR) using pooled instead of individual FEC is the lack of variability in the results. Latest WAAVP guidelines for diagnosing anthelmintic resistance using individual FEC in ruminants can be found here. It is also important to remember that timing of post-treatment FEC is dependent on the molecule that is used (table 1).


Anthelminthic drug class

Time interval (days) for follow up sampling 9–11


7 - 10


10 - 14

Macrocyclic Lactones

14 - 17


17 - 21

Several drugs at once


Table 1. Recommended interval between drug administration and follow up sampling for a fecal egg count reduction test (FECRT)  

Restricting FEC determination to a single fecal analysis after anthelmintic treatment, although informative, may lack of accuracy. Firstly, the absence of any information regarding the levels of infection prior to treatment can be problematic. This can be a critical information as the anthelmintic efficacy will not be interpreted similarly if pre-treatment FEC are low (typically below 300 EPG in sheep), or high (above 1 000 EPG in sheep). The second factor to take into consideration is the natural variation in FEC over time. This will largely be ignored if a control (untreated) group is not present 10. This control group should regroup similar animals to the treated group (same herd and pasture management). Therefore, to test the efficacy of wormers the ideal scenario is to perform pooled FEC with a control group and to do it just before treatment and after treatment at the recommended time intervals.

To calculate the levels of efficacy of wormers, the following formulae should be followed:

FECR = (1-EPGt/EPGc) x 100

Where EPGt is the FEC value for the treated group and EPGc is the FEC value for the control group. Efficacy of the tested molecule should then be considered normal when FECR >95% and reduced otherwise. When efficacy is below this threshold, confounding factors should first be excluded before resistance is declared. Confounding factors are numerous, highly likely to occur in farm environments, and rarely possible to control12. They can originate from differences in the pharmacokinetic behavior among drugs4, parasite species, host types and individuals that affect the therapeutic anthelmintic response, helminth demographics affecting test repeatability and technical errors. Repeated assessment is a good approach to detect trends and reach more robust conclusions. In case of resistance, if possible, identifying which GIN species have become tolerant to the molecule is of interest. In any case, identification of resistance should lead to rethinking parasite management to integrate different tools, like pasture rotations and selection for more GIN-resistant animals. These steps cannot be taken alone for the farmer, and the veterinarian can contribute to offer tailored management options.

 fecal egg count

Points to consider when analyzing fecal egg count reduction test (FECRT) results

Testing to assess dewormers efficacy should be strongly considered in any sheep or goat herd health management plan. Results of a FECRT, as is true for any diagnostic test, should be interpreted according to the clinical context, and bearing in mind confounding factors specific to this method.

It’s good to have a general knowledge of the resistance levels in the country we work in, and in which host species and what specific GIN. Gathering information about GIN management practice on a farm also helps define the chances of resistance occurring: how often are animals treated for GIN? Are diagnostic FEC regularly run? Have animals been bought in the last 5 years? Those are all relevant questions to have in mind when interpreting FECRT results.

Every step of the FECRT is important to ensure reliable results. It’s indeed very important that quality of the anthelminthic used is correct, and the product has not expired. Adapted formulation should be used, e.g. injectable formulation should be preferred over topical solutions in small ruminants as the latest are known for their low plasmatic disposition in goats and sheep 4,14. Other important considerations are linked to sample treatment, as conservation and timing is particularly important when running FEC for GIN.

Finally, communicating the results rapidly, and offering a way forward for farmers as far as integrated parasite management, can greatly enforce a veterinarian and farmer relationship.

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2.  Rose Vineer, H. et al. Increasing importance of anthelmintic resistance in European livestock: creation and meta-analysis of an open database. Parasite 27, 69 (2020).

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8. Raynaud, J.-P., William, G. & Brunault, G. Etude de l’efficacité d’une technique de coproscopie quantitative pour le diagnostic de routine et le contrôle des infestations parasitaires des bovins, ovins, équins et porcins. Ann. Parasitol. Hum. Comp. 45, 321–342 (1970).

9. COMBAR. Fecal Egg Count Reduction Test (FECRT) Protocol _ Gastrointestinal Nematodes - Sheep and Goats. combar-ca.eu (2021).

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14. Bordes, L., Ticoulet, D., Sutra, J. F., Lespine, A. & Jacquiet, P. Lack of efficacy of topical administration of eprinomectin against gastrointestinal nematode in a French dairy sheep farm: A case of underexposure of worms. Vet Record Case Reports (2022) https://doi.org/10.1002/vrc2.435.

Sophie Jouffroy (PhD candidate in small ruminants parasitology)

About the author

Graduated from National Veterinary School in Toulouse (ENVT, France) in 2017. After working 2 years as a mixed practitioner in central France, completed a Master of Science degree in Integrated Management of Animal Tropical Disease through ENVT and Cirad (Montpellier). After several months as a practitioner in Eastern France, joined CEVA as a PhD student through the industrial agreement for training through research (CIFRE) system. In the last year of doctorate, working in the academic setting of ENVT on resistance to eprinomectin in strongyle of dairy sheep. Junior researcher interested in clinical and applied research in small ruminants.

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Damien Achard

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