1 Nov 2025
Researchers recommended farmers implement a range of management practices, including the introduction of susceptible parasites to dilute those with resistance.
Chris Simpson
Job Title
A common deworming treatment for sheep and goats is no longer effective due to a buildup of anthelmintic resistance, a new study has shown.
Researchers from the Foundation for Food and Agriculture Research (FFAR) and the Cummings School of Veterinary Medicine at Tufts University conducted faecal egg count reduction (FECR) tests on small ruminants across southern New England and found reduced efficacy of fenbendazole.
The authors said the findings revealed the need for improved use of parasite-management tools and strategies to combat increasing resistance.
Researchers tested faecal egg counts (FEC) of 110 animals (80 goats and 30 sheep) across 19 farms in Connecticut, Massachusetts and Rhode Island before and after administering fenbendazole.
They found an average FECR of 41%, well below the minimum clinically recommended threshold of 90%.
Just 14 (12.7%) of the animals met that threshold, while 96 (87.3%) had a failure of treatment; all 19 farms had at least one animal that did not respond to the treatment.
Of the 96 that did not respond, 34 animals (35%) actually had a higher FEC following administration of fenbendazole.
The researchers also found there was no link between moving animals between farms and the dewormer’s efficacy.
They recommended producers submit their annual screening of FECs in mid-June rather than in the spring to get a more accurate representation of the parasite burden their animals will face during the warmer and more humid summer months.
As part of the study, producers also completed a survey in which they highlighted difficulties estimating their animals’ bodyweights to ensure correct dosage; it was observed that the individual weights were higher than the producers had estimated prior to the study.
The researchers noted that under-dosing can contribute to the development of resistance and urged farmers to invest in scales to ensure accurate doses are administered.
They also recommended implementing rotational grazing or taking animals out of pasture where possible to disrupt parasite lifecycles and prevent overgrazing, which can also increase the likelihood that animals ingest parasite eggs.
Lead author Eleanor Kharasch, a FFAR veterinary student research fellow, said: “An important takeaway is that we cannot deworm our way out of parasite resistance.”
She continued: “There is a possibility that fenbendazole can work again in the future if we opt to improve management practices and minimise deworming.
“If we just use the next drug with less resistance, we eventually will create resistance to that drug and so on; in some circumstances, veterinarians may encounter cases where all of our dewormers don’t work.”
Dr Kharasch noted that while there are alternatives to fenbendazole, “it is extremely important that producers know that those are not the solution to the parasite resistance we are seeing.
“Resistance in a parasite population can actually be reversible if we improve our management practices and increase the amount of parasites in our animals that are still ‘susceptible’ to our treatments; this will dilute out the resistant parasites when we allow non-resistant parasites to ‘contaminate’ the pastures to outnumber any resistant ones.”
