Making Sense of the Claim: Is 92-96% Really a Reasonable Efficacy for Use of Fenbendazole in Vet Medicine?
Fenbendazole veterinary use is the use of fenbendazole (a benzimidazole class anthelmintic) to treat parasitic nematodes in domesticated animals with guidance from a veterinarian. The reported efficacy of 92–96% indicates that treated cattle or goats have lower worm burdens (or faecal egg counts) by 92–96% specifically for Haemonchus contortus after treatment.
In real studies, the picture is mixed: some high efficacy in ideal conditions, but also many instances of limited efficacy via resistance.
In cattle, one study in Haryana, India reported 100% faecal egg count reduction (FEC reduction) following treatment at 5 mg/kg of fenbendazole of two groups of animals where pre-treatment egg counts were >150 EPG and Haemonchus was the predominant species. (ResearchGate) In other early studies carried out in calves [54-57] >99% effectiveness was obtained with fenbendazole-medicated feed blocks (5 mg/kg) against adult and immature stages of H. contortus. (PubMed) Azura benchmarks show that under favorable conditions, the “92-96%” benchmark can be achieved or exceeded. But there are numerous instances in which effectiveness can be less than that threshold because of resistance, poor control, or misuse.
Why and How Fenbendazole Treatment Efficacy Varies: Resistance, Dosage & Management
Resistance in Haemonchus contortus undermines fenbendazole’s power
Resistance development: Overuse, or underdosing, with benzimidazoles (the drug family of fenbendazole) can lead to the selection for resistant worms. Several studies show:
Benzimidazole resistance (AR) in approximately 31% of farms was detected by researchers on goats in Mozambique when using 5 mg/kg fenbendazole. Resistant worm species involved Haemonchus contortus. (PMC) In Portugal, where fenbendazole was frequently used on farms, FECRT (faecal egg count reduction test) was indicative of only 48-84% reduction — a far cry from the desired 92-96% in mixed goat-sheep flocks. (MDPI)
These examples demonstrate that treatment with fenbendazole does not produce 92-96% efficacy under all circumstances when resistance alleles have reached high frequencies.
Dose, formulation and species do matter for achieving good efficacy
Correct dose and correct route are necessary.
The recommended dosage of fenbendazole in cattle/goats for Haemonchus contortus is 5 mg/kg body weight given orally. Increased dosages or alternate dosing regimens are occasionally employed when resistance is suspected. Feed blocks providing 5 mg/kg over several days achieved >99% in calves. (PubMed) Combinations of anthelmintics (e.g. fenbendazole + ivermectin) have been evaluated in goats to overcome partial resistance. In a goat trial in Haryana, the combined product showed ~98.6% efficacy at 12 days post-treatment. (LUVAS)
Also, the formulation (paste, suspension, feed block) and ensuring stock actually get the correct dose is important.
Efficacy level is supported by management factors & best practices
To achieve and maintain 92-96% drug efficacy with fenbendazole use in veterinary practice several non-drug actions are required.
Selective treatment: Not treating all animals but only those with high FEC or signs of disease is one way to put less selection pressure.
Rotate drug classes: Don’t keep rotating with fenbendazole but only to other drug classes. The following drugs (drugs rotated into) are good for at least 18 months from the last use of any drug in the family.
Refugia: not treating (refuge) some worm population so susceptible alleles can continue to exist and therefore dilute out resistant ones.
FECRT Monitoring: Frequent testing (every 1-2 years or less if failure is suspect) is critical.
Inquire about underdosing: Always determine the weight accurately. Goats may metabolize certain medications more quickly, and wrong dosing can result in a less than satisfactory effect.
A classic error: assuming a single dose passed down the rumen in all cases — even correctly dosed, 50-80% efficacy is the norm on resistant farms unless it’s effectively applied with management changes.
Fenbendazole for Goats and Fenbendazole for Cattle Efficacy Against H. contortus
What does the evidence say about fenbendazole for goats
Goats are frequently more susceptible to insufficient efficacy as a result of underdosing, rapid metabolism, and regular reinfection.
The Mozambican experiment: some farms with 100% susceptibility and some with resistance; overall, suboptimal outcomes on the average of many. In Haryana goat farms 79% decrease in FEC was observed with fenbendazole alone and additive to other drugs it showed better results.
Therefore, although 92-96% might be plausible under well-managed, low-resistance conditions, many goat enterprises achieve lower efficacy.
Here’s what the science says for fenbendazole for cattle
In some studies cattle appear to have more consistent reactions, however resistance does occur.
The Haryana cattle trial revealed 100% efficacy of 5 mg/kg fenbendazole (FBz) under field conditions, when Haemonchus spp. were predominant. The calf feed block experiment supported >99% removal of adult and immature nematodes of H. contortus. And so 92-96% is more practical in cows, under ideal circumstances.
The Role of Fenbendazole for Cats, Dogs, Chickens and Other
Fenbendazole in cats: used to treat roundworms, hookworms, and sometimes Giardia. Dosing varies based on indication, and efficacy is usually good for non-resistant parasites. However, data against Haemonchus is irrelevant because Haemonchus is a parasite of ruminants.
Fenbendazole in dogs: similar; used for some nematodes and cestodes. The correct dose and safety of the product are well known, but it is not effective for Haemonchus.
Fenbendazole in chickens: generally used in feed or drinking water for Ascaridia, gapeworms, etc., but not for Haemonchus.
These examples demonstrate that while fenbendazole is broad-spectrum, its effectiveness is extremely dependent on the fit between drug, dose, species, and parasite.
Expert-Guidance for 92-96% Efficacy
Diagnostics before and after treatment
In addition to that, carry out FECRT (Faecal Egg Count Reduction Test) from time to time to ensure high efficacy. If reduction is less than ~90-95%, interpretation should assume loss of efficacy or resistance. Document the species composition in pre-treatment populations (e.g., using larval culturing) so that you know if H. contortus is one of the predominant species.
Strategic dosing
Where partial resistance is known to occur, administer fenbendazole in combination with a product of another anthelmintic class, rotationally rather than concurrently, or treat pregnant animals only. For instance, resistant goats only showing <80% efficacy when treated with fenbendazole can reach 100% if combined with ivermectin or closantel (according to the susceptibility).
Good herd/flock management
Feed well and minimize stress, all animals with good immunity cope better. Exposure is reduced by management of pastures (rotational grazing, resting pastures). Prevent “set pasture, overgrazed” conditions that only magnify worm loads.
Warning Scenarios – Evidence of what happens with Efficacy Loss below 92%?
When fenbendazole in veterinary usage produces only 50-80% reduction (as observed with goat-farm studies) a number of bad things happen:
Clinical signs (anaemia, weight loss, decreased milk production) persist in infected animals.
Economic losses continue to rise: more treatments, labor, death.
When animals are underdosed or misdosed, genes of resistance spread more swiftly.
Ctrl+F to find other classes used too much instead → risk of MDR nematodes.
So to administer fenbendazole when its efficacy is waning, without any control of such a situation, warrants the taking of chances.
Is 92-96% Efficacy a “Good” Threshold?
Yes if the farm has low resistance (animals weighed, drenched at 5 mg/kg or adjusted for requirement & good management such as nutrition, diagnosis and pasture).
However, in many practical situations, especially in small ruminants/goats, the efficacy is between 60 and 90% due to resistance, under-dosing, or drug quality problems.
So 92-96% is a good destination, but not always an assured one.
Reality check for producers & vets
Before using fenbendazole treatment, test whether H. contortus or other strongyles are susceptible in your herd/flock. Always dose by the actual weight — better too much than too little. Maintain detailed records of treatment (what drug was used, when, how much and subjective effectiveness).
Rotate among the drug classes and possibly use combinations where permissible, said Dr. Cecil. HRZ, pasture recovery, and do not blanket. The key to management of HSB is using refugia. Too many refuges are treated with unnecessary blanket treatments. Post-treatment examination – improving condition, EPG lowered by quite a bit, not just what you can assume after treatment.
Key Research Statistic
For example, one of the most recent studies to also illustrate the phenomenon showed that replacement of a majority resistant population in Haemonchus contortus with a susceptible strain returned efficacy for fenbendazole (a benzimidazole) back up to 95.2% FECRT compared from ~59.3% when the population was predominantly resistant (Jacob et al., 2017).