What Is Anthelmintic Resistance?
Anthelmintic resistance occurs when a population of worms is no longer killed effectively by a drug that previously worked against them. It is not the animal that becomes resistant — it is the parasites themselves. Through natural selection, worms with genetic mutations that allow them to survive drug exposure reproduce and pass those traits to offspring. Over generations, resistant worms can come to dominate a parasite population, rendering a previously effective treatment partially or wholly ineffective.
This is not a theoretical concern. In sheep, horses, and goats, anthelmintic resistance is a serious and well-documented crisis. The question is: to what extent is it affecting companion animals, and should pet owners be worried?
The Livestock Situation: A Cautionary Reference Point
To understand the companion animal picture, it helps to grasp how severe resistance has become in production animals. In sheep globally, resistance to benzimidazoles (the drug class that includes fenbendazole and albendazole) is now nearly universal. Resistance to macrocyclic lactones (ivermectin, moxidectin) and to levamisole is widespread. Many sheep flocks in Europe, Australia, and North America now harbour worm populations resistant to three or more drug classes simultaneously — so-called multi-drug resistance.
The drivers of this crisis were predictable in hindsight: high treatment frequency, underdosing, failure to monitor efficacy, and movement of animals without quarantine and treatment protocols. The companion animal sector has some of these same risk factors, which is why parasitologists are paying close attention.
Resistance in Companion Animals: What the Evidence Shows
Benzimidazole Resistance in Dogs and Cats
The most studied area of concern in companion animals is resistance to benzimidazoles, particularly in Toxocara canis (the dog roundworm). Laboratory studies have identified benzimidazole resistance gene variants in Toxocara canis populations in several countries. However, the clinical significance of this is not yet clear — the presence of resistance genes does not necessarily mean treatment is failing in practice.
Some studies have reported reduced efficacy of fenbendazole against Uncinaria stenocephala (a hookworm) in European kennels, suggesting that high-frequency treatment in densely housed animals may be selecting for resistance faster than in the general pet population.
For now, the evidence of clinically significant benzimidazole resistance in companion animals is emerging but not yet conclusive. This does not mean complacency is appropriate — it means monitoring matters.
Macrocyclic Lactone Resistance: Heartworm in the USA
The most concrete and alarming example of resistance in companion animals comes from the United States, where macrocyclic lactone (ML) resistance in Dirofilaria immitis (heartworm) has been confirmed. Dogs receiving monthly preventives containing ivermectin or milbemycin oxime have developed heartworm infections despite consistent treatment — a finding that was initially disbelieved but has since been confirmed through rigorous controlled studies.
This resistant strain, sometimes called the MP3 strain, was likely selected in areas of the Mississippi Delta where heartworm transmission pressure is extremely high and the same drug classes have been used for decades. It represents a genuine failure of a previously reliable prevention tool.
Macrocyclic Lactone Resistance in Europe: Current Risk
The current risk of ML-resistant heartworm in Europe appears low. European Dirofilaria immitis populations have not been under the same sustained selection pressure as those in the United States. However, international travel — both of people with pets and of rescue dogs imported from higher-risk countries — creates pathways for resistant strains to enter Europe. ESCCAP recommends ongoing surveillance and caution, and European veterinary parasitologists are monitoring the situation closely.
Responsible Use: The ESCCAP Approach
ESCCAP emphasises that responsible anthelmintic use is the cornerstone of resistance management in companion animals. Key principles include:
- Treat when necessary, not by reflex: Risk-stratified treatment protocols are preferable to blanket high-frequency worming for all pets regardless of exposure.
- Use the right drug at the right dose: Underdosing is one of the fastest routes to selecting resistant worms. Always use weight-appropriate doses.
- Do not rotate drug classes unnecessarily: In livestock, rotation was once thought to delay resistance, but the evidence now suggests it may accelerate multi-drug resistance by exposing worm populations to multiple selection pressures. For companion animals, using the most effective available drug for the specific parasite is preferred over reflexive rotation.
- Monitor treatment efficacy: Faecal egg count (FEC) testing before and after treatment is the standard way to detect reduced efficacy. If worm egg counts are not falling by at least 95 per cent after treatment, resistance should be suspected.
Faecal Egg Counts: Monitoring Efficacy in Practice
Faecal egg count reduction testing (FECRT) is the practical tool for detecting resistance in a given animal or environment. A fresh faecal sample is tested before treatment; another is taken ten to fourteen days after treatment. A significant reduction in egg counts confirms the treatment is working. Persistently high post-treatment counts — particularly in kennels, breeding establishments, or shelters — may indicate resistance and should prompt a veterinary investigation.
For individual pet owners, FEC testing is most valuable in dogs with suspected treatment failure or in high-risk environments such as rescue centres. It is less routinely necessary in the average household pet with a well-managed protocol.
Why Vet-Prescribed Treatment Remains Important
One consequence of the resistance threat is that it reinforces the importance of using licensed products at appropriate doses under veterinary guidance. Over-the-counter products sold in supermarkets and unregulated online marketplaces are often based on older compounds (such as piperazine) with a narrower spectrum of activity. They are also more likely to be underdosed, which is a direct driver of resistance selection.
Products such as Milbemax, Drontal, Advocate, and Profender are licensed veterinary medicines formulated to deliver precise, effective doses. They come with regulatory oversight of efficacy claims that unregulated products do not. Using licensed products — obtained from your vet or reputable retailers such as Zooplus in the case of non-prescription items — is both clinically and ethically the right approach.
The Future of Parasite Control
Research into new anthelmintic classes for companion animals is ongoing, though the development pipeline is far less active than for human infectious diseases. Novel targets and formulations — including combination products that hit multiple biological pathways simultaneously — may help slow resistance development. Vaccines against Toxocara and other species are in early research stages.
In the meantime, the most effective tools available are good monitoring, risk-based treatment protocols, and consistent use of licensed products at correct doses — under veterinary supervision wherever appropriate.
Key Takeaways
- Anthelmintic resistance is well-established in livestock and an emerging concern in pets
- Benzimidazole resistance genes have been detected in Toxocara canis but clinical impact remains uncertain
- ML-resistant heartworm is confirmed in the USA; European risk is currently low but being monitored
- Responsible use means risk-stratified treatment, correct dosing, and monitoring efficacy via faecal egg counts
- Always use licensed veterinary products and consult your vet for a tailored protocol
Written by Sarah Bennett, animal health writer at ForPetsHealthcare.