How Does Anthelmintic Resistance Develop?

This post may contain affiliate links. If you click one, I may earn a commission at no cost to you. As an Amazon Associate, I earn from qualifying purchases.

Key Takeaways

  • Anthelmintic resistance happens when worms become resistant to deworming medicine.
  • It is caused by using dewormers too much or incorrectly.
  • Genetics of the worms also play a role.
  • To prevent resistance, dewormers need to be used carefully.
  • Resistant worms can pump medicine out or digest it before it kills them.


Parasite infections are a big problem for livestock and horses around the world. Deworming medicines called anthelmintics help control these infections. But sometimes the parasites become resistant to the medicines. This means the medicines stop working against them. Anthelmintic resistance has happened with different types of worms and dewormers on many continents. This article will look at how anthelmintic resistance develops in detail.

Understanding why and how resistance happens is important. Then steps can be taken to slow it down. The content here will cover background facts on the topic. It will analyze the main factors that lead to resistance emerging. Key points like treatment practices, genetics, and parasite biology will be comprehensively discussed. The goal is to provide a deep look at the development of anthelmintic resistance. This will help readers to gain a thorough understanding of this major problem.

The article draws on scientific research and expert sources. The aim is to educate readers on why resistance occurs and how it can be prevented or delayed. With this knowledge, proper deworming practices can be implemented. This will help keep valuable dewormers working for as long as possible. Read on to learn all about the development of anthelmintic resistance in livestock and horses.

How Does Anthelmintic Resistance Develop?

What is anthelmintic resistance?

Anthelmintic resistance happens when worms no longer respond to deworming treatments. The medicines become ineffective at killing the parasites. This allows the resistant worms to survive and pass on resistant genes. Resistance can develop to any type of dewormer, including benzimidazoles, imidazothiazoles, macrocyclic lactones, and others.

What causes anthelmintic resistance to develop?

There are several key factors that lead to anthelmintic resistance:

Frequent deworming

  • Dewormers put selection pressure on worms.
  • Frequent treatment kills the susceptible worms.
  • This allows resistant ones to take over the population.
  • The more often worms are exposed, the faster resistance can build.


  • Underdosing provides a major advantage to resistant worms.
  • They survive while susceptible worms are killed.
  • Underdosing includes underweight dosing and incorrect dosing.
  • It can quickly increase resistance frequency.


  • Some worms have genes that help them resist dewormers.
  • These genes can spread through worm populations.
  • Worm species and isolates vary in their genetic potential for resistance.

Treatment practices

  • Treating all animals in a group breeds resistance.
  • But leaving some worms untreated allows susceptibility genes to remain.
  • Targeted selective treatment helps slow resistance.

How does anthelmintic resistance work biologically?

Resistant worms avoid being killed by dewormers mainly through:

  • Changes that allow faster drug breakdown. This destroys the medicine before it can kill them.
  • Upregulating efflux transporters. These pump the drug out of their cells before it takes effect.

These biological changes are produced by genetic mutations. Worms with useful mutations are naturally selected under deworming pressure.

How can anthelmintic resistance be prevented or delayed?

  • Reduce unnecessary deworming to limit selection pressure.
  • Do fecal testing to only treat animals that need it.
  • Rotate between drug classes and don’t overuse any one dewormer.
  • Make sure to dose accurately based on weight.
  • Combine drugs from different classes when treating.
  • Limit the spread of resistance genes by quarantining new animals.
  • Monitor fecal egg count reduction for signs of resistance.
  • Work with your veterinarian to design a sustainable plan.

Frequently Asked Questions

How fast can anthelmintic resistance develop in worm populations?

The speed at which anthelmintic resistance develops depends on several factors. With intensive, frequent, and improper deworming, resistance can emerge quite rapidly. Some worm species also have a higher genetic potential for resistance. However, when dewormers are used carefully and appropriately, development of resistance can be significantly slowed. But once present, resistance is difficult to reverse.

Do all types of livestock and horses get anthelmintic resistance?

Yes, anthelmintic resistance has been reported in many host species. These include sheep, goats, cattle, horses, pigs, and poultry. Different classes of dewormers are affected. Benzimidazole resistance is very common. Ivermectin resistance is increasing in parasites of cattle, sheep, and horses. All livestock and horses are at risk if dewormers are misused.

Can combination dewormers prevent resistance in livestock parasites?

Using multiple classes of dewormers together can help delay resistance. The worms are less likely to have resistance genes to different drugs. Combinations hit the worms with different modes of action. This reduces survival of any resistant mutants present. But combination drugs still put selection pressure. So they must be used carefully and appropriately.

What is targeted selective treatment for livestock parasite control?

Targeted selective treatment involves only deworming animals that need it. Fecal egg counts are done to identify animals needing treatment. Those with low counts are left untreated. This maintains a population of worms susceptible to the dewormer. It slows resistance by reducing selection pressure.

How do you know if livestock or horses have anthelmintic resistant worms?

Fecal egg count reduction testing is the main method to check for anthelmintic resistance. Fecal samples are taken before and after deworming. If the treatment reduced egg counts by less than 90%, there is anthelmintic resistance present. PCR tests also help detect resistance genes directly in worm larvae from feces.


Anthelmintic resistance is a major challenge for sustainable parasite control in livestock and horses. Understanding how it emerges is key to preventing its spread. Responsible deworming practices can slow the development of resistance. But vigilance is needed, as resistance can arise rapidly with misuse. Working with veterinarians to design targeted, integrated treatment programs is crucial. With proper management, the useful lifespan of available anthelmintics can be extended as much as possible.

About The Author

Scroll to Top