To Worm or Not to Worm: Anthelmintic Resistance in Dogs

This month is National Pet Month and across the UK people are celebrating the support, love, and affection we receive from these wonderful animals. We all want to keep our pets healthy and regular worming treatment is considered part of that. But is it really the best option? Can repeated treatment of worms lead to wide-spread anthelmintic resistance in dogs as it has in livestock?

Which worms can dogs catch?

Domestic dogs can catch a number of different types of worms, from intestinal worms to worms that live in the heart or lungs. The geographical distribution of these parasites varies with species. So some parts of the world are more inclined towards particular worms. For example, heartworm, Dirofilaria immitis, is not found in the UK. It is of major concern in the US, Australia and more southern parts of Europe, however. The primary intestinal worms in the UK are roundworms or ascarid worms, including Toxocara canis, and tapeworms. Hookworms and whipworms are found in mainland Europe and sometimes crop up in UK dogs.

What is anthelmintic resistance?

Dogs can pick up worms through eating infected soil, other animals faeces, or through contact with the environment.
Dogs can pick up worms through eating infected soil, other animals faeces, or through contact with worm larvae in the environment.

Anthelmintics are the drugs we use to treat helminths, or worms. When you treat an infected animal for a worm infection, some of the worms might have a genetic advantage that lets them survive.1 These worms will then be able to multiply without as much competition and so propagate the genes that let them survive the treatment. If they then pass on to another host animal, if that animal is treated with the same drug, it will not be as effective and might be insufficient to stop damaging symptoms. Eventually, worms might evolve that have enough resistance that the treatment become completely useless.

The anthelmintic treatment acts as a strong selection pressure which pushes the worms to evolve resistance. This is the same general process which causes antibiotic resistance in such infamous pathogens as MRSA and other so called ‘superbugs’.

While anthelmintic resistance in dogs has been far less rapid than in livestock, there is emerging evidence that this resistance is starting to develop. In the canine hookworm, Ancylostoma caninum, vets and scientists have discovered resistant populations in the US and Australia.2

So what is the problem?

While evolution against selection pressures is a natural process, human intervention is causing these changes to occur at a much faster rate. In addition, this problem is compounded by the way which we use these drugs. We make this problem worse by:

  • Over-using anti-parasite drugs
  • Under-dosing infected animals
  • Using non-specific, broad-spectrum wormers

If worms develop complete resistance to all classes or families of anthelmintics, eventually we will have no way of treating infections when they occur. Effective new drugs are difficult to discover and develop and so our anti-parasite arsenal could become useless.

How do we treat worms?

Worms can transfer from dogs to their puppies and anthelmintic resistance can make treating infections difficult.
Worms can transfer from dogs to their puppies and anthelmintic resistance can make treating infections difficult.

In the UK, recommendations for worm control vary. Many vets and other organisations, for example the ESCCAP, recommend regular worming every 1 – 3 months. With the growing threat of anthelmintic resistance, it is possible that this prophylactic style of treatment will start to be replaced by a more bespoke approach to deworming.

Faecal Egg Counts (FECs) are commonly used in livestock animals to determine worm burden and so inform parasite treatment. While it is currently not particularly common in the UK for dog owners to use this method to see whether their pets have worms, it is becoming more widely adopted.

With a better understanding of how many worms and the species of the worms infecting the dog, a targeted dewormer can be given. This reduces the risk of anthelmintic resistance developing and makes sure that any problems are treated efficiently.


  1. Matthews, J. B. Anthelmintic resistance in equine nematodes. International Journal for Parasitology: Drugs and Drug Resistance 4, 310–315 (2014).
  2. Jimenez Castro, P. D. et al. Multiple drug resistance in the canine hookworm Ancylostoma caninum: an emerging threat? Parasites & Vectors 12, 576 (2019).