Wary about wormers

Wary about wormers

Health & Safety

There are some drugs that can have adverse effects on certain species and breeds. For example, dogs can tolerate paracetamol while it will kill cats. For the purpose of this Pets4Homes article, we are covering animal drugs, rather than human drugs. For this article, we are looking at one drug in particular - Ivermectin.

So, what is Ivermectin?

This drug is part of a whole spectrum of drugs called avermectins. They are used because they an effect on the cells of parasites such as worms, mites, and insects. They work by binding themselves to receptors in cells, specifically the type of receptor that maintains the amount of chloride that enters the cell. Ivermectin causes this receptor to work differently, meaning chloride floods into the cell, which causes paralysis and ultimately death.

Ivermectin is used because most of the cells in the body of a mammal have a different type receptor to these parasites. It means Ivermectin does not effectively bind to the cells. Ultimately, for a normal dose in a healthy dog, Ivermectin is a safe choice of drug.

Giving it by mouth, as an injection under the skin or even into the bloodstream, does not cause problems. The only receptors that it would work on are in the brain, so injecting it directly into the brain, (not that anybody would) would result in the same effects as a parasite.

How is it stopped from getting to the dog’s brain then?

You may think because the drug is in the bloodstream, it would get into the dog's brain. However, dogs, just the same as all mammals, have a defence mechanism. It is a specially developed membrane which encompasses the brain – this membrane is called the Blood-Brain Barrier.

The membrane works like a filter, by not allowing harmful chemicals from entering and getting into the brain. If these chemicals do get through, they could damage very sensitive nerve cells.

So, if there is a Blood-Brain Barrier, why is it a risk to collies?

Unfortunately for collies, there is a problem with the Blood-Brain Barrier – it is actually a genetic mutation known among other things as ABCB1. This mutation means the Blood-Brain Barrier does not do its job efficiently. Chemicals can be let through the barrier, although only certain types – one of these types being Ivermectin.

So, are all collies affected?

The simple answer to this is no, not all collies are affected with this problem genetic mutation of the Blood-Brain Barrier. The issue is, many are – in research, it is believed that as many as 75% of herding type dogs may carry this mutation. This is why most vets should advise owners of collies, and herding dogs against the use of Ivermectin – they just don't know if the dog is can be affected.

So, what would I look for?

If a collie dog is overdosed with Ivermectin, because of the complexity of the brain compared to worms or fleas, it is highly unlikely the dog will become paralysed. Typically, there would be symptoms in the dog similar to the following:

  • Behavioural changes – normally lethargy or extreme sleepiness, they may also not respond to you normally.
  • They may be very unsteady on their feet, wobbling and finding it difficult to stand up.
  • They made drool excessively.
  • Vomiting is another symptom.

Of the more serious signs:

  • A slow heart rate which in severe cases, the drug can actually stop the heart.
  • A dog may start displaying tremors or even unusual twitching.
  • Seizures are one of the most dangerous symptoms.
  • In extreme cases, the dog may also go into a coma.

What can be done to help?

With normal poisoning and most toxic cases, the cause can be managed with the veterinary team by treating the symptoms with drugs that control any seizures, and medication to treat other symptoms in the body. This will be the plan until the body can deal with the poison itself.

Unfortunately, Ivermectin works differently, it is excreted from the liver and into bile. The bile then enters the intestines, where the drug is then reabsorbed back into the animal’s body. This reabsorption from the bile and liver is termed as enterohepatic recirculation. It effectively means because the process is in a circuit, the body will take a long time to remove the poison.

Because of this, the treatment differs slightly, and the dog would be involved in a specific therapy to help with the symptoms. This may include all or some of the following:

  • Intensive care therapy, nursing, and monitoring.
  • Fluids are given intravenously.
  • Drug therapy to prevent the dog from fitting.
  • Medication to combat vomiting.
  • In severe cases of the poisoning, where the dog is not responding as well as it is hoped, they may also be put into a medically induced coma.
  • Drugs that act as an antidote are also given. To aid the removal of Ivermectin from the bloodstream, a chemical lipid (a fatty chemical) is put into the bloodstream in a procedure known as intravenous lipid infusion.

This therapy reduces the time the Ivermectin is being reabsorbed and affecting the animal.

What else can I do to ensure my dog is safe?

Worming is very important, for a number of reasons, but that is another article entirely. The main thing is to check that by using antiparasitic treatments, you do not overdose your dog. In the same family of drugs as Ivermectin, are drugs called selamectin, moxidectin and milbemycin – these are all common flea and worm treatments. They are also regarded safer for use in collies because they have less receptor action than Ivermectin.

Although they will go under different names, according to the manufacturers that they are made by, it is very important to check the actual drug ingredient. It is easy to still overdose a dog with the genetic mutation, if you use a flea treatment containing one of the drugs and then a wormer containing, for example, milbemycin. If these are used together, you could still put your dog into an overdose situation. Always speak to your vet if in doubt.


The safety margin of these drugs has been tested, however always err on the side of caution, and never risk giving your collie or herding dog any preparation containing Ivermectin. If your vet recommends a product you suspect that may contain it, by all means, question it with them.



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