Why shouldn’t my collie have ivermectin wormers?

Ivermectin is a very commonly used wormer, suitable for horses, cattle, sheep, birds, and a wide range of small mammals. It is occasionally used in dogs – but never in collies and other herding breeds.

What is Ivermectin?

Ivermectin is part of a family of drugs called the avermectins (which, in turn, are one variety of macrocyclic lactones). They work by binding to receptors in worm, mite and insect cells which control how much chloride enters the cell. The drug causes chloride to flood into the cell, causing paralysis and death.

That sounds horrible! Why do we use it in our pets if it’s that nasty?

Because the cells in most of a mammal’s body have a different type of receptor, and ivermectin doesn’t bind very well to it. So, at normal doses in normal dogs, it’s a very safe drug. The one caveat is that you mustn’t inject it into the brain (not that most people would…), as there are receptors in the brain that the drug does activate – but giving it by mouth, or by injection under the skin (or even into the bloodstream directly) is fine.

So what stops it getting into the brain, if it’s in the dog’s bloodstream?

All mammals, including dogs, have a special membrane surrounding the brain, called the Blood Brain Barrier. It works to prevent nasty chemicals getting into our brains and causing problems with the sensitive nerve cells there.

So there’s something unusual about the Blood Brain Barrier in collies then?

Yes, exactly. There is a particular genetic mutation, called MDR1? (also known as ABCB1, just to confuse people…) that means the barrier doesn’t work properly, and allows certain types of chemical through. One of these types is (you’ve guessed it!) the macrocyclic lactones – including ivermectin.

So all collies have this mutation do they?

No – but lots (perhaps as many as 75%) of them do. And a wide range of other dogs also have high incidence of this mutant gene – in fact, we’d generally advise against the use of ivermectin in any herding-type dog.

So, what problems does it cause? Do they become paralysed too?

Not normally – a dog’s brain is a lot more complicated than that of a worm or a flea! The typical symptoms seen include:

Altered behaviour – typically sleepiness, lack of normal responses, or coma.
Wobbliness and difficulty standing up.
Drooling and vomiting.
Slow heart rate and breathing (in some cases, the heart may even stop).
Tremors or twitches.
Seizures.

Can it be treated?

In most cases, toxicity will be managed symptomatically – with drugs to control the seizures and other symptoms until the body can deal with the poison. However, this particular drug is excreted by the liver into the bile; the bile then moves into the intestine, and the drug is then reabsorbed into the body. This is known as enterohepatic recirculation, and means it takes the body a long time to remove it.

As a result, the preferred treatment involves symptomatic therapy (intensive care nursing, intravenous fluids, drugs to prevent seizures and vomiting, and in severe cases medically inducing a coma), plus a specific antidote. This is a treatment called “Intravenous Lipid Infusion”, where a fatty chemical is injected into the bloodstream to “mop-up” the ivermectin.

Are there any other drugs I should avoid? What about selamectin and moxidectin – they sound very similar…

They sound similar because they’re part of the same family. So is milbemycin oxime (another common wormer). However, these drugs are safer in collies (and other affected dogs), because they have an even lower affinity for mammal receptors. However, it is very important that if you are using them, you give exactly the right dose – because it’s much easier to overdose an MDR1? dog than a normal one. For example, if you give your dog an anti-flea spot-on containing moxidectin, you mustn’t give a wormer containing milbemycin at the same time, because together, they could result in an overdose.

If in doubt as to what drugs you can mix, have a chat with one of our vets about it!