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Toxicities that cause disease and death of calves are luckily few and far between. When they do occur – sometimes from a simple mistake – it is extremely upsetting for those involved. But ­ if we can use the bad as a timely reminder, we can help ensure it doesn’t happen to anyone else.

Recently we’ve seen abamectin toxicity, an active ingredient in several drench products (pour on, oral and injectable).

Abamectin kills worms by interfering with their nerve function and paralysing the muscles of their mouth and body wall. Abamectin doesn’t affect the nerves of the body of mammals. The only nerves it can affect are in the brain and the brain is well-protected from the blood stream: substances such as toxins or bacteria can’t get through (it’s called the blood-brain barrier and this video explains it well).

screen shot of drawing the blood brain barrier

However, in young calves this blood-brain barrier is thought to be immature and more permeable to drugs such as abamectin. If it gets into the brain, abamectin can cause clinical signs such as depression, unsteadiness, recumbency, muscle twitching, drooling, tongue paralysis, droopy ears, blindness and, in the worst-case scenario, death. There is no treatment available other than supportive care while the calves hopefully recover in 2–14 days.

As such, abamectin products should not be used in calves less than 16 weeks of age (they should also not be administered to lambs less than 6 weeks of age).

Avoiding toxicity: reading the label

The labels on products such as drenches, worm tablets, fly and flea treatments are crowded as there’s lots of information that they are required to contain. That information includes things such as:

  • Product name
  • Active ingredient
  • Reason for use
  • How the product is given (dose rate and where it is administered)
  • Date of manufacture, batch number and expiry date
  • How to store and dispose of the product
  • Withholding periods
  • Trade advice (export slaughter intervals)
  • Restraints, contraindications and precautions

It’s this last point that is worth talking about some more…


A restraint is an absolute restriction placed on the use of a product. It aims to manage a risk associated with using the product. This risk may be an animal, human, public health or environmental safety concern that has been determined during evaluation of the product. For example, some products may have the restraint “Not for use in food producing animals” to avoid the risk of residues for people.


Contraindications indicate scenarios when the product should never be given: when there is evidence that toxicity will occur in certain situations or physiological states of the target animal. Example contraindications include:

  • Do not administer to pregnant cows
  • Don’t administer at the same time as product X
  • Don’t administer to puppies under 6 weeks of age
  • Do not treat animals in extremely hot weather


Precautions are bits of advice that minimise the risk of any side-effects when giving the product as per the label advice (at the correct dose and at the correct age). Product disposal and first aid instructions can also be included in this precautions section. For example, a product might say:

  • Don’t retreat within Y days
  • Use in well-ventilated areas or outdoors
  • Ensure animals have access to water prior to treatment
  • Weigh animal correctly to avoid over or under dosing

For any product that you purchase for your stock or household animals, make sure you look for these three categories on the label and follow their advice.

This calf was drenched with an abamectin pour-on when it was too young. It is having difficulty seeing and swallowing because abamectin was able to cross into its brain.