Many dogs have a tendency to eat things they shouldn’t when out and about, including horse and livestock manure. Although it’s one of the least desirable habits among our four-legged friends, most breeds of dog that eat a small amount of manure will not become ill. But there is a risk of toxicity due to chemicals in worming medications which will be passed in the faeces, and for this reason dog owners – particularly those with certain ‘at risk’ breeds (see below) – should avoid letting their pets consume it.
Why can horse manure be dangerous to dogs?
Horse worming treatments often contain a chemical called ivermectin which is effective against many different parasites across a range of species. It is also used as a wormer in cattle and sheep. Outside of the UK, some worming medications for dogs include the chemical but in very low doses managed by a vet that cause no harm.
There is a far higher concentration of ivermectin in horse and livestock worming treatments and this can passed in manure for days after the animal has taken the medication. This can be toxic to dogs that eat it, but the risk depends on the breed of dog, when the animals were treated and the amount consumed.
What breeds are most at risk from ivermectin poisoning?
Any dog that consumes a large amount of horse poop containing the chemical could become very ill, but a percentage of dogs of certain breeds have a gene mutation which predisposes them to toxicity from ivermectin at low levels. These include collies, Shetland sheepdogs, Australian shepherds, Old English sheepdogs, long-haired whippets, merle Pomeranians and possibly other herding breeds as well as those with white feet. A test for the gene mutation which puts dogs at high risk can be done so you can check whether your pet is at risk. Toxicity in breeds that don’t have the genetic predisposition to being affected by ivermectin in low doses is very rare.