Almost every barn has a cat and a dog or two hanging out. They help with rodent control, provide company for the horses and security. But having our small companion animals at the barn or sharing medications can be harmful, even deadly.
This spring, a Border Collie lapped up the dribbles from a horse's dose of apple-flavored ivermectin dewormer. A day or so later, the dog's owner realized her top agility dog couldn't see. The dog went directly to a veterinary college for exam and a grave prognosis was given. Heartbroken, her owner faced the end of a fun career they both loved.
A few days later, it appeared the dog was starting to regain vision. In fact, her vision is almost totally back—the problem not being a true loss of vision but a side effect of ivermectin toxicity in a dog with the MDR 1 mutation. She was a lucky dog.
In another case, an Australian Shepherd snacked on some manure the day after the horses in the barn were dewormed with an ivermectin product. That dog died.
While many dogs can handle ivermectin, there are breeds that have a genetic mutation that leads to health problems and even death (see Dog Breeds With MDR 1 Mutation).
The case of the Border Collie above was an accidental poisoning with a dog getting a tiny bit of the ivermectin paste intended for the horse. Many horse owners try to save on costs by using equine ivermectin paste for their dogs for heartworm prevention, but it's nearly impossible to get the dosing right using the paste.
If you own a dog with an MDR 1 mutation, or to be safe if you own a dog of one of the sensitive breeds, keep the dog away from all manure for 48 hours after deworming your horse with any ivermectin, selamectin, milbemycin or moxidectin product. There are no set guidelines, but some drug will pass through your horse, so this is an extra precaution. Dispose of used paste tubes carefully and clean up any dribbles from your horse.
Acepromazine and butorphanol are other drugs that can cause problems in MDR 1 dogs, as well as loperamide (Imodium).
Not Just Dogs. Some medications are used for both small animals and horses, but generally the dosages will be different. It may seem economical to buy a horse-size antibiotic and break up the pill, but that doesn't always work. Many medications aren't distributed evenly throughout a pill for example. If there is no scoring, that's likely why. If you break that pill in half, the actual dose of medication may not be 50/50 but more like 25/75. You could accidentally overdose your pet.
Other drugs may be approved for both horses and dogs but must be used with care. Many horses do quite well on phenylbutamine (bute), but dogs often have gastric upsets with this drug. Using your horse's version for your lame dog could cause serious problems.
On the other hand, maybe you got a deal buying a bottle of small-animal-size antibiotic from your vet. If your horse needs the exact same antibiotic, you might use the small-animal version to dose him. But instead of grinding up one large pill with a mortar and pestle to then mix with molasses or another taste tempter (see Horse Journal July 2011), you could grind up 40 small pills.
Always verify with your veterinarian that it is the EXACT same medication. Many drugs have similar names but different actions. Generally, the cost ends up being more anyway since you need to use so many of the smaller pills to make up a horse-sized dose.
Many medications are ineffective if given at the wrong dose. Underdosing with antibiotics can contribute to antibiotic-resistant strains of bacteria. Antibiotics shouldn't be used in any animal unless your veterinarian has prescribed them.
Overdosing with any medication can lead to toxicity (Equine Products and Pets). Any medication that is not FDA-approved for a certain species should only be used under veterinary guidance.
If your barn has a goat companion, you should know that many horse supplements, special salt blocks and even some grains, contain extra copper, which can be fatal to sheep and goats. While both horses and goats are herbivores, their digestive tracts are different. This can also lead to problems with other medications as well.
Similar or worse problems can occur giving horse medications to cats. Millions of years ago, cats lost the gene for a liver enzyme that metabolizes many drugs. This is why a single acetaminophen tablet (Tylenol) can kill your cat. Basically we never recommend using any medication for a cat that doesn't specifically say "safe for cats." That includes fly sprays and topicals with permethrins, some essential oils, etc.
Bottom Line. Overall, you are better off keeping the medications for your horse and your companion animals separate. One emergency visit will negate any savings you hoped to gain by using the other species' medication.