Horses are naturally careful eaters and have evolved to distrust unpleasant-tasting and potentially poisonous plants in their diet.
Unfortunately, many medicinal drugs also have a bitter taste. The phrase, “He won’t touch that stuff in his feed,” is one that all vets are used to hearing. When the nasty taste comes from a potentially vital antibiotic or painkiller, it is up to the owner and vet to find a way to get the treatment down.
Giving medicine by mouth is not as straightforward as it may seem. Before it can take effect, the medicine first needs to be absorbed from the stomach or small intestine. How quickly this happens depends on how much food is in the stomach at the time; a full stomach will slow drug absorption, and can also mean that the medicine only reaches a lower, less effective level in the blood.
Once it is in the bloodstream, the drug is carried to the parts of the body or the organs where it is needed. This is why vets often give an initial dose of a medication directly into the vein, ensuring a much quicker onset of action. It is also the reason detomidine gel, an oral sedative marketed as Domosedan or Dormosedan gel, comes with the apparently puzzling instruction that it must be placed under the horse’s tongue. This is because it can then be absorbed straight through the mucous membranes and into the rich network of blood vessels on the floor of the mouth, a route that bypasses the stomach.
Most medicines are swallowed, however, which means they have to survive the effects of the stomach acid and digestive enzymes with their chemical properties unaltered. “Oral bioavailability” is the term used to describe how much of a drug that is taken by mouth will actually be active within the body.
Phenylbutazone (bute) for example, has a high oral bioavailability of 90%, which means that almost all the drug that is eaten will be effective. By contrast, omeprazole, the standard treatment for stomach ulcers, has an oral bioavailability of only about 10%. This means that giving it by mouth is inefficient — much more of the drug has to be given, increasing the cost significantly.
Many over-the-counter feed additives also have unknown or questionable absorption rates. For example, owners of older horses feed a glucosamine supplement to help combat joint stiffness. There are studies which appear to show that this is beneficial, but the availability of in-feed glucosamine can be as low as 6%.
Hard to swallow
The best way to dose a horse who does not want to eat his medicine depends on the particular drug’s formulation. One of the easiest methods of administering medicine is as a paste, which is why wormers in particular usually come pre-loaded in a syringe.
There is still a knack to getting a syringe of paste into a reluctant patient. First, make sure your horse hasn’t got a mouthful of hay or feed. Then stand at his near shoulder, facing forward, put your right arm under and around his head and hold the muzzle just above where the noseband goes.
Slide the syringe into the corner of his mouth with your left hand, getting it well back in the mouth to avoid shooting the contents straight out the other side. Keep his head up afterwards.
Coating the barrel of the oral dosing syringe in honey will help some horses to accept it, although others are so reluctant that the process will always be a struggle.
Paste might be relatively easy to give to most horses, but producing a paste or gel is expensive. They also tend to be less stable (the ability of a drug to retain its potency and appearance over time), with a shorter shelf life. This is why most drugs given daily for weeks, or months, tend to be in tablet or powder form.
Tablets are a good drug delivery system for humans and dogs, because a tablet is typically swallowed whole, and if the active ingredient has an unpleasant taste, the tablet can be sugar-coated to hide it. Horses grind up their food, however, before swallowing. Something suspicious in a mouthful of feed can soon be singled out and rejected.
Down the hatch
Bute, typically prescribed in powder form, has a notoriously bitter taste. Some horses have an ability to detect it in their feed that would match a sniffer dog.
Owners’ stratagems to get bute into a reluctant horse are many and ingenious. Some of the more successful methods include:
● Mixing the drug in with a molassed feed.
● Making a sandwich. Spread a slice of bread with honey, jam or Bovril and sprinkle the bute on. Add another slice and serve.
● Squirting the drug down. Bute doesn’t dissolve in water, and a bute and water mix is easily spat out. A better way is to mix the powder in a tablespoonful of ready-made custard and load it into a dosing syringe. The custard coats the horse’s mouth and tongue, making it hard to spit out, and the sweet taste helps disguise the bute.
● Opting for the alternative: suxibuzone, marketed as Danilon. The active ingredient is a “pro-drug”, which is converted within the body and broken down to bute by the liver. Danilon is marketed as being more palatable than bute in feed, while still having the same clinical effect.
If a medicine is stable enough to be given by mouth, it is unlikely that adding warm water or sweeteners will affect its action. Your vet will advise you if there are any important instructions about how a medicine should be given.
Cushing’s disease is now the most common reason for a horse to be on long-term medication. The condition responds well to treatment with the drug pergolide (marketed as Prascend), but it involves a daily tablet for the rest of the horse’s life.
Although pergolide does not taste as unpleasant as bute, some horses become very skilled at eating all their feed and leaving a single — very expensive — tablet behind. This problem can be compounded by the fact that one of the more common side effects of pergolide is a temporary reduction in appetite.
So, what is the best way to persuade a horse to take it?
The manufacturers recommend that the tablets are mixed with molasses in a feed, or dissolved in a small amount of water or sweetener and syringed down. The familiar hole in a carrot or apple works for some (pictured), while other owners push the pill into the side of their horse’s mouth. A pill given this way is hard to spit out and will quickly dissolve. It’s important not to crush the tablets because pergolide dust can be an irritant if handlers breathe it in or get it in their eyes.
For owners who are still struggling, there is fresh hope in the recent development of a pergolide paste containing molasses to make it more palatable. It was well accepted by the horses in a small trial, but at the moment the paste form is only available as an “extemporaneous preparation”, or “special”.
This means that it has to be prepared individually for each animal, based on a veterinary prescription. A vet can prescribe a “special” if they are satisfied this is the only way the horse will get the correct level of medication.
Ref Horse & Hound; 28 March 2019