Remember the child’s horse, recently purchased, dramatic change in temperament? Our exam provided a reason for the change in temperament, as the horse had lost vision. But why? Any number of tests you might perform will not give you an answer. The lesson here is to take a thorough history. Ask the right question and the door will open. This month’s prize goes to Holly Anderson. She asked if the horse was dewormed or vaccinated by the new owner.
Here’s Holly, looking pleased with the good weather, her correct answer, and her spectacular new vest!
Of interest in this case is the de-wormer. In the good old days, we would tube worm horses with an organophosphate. That was because the effective dose was also a toxic one – if the horse didn’t get a little bit ill, the parasites probably were not eliminated. The only way to control the dose was by stomach tube. Paste worming was too variable. Today, owners de-worm their own horses, and often consider the products completely safe and non-toxic. Although “safer” than what was available in the past, they are definitely still toxic. If we look at our diagram of living things that might eat your horse, note that nematodes and insects (the parasites we want to eliminate) are multicellular, complex organisms with a biochemistry very similar to horses. A general rule of thumb is that the more like us it is, the harder it will be to kill without also making us sick. That’s why cancer is hard to cure and chemotherapy and radiation have so many side effects.
Today’s anthelmintics (de-wormers) have large safety margins, but they vary. Anthelcide has a 50 fold safety margin, strongid a 20 fold margin, ivermectin a 5 fold margin. You know those little wheels on the paste wormer that allows you to select the weight of your horse? That’s not just so you can save money by not using more product than you need to. It is also to keep you from administering a toxic dose. This very small horse was given a full tube of ivermectin – enough for a 1200# animal, and exceeding the safety margin for the drug.
So how does ivermectin work, and why these side effects? Well, the drug interacts with gaba receptors in the central nervous system of the parasites, causing paralysis. Does your horse have a central nervous system? Of course. Does your horse have gaba receptors. Yes. Then what gives? As we mentioned earlier, as horse eating creatures get larger and more complex, we need to be increasingly clever to kill them without hurting the horse. In this case, we take advantage of something called the blood brain barrier which your horse has, and the parasites do not. Ivermectin will reach high concentrations in the blood, enough to kill the parasites, but will not reach high concentrations in the cerebral spinal fluid (CNS) which bathes the brain and spinal cord where your horse’s gaba receptors are located. If this barrier is compromised (by some diseases), or if you give too much and blood concentrations are too high, it will leak across the barrier and affect the central nervous system as it did in this horse.
While this was going on, the biopsy results came in. Let’s take a look!