"The first is that ivermectin is more effective against heartworm. Now, just for the record, ivermectin is a neurotoxin derived from bacteria, and its mechanism is very general (it binds to and activates a specific chloride channel in the brain, and if your nervous system is no longer in charge of activating that channel then your nervous system no longer works [...]. So, like, if you inject it into a human's brain the human will die for the same basic reasons that an ivermectin treated heartworm will die. Of course, the reason this chemical works as an anti-parasitic drug is because we have a blood brain barrier that keeps it out of the central nervous system while worms and insects do not. Not because they don't have a brain though, the essential difference is that we have genes coding for membrane proteins slash multi drug resistance proteins which bind ivermectin-like drugs and actively remove them from the central nervous system (in this context the term "blood brain barrier" is actually just code for the existence of these proteins. The anatomical barrier between the blood and the brain has very little effect on how these kinds of drugs behave.)
Now, selamectin is basically just the same chemical backbone as ivermectin, but with a small change to one of the functional groups. That means that for the most part it binds to the same proteins that ivermectin does, but with slightly different affinities and therefore different outcomes. This kind of drug design is ludicrously common, it's a good way to find drugs that are more effective and have less side effects, but it is also a good way to get a new patent on what is effectively an old drug. Clinical research is just filled with examples of these new drugs that turn out to be less effective than the drugs they're derived from, but which still thrive on the market because the company is willing to put a significant amount of money into marketing them and getting them approved for uses that the old drug hasn't been approved for. In this latter case, the old drug might be better for those new uses as well, but there are less people willing to pay for those studies. Now, this doesn't have to be a problem, I mean, evidence based medicine requires evidence, and the whole point of the patent system is to give companies a profit motive for producing that evidence. Really, when old drugs turn out to be superior to new drugs the problem is that we a)aren't providing companies with a profit motive for producing that evidence, and b)aren't spending public funds on research.
Anyway... So okay, ivermectin is a very old medicine, we have a ton of evidence on how it works in humans, and it's off patent so Merck is willing to donate massive quantities to developing countries. There's not much of a financial reason for finding a competitor there.
However... Some sheep dogs, especially collies, are by virtue of being heavily inbred very likely to have an inherited disorder where one of their blood brain barrier proteins is broken, and without that ivermectin gets into their brain and, well, plays out its role as a neurotoxin and kills them. So, there was a big reason to find competing drugs for veterinary use, and Pfizer came out with selamectin, a collie safe version of ivermectin, and because it's a new and therefore patentable medicine they also went through the trouble of preparing evidence that selamectin is good against a bunch of other parasites. Now, some public research shows that ivermectin and other older products are also good against the same range of parasites, but there's not much money in proving that so those studies are small and unreliable. So anyway on top of this Pfizer also turned it into a topical formulation and showed that the topical formulation kills fleas. So, they end up with a unique dog specific product and a huge marketing blitz.
Now, there is one issue with this, that's more of a regulatory issue. So, getting approval to use a chemical near humans is significantly easier than getting approval to use a chemical on humans, and recently we've been seeing a lot of studies showing that some classes of parasite killing neurotoxins for use on pets correlate with problems in the pet owners. For example, I think there's been a few studies finding a significant correlation between families that have used pyrethrin based tick/flea dog shampoos and families that have autistic children."
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