The reason why so many claims made by defenders of vivisection are unfounded or plainly false is, I think, the following.
They make the mistake of thinking that "post hoc, propter hoc" (after that, therefore because of that).
For example, they may say that a certain cure or drug has been found "because" of animal experimentation, when in fact it could be that it was simply found "after" (unfortunately) time and money was devoted to animal experimentation.
The example of effective rehydration for diarrhea (mentioned some time ago in a letter to Peter Singer in The New York Book Review) seems a good one.
Bruce Max Feldmann says in answer to the letter to Singer:
"Rothman claims that oral fluid rehydration of Third World diarrheas is a treatment ‘based on many years of animal experimentation.’ To the contrary, in the three seminal papers on oral fluid rehydration for severe human diarrhea there is not a single reference to oral fluid rehydration experiments in laboratory animals with diarrhea. What really happened was that some more-creative-than-average health professionals said to themselves: ’Hey, wait a minute. Third World people are dying right and left from diarrheas. And intravenous fluids and fluid administration equipment necessary to save their lives are not affordable. So why not at least try oral fluids, even though we've been taught that they aren't much use in severe diarrhea. Maybe they'll help.’
"Well, oral fluids did help—a lot; tens of thousands of lives have been saved as a result. So Rothman's example to argue the importance of animal research illustrates precisely the opposite point — Singer's point: more of the world's limited medical resources should be allocated to immediate human life-saving efforts and to non-sentient animal research; less resources should be expended on animal research of questionable ethics and dubious value."
Here's a good example of how probably someone had jumped to the conclusion that a treatment had been found due to animal experimentation, because maybe there had been considerable resources devoted to animal experiments, but the actual solution was found in another way.
So a link which did not exist was established.
I suspect many cases will be of the same kind.
Post hoc, propter hoc is a very common fallacy.
We tend to assume that, if a fact follows another fact, the second was caused by the first.
See, for example, the idea that psychotherapy "cures" only because people after some time feel better: they probably would anyway (spontaneous remission).
The way vivisection apologists talk about animal experiments sometimes is a bit like this.
Suppose that someone, a traveller, has taken a long and tortuous route to get somewhere, not knowing that there was in fact a simpler, direct, shorter one.
He may then say that it was only thanks to that long route that he got to his destination.
Well, it's true. But the fact that he actually got to his destination through that route says nothing about alternative routes he might have taken which could have been more effective.
In the case of animal experimentation, furthermore, in many cases the link between the route taken and the results achieved is not so obvious but is on the contrary highly speculative.
When alternative methods are looked for, they are often found: I said “often”, but I would say “always”.
A well known example. Years ago the campaigner Henry Spira tackled Revlon over their use of rabbits to test cosmetics for potential eye damage, and exerted enough pressure to persuade the company to put $750,000 into the search for alternatives. Having seen the public relations disaster that Revlon had narrowly averted, Avon, Bristol-Myers and other major American cosmetics corporations soon followed suit. Though it took ten years for the research to yield the desired results, they did find what they were looking for: alternative methods. And so many cosmetics corporations can now truthfully state that their products are not tested on animals.