If i could give (one) doctor's view:
White tail spider bites are a bit like the Abominable Snowman crossed with a Snow Leopard. Either the thing is out there and mighty hard to find using conventional techniques, or it's not there at all.
My understanding is that the CURRENT state of knowledge is that the White Tail Spider does NOT have a necrotising venom, but that the cause of the severe pain, swelling, ulcers etc is a soil bacterium (bug for the non-medical). The bacterium could be present on the jaws of the WTS or on other spider species or possibly on non-spider species, or maybe even just in the soil and getting into a cut.
This "current state of knowledge" only dates back to the article reported above - before that the WTS was a "known" cause of ulcers. Many GPs would not be aware of the change (though most would).
Prior to groundbreaking research from two WA doctors, everybody in the world knew that stress was the leading cause of stomach ulcers. If you spoke to your GP about your ulcer in the 1980's he (they were almost all male, then) would have told you to reduce your stress. Now she (most medical graduates now are female) would prescribe a course of antibiotics. Most stomach ulcers are due to a bacterial infection.
The moral here is - just because your doctor tells you something doesn't mean it's true. And if it's not true they may still be giving you the best knowedge available at the time, though later evidence proves them wrong.
Maybe some day a WTS which DOES cause ulcers will be found. But despite what people have been told, the best evidence at present says it's a bacterium, possibly (though not certainly) a hitch-hiker.
