Cat's leg swollen.

Hi there. I've spent an hour or more doing research about this online, and I'm at a loss. So I seek your help.

I work for a large animal rescue. We are in an area with snakes, spiders, scorpions, etc. All kinds of fun stuff.

Recently, it appeared a cat got bitten by something. That's the best we can figure. Cat beds and linens were checked, no spiders, scorpions, or snakes were found. No bite marks were found. The affected area had swelling and the skin turned a dark purple. The cat got so sick, he had to be crossed over. Whatever bit this cat (if it was a bite), killed it. Another cat showed up with the same symptoms. His leg was irrirritated, and within 30 minutes of calling a vet tech, was extremely swollen. They shaved his leg, again could find no bite marks, but again the leg turned dark purple. This cat recovered, and seems to be fine now.

But, obviously, I'm concerned. I have found some info online about heart stuff, but I find it difficult to believe that both cats had heart problems at the same time, right during wasp/bee/spider/scorpion/snake season. All of these dangerous creatures have been spotted around, just not at these particular times, and no discernable bite marks were found. I cannot find anything on the Internet about a cause for swelling and the skin turning a dark purple. I'm concerned about other cats, and what could possibly be going on. I certainly understand it's hard to tell without being here, but you're kind of my last hope for any ideas at this point. The rescue's vets are stumped, too, and that's after actually seeing/examining the cats! We've already lost one precious baby, don't want to lose any more.

Thank you.