PHOENIX -- More than a half-dozen people have been bitten by rattlesnakes in the last two weeks, and doctors are now warning that the venom is becoming more poisonous.

Russ Johnson nearly lost a finger after a severe rattlesnake bite eight years ago.

"It was just like wave after wave of pain as venom pulsed through the body," Johnson said.

Johnson still works with reptiles at the Phoenix Herpetological Society, but he said he's more careful now.

"I've broken eight bones and to be honest with you, after I got to the hospital, I'd rather have broken all eight bones again," Johnson said. "I did ask them to cut the finger off, but by then it was too late. It was already up to my elbow.

Valley doctors are starting to see more patients like Johnson, becoming critically ill after rattlesnake bites, compared to just a few years ago.

"These patients commonly have very low blood pressure. Many times they're lethargic or unconscious," said Dr. Steve Curry of Banner Poison Control.

Similar trends are appearing in southern Arizona, Colorado and southern California.

"The reason for this isn't clear to us," Curry said. "Snake venom can't just change across the country all at once."

Whatever the reason, Curry says rattlesnake bites are becoming more dangerous. That means it's even more important for someone who's bitten to get medical help right away.