For months, the veterinary center in West Point, Miss., had watched its supplies of the drug dwindle. Dr. Karen Emerson, the veterinarian who owns the hospital, started the year with one 500-milliliter bottle of ivermectin, which she uses to kill parasites in dogs, chickens and other patients. But as the bottle emptied and her staff tried to find more, they were able to obtain only a 50-milliliter vial. Everyone else told them: None available.

So Dr. Emerson began rationing the medicine to give to snakes and other exotic animals for which she had no other deworming treatment. She told dog owners to pay for a more available replacement drug that can cost seven times as much…

The dearth has led some farm owners, ranchers and veterinarians to switch to generic or more expensive alternatives for their animals. Others have turned to expired ivermectin or quietly stockpiled the drug when they could. Many were alarmed.

“I’m pretty worried,” said Marc Filion, the owner of Keegan-Filion Farm in Walterboro, S.C., which uses the drug for his 400 pigs and 25 cattle. If he couldn’t treat his pigs with the medicine when they were 5 weeks old, he said, they could develop diarrhea and might need to be killed.