Now parasitic worms that prey on livestock are becoming drug-resistant, too, according to University of Georgia parasitologist Ray Kaplan.
Just as with bacteria, bugs and weeds, some worms have evolved through a process of natural selection — worms with a genetic mutation that gives them resistance survive and reproduce — helped along by drug overuse.
“This process has been going on for quite a few decades. But what’s changing is that now we’re seeing resistance to multiple drugs at the same time,” said Kaplan, a professor in the UGA College of Veterinary Medicine’s infectious diseases department.
The parasitic worms for the most part are not a threat to human health, but invade animals that browse on grass or undergrowth.
Sheep and goat herds are showing the worst effects, but cattle and even horses are also suffering. Drug-resistant worms seem most prevalent in areas with intensive production agriculture, like the United States, Latin America and Australia, but resistant worms are showing up elsewhere.
“This is a worldwide problem,” Kaplan said.
Drugs that combat worms were a great boon to farmers when they were first introduced about half a century ago. Animals treated with anti-parasitic drugs grew faster and healthier, increasing farmers’ profits.
“Farmers became more and more reliant on these drugs” as they got cheaper, Kaplan said. “Over time, the use of them just became a habit.”
For a while, farmers could compensate when worms became resistant to a particular drug by switching to another. But that option is disappearing.
“We’re already seeing the worst-case scenario playing out,” Kaplan said. “In goats particularly, which have the worst problems with parasites and drug resistance, we quite frequently see farms that have parasite resistance to all de-wormers.
“Some of these farms reached the point where they no longer could control the effects of the parasites and decided to go out of business.”
But farmers have other options besides drugs to combat parasitic worms, Kaplan said.
One is to periodically rotate herds to new pastures.
Worm eggs are passed from animal to animal through manure, and develop into worms once ingested. New grass has fewer parasites, and many of the parasites left behind in the old pasture will die, Kaplan said.
Most animals infected with worms have just low-level infections, and farmers might be better off in the long run just accepting that low level rather than trying to eliminate all infections, which helps the worms develop resistance, he said.
And more trouble is on the horizon.
Heartworms, a parasite that affects dogs, are also developing resistance to drugs, Kaplan said.
So far, resistant worms are found mainly in the Mississippi Delta area, where there are lots of mosquitoes. Heartworm infection in dogs is high along the Mississippi River corridor from the southern tip of Illinois down through Louisiana and Mississippi.