Picture an open pasture with woolly sheep grazing on lush green grass. Idyllic, right? What’s not so idyllic is that inside some of these animals’ guts, deadly parasitic worms could be thriving, sucking blood from the animals’ stomachs and depriving them of nutrients. Researchers around the world have reported finding drug-resistant roundworms—some resistant to multiple classes of standard treatments—in every type of livestock host, including sheep, cattle, and goats.
Outwardly, an animal may look healthy, leading a farmer to believe that regular treatment is keeping parasitic worms at bay. Inwardly, though, worms that have mutated to shield themselves from common drugs are shedding eggs that will hatch to create the next generation of resistant worms. The sheep spread the eggs through their feces, leaving behind infective larvae on the verdant pasture.
“The consequence is that a farmer will come to see a veterinarian because he has a whole lot of sick or even dead animals lying around his paddock,” says David Leathwick, principal scientist for the New Zealand government research group AgResearch. The farmer will ask, “What’s wrong with my animals?” Leathwick says. “The answer is, ‘Well, you just treated them, but it didn’t work, and you’ve had drug resistance for years.’ ”
Parasitic roundworms infect livestock, crops, companion animals, and humans worldwide. The advent of modern antiworm drugs, or anthelmintics, starting around the 1960s gave doctors, veterinarians, and farmers the tools to effectively treat devastating infections. For decades, anthelmintics have kept livestock safe from blood-sucking roundworms and helped halt the spread of parasitic-roundworm-induced river blindness among humans in developing countries. But the blanket use of these compounds has contributed to a global rise in roundworms that exhibit drug resistance.
The problem is especially bad in sheep and goats. And roundworms that are resistant to drugs developed less than a decade ago have already appeared on sheep farms. As drug resistance slowly creeps into the U.S. cattle industry and arises in human infections around the world, farmers, veterinarians, doctors, and scientists are seeking new treatment strategies.
Ivermectin, the gold standard
Out of the handful of anthelmintics used against roundworms, otherwise known as nematodes, one stands apart in terms of potency and success: ivermectin. Not only does ivermectin kill roundworms in infected plants, animals, and humans, leaving the host organisms unscathed, but it also kills other types of parasitic worms. And it can treat infections caused by arthropods, such as lice, ticks, and mites, while causing limited side effects.
Although researchers are still working to fully understand ivermectin’s mechanism of action, they do know that it binds to an array of chemically triggered ion channels, called ligand-gated ion channels, that are nestled into the membranes of nerve and muscle cells in roundworms. This “basket of tricks” likely explains how ivermectin can target multiple parasite species and life stages, says Ray Kaplan, a veterinarian and professor of parasitology at the University of Georgia.
The compound binds most strongly to glutamate-gated chloride channels, which regulate the passage of chloride ions into and out of cells. By strongly activating the channel, ivermectin induces muscle paralysis and death in roundworms.
Debuted by Merck & Co. as a livestock drug in 1981, ivermectin was the product of an international collaboration among public and private institutions. Satoshi Ōmura at Kitasato University in Tokyo first discovered avermectins, compounds produced by the soil bacterium Streptomyces avermitilis that have powerful activity against nematodes. William C. Campbell, who worked for the Merck Institute for Therapeutic Research at the time, then studied the efficacy of the avermectins and their derivatives, including ivermectin, against parasites in animals.
Ivermectin became a boon to livestock producers around the globe and was approved for use in humans starting in 1987. Since then, Merck has been donating ivermectin—“as much as needed, for as long as needed,” according to the company’s website—to help fight river blindness, a roundworm infection afflicting millions of people in the developing world. In 2015, Ōmura and Campbell were jointly awarded one-half of the Nobel Prize in Physiology or Medicine for their discoveries and the impact they’ve had on society.
The booming success of ivermectin for livestock parasite control was so great that it changed animal husbandry practices worldwide. Modified drug formulations led to longer-lasting anthelmintics that were easier to administer, dramatically reducing labor costs. “When you talk about worm control to a producer,” Leathwick says, “their first, second, and third thoughts are all around the use of drugs.” In the same way that overuse of antibiotics has led to antibiotic resistance, once blanket anthelmintic treatment became the norm among farmers, it was only a matter of time before resistance to those drugs cropped up.
Instead of treating only animals with extreme infections, livestock producers now treat the entire flock. This practice helps support animal health and boost growth, but it also eliminates important genetic refugia, or populations of worms lacking the gene mutations that confer drug resistance. These susceptible worms help dilute the effects of resistance-causing genes in the overall worm gene pool, thereby keeping drug resistance at bay for longer.
Drug resistance is “widespread in cattle in South America and in sheep and goats the world over,” says Timothy G. Geary, director of the Institute of Parasitology at McGill University. It’s even showing up in horses, he adds. “There’s a legitimate concern about our ability to manage these things.”
Nearly all standard anthelmintics have stopped working against sheep parasites, according to Nick Sangster, a program manager for Meat & Livestock Australia. Newly developed drugs are often the only treatment option, Sangster adds. And resistance to the aminoacetonitrile derivative monepantel has already appeared in worms, only seven to eight years after the drug’s introduction. “It’s a ticking time bomb,” he says of the growing drug-resistance problem.
Although there is definitely a need to look for new drugs, Geary points out that “the bar has been set pretty high” by ivermectin and similar drugs. New drugs must be active against multiple parasite species yet adhere to stringent health and safety regulations. “Basically, we’ve evolved the market to think that one product gets all worms,” Geary says. “It’s a very challenging market.”
After the rise of widespread antibiotic resistance, many academic scientists stepped in to help fill gaps in basic research and new drug development. A similar movement to aid in the search for novel anthelmintics is slowly taking shape today.
Given the challenges of working with parasitic worms—their complex life cycle cannot be reproduced in the lab without a host animal—many researchers have turned to Caenorhabditis elegans, a nonparasitic nematode and, thus, a model worm. Their goal: Identify vulnerable proteins and biochemical pathways in the worms and then find compounds that target them.
Parasitic roundworms have exhibited resistance to all these classes of drugs.
|Drug class||Drug examples||Class discovery date||Cellular mode of action||Drug effect||Mechanism of resistance|
|Benzimidazoles||Albendazole, thiabendazole||1961||Attacks scaffolding protein β-tubulin||Impaired movement and reproduction||Worms substitute tyrosine for phenylalanine in β-tubulin|
|Imidazothiazoles||Levamisole, morantel, pyrantel||1970||Activates nicotinic acetylcholine receptor||Paralysis||Unknown|
|Macrocyclic lactones||Avermectin, ivermectin||1974||Binds to glutamate-gated chloride and other ion channels||Potent, persistent paralysis and death||Unknown|
|Cyclic octadepsipeptides||Emodepside||2002||Possibly binds to calcium-activated potassium channel||Paralysis||Unknown|
|Aminoacetonitrile derivatives||Monepantel||2008||Binds to some nicotinic receptor subunits||Paralysis||Unknown|
Sources: satoshi-omura.info, wormbook.org
Some researchers, like Laura L. Kiessling of Massachusetts Institute of Technology, focus on a protein target already known to be essential to the worms and search for novel chemicals to disrupt its function. Kiessling, a professor of chemistry, had originally set out to target bacteria by attacking the enzymes that create key polysaccharide chains, or glycans, on their protective outer surface. Her team found that inhibiting the enzyme uridine 5′-diphosphate galactopyranose mutase (UGM) in Mycobacterium microbes slowed their growth. Because glycans, including those produced by UGM, are also crucial components of roundworms’ protective sheaths, or cuticles, Kiessling thought these UGM inhibitors could provide a novel, nonmammalian drug target.
Unlike the drugs that treat roundworms by binding ion channels and knocking nerve and muscle cells out of commission, drugs targeting the cuticle would disrupt the organisms’ motion and potentially kill worms and larvae. Disrupting the cuticle could also help other drugs reach their target, Kiessling says.
Kiessling’s team identified 14 small-molecule inhibitors of UGM. Although the compounds inhibited UGM activity when tested against the enzymes directly, they were not effective against live C. elegans. The worms’ powerful detoxification enzymes targeted the carboxylate groups on the compounds and rendered them inactive. However, a single functional group swap, replacing the carboxylate group with an acylsulfonamide, created “surrogate” compounds that circumvented the detoxification enzymes and disabled and killed C. elegans (ACS Chem. Biol. 2017, DOI: 10.1021/acschembio.7b00487).
The next steps include testing these compounds against parasitic nematodes and ensuring their safety in animals and humans, Kiessling says, although the side effects should be limited because mammals do not possess the UGM enzyme.
She is equally excited about the potential to use carboxylate surrogates as a general mechanism to activate other compounds. “I think it could be really useful for other people that are trying to hit targets in nematodes and running up against their detoxification mechanisms,” Kiessling says.
Another route to novel anthelmintics goes beyond screening compounds against predetermined targets. “We’re not assuming that any one protein family in particular is a good target,” says Peter Roy, a professor of molecular genetics and pharmacology and toxicology at the University of Toronto. “We’re just blasting the worm with compounds and letting the worm tell us what it doesn’t like.”
In a screen of more than 67,000 compounds, Roy’s team homed in on a set of 30 distinct compounds that killed C. elegans and two parasitic nematode species but did not harm zebrafish or a human cell line (Nat. Commun. 2015, DOI: 10.1038/ncomms8485). By exposing C. elegans to chemical mutagens, the team also generated 19 million genetic mutations in the worm and then exposed the organism to various compounds to observe how easily resistance could arise, a practice that also helps elucidate which proteins or pathways are targeted. The scientists zeroed in on one family of promising molecules to determine how it killed the nematodes. The compounds disrupted an essential component of the electron transport chain in mitochondria, cells’ energy factories. Only nanomolar concentrations were needed to achieve this effect, and the compounds showed limited risk of inducing resistance.
One of the key areas for future drug development, according to Roy, is gaining access to novel chemicals. “Most of the stuff that’s commercially available that we can get our hands on has been screened up the wazoo,” he says. A worldwide, open access library of new structures “would be a fantastic public effort.”
As research into novel anthelmintics pushes forward, scientists and veterinarians agree that dramatic changes in drug use practices are needed to help stem the spread of resistance.
We need a “philosophical shift” away from the blanket drug use model, one that includes a more holistic approach to parasite control and sustainable use of new drugs, Kaplan says. For example, although scientists have yet to uncover the mechanism behind it, adding certain tannin-rich plants to animals’ diets can naturally help control parasite infections, he says.
Another nonchemical practice includes labor-intensive field and grazing rotations to limit exposure to infective worm larvae. Eggs take about four to five days to reach the infective larval stage, so moving sheep to a new pasture helps reduce the risk of infection, Meat & Livestock Australia’s Sangster says. This requires extra fencing and more field hands, however, and sheep cannot return to the first field they grazed in for 60 days, he adds.
Rather than treating all animals in a flock, farmers should instead target animals with the worst infections for treatment, Kaplan says. In general, about 20% of the animals will harbor roughly 80% of the parasites. Comparing the mucus membranes of animals’ eyes to a specialized color chart can help farmers identify anemia, a telltale side effect of the most important parasitic worm infection in sheep and goats.
AgResearch’s Leathwick and his team have extensively studied the use of drug combinations—multiple drugs that target the same worm species—to manage parasite infections in sheep. Worms attacked on multiple fronts are less likely to develop resistance to any one drug. Leathwick’s team is also beginning to delve into new gene-sequencing technology to find more potential drug targets. Leathwick is excited about the increasing feasibility of finding such a physiological trigger and working with a “clever chemist” to target it.