Crystal proteins, already used on crops, show promise against Haemonchus contortus in ruminants
This article is more than 3 years old
A protein made by common soil bacteria may help treat a widespread parasitic problem in ruminants, according to a recent study.
A related study also indicates the substance is highly toxic to a hookworm that parasitizes humans. But any commercial products—in veterinary or human medicine—likely remain years away.
An article published in November in the International Journal of Parasitology: Drugs and Drug Resistance describes promising results for a Haemonchus contortus treatment in sheep that uses crystal proteins contained in the cell walls of killed Bacillus thuringiensis bacteria. Inside a ruminant’s digestive system, H contortus ingests dead bacteria containing the crystal proteins, which bind with the nematode’s intestinal cells and kill the nematode.
Joseph F. Urban Jr., PhD, supervisory microbiologist for the U.S. Department of Agriculture’s Agricultural Research Service, was among collaborators on the studies involving H contortus and the human-parasitizing hookworm Ancylostoma ceylanicum. He described the results as the most encouraging he has seen since development of ivermectin.
Dr. Urban noted that similar crystal lysate powders have been used as insecticides in farming for decades, and home gardeners dust their tomatoes with such products. Some transgenic food crops are modified to produce crystal proteins, he said.
The study involving H contortus in ruminants was a collaboration among researchers from the USDA Agricultural Research Service, the University of Massachusetts, Virginia-Maryland College of Veterinary Medicine, the University of California-San Diego, Worcester State University, Utah State University, and the University of Rhode Island. A USDA announcement indicates the experimental treatment caused dramatic reductions of parasites in infected sheep without any observed harm to the sheep.
Dr. Ray M. Kaplan, professor of parasitology at the University of Georgia College of Veterinary Medicine, said the crystal protein–based products are promising but likely years away from availability because of the need for further research on aspects such as dosing and delivery. If they become available, farmers and veterinarians would need to treat them with care and integrate them into parasite management plans that include adjunctive management. Otherwise, selection for drug resistance could reduce the effectiveness of these products just as has happened with approved anthelmintics, he said.
Dr. Kaplan has been collaborating on ongoing studies on use of the same crystal proteins to treat hookworms in dogs, with encouraging results so far.
Cry5B and IBaCC
The International Journal of Parasitology article indicates researchers experimentally infected goats and sheep with H contortus and administered each species a different treatment in oral suspension. Both treatment substances were forms of B thuringiensis crystal protein 5B, abbreviated as Cry5B. B thuringiensis naturally releases the crystals at the same time it releases spores.
The researchers tested this naturally occurring form of Cry5B in goats and found no effect on H contortus. But the research team found that Cry5B can become an efficient anthelmintic against H contortus larvae in sheep when the B thuringiensis are killed and the crystals remain contained within the dead bacteria’s cell walls.
That active pharmaceutical ingredient is named IBaCC—inactivated bacterium with cytosolic crystal.
Sheep administered IBaCC had 72% lower parasite burdens relative to experimentally infected controls, and their fecal egg counts dropped 88%-96%. The study authors attributed the sharper drop in egg counts to a 96% reduction in female worms, versus a 60% reduction in males.
Raffi V. Aroian, PhD, a professor of molecular medicine at the UMass Medical School and one of the lead investigators for the studies on H contortus and A ceylanicum, suspects the different formulations explain most of the differences in the results between sheep and goats, and the International Journal of Parasitology article suggests the IBaCC version may keep more crystal proteins intact through the rumen before they enter the abomasum. The article also states that the differences may relate to factors such as the higher volumes and numbers of doses given to the sheep and differences between the sheep and goats themselves.
But Dr. Aroian said a yet-unpublished study shows that, contrary to statements in the International Journal of Parasitology article, smaller doses of IBaCC in sheep were effective against H contortus.
A scientific article published in December 2020 in Antimicrobial Agents and Chemotherapy—authored by some of the same researchers as the H contortus article—describes the development of IBaCC and indicates IBaCC was effective at killing human-parasitizing A ceylanicum hookworms in tests using hamsters. The article states that crystal proteins are considered nontoxic to vertebrates even at high doses and that people have used crystal proteins for more than 60 years to control caterpillar, beetle, black fly, and mosquito populations.
“IBaCC promises new hope for a new arsenal of anthelmintics against the most common parasites of humans and animals,” the article states.
Widespread resistance to existing drugs
Dr. Dante Zarlenga, a microbiologist in the ARS Animal and Parasitic Diseases Laboratory and one of the International Journal of Parasitology article co-authors, said in a message that drug-resistant H contortus is present on almost all sheep and goat farms in the U.S., and the parasites on farms are particularly resistant to ivermectin and its derivatives. Dr. Zarlenga also expressed doubt anyone was conducting large-scale studies or monitoring of parasite levels, parasite species, and drug resistance.
Dr. Kaplan said Haemonchus nematodes are ubiquitous, and he doubts a sheep or goat can go through the warm months of the year on a U.S. pasture without becoming infected. He is finalizing a manuscript on a retrospective study of resistance prevalence during 2000-16 on hundreds of farms.
Dr. Kaplan said his data indicate most farms appear to be down to one anthelmintic drug that works well, and some have none. The problem is worst in the Eastern U.S., he said.
Dr. Kaplan suspects parasites would develop resistance to Cry5B more slowly than they have to existing anthelmintics because it kills worms through a different mechanism of action than the existing drugs. But he expects widespread use could accelerate that selection process.
He cautioned that veterinarians and farmers still need to shift away from the idea they can eliminate parasites through pharmaceuticals and toward the recognition that parasites need management that includes adjunctive methods. Those include implementing the concept of refugia to leave some animals untreated, reducing drug treatments, monitoring pasture height and forage quality, maintaining lower stocking densities, and adding, say, tannin-rich plants or copper to ruminants’ diets.
Even with a new anthelmintic class, Dr. Kaplan doubts farmers will see another era when monthly dewormers alone give them healthy, fast-growing, high-density herds.
Dr. Anne Zajac, a parasitology professor at Virginia-Maryland who was among the International Journal of Parasitology article authors, said the studies conducted so far on the crystal proteins showed the product’s activity against parasitic nematodes, and further studies will help pinpoint the best doses and formulations. The next steps toward Food and Drug Administration approval include toxicological studies, although she said no studies have shown toxic effects of crystal proteins on vertebrates.
Dr. Zajac expects existing knowledge of how to produce crystal proteins as insecticides will aid production of similar products as commercial anthelmintics. Because Cry5B is one member of a family of crystal proteins under investigation, she expects current studies could produce multiple related products, some of which may be more effective than Cry5B.
Dr. Aroian said his laboratory is testing at least a half-dozen crystal proteins for effectiveness, although Cry5B is the furthest along. He and fellow UMass Medical School professor Gary Ostroff, PhD, plan to develop the products for humans and livestock in parallel investigations, and he wants to start work with the FDA to get the approvals needed for further clinical studies. He’s searching for commercial partners, and he sees a need for the product among sheep producers in particular.
“We really are out there to make a difference in terms of helping people cope with these parasites, and we think that holds for humans and for livestock,” Dr. Aroian said. “This thing has tremendous potential to really help.”