Scientists at Yale and the University of California-San Diego (UCSD) have discovered that a natural protein produced by Bacillus thuringiensis, the bacterium sprayed on crops by organic farmers to reduce insect damage, is highly effective at treating hookworm infections in laboratory animals.
Their discovery, detailed in the Sept. 25 online edition of the Proceedings of the National Academy of Sciences, could pave the way for the development of more effective treatments for hookworm and other soil-transmitted nematode infections, which are a major global health problem in developing countries. Many of the nearly two billion people worldwide infected with these intestinal parasites are children, who are at particular risk for anemia, malnutrition and growth delay.
The UCSD-Yale team found that a protein produced by the bacterium Bacillus thuringiensis, or Bt, when given orally to laboratory animals infected with hookworm was as effective as mebendazole in eliminating the parasites, curing anemia, and restoring weight gain in the hamsters. Mebendazole is one of the drugs currently recommended to treat infections in humans. The scientists also discovered that this protein, called Cry5B, targets both developing, or larval, stages and adult parasites, and impairs egg excretion by female worms.
Hookworms cause anemia and weight loss by attaching to the intestine and feeding on their host's blood and nutrients. The researchers report that this naturally-produced protein has the potential to substantially improve this global health problem because it is safe to humans and other vertebrates and can be produced inexpensively in large quantities.
"Our ability to control parasitic nematode infections with chemotherapy on a global scale is dependent on the availability of medicines that are safe, effective and inexpensive to manufacture," says Michael Cappello, one of two principal authors of the study and a professor of pediatrics and epidemiology and public health at Yale School of Medicine. "We believe that Bt crystal proteins not only meet, but exceed these essential criteria."
This discovery is considered particularly relevant in light of concerns about the potential resistance in human intestinal nematodes to currently available medications. "There are few new agents under development for the treatment of hookworm and other intestinal parasite infections," says Raffi Aroian, an associate professor at UCSD and co-principal author of the study. "Crystal toxins are safe to humans, mammals and other vertebrates. And it might be possible to improve the efficacy of current treatments by giving mebendazole and Cry5B simultaneously."
Other authors of the study are Richard Bungiro and Lisa Harrison at Yale School of Medicine and Larry Bischof, Joel Griffitts and Brad Barrows at UCSD.
Aroian and his UCSD colleagues discovered five years ago that the roundworm C. elegans and other nematodes are susceptible to the effects of Cry5B, then known primarily as an insecticide. The toxin forms tiny holes in the membranes of the cells of nematodes and insects. However, since the toxin cannot bind to the cells of mammals or other vertebrates, they do not hurt humans.
"Crystal proteins had been used for decades by organic farmers who sprayed their crops with Bt to kill insects," says Aroian. "Until now, however, no one has used a purified Cry protein to treat a parasitic nematode."
At a meeting of the BurroughsWellcome Fund, Aroian met Cappello, a pediatric infectious diseases specialist who studies hookworm. They decided to collaborate on a project to see if crystal proteins could be effective against hookworm infections. Three years ago, Aroian and his colleagues purified Cry5B toxin and sent it to Cappello at Yale, who then tested the compound in a laboratory model of hookworm infection.
"It worked on the first day," says Aroian. "Laboratory animals treated with Cry5B survived a lethal hookworm infection, and showed no side effects from the medication."
Colleagues in Cappello's lab then demonstrated that Cry5B was comparable to mebendazole for treating hookworm infection in laboratory animals. Additional studies also determined stages in the life cycle of the parasite that were most susceptible to Cry5B and what concentrations were most effective.
"These experiments confirmed that the mechanism of action of Cry5B in Ancylostoma hookworms appears to be identical to that for other nematodes, including C. elegans," says Cappello. "This suggests that crystal proteins will likely have activity against a broad range of parasitic worms, and could be used to treat children who are often infected with multiple intestinal parasites."
The study was supported by grants from the National Institutes of Health and the Burroughs Wellcome Fund.
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