Think about the last time you heard of someone you knew getting a parasite infection. It’s probably never—at most, maybe you had a friend who got “beaver fever,” or Giardia, from accidentally swallowing too much lake water. People living in the U.S. and other developed countries are blissfully unaware of the existence of parasite infections because we have sanitation and safety regulations that keep our food, water, and living environments relatively clean. However, this is not the case in many parts of the world. For people living in areas with poor sanitation and a lack of clean drinking water—mainly in tropical and subtropical regions—parasite infections are a common and pervasive health concern. Intestinal nematode parasites, a kind of multicellular worm, have the most widespread infection rates by far, affecting an estimated 1.5 billion people, or 24% of the global population. There are many species of parasitic nematodes, but hookworms are one of the most prevalent types, infecting roughly 480 million people globally.
Damien O’Halloran, a professor and researcher in the Department of Biological Sciences at The George Washington University, has dedicated his time to better understanding these hookworms. Recently, he has been tackling a looming health risk that threatens to dislodge years of progress in managing parasitic worm infections all over the world: anthelmintic resistance. Anthelmintics are a class of drugs used to treat infections caused by parasitic worms, or “helminths,” and other internal parasites. O’Halloran has specifically examined resistance in the canine hookworm Ancylostoma caninum, knowledge he hopes can help better understand the primary hookworm species that infect humans, Ancylostoma duodenale and Necator americanus.
Hookworms: The Real Vampires
As with many other soil-transmitted parasitic worms, hookworms enter people through their skin, usually the feet, and travel through the bloodstream and into the lungs. The larvae are then coughed up, swallowed, and go on to mature in the intestines. The name for these worms comes from their hook-shaped heads, which is how they latch onto their host’s intestinal walls and feed on their blood. Hookworms lay their eggs in the intestines of their hosts, which are then passed on when an infected person defecates. People contract hookworm when infective larvae in contaminated soil penetrate the skin, often through bare feet. While some soil-transmitted helminths can also be acquired through ingestion, hookworm infection primarily happens through skin penetration. It causes anemia and protein deficiency, among other symptoms that cause gastrointestinal discomfort, which can be seriously detrimental if left untreated, and even more so upon repeated infection. Hookworms regularly and repeatedly infect young children, which can lead to slowed growth and mental development. Pregnant women are also at high risk for serious health effects from hookworm infection, as the blood loss it causes can harm the development of the fetus.
How Anthelmintic Resistance Begins
For many years, the World Health Organization’s approach to reducing the burden of infection has been mass drug administration (MDA) programs, first introduced in 2001. It is most highly encouraged in endemic areas of the world, including sub-Saharan Africa, China, and South America. The program prescribes the treatment of high risk populations with anthelmintics without individual diagnosis, meaning everybody in that population will receive medicinal treatment whether or not they actually have an infection. These programs have mainly targeted school-aged children and pregnant women. The goal of an MDA program is to reduce infection rates on as large of a scale as possible with the limited resources often at hand.
However, there are downsides to this kind of approach. You’ve likely heard of antibiotic resistance, or the overuse of antibiotics that makes them ineffective—and the case with anthelmintic resistance is no different. The overuse of anthelmintics leads to certain mutations in worm populations of traits that allow these worms to survive exposure to a drug that is meant to kill them. Anthelmintic resistance is already familiar to veterinarians and farmers alike, as intensive deworming of livestock animals has led to a widespread resistance to the drugs used to treat them. We are also now seeing widespread resistance to anthelmintics in dogs, a result of intense deworming in racing greyhounds that spilled over to companion dogs. Resistance has now been confirmed against all three major anthelmintic classes: benzimidazoles, pyrantel, and macrocyclic lactones, raising serious concern about the long-term viability of current canine parasite-control strategies. O’Halloran explains that greyhound dogs and farm animals have historically been subjected to intensive single-dose deworming with multiple drug classes, while simultaneously experiencing continuous exposure to infective environmental larvae. This combination of intense treatment, optimal environmental conditions for infection, and sustained high transmission creates a powerful positive selection of resistance alleles, or the genes that encode for resistance mechanisms. Luckily, anthelmintic resistance in hookworms that infect humans has not been widely observed… yet.
A Threat Ignored
Many scientists, including O’Halloran, only see the outbreak of resistance in humans as a matter of time. “Once it takes hold, it doesn’t go back,” O’Halloran said. Two years ago, a group of scientists projected that under the current MDA strategy, resistance would evolve within a decade. O’Halloran identifies this as the major reason anthelmintic resistance is not being addressed by pharmacological institutions: there is no incentive for drug companies to develop new anthelmintics because the issue is only “theoretical” in humans right now. This is why his work in the Biology Department at George Washington University is especially important: he is able to pursue this issue that poses a future threat. O’Halloran’s research is trying to anticipate the problem before it even becomes one.
Running Out of Time
While a widespread outbreak of anthelmintic resistance has yet to affect humans, localized cases have already been found. For example, resistant alleles were observed in hookworms isolated from children in Ghana in a 2018 study. The ratio of resistance to wild-type alleles, or the alleles not associated with any resistance, was high in children who were exposed to anthelmintics, suggesting drug exposure increases mutations associated with resistance.
The class of drugs used to treat hookworm infections in both dogs and humans are called benzimidazoles. They target β-tubulin, a protein that forms microtubules, which are filaments that build part of the cell’s cytoskeleton. Hookworms have slightly different versions of these proteins than humans, so the drugs have little effect on humans but can successfully eliminate hookworms. In animal populations, the mutations observed in resistant hookworms prevent the drugs from targeting these cell-skeleton proteins in the hookworm, making the drugs ineffective. These same mutations were observed in the resistant hookworm isolated from humans in the study from 2018, confirming scientists' warnings about the impending risk of resistance in human populations. At the time the study was published, the frequency of the mutant alleles was low in the population it sampled from. Now, eight years later, there is concern that the frequency of these alleles could have increased over time with the growing use of anthelmintics in human populations and will only continue to worsen.
New Approaches to Anthelmintic Drug Discovery
O’Halloran’s lab is taking a new approach to tackling anthelmintic resistance: finding drugs already used in humans that could be repurposed as anthelmintics. This strategy sidesteps the laborious and costly process of synthesizing new drugs and completing multiple rounds of clinical trials to ensure that they are safe to use, something most universities do not have the capacity to do. O’Halloran’s lab is instead utilizing a machine learning model that can identify drugs with anthelmintic properties, which are then screened using simple drug-assay tests—treating eggs and larvae with the drug to see the effects on development. Flutamide, a drug currently used for treating prostate cancer, was among the most effective drugs found so far. His lab is currently working on uncovering its mechanism of action, which is the biological pathway that the drug exploits to disrupt hookworm activity. They found that flutamide inhibits egg hatching and larval development, which is promising, but for a new anthelmintic drug to be successful in humans, it will need to kill the adult worms that live inside them. O’Halloran sees flutamide as a way to get a “foot in the door” into uncovering a new pathway in hookworm that scientists could utilize. He explained that “if we know the pathway of specifically how [flutamide] works, we can explore other checkpoints in that pathway,” with the goal of finding a drug that kills adult worms. O’Halloran also hopes to expand the machine learning drug screen, as their previous screen only looked at a subset of 2470 FDA approved drugs. Casting a wider net would increase the likelihood of finding more effective potential anthelmintics.
Forward Thinking
If successful in identifying repurposed anthelmintics, O’Halloran hopes that his research will be picked up by the pharmaceutical industry “to be taken to the next steps” as he continues to find drugs with anthelmintic properties. Pharmaceutical companies have the resources and bandwidth to repurpose a drug as an anthelmintic, a much shorter process than what is required for a novel drug. We can’t afford to ignore the potential threat of anthelmintic resistance, even if it does not affect our health today or even tomorrow. Researchers are fortunate to be a few steps ahead of these persistent parasites, and with the continued work of scientists like O’Halloran, we can successfully fight anthelmintic resistance before it takes hold in humans.