The greyhound racing sector has been linked to the emergence of drug-resistant hookworms, which have the potential to infect both canines and humans.
Backstory
In 2017, Ray Kaplan, a veterinarian and parasitologist from the University of Georgia, began receiving emails from colleagues across the United States seeking assistance with drug-resistant parasitic infections in dogs. The infections were caused by hookworms, a group of roundworm species that can affect both animals and humans. Although Kaplan's expertise lay in livestock animals rather than pets, the descriptions of the infections in the emails resembled cases of drug resistance that he had previously studied in livestock parasites. The global problem of drug resistance in parasites has emerged due to the excessive use of antiparasitic drugs in sheep and goats, as well as the trade of these animals. If drug-resistant parasites were spreading among dogs, it would be a cause for concern. However, Kaplan was unaware of the true extent of the problem until he started investigating it.
Over the past few years, Kaplan and his colleagues conducted a series of studies that explored the origin, evolution, and spread of drug-resistant hookworms in dogs. Their research implicated the greyhound racing industry as a contributing factor to the rise of these highly resistant parasites. Greyhound racing, once a popular sport in the United States, is now nearly extinct. Nonetheless, it may have left a dangerous legacy that poses a risk to all dogs. The discoveries made by the researchers also serve as a warning regarding the management of human parasite infections. Roundworms are widespread in the animal kingdom, and hookworms, named for their hook-shaped mouthparts, latch onto the intestinal wall of their hosts to feed on blood. In dogs, the most common species of hookworm is Ancylostoma caninum. Adult hookworms reside in the gut, and their eggs are spread through faeces. Dogs can become infected when they come into contact with the larvae while walking or playing in contaminated areas or by ingesting faeces containing the larvae.
Veterinarians are aware that A. caninum infection can be potentially fatal for puppies. The worms consume a significant amount of blood, which can lead to death due to blood loss in young animals. Puppies are particularly vulnerable because they can acquire high numbers of larvae through the milk of an infected mother.
Investigation into drug resistance in infections
Typically, hookworm infections are treated with one of three classes of antiparasitic drugs: benzimidazoles, macrocyclic lactones, or tetrahydropyrimidines. To determine if dog hookworms were resistant to any of these drugs, Kaplan initiated a series of painstaking in vitro and in vivo tests using hookworm samples from three dogs with persistent infections, including a greyhound named Worthy.
Greyhounds are notorious for developing resistant hookworm infections. Veterinarians previously attributed this tendency to A. caninum's ability to remain dormant in the host's tissue and reemerge after the initial infection had cleared—a phenomenon known as the larval leak. However, Kaplan's initial results, published in 2019, revealed that the worms in the three dogs he tested were resistant to all three main classes of drugs used to treat hookworm infections. He also tested for benzimidazole tolerance in worms from another greyhound living with Worthy and from another retired greyhound, both of which had a history of stubborn infections, and found that they were resistant as well. These findings, combined with the reputation of greyhounds regarding hookworms, led Kaplan to suspect that something was occurring on greyhound farms. Further investigation revealed the perfect combination of factors that promoted the evolution of drug resistance.
Escalating concerns
He also tested for benzimidazole tolerance in worms from another greyhound living with Worthy and from another retired greyhound, both of which had a history of stubborn infections, and found that they were resistant as well.
These findings led Kaplan to suspect that something was occurring on greyhound farms.
In the early 1990s, greyhound racing in the United States reached its peak, with greyhounds competing in numerous tracks across 19 states. According to the National Greyhound Association, 39,139 new dogs were born on greyhound farms in 1993. However, the sport began to decline in the early 2000s due to pressure from animal welfare organizations and the enactment of bans in many states. By 2020, the number of new greyhounds born on farms had plummeted to 4,898 per year. Presently, dog racing is illegal in 42 states, with only two active tracks remaining, although approximately 100 breeding farms are still in operation. During the sport's heyday, greyhound farms raised large numbers of dogs and regularly treated them with dewormers, regardless of whether they had an active infection, to maintain their optimum health. "That's exactly what you don't want to do if you want to avoid resistance," explains hookworm expert John Hawdon of George Washington University, who independently reported drug-resistant hookworms in another former racing greyhound in 2019.
The constant exposure to drugs meant that any surviving worms had a reproductive advantage and would dominate the next generation. Additionally, the exercise pens for these dogs were typically set up on sandy or dirt surfaces, which provide an ideal environment for the development of hookworm larvae. After the dogs defecate in the pens, the hookworm eggs hatch and the larvae eventually reach their infective stage within five to ten days. Consequently, the dogs were repeatedly exposed to high levels of infective larvae, which further contributed to the selection of drug-resistant hookworms.
As a result, the greyhound farms became breeding grounds for highly resistant hookworms. When retired racing greyhounds were adopted as pets, they brought these drug-resistant parasites with them into new environments. Once in residential areas, the infected greyhounds contaminated the soil with hookworm eggs, posing a risk to other dogs that came into contact with the contaminated areas. This led to the spread of drug-resistant hookworms beyond the greyhound population, affecting a broader range of dogs across the United States.
The ramifications of drug-resistant hookworms are a cause for concern. Infected dogs may experience chronic gastrointestinal issues, anaemia, weight loss, and general debilitation. Treatment options become limited when the parasites are resistant to multiple classes of antiparasitic drugs. Controlling the spread of drug-resistant hookworms requires a comprehensive approach, including improved sanitation practices, strategic use of antiparasitic drugs, and increased awareness among dog owners and veterinarians.
It's important to note that the information provided here is based on the knowledge available up until September 2021, and there may have been further developments in the understanding and management of drug-resistant hookworms in dogs since then.