Hookworms Resistant to Treatments
Hookworms Resistant to Treatments
Hookworms are one of the most common intestinal parasites affecting dogs, and Ancylostoma caninum is the principal species causing canine hookworm disease worldwide. In the United States, veterinarians rely on three antihelminthic drugs to treat hookworms, but a study by University of Georgia (UGA), College of Veterinary Medicine, researchers, recently published in the International Journal for Parasitology: Drugs and Drug Resistance, shows that the parasites have developed resistance to these medications.
Hookworm transmission
Using their hook-like mouths, adult hookworms attach to a dog’s intestinal lining, where they feed off the dog’s blood. The female hookworm produces numerous eggs per day, which are shed in the dog’s feces, contaminating the environment. In 5 to 10 days from being shed, the eggs will develop to the infectious third stage larvae (L3). Transmission to other dogs can occur via three routes.
- Ingestion — When L3 hookworms are ingested, they travel to the small intestine, where they enter the glands in the intestinal wall. After several days, they emerge and mature to the adult stage. Dogs ingesting insects or rodents infected with L3 can also become infected.
- Skin penetration — L3 hookworms can penetrate the skin and travel via the blood to the lungs, where they are coughed up and swallowed, and make their way to the small intestine. L3 hookworms can also enter the alveolar blood vessels and travel to the muscles or organs, becoming encysted. These arrested larvae can later become reactivated if conditions are appropriate, including during transmammary transmission, or larval leak. Larval leak occurs when L3 in muscles and the intestinal wall are reactivated to become adults, after other adult hookworms are eliminated.
- Transmammary — Arrested L3 hookworms can become reactivated during parturition, and transmitted to puppies through the mother’s milk.
Clinical presentation
Once a dog is infected, hookworms feed on the dog’s blood, resulting in blood loss. Disease can manifest in four forms:
- Peracute — This form is typically seen in newborn puppies, who at approximately a week old, become infected by transmammary transmission, causing dramatic anemia. The condition is life-threatening, and requires aggressive supportive care and blood transfusions.
- Acute — This form affects older puppies, and causes less severe anemia, but can also be life-threatening, if not treated.
- Compensated — Some infected adult dogs can appear clinically normal.
- Decompensated — This form typically occurs secondary to another disease process in adult dogs, who can become anemic and exhibit signs.
Zoonotic potential
Hookworms can also infect humans, most commonly by penetrating the skin. In humans, hookworms typically remain confined to the outer skin layers, causing cutaneous larva migrans. Once under the skin, the parasites create two to three millimeter wide, snakelike tracts in the area surrounding the penetration site. The lesions are intensely pruritic. The sites most commonly infected include the feet, hands, and knees. In some cases, the larvae migrate to the intestine, causing enteritis, and can also penetrate other organs, including the eyes, where they may cause blindness.
Antihelminthic treatment options for hookworms
Veterinarians in the United States currently rely on three anthelmintic medications to treat hookworms in dogs.
- Fenbendazole — Fenbendazole is a benzimidazole antiparasitic drug that works by disrupting the parasite’s cell structure, and preventing nutrient uptake.
- Moxidectin — Moxidectin is a second generation macrocyclic lactone, and works by blocking neuronal signal transmissions in parasites, causing paralysis, and expulsion from the body.
- Pyrantel pamoate — Pyrantel pamoate is a tetrahydropyrimidine that acts as a depolarizing neuromuscular blocking agent, causing the parasite to release its hold on the intestinal wall, and to be expelled from the body.
Hookworm resistance
For their recent study, the UGA researchers analyzed fecal samples from current and former racing greyhounds. Dog race tracks are particularly conducive to hookworm spread, because numerous dogs are housed in close proximity, and the sandy soil at the facilities is an ideal parasite breeding ground. Dogs in this environment are typically dewormed frequently, every three to four weeks. Fecal samples were taken from greyhound adoption kennels, three veterinary practices associated with adoption groups, and an active racing kennel. Four out of every five greyhounds tested positive for hookworms, and more could be infected if larval leak is considered. After hookworm treatment, the dogs were tested again, and their infection levels remained high.
In racing dog breeding farms and kennels, numerous dogs can be infected, hosting large parasite loads. These conditions permit the parasites to develop mutations, which allow them to become resistant to deworming medications. Frequent deworming helps these parasites survive and pass the mutation to their offspring. Over time, the drug-susceptible parasites will be killed, and the resistant parasites will predominate.
Complicating matters, most dogs are not tested after treatment, so the hookworm infection is not noticed until the dog’s infection is significant, leading to anemia. Testing by the researchers revealed that the parasites were resistant to all three commonly used antihelminthic drugs. As these dogs are adopted, they spread these resistant worms to other pet dogs, making areas such as dog parks unsafe. Suspected cases are now emerging in non-greyhound dogs across the United States. As the resistant hookworms spread, they could become a problem for humans, as well.
Hookworm task force
To address this concerning issue facing canine and human health, the American Association of Veterinary Parasitologists (AAVP), recently formed a national hookworm task force of 25 members, including veterinarians and parasitologists from academia, industry, and government. They will develop guidance for veterinary practitioners to diagnose and treat multi-antihelminthic drug resistant (MADR) Ancylostoma caninum. Current recommendations for veterinarians include:
- Test — No validated test for MADR A. caninum is currently available, so dogs who test hookworm-infection positive should have a fecal egg count reduction test (FECRT) performed, to compare the fecal egg counts before treatment and at 10 to 14 days after treatment. The dewormer is likely not effective if hookworm fecal egg counts do not decrease by 75 percent or more.
- Fecal removal — Dog owners should be counseled to immediately remove fecal material, because proper disposal can protect dogs from reinfection, and prevent further spread of MADR hookworms.
- Emodepside — Emodepside is a cyclic depsipeptide antiparasitic drug approved for use in cats in the United States. MADR hookworms appear susceptible to this drug, which acts at the parasite’s neuromuscular junction, resulting in paralysis and death. Since emodepside is not approved for dogs, only licensed veterinarians should prescribe and administer the drug.
Hookworm resistance is a serious issue affecting dogs and humans, but the quick formation of a national task force will hopefully provide insight to help combat this concerning problem, to protect dogs and their owners.
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