In a presentation yesterday at the 2019 ACVIM Forum in Phoenix AZ, Dr. Ray Kaplan from the University of Georgia gave a somewhat scary talk about the emergence of multidrug-resistance in the hookworm Ancylostoma caninum.
Resistant parasites don’t tend to get as much attention as resistant bacteria, but they can present similar challenges. The concern is that there is now pretty solid evidence of hookworms that are resistant to most or all approved drugs typically used to treat these parasites. It’s thought to mainly have developed in greyhound breeding and racing kennels, where there’s a lot of dewormer use and a lot of infection pressure, creating a perfect environment for emergence of resistance.
Resistance poses a particular risk to puppies, since hookworm infestations can kill young dogs. Adult hookworms live in the small intestine, where they attach to the intestinal wall and suck blood from the host. They are voracious feeders and large burdens can cause serious (or even fatal) problems because of the amount of blood that is lost. It’s also a concern from a public health standpoint – people don’t get the intestinal infection, but they can develop a condition called cutaneous larva migrans, in which the parasite larva in the environment penetrate and burrow though the skin, causing an extremely itchy rash that persists until the parasite ultimately dies or is killed. The same drug classes are used to treat this infection in people and dogs. In people, the infection typically will go away on its own even without treatment, but it can be very uncomfortable for a few weeks or months waiting for a resistant infection to naturally die out.
The scope of the resistant hookworm issue isn’t clear, but it’s something to be aware of. We commonly see recurrent hookworm infections because of what’s call “larval leak.” When a dog is infected with hookworms, they migrate throughout the body and can become dormant. After deworming eliminates the intestinal infection, these larvae can re-activate, make their way to the gut, and re-establish the infection. That’s not resistance, it’s the biology of the worm and is why repeated treatments are often needed.
Resistance is when the intestinal worms aren’t killed by the dewormer. Resistance is most easily detected by doing a fecal egg count reduction test (FECRT). That involves testing a fecal sample before and after deworming (before there’s enough time for larval leak), to see how much of a reduction in egg count occurred (which should correspond to the proportion of adult worms that were killed by the treatment).
Individuals and groups adopting greyhounds from US breeding and racing facilities should be particularly aware of the risk of resistant hookworms. Performing a fecal egg count reduction test should be a routine practice on newly obtained greyhounds, and is never a bad idea in any dog that is found to have a hookworm infestation.
The other big consideration is reducing exposure. Resistant parasites are another reason why it’s important for people to be responsible and pick up their dog’s feces. The less fecal contamination there is in the environment, the less exposure there will be of dogs (and people).