A Rare, Cancer-Like Parasite Is Emerging in North America

A Rare, Cancer-Like Parasite Is Emerging in North America
An adult Echinococcus multilocularis tapeworm isolated from a fox in Hungary. (Image: Wikimedia Commons)

A cancer-like parasitic disease caused by certain tapeworms has squirmed its way into North America, researchers in Canada warn. Their recent review shows that cases of the still very rare disease have started to rise in the province of Alberta and elsewhere over the past few years, in both humans and animals. They also present evidence that these parasites were likely brought over by dogs from Europe and have now definitely made a new home here.

The disease is called alveolar echinococcosis (AE) and is caused by infection from the tapeworm Echinococcus multilocularis. These worms can infect several species of mammals, but their definitive hosts (hosts for their adult and sexually reproductive stage of life) are wild canines and domestic dogs. People are usually infected through the ingestion of microscopic eggs from eating contaminated food or handling other infected animals, and we then become a dead end for the tapeworm.

Unfortunately, their eggs still cause trouble for us, since they can burrow into our organs, typically the liver, and turn into self-sustaining parasitic growths, becoming a sort of external tumour. It can take years of infection for the symptoms of AE to emerge, which often mimic liver cancer and include upper abdominal pain, general weakness, as well as weight loss. Without treatment — usually the surgical removal of the growths and/or a life-long course of antiparasitic drugs — AE is eventually fatal.

Human AE is rare but endemic in parts of Europe and Asia, but it had almost never been seen in North America until recently. Before the 2010s, there were a total of two victims ever clearly documented on the continent: a case from Manitoba, Canada in 1928, and a second from Minnesota in 1977. Other research has suggested that these worms may have lived in the northwestern parts of the continent for a while now, occasionally sickening Indigenous people who came across them, but never in huge numbers. Now, researchers from the University of Alberta and elsewhere say that the situation has taken a turn for the worse.

In a review of cases published this March in The American Journal of Tropical Medicine and Hygiene, the scientists say they’ve documented 17 cases of human AE in Alberta alone between 2013 and 2020. All of the cases were treated with antiparasitic drugs, which are used when surgery alone can’t remove the growths safely. Unfortunately, one person died following complications from surgery. Most of these victims owned dogs, and the scientists point to other research showing that cases of wildlife AE (often in coyotes) have increased in the area as well during the same time period.

These newer cases, based on a genetic analysis of the tapeworms found in the patients, seem to trace back to a strain that probably hitched a ride from dogs originally infected in Europe — a strain seemingly more capable of causing illness in humans and animals than any previously existing populations in the area.

“It is unequivocally new as a human disease in the western hemisphere. The explanation very clearly seems to be the introduction of the more virulent European strain of the parasite into our wildlife ecology,” lead author Stan Houston, an infectious diseases expert at the University of Alberta, told Gizmodo in an email. “Why it is most apparent in Alberta, by far, at the present time, is somewhat speculative. I think some mix of factors of where the parasite was first introduced and/or favourable wildlife ecology are most likely.”

Though Alberta does seem to be the current hotspot for these worms, Houston has heard from other researchers about their recently discovered AE cases, too, including two human cases in the U.S. And he’s certainly worried that the worms could spread farther across North America. “Of course it could, and in fact, what we know so far suggests that the parasite has been remarkably successful, achieving considerably higher prevalence in Alberta coyotes than in its natural reservoir, the red fox in Europe,” he said.

Human AE thankfully remains a relatively rare disease, with an estimated 18,000 cases annually, mostly in China. And because AE ultimately kills its victims if untreated, Houston doesn’t suspect that there are many more hidden cases than what scientists have found so far. Simple good hygiene, like washing your hands after handling your dog, especially if you live in a more rural area where coyotes live, and washing food grown locally in coyote-rich areas, can further lower the risk of infection.

Uncommon as it is, the incidence of AE does appear to be increasing in some parts of Europe, too, possibly due to the effects that climate change and urbanisation have had on the worms’ host animals. And the lack of symptoms upon initial infection can also delay diagnosis to the point where surgery is no longer an option (in eight of the 17 cases found in Alberta, the growths were found by complete accident during other testing). So Houston and other researchers plan to keep an eye on these tapeworms. They’re next conducting a study of patients in Alberta who had their livers biopsied but seemingly had no cancer, to see if any of them may have had AE instead.