Echinococcosis: Emerging US Public Health Threat?

A recent study suggests that Echinococcus spp. tapeworms, in particular, E. canadensis, may be under-recognized in the United States.

Jacey Roche Cerda, JD, MPH, Colorado State University, Fort Collins, Colorado, and colleagues evaluated previous studies of Echinococcus spp. and echinococcosis in humans in North America.

They published the results of their study in the February 2018 issue of Emerging Infectious Diseases, the Centers for Disease Control and Prevention’s monthly peer-reviewed public health journal.

"Although well studied globally, the current presence, prevalence, and transmission dynamics of Echinococcus spp. tapeworms in the contiguous United States are currently unknown," the authors write. "Substantial research was conducted between the early 1930s and the 1980s; however, very little research has occurred in the past 3 decades."

According to the authors, Echinococcus spp. currently infect 2 to 3 million individuals worldwide, and these infections are associated with annual economic losses ranging from $200- to $800-million.

Studies also show that infection with these zoonotic tapeworms appears to be increasing, reemerging, and geographically expanding across Europe, Asia, Africa, and the Americas (predominantly in Latin America), with more than 200,000 new cases occur each year.

As a consequence, the researchers emphasize that scientists, veterinary and human medical professionals, and other public health officials need a more complete picture of Echinococcus spp. cycles in the United States, particularly because echinococcosis is not a reportable disease.

In an interview with Contagion^®, Ms. Cerda explained that she and her colleagues chose to pursue this study because of the increased attention to the potential public health threat of Echinococcus spp. via transmission from wildlife hosts.

“Previous work has shown that fear of disease from wildlife can decrease public support for conservation, even if human actions are contributing to disease spread,” she said. “We wanted to better understand this threat, if any, and whether or not public health risk truly was increasing.”

The researchers analyzed data on Echinococcus spp. and echinococcosis in the United States and Canada.

These data indicate relatively stable E. canadensis cycles and transmission dynamics in the Arctic and sub-Arctic regions of North America, they write. For example, one study in Alaska showed that approximately 30% of 200 wild canids were infected with E. canadensis tapeworms. Likewise, another study found infections in 37% of 191 wolves sampled across Canada.

E. multilocularis tapeworms are similarly endemic in Alaska and Canada. One study in Alaska showed heavy infections (mean infection rate, 77%) in Arctic fox (Vulpes lagopus) and their small mammal prey species (infection rate range, 10% to 80%). And urban coyotes and domestic dogs have been demonstrated to be both definitive and aberrant intermediate (dog) hosts in various regions of Canada.

Cases of alveolar echinococcosis (AE) and cystic echinococcosis (CE) in humans have been shown to be prevalent in both Alaska and Canada. From 1940 to 1990, 300 cases of CE were reported in Alaskan Native Americans, with only 3 more cases reported between 1990 and 1999. Most AE cases in Alaska also arose in Native American populations, with 54 cases reported from 1947 to 1986 and none from 1986 to 2010. From 2010 to 2014, 5 cases were reported, although the authors note that these cases were more likely to be CE.

AE and CE are less prevalent in Canada than in Alaska, with data from indigenous populations indicating exposure to CE ranging from 0% to 48%. AE cases in Canada are even more rare, with only 1 autochthonous case reported before 2013. Data indicate 12 to 16 additional cases arising after 2013, although these numbers may be under-represented, especially given the strongly endemic regions present in Canada, the authors stress.

According to Ms. Cerda and colleagues, in the contiguous United States, E. multilocularis tapeworms are also endemic in several northern states. One study examined almost 8,000 definitive and intermediate host mammals from eastern Montana, North and South Dakota, Minnesota, and Iowa, from 1965 to 1969, and found that 8.5% of 1,540 red foxes and 4.1% of 171 coyotes were infected.

Studies in the 1980s and 1990s further extended the range of E. multilocularis infections into Nebraska, Illinois, Wisconsin, Michigan, and Ohio.

In 2000, a study in South Dakota indicated that 74.5% of foxes and 44.4% of coyotes were infected at the time of trapping. However, analysis of blood samples from 115 South Dakota trappers showed no evidence of human infection.

“Only 1 autochthonous case of AE has been confirmed in the contiguous United States, and that occurred in a 56-year-old woman from Minnesota,” the authors state. The woman was thought to have been infected through interaction with pet dogs that ate rodents on the family farm.

“The main point that we hope readers take away from our paper is that, although there may be a potential public health risk, human cases are extremely rare despite multiple Echinococcus species being present in North America for several decades, and human infections are easily prevented, no matter what wildlife are in the area,” said Ms. Cerda.

To reduce the public health risks, Ms. Cerda recommended that individuals regularly deworm their pets and practice good hygiene if handling any wild animal carcasses or feces.

“In terms of future studies, we believe there is a need for much more extensive data on the true presence and prevalence of the tapeworm in canid populations across North America,” she concluded. “Our ongoing research is currently addressing these questions.”

Dr. Parry graduated from the University of Liverpool, England in 1997 and is a board-certified veterinary pathologist. After 13 years working in academia, she founded Midwest Veterinary Pathology, LLC where she now works as a private consultant. She is passionate about veterinary education and serves on the Indiana Veterinary Medical Association’s Continuing Education Committee. She regularly writes continuing education articles for veterinary organizations and journals and has also served on the American College of Veterinary Pathologists’ Examination Committee and Education Committee.

Feature Picture: Man's arm showing positive skin test for hydatid disease (echinococcosis). Feature Picture Source: CDC / Public Health Image Library / Dr. I. Kagan