Even before advances in molecular genetics, researchers recognized the form of E. granulosus cycling between wild canids and their ungulate prey species in Alaska and Canada as distinct and referred to it as the northern biotype (11). The northern biotype species is now recognized as E. canadensis. In the boreal regions of North America, E. canadensis are ubiquitous parasites of wild canids and ungulates from the western coast of Alaska through all territories and provinces of Canada, exclusive of its east coast (11–13). Caribou (Rangifer tarandus), moose (Alces alces), and elk (Cervus canadensis) appear to be the most important intermediate hosts, although other ungulate species may become infected. Wolves (Canis lupus) and domestic dogs are the most important definitive hosts; however, coyotes (Canis latrans) are also competent (11). Rausch’s (11) comprehensive review of E. canadensis tapeworms provides extensive information regarding the strongly endemic cycles he and others observed from the 1950s to the late 1990s throughout Alaska, Canada, and other Arctic countries. Likewise, readers are directed to Schurer et al.’s (14,15) reviews of ungulate and wolf infections.

All 3 reviews indicate fairly stable E. canadensis cycles and transmission dynamics in the Arctic and sub-Arctic regions of North America. For example, Rausch reported that in a randomly collected sampling of 200 wild canids in the Brooks Range of Alaska, ≈30% were infected with E. canadensis tapeworms. This finding is similar to Schurer et al.’s (15) recent report indicating infections in 37% (71/191) of wolves sampled across Canada. Rausch (16) found 24% (24/101) of moose infected in an agricultural region of southern Alaska, and 4% (1/23) of moose infected in the Anchorage area. Schurer et al.’s (14) review of ungulate infections in Canada revealed prevalence of 0%–73% in elk, 1–21% in caribou, 11–38% in moose, and 0.3%–44% in white-tailed deer (Odocoileus virginianus), depending on sample location.

E. multilocularis tapeworms are similarly endemic in Alaska and Canada, spanning from the northern Arctic regions south into the rural and urban southern provinces of Canada, where urban coyotes and domestic dogs have been confirmed as both definitive and aberrant intermediate (dog) hosts (7,13,17,18). As with E. canadensis, Rausch et al. (19) performed important work related to E. multilocularis in Alaska, with a particular focus on St. Lawrence Island. Their work revealed heavy infections in Arctic fox (Vulpes lagopus) and their small mammal prey species. The mean rate of infection for 1,579 fox examined was 77%, whereas infection in prey animals could range from 10% to 80% (19). On St. Lawrence Island, dog necropsies also revealed a 12% prevalence of infection (20).

Gesy et al.’s (21) recent review of E. multilocularis tapeworms across Canada demonstrated coyotes, red fox (Vulpes vulpes), wolves, and Arctic fox were all competent hosts, with prevalence differences related to sampling location. For example, their report indicated 37% (10/27) of coyotes and 17% (1/6) of red fox were infected in Quesnel, British Columbia (21). Three dogs in Canada were also recently found to be infected with cysts (7,13,17). The high prevalence in wild hosts along with the new intermediate infections in urban pet dogs has increased concern that the geographic reach of E. multilocularis tapeworms is expanding within Canada. As a result, Massolo et al. (22) suggest that much more research is necessary to understand potential public health risks associated with alveolar echinococcosis (AE) in North America.

Both AE and cystic echinococcosis (CE) have been reported in Alaska and Canada, with most infections occurring in Native American populations in Alaska. During 1940–1990, a total of 300 cases of CE were reported in Alaskan Native Americans, with only an additional 3 cases between 1990 and 1999 (23). Pathogenicity of CE in Native American patients appeared to be fairly benign, with smaller, asymptomatic cysts that often resolved without the need for surgery (23). However, Castrodale et al. (23) reported 2 unusually severe cases of CE in Alaska in 1999, in a 51-year-old Caucasian woman and a 17-year-old Native Alaskan woman. The cysts from the latter patient were confirmed as belonging to the species E. canadensis (G8) (23). Most AE cases in Alaska also arose in Native American populations, with St. Lawrence Island as an infection hotspot. Jenkins et al. (13) reported 54 human cases during 1947–1986, none during 1986–2010, and potentially 5 during 2010–2014, although the latter infections were more likely to be CE.

Human AE and CE cases in Canada are much less prevalent than those in Alaska. Serosurveillance of indigenous populations in the Saskatchewan, Nunavut, Quebec, and the Northwest Territories of Canada indicated exposure to CE of 0%–48% (20). CE infections in nonindigenous patients are rare and often only incidentally reported. AE cases in Canada are even more rare, with 1 autochthonous case reported before 2013 (20). Subsequent reviews have found 12–16 additional cases; however, Deplazes et al. (20) suggest these numbers are under representative, given the strongly endemic regions present in Canada.

Far fewer studies have researched the Echinococcus spp. tapeworms present in Mexico; however, the primary species appears to be E. intermedius (G7), cycling between dogs and pigs (20). The species E. granulosus s.s. (G1) and E. ortleppi (G5) were also previously reported (20). Prevalence in slaughtered pigs is low, with reports of 0.27%–6.5%. Deplazes et al. (20) provide more extensive information regarding infections in Mexico.

E. multilocularis tapeworms are also historically endemic in several northern states. During 1965–1969, Leiby et al. (40) examined 7,898 definitive and intermediate host mammals from eastern Montana, North and South Dakota, Minnesota, and Iowa. Among the definitive hosts, the authors found that 8.5% (131/1,540) of red fox and 4.1% (7/171) of coyotes examined were infected. In North Dakota, 55.3% of red fox were found infected in 1965, while only 7.3% were infected in 1968. No domestic cats (n = 35) or dogs (n = 88) were found to be infected (40). However, in a subsequent report, Leiby and Kritsky (41) reported that 2 adult house cats from a homestead in North Dakota were infected with E. multilocularis tapeworms in 1971.

Of the numerous potential intermediate hosts examined by Leiby et al. (40), only 3 species, the deer mouse (Peromyscus maniculatus), meadow vole (Microtus pennsylvanicus), and house mouse (Mus musculus), were found to have E. multilocularis cysts. Infected deer mice were found in all 5 states, with the highest number, 202/4,209 (4.8%), in North Dakota. Infected meadow voles were found only in Iowa and North Dakota, with 1.9% infected (20/1,033). Only 1/91 (1.1%) house mice examined, also from North Dakota, was infected. The reported infections may be an overly conservative estimate because data were obtained through visual examination of the carcasses for cysts; early or latent cysts may be too small for gross visualization.

In the 1980s, Ballard et al. (42,43) extended the known range of E. multilocularis tapeworms into Nebraska, Illinois, and Wisconsin, further indicating the endemic cycle in the north-central United States. Ten (27%) of 36 foxes were infected in Nebraska, 4 (10%) of 40 in Illinois, and 6 (8.3%) of 72 in Wisconsin (42). In the mid-1990s, the known range expanded again when E. multilocularis infection was found in Michigan and Ohio (44,45).

In 2000, Hildreth et al. (46) reported the results of a combined study of both wild canids and human trappers in South Dakota. The intestines of foxes trapped between 1987–1991, and of coyotes trapped between 1990–1991, were examined for the presence of E. multilocularis tapeworms. Results showed that 74.5% of foxes and 44.4% of coyotes were infected at the time of trapping. Because trappers are a population at extreme risk for AE, given their constant and extended contact with potentially infected wild canids, blood samples were obtained from 115 attendees of the South Dakota trappers meetings in 1990 and 1991. However, despite their high risk, none of the trappers showed evidence of 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 (47). The patient had never lived outside of Minnesota, and her travel history included only California, Hawaii, Florida, and Manitoba, Canada. Gamble et al. (47) asserted that she was most likely to have been infected through interaction with pet dogs allowed to consume rodents on the farm where she grew up.

In the past 3 decades, there has been little focus on Echinococcus spp. tapeworms in the contiguous United States in humans or animals. The apparent lack of concern regarding the potential public health threat posed by echinococcosis is probably due to the rarity of reported human cases. Similarly, since the 1980s, there have only been a handful of reports on the presence and prevalence of Echinococcus spp. tapeworms in domestic and wild animals in the lower 48 states. The lack of human cases may reflect low exposure to infected definitive hosts of either domestic or sylvatic origin. Alternatively, it may reflect lack of detection, misdiagnosis, or failure to publish literature regarding new AE and CE cases. Regardless of the reason, however, there is a legitimate concern that the public health risk may increase within the United States in the future, given the continued expansion of human infection in Europe and Asia, as well as an apparent expansion of the range of E. multilocularis tapeworms in Canada.

Current surveillance and basic scientific understanding of Echinococcus spp. tapeworms and echinococcosis in the United States is particularly lacking. In addition, there are many outstanding questions, including the presence, prevalence, and geographic distribution of Echinococcus spp. tapeworms across the contiguous United States; geospatial, ecologic (both biotic and abiotic), and host population variables influencing sylvatic echinococcosis dynamics; and perhaps most important, what risk these sylvatic and domestic cycles pose to human health. As noted by Massolo et al. (22), Canada appears to be experiencing an increase in the presence and prevalence of E. multilocularis tapeworms, with spillover events starting to occur. If this asserted expansion is true, a similar expansion may be occurring in the United States. We support increased surveillance of Echinococcus spp. tapeworms in the United States to answer these questions and in particular to focus on the existing and potential public health risks associated with endemic Echinococcus spp. tapeworms. To enhance data collection on current and past cases, and further define the human burden of echinococcosis, we support and recommend the use of historically underused data sources, such as the National Inpatient Sample (48). We believe addressing the described data gaps is very important not only because of potential spillover from sylvatic cycles but also because echinococcosis is not reportable in either animals or humans in the United States and the United States does not screen for these types of infections in imported animals. If appropriate surveillance occurs that comprehensively analyzes current endemic cycles, then effective and efficient detection of new or expanding cycles or spillover events would be much more likely to occur, leading to a decrease in potential human cases or other negative public health outcomes.