Echinococcus spp. Tapeworms in North America

Jacey Roche Cerda; Danielle Elise Buttke; Lora Rickard Ballweber


Emerging Infectious Diseases. 2018;24(2):230-235. 

In This Article

Echinococcus Spp. Tapeworms and Ce in the Contiguous United States

Domestic Cycles

Historically, several genotypes of E. granulosus s.l. tapeworms were considered endemic and regularly cycling in the Mississippi Valley, the mid-Atlantic states, and multiple western and southwestern states. Franklin and Ward[24] reported that hydatid cysts were found in hogs and cattle in Virginia, Oklahoma, Arkansas, and Louisiana before 1951. They then reported the first discovery in Mississippi, finding that 2 of the 50 dogs they examined in 1951 were infected with E. granulosus s.l. tapeworms. Subsequently, Ward[25] reported that 6% of 8,066 slaughtered hogs in Mississippi were infected in 1956 and that 4 of the 9,300 dog intestines he examined over 8 years were infected with 300–700 adult E. granulosus cestodes each. At that time, 15 dogs had been confirmed with E. granulosus infection, with 5 of them from Mississippi.[25] Given our current genetic understanding of Echinococcus spp., the species cycling in Mississippi was most likely E. intermedius (G7). Infections were also reported in dogs from New York, Kentucky, Georgia, and Tennessee, as well as Washington, DC.[25] However, we were unable to find the originally cited papers to report the intermediate hosts involved and thus cannot comment on what species were present in those states.

Throughout the 1900s, an endemic dog/sheep cycle of CE also existed in the Central Valley of California. Through examination of the organs of 22,720 sheep carcasses during December 1967–June 1968, Sawyer et al.[26] found 1,100 infected sheep. Tracebacks conducted where possible demonstrated that infections could range from 5% to 99% of a producer's entire herd.[26] Some of the infected herds originated in Idaho and Utah, while the rest originated from 8 ranches in 4 California counties.[26] For the producers with the highest herd prevalence, Sawyer et al.[26] investigated the ranches, took medical histories of the families, tested the onsite dogs for taeniids through arecoline purging, and tested live sheep serum for evidence of infection. Resident domestic dogs and sheep were infected on 3 of the 4 ranches evaluated, implying local transmission. Local transmission indicates that the dogs were probably allowed to consume viscera of infected sheep, with subsequent egg contamination of the human environment through dog infections.[27] Across the same 4 ranches, 3 humans also harbored hydatid cysts; however, only 1 of these cases was confirmed as acquired within California.

In Utah, Loveless et al.[28] investigated dog and sheep infections during 1971–1976, finding an overall prevalence of 11.3% (95/839) in dogs and 9.8% (877/8,994) in sheep. A subsequent study by Anderson et al.,[29] with data covering 1971–1982, revealed a prevalence of 9.7% (109/1,120) in dogs and 7.1% (1,116/15,775) in sheep. During 1944–1980, Utah had the highest density of autochthonous human cases in the United States, with 50 persons infected.[28] A similar dog/sheep cycle was established in New Mexico and Arizona.[30] In those areas, human infections were reported during 1969–1976, revealing 21 confirmed autochthonous cases. Nineteen of the cases occurred in Native Americans of the Navajo, Zuni, and Santo Domingo tribes.[30] The source of infection in all of these areas appeared to be contamination of the human environment by infected dogs. Risk factors for human infection included sheepherding behavior; companion-animal ownership; home slaughter of sheep; allowing dogs to consume raw viscera; and contact with dogs, sheep, and swine.[31,32] Although not confirmed, our current understanding of Echinococcus spp. tapeworms suggests that the organism causing the California, Utah, New Mexico, and Arizona cycles was most likely E. granulosus s.s.

More recent focal studies have revealed that cycles of E. granulosus s.l. continue to occur throughout the United States. For example, in 1994, Hoberg et al.[33] reported an unusual case of 3 cysts in a 14-year-old Thoroughbred horse that lived in Virginia and Maryland. The cysts were not sequenced to determine the genotype; however, those authors assert it was probably related to the species E. equinus (G4), as previously found in horses in Europe and Asia.

Sylvatic Cycles

Throughout the mid-1900s to late 1900s, a sylvatic coyote/deer cycle occurred concurrently with the domestic sheep/dog cycle in California.[34,35] Brunetti and Rosen[34] analyzed 2,049 deer carcasses collected between 1945–1969 and found an overall infection rate of 1.3%. The infected animals were concentrated in 8 counties in the Central Valley of California. Although unknown, the species involved in the sylvatic cycle may have been E. canadensis given the host species involved.

Riley[36] reported a sylvatic cycle between moose and wolves in Minnesota in the 1930s. Ramsey[37] reported that E. canadensis infection was found in a moose in northwestern Montana during 1976–1983. In 2009, Foreyt et al.[38] reported the presence of E. canadensis infections in wolves, elk, mule deer (Odocoileus hemionus), and a mountain goat (Oreamnos americanus) in Idaho and Montana; however, the specific genotypes (G8 or G10) were not delineated. In 2012, Lichtenwalner et al.[39] described the finding of an E. canadensis (G8)–infected moose in Maine.

The several sylvatic cycles reported in California, Montana, Idaho, and Minnesota indicate that E. canadensis tapeworms have been sporadically endemic in wild canid/ungulate populations across the United States since the first discovery in the early 1900s. With no further cases reported, the coyote/deer cycle appears to have ended in California. However, the cestode now appears to be reemerging in a wolf/ungulate cycle in the western United States.