We most often think about zoonotic tapeworm infections in association with cattle and swine: humans serve as the definitive host for Taenia saginata, the beef tapeworm, and can serve as either the definitive host for Taenia solium (the pork tapeworm) or can be infected with the tissue cyst form of this tapeworm (leading to cysticercosis).

Dogs and wild canids are the definitive host for Echinococcus tapeworms, but the zoonotic potential of Echinococcus needs to be addressed and appreciated. Although tapeworms are not normally considered a serious threat to human beings, the Echinococcus spp. pose a special concern because humans serve as aberrant intermediate hosts, developing potentially aggressive and damaging tissue cysts.

There are two distinct species of Echinococcus, each associated with specific definitive and intermediate hosts: (Click here to see a schematic of these relationships.)

Echinococcus granulosus:

Domestic dogs are the most common definitive host for E. granulosus and sheep serve as the normal intermediate host. Echinococcus granulosus is most prevalent in the sheep-raising areas of the Western U.S.

Infection of humans with E. granulosus leads to the development of "hydatid cysts."

• Hydatid cysts are slowly enlarging, mass lesions producing pain and potentially physical obstruction/pressure on surrounding organs.
  • They develop over a period of months to years.
• If the cyst ruptures, "daughter" cysts may be released that spread elsewhere in the body.
  • Most cysts develop in the liver or lungs.

• (Click here to see an MRI scan of a hydatid cyst.)
• (Click here to see a hydatid cyst being surgically excised.)

Echinococcus multilocularis:

For this species, foxes (and to a lesser extent coyotes, wolves and other wild canids) are the definitive hosts and rodents (voles and deer mice) serve as the normal intermediate hosts. Echinococcus multilocularis is most prevalent in the far north. Domestic dogs are increasingly being implicated as hosts in Alaska and there is great concern for further southward progression of this parasite and extension into the domestic dog population of the continental U.S., particularly the northern Midwestern states.

Infection of humans with E. multilocularis leads to the development of "alveolar hydatids."

• These are much more aggressive lesions than hydatid cysts because the organism reproduces asexually by lateral budding and extension into surrounding tissues, rather than into the cyst itself.
  • Fatality rates in infected humans approach 50%.

Treatment of hydatid disease in humans:

Treatment of hydatids in humans may be medical or surgical, depending on the extent and accessability of the lesions. Prior to surgical excision, the lesions are often aspirated and injected with a protoscolicidal agent (e.g., 95% alcohol) to reduce the chance for leakage of anaphylactogenic or infectious contents during surgery.

Diagnosis of Echinococcus infestation in dogs:

Diagnosis of Echinococcus infestation in domestic dogs is difficult:

• Adult Echinococcus tapeworms are extremely small (1-6mm in length). Therefore, it is unlikely that individual proglottids (segments) will be discernible.
• The eggs of Echinococcus spp. cannot be differentiated from those of the common Taenia tapeworm of dogs.
  • Therefore, particularly for dogs that live in or have recently visited the northern Midwest, Canada or Alaska, any Taenia-like eggs should be considered possible evidence of Echinococcus infection.

Treatment of Echinococcus infestations in dogs:

Given the difficulty in differentiating infection with Echinococcus vs. Taenia, it is very fortunate that praziquantel (Droncit®), the commonly used anti-tapeworm anthelmintic for dogs, is also highly effective against Echinococcus.

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