Echinococcus spp

Synopsis

CAPC Recommends

• Treating intestinal Echinococcus spp. infections in dogs with praziquantel at dose of 5 mg/kg either orally or injectable.
  
• Educating owners of the risk of Echinococcus spp. to humans, emphasizing prompt fecal removal and disposal to reduce the environmental risk of the parasite to pets and people.
• Using PCR tests offered at diagnostic labs to molecularly differentiate Echinococcus spp. eggs from Taenia spp. eggs (—often referred to as Taeniid type eggs) when appropriate.

Species

Canine

Echinococcus granulosus sensu lato

• E. canadensis
• E. granulosus sensu stricto

Echinococcus multilocularis

Feline

Echinococcus multilocularis

Overview of Life Cycle

• Echinococcus spp. are cyclophyllidean cestodes that have indirect life cycles requiring specific intermediate hosts. Most Echinococcus spp. are maintained in sylvatic cycles with wild carnivore definitive hosts and ungulate or rodent intermediate hosts, but E. granulosus can also be found in domestic cycle involving domestic dogs and sheep.

• Definitive hosts infected with adult Echinococcus spp. shed gravid proglottids in their feces. When the eggs are consumed by the appropriate intermediate host, large hydatid cysts develop. Dogs and cats are usually infected when they ingest these cysts during predation or scavenging.

Stages

• Adults of Echinococcus spp. are very small and contain very few body segments (E. granulosus: 2-7mm consisting of 3-4 segments, E. multilocularis: 1.2-4.5mm consisting of 4-5 segments).
• Adults of E. granulosus are found primarily in the anterior portion of the small intestine, while E. multilocularis is found mainly in the mid to distal portion of the duodenum.
• Eggs, contained in proglottids, are passed into the environment via the feces of definitive hosts and are immediately infective to intermediate hosts. Infectious eggs contain a hexacanth embryo and are morphologically indistinguishable from those of Taenia spp.
• Once eggs are ingested by an intermediate host, oncosphere larvae are released and penetrate through the layers of the intestinal wall, where they are then transported through the blood or lymph vessels to the liver, lungs, or other organs. Oncosphere larvae then develop into hydatid cysts (metacestode larvae).
• Unilocular hydatid cysts of E. granulosus are large fluid-filled bladders with internal budding of protoscolices which are mainly found in the liver and lungs of infected intermediate hosts.
• Multilocular (alveolar) hydatid cysts of E. multilocularis develop primarily in the liver and aggregate forming alveolar structures composed of numerous cysts of irregular shapes. Multilocular hydatid cysts have the capacity to proliferate by external budding and form root-like protrusions of tissue, enabling the cestode to progressively invade host organs. They are highly invasive and mimic malignant neoplasms in the tissues of infected intermediate hosts.
• The definitive host becomes infected when it ingests the protoscolices within hydatid cysts in an intermediate host.

Disease

• Adult Echinococcus spp. in the lumen of the small intestine cause no disease in the majority of dogs and cats. Dogs have been occasionally reported to have bloody diarrhea.
• Dogs can also develop alveolar echinococcosis, in which they serve as aberrant intermediate hosts and develop multilocular hydatid cysts. A number of cases have been reported in North America, which is outside of the historical endemic range. These cases are thought to follow either ingestion of a large number of eggs from the environment or intestinal infections in which the adult E. multilocularis release eggs into the intestinal lumen.
• Recently, alveolar echinococcosis associated with hemorrhagic masses in the liver has been described in a number of dogs in Canada (Alberta, British Columbia, Manitoba, Ontario, and Saskatchewan) and the United States (Washington, Missouri, Virginia). Research to understand this phenomenon is ongoing.

Prevalence

• E. granulosus has a worldwide distribution, with the highest prevalence in parts of Eurasia, north and east Africa, Australia, and South America. Infections have also been reported in dogs in North America, albeit at a lower prevalence.
• E. multilocularis is endemic to the northern hemisphere, where its range includes western Europe, parts of the near East, Russia, and the central Asian Republics, China, northern Japan, and Alaska.
  • Prevalence in these areas ranges from 1-12%.
• Infections have also been reported in Canada and the United States with increasing cases outside of the historic range.
  • These cases include western areas of British Columbia, Canada, Oregon, Washington, and California as well as a southern expansion to Nevada, Colorado, Kansas, Missouri, Texas, Florida, and Virginia.
  • This expansion of the endemic range appears to be linked to European haplotypes (strains) which may be more transmissible to domestic dogs.
• Younger dogs are more commonly infected with Echinococcus spp. than older dogs.

Host Associations and Transmission Between Hosts

• E. multilocularis can infect both dogs and cats, following ingestion of rodents with multilocular hydatid cysts.
  • E. multilocularis is typically maintained in a sylvatic cycle with foxes as definitive hosts and smaller mammals, such as rodents, as intermediate hosts. Coyotes, wolves, and raccoons can also serve as definitive hosts with recent studies reporting a high percent of coyotes infected. Cats and dogs act as definitive hosts when they acquire the infection from rodents. However, cats are thought to serve as less optimal hosts for E. multilocularis than dogs.
• E. granulosus is only known to infect dogs and wild canids, following ingestion of cysts in ungulate viscera.
  • There are two strains of E. granulosus in North America.
    • A sylvatic strain which is endemic in Alaska and parts of Canada and cycles in wild canids and cervids.
    • A pastoral strain, which cycles through domestic dogs and sheep (goats, cattle, pigs, horses, and camels can also serve as intermediate hosts), and has been reported in Arizona, California, New Mexico, and Utah.

Prepatent Period and Environmental Factors

• The prepatent period of Echinococcus spp. may be as long as 1-2 months.

• Sexual maturation is reached within 4-5 weeks and eggs or gravid proglottids are shed in the feces

Site of Infection and Pathogenesis

• Echinococcus spp. adults in the small intestines of dogs or cats do not typically cause significant disease, although recent case reports have documented dogs with severe, bloody diarrhea. The zoonotic risk this parasite creates warrants treatment regardless of clinical signs exhibited.
• Dogs that develop multilocular hydatid cysts with associated hemorrhagic lesions and organ failure have a poor prognosis.

Diagnosis

• Due to the small size of Echinococcus spp. adults and their low number of body segments, it is extremely unlikely for proglottids to be observed grossly.
• Fecal flotation with centrifugation
  • Diagnosis of patent Echinococcus infections via centrifugal fecal flotation can be challenging as eggs are quite dense. Click here to see Fecal Examination Procedures. Examine for taeniid eggs.
  • The use of sugar solution and centrifugation can increase the sensitivity of Echinococcus egg detection although the sensitivity remains low.
  • Eggs of Echinococcus spp. cannot be differentiated morphologically from those of Taenia spp.
  • Due to the zoonotic potential, similarity to Taeniid eggs, and low infective dose, extreme caution should be used when processing samples suspected of Echinococcus infections. This may include working or hunting dogs that have access to wildlife or offal.
• PCR on feces
  • Due to the need to differentiate taeniid eggs, several diagnostic laboratories have PCR to molecularly identify Echinococcus multilocularis and E. granulosus.
• Diagnosing alveolar echinococcus in dogs
  • The multilocular cysts may be mistaken for hepatic cancers on radiographs and ultrasound.
  • Fine needle aspirate of the cysts can be evaluated to look for the protoscolices.
  • PCR on fine needle aspirates is commercially available.
  • Care should be taken when doing the FNA, as releasing protoscolices from the cyst can seed other areas of the abdomen, exacerbating the disease.

Treatment

• Praziquantel is approved at 5mg/kg orally or subcutaneously for elimination of intestinal stages of adult Echinococcus spp. in dogs.
• Canine alveolar echinococcosis carries a guarded prognosis.
  • When clinical condition allows, alveolar echinococcosis in dogs can be treated with surgical resection of the mass and extra-label use of albendazole (10 mg/kg every 24 hours).
  • Praziquantel is also advised, in conjunction with albendazole, if intestinal stages are suspected. Elimination requires very aggressive therapy for a prolonged period (months to years). See Haller M et al. 1998. Surgical and Chemotherapeutic Treatment of Alveolar Echinococcus in a Dog. J Am Anim Hosp Assoc. 34: 309-314.
  • Due to concerns about bone marrow suppression with the use of albendazole in dogs, close supervision by a veterinarian and routine complete blood counts are recommended throughout therapy.
• Treatment of Echinococcus spp. in dogs and cats must be combined with management changes to prevent ingestion of vertebrate prey species; in the absence of these changes, re-infection is likely to occur.

Control and Prevention

• Prevention of predation and scavenging activity by keeping cats indoors and dogs confined to a leash or fenced in a yard will limit the opportunity for acquiring infection with Echinococcus spp. via ingestion of infected intermediate hosts.
• In dogs that are allowed outside or that are known to have predatory behavior, a heartworm preventive containing praziquantel will routinely treat infections with Echinococcus spp. and other cyclophyllidean cestodes.
• Dogs and cats should be treated with praziquantel before transporting them to a new, non-endemic area to avoid spreading Echinococcus spp.

Public Health Considerations

• Although the overall risk of human infection with Echinococcus spp. in North America is low, dogs and cats infected with these tapeworms create a zoonotic risk. The eggs shed in the feces of an infected dog or cat are immediately infectious to the intermediate host.
• Humans are susceptible as intermediate hosts to E. granulosus. The large, slow growing, thick-walled unilocular hydatid cysts usually develop in the liver and lungs and are associated with pressure atrophy and impaired organ function due to their size. Surgical removal is necessary to effectively treat. The sylvatic strain appears to result in a less pathogenic disease in humans than the pastoral strain.
• Humans may also serve as intermediate hosts of E. multilocularis. Due to the rapid, invasive growth of the multilocular hydatid cysts associated with alveolar echinococcosis, this disease is more severe in people and even with surgical resection followed by anthelmintic treatment, potentially fatal.
• To prevent zoonotic infections in areas where E. granulosus is endemic, routine monthly deworming of dogs with praziquantel should be practiced. In areas where E. multilocularis is known to be present, treatment of dogs and cats every three weeks with praziquantel is recommended.

Selected References

• Conboy G. 2012. Cestodes of dogs and cats in North America. Vet Clin North Am Small Anim Pract 39:1075-1090.
• Deplazes P, van Knapen F, Schweiger A, and Overgaauw PAM. 2011. Role of pet dogs and cats in the transmission of helminthic zoonoses in Europe, with a focus on echinococcosis and toxocarosis. Vet Parasitol 182:41-53.

• Jenkins E, Volappi T, Malone CJ, Germitsch N, Virtanen JP, Oksanen A, Bessell E, Lundström-Stadelmann B, Frey CF. Changing distribution, diversity, and health impact of Echinococcus multilocularis in Europe and North America: Comparison, connections, and opportunities. Adv Parasitol. 2025;128:159-253. doi: 10.1016/bs.apar.2025.07.003. Epub 2025 Sep 9. PMID: 40947161.

• McManus DP, Gray DJ, Zhang W, and Yang Y. 2012. Diagnosis, treatment, and management of echinococcosis.  BMJ Jun 11; 344:e3866.
• Skelding A, Brooks A, Stalker M, et al. Hepatic alveolar disease in a boxer dog from southern Ontario. Can Vet J 55:551-3, 2014.