Echinococcus spp
Species
Canine
Echinococcus granulosus
Echinococcus multilocularis
Feline
Echinococcus multiloculari
Overview of Life Cycle
Echinococcus spp. are cyclophyllidean cestodes that have indirect life cycles that require 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 egg-laden 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 protoscoleces and 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 from 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 metastatic neoplasms in the tissues of infected intermediate hosts.
- The definitive host becomes infected when it ingests the protoscoleces within hydatid cysts in an intermediate host.
Disease
Adult Echinococcus spp. in the lumen of the small intestine are not known to cause any disease in dogs or cats.
Occasional cases of alveolar echinococcosis, in which dogs serve as aberrant intermediate hosts and develop multilocular hydatid cysts, have been reported in central Europe. 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 small number of dogs in Canada, including Ontario and British Columbia. 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 upper Midwestern states in the United States.
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. In some endemic areas, coyotes, wolves, and raccoons also serve as definitive hosts. 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, and 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 intestine of dogs or cats do not typically cause significant disease, even in those with very high worm burdens. However, the zoonotic risk this parasite creates warrants treatment.
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 sp. adults and their low number of body segments, it is extremely unlikely for proglottids to be observed grossly.
Eggs of Echinococcus sp. cannot be differentiated morphologically from those of Taenia sp.
Coproantigen enzyme-linked immunosorbent assay (CELISA) may be available, or PCR can be used for differentiation of species.
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.
To limit opportunities for re-infection, treatment of Echinococcus spp. in dogs and cats must be combined with management changes to prevent ingestion of 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.
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.
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.
Species
Canine
Echinococcus granulosus
Echinococcus multilocularis
Feline
Echinococcus multiloculari
Overview of Life Cycle
Echinococcus spp. are cyclophyllidean cestodes that have indirect life cycles that require 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 egg-laden 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 protoscoleces and 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 from 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 metastatic neoplasms in the tissues of infected intermediate hosts.
- The definitive host becomes infected when it ingests the protoscoleces within hydatid cysts in an intermediate host.
Disease
Adult Echinococcus spp. in the lumen of the small intestine are not known to cause any disease in dogs or cats.
Occasional cases of alveolar echinococcosis, in which dogs serve as aberrant intermediate hosts and develop multilocular hydatid cysts, have been reported in central Europe. 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 small number of dogs in Canada, including Ontario and British Columbia. 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 upper Midwestern states in the United States.
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. In some endemic areas, coyotes, wolves, and raccoons also serve as definitive hosts. 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, and 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 intestine of dogs or cats do not typically cause significant disease, even in those with very high worm burdens. However, the zoonotic risk this parasite creates warrants treatment.
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 sp. adults and their low number of body segments, it is extremely unlikely for proglottids to be observed grossly.
Eggs of Echinococcus sp. cannot be differentiated morphologically from those of Taenia sp.
Coproantigen enzyme-linked immunosorbent assay (CELISA) may be available, or PCR can be used for differentiation of species.
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.
To limit opportunities for re-infection, treatment of Echinococcus spp. in dogs and cats must be combined with management changes to prevent ingestion of 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.
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.
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.