Echinococcosis is a zoonotic disease caused by tapeworms in the genus Echinococcus. The organisms live in the intestines of the definitive hosts as adults, and in the internal organs of intermediate hosts as cyst-like larvae. Some species of Echinococcus circulate in domestic animals; others occur in wildlife (sylvatic) cycles or have both domestic and sylvatic cycles. Adult tapeworms are usually carried asymptomatically in their definitive hosts, which can include dogs, cats and wild carnivores.
Dogs are particularly important in zoonotic transmission due to their defecation habits and close relationships with humans. Many mammals and marsupials, including humans, livestock, pets and wildlife, can act as intermediate hosts. Intermediate hosts are usually asymptomatic in the initial stages, but the growing larvae, which often develop in the liver or lungs, may eventually cause illness and sometimes death.
Echinococcosis is caused by members of the genus Echinococcus, parasitic tapeworms in the family Taeniidae, subclass Cestoda. Larval infections in the intermediate host are also called hydatid disease. Species known to affect humans or domestic animals include E. multilocularis, the members of the E. granulosus sensu lato complex, E. shiquicus, E. vogeli and E. oligarthrus. The name of the last organism can differ between publications: while it has been called E. oligarthrus since the 1920s, some authors now argue that this was based on a misspelling and is inconsistent with naming practices, and it should be called E. oligarthra.
Until the last 10 years, the E. granulosus s.l. complex was considered to be a single species divided into 9 or 10 strains or genotypes. Some of these genotypes have now been elevated to species. They include E. granulosus sensu stricto, which comprises the former G1, G2 and G3 strains; E. canadensis, which incorporates the G6, G7, G8 and G10 strains; E. equinus (G4); E. ortleppi (G5); and E. felidis (the lion strain). Some have proposed further dividing E. canadensis into two or three species, but this is controversial. A G9 strain, which was isolated from a human case in Poland, is now considered to be a variant of G7. There might be additional species of Echinococcus, particularly among wildlife.
Echinococcus larvae develop differently in the intermediate host, resulting in diseases with different clinical courses. The disease caused by E. multilocularis is called alveolar echinococcosis. Cystic echinococcosis is caused by the members of the E. granulosus s.l. complex. E. oligarthrus and E. vogeli cause polycystic echinococcosis (or neotropical polycystic echinococcosis), based on their larval forms in their usual intermediate hosts. Others use the term unicystic echinococcosis for E. oligarthrus and polycystic echinococcosis for E. vogeli, after the forms of the larvae seen, to date, in human cases.
Echinococcus granulosus s. s.
Dogs are usually the definitive hosts for E. granulosus s.s. in the domestic cycle. Sylvatic cycles are perpetuated by wild canids including various species of foxes, dingoes (Canis lupus dingo), golden jackals (C. aureus), wolves (C. lupus) and hyenas. This organism has also been detected in genets and wild lions (Panthera leo). Sheep are thought to be the most important intermediate hosts, but infections have been found in many other species including goats, cattle, water buffalo, yaks, pigs, wild boar, warthogs (Phacochoerus spp.), camelids, horses and other equids, cervids and other wild ungulates, European hares (Lepus europaeus) and cats. Macropod marsupials such as wallabies (Macropus spp. and Wallabia spp.) and kangaroos (Macropus spp.) are important in a wildlife cycle in Australia.
The definitive hosts for E. equinus are canids, including dogs. Infected lions and black-backed jackals (C. mesomelas) have been found in parts of Africa. Equids such as horses, donkeys and zebras are the major intermediate hosts. A few sterile E. equinus cysts were found in experimentally infected sheep. One attempt to infect rhesus macaques (Macaca mulatta) failed, which led to the belief that this organism does not affect primates; however, it was recently found in a captive red ruffed lemur (Varecia rubra) and a ring-tailed lemur (Lemur catta).
The definitive hosts for E. ortleppi are canids. Known hosts include dogs and jackals. Cattle are important intermediate hosts, but E. ortleppi has also been reported in water buffalo, sheep, goats, pigs, camels, a zebra, oryx (Oryx gazella), a captive Philippine spotted deer (Rusa alfredi), at least two nonhuman primates (a captive ring-tailed lemur and a captive red-shanked douc langur, Pygathrix nemaeus) and a captive crested porcupine (Hystrix cristata).
Dogs and other canids are the definitive hosts for E. canadensis. Wolves are thought to be major hosts in some sylvatic cycles, but other wild canids may be regionally important. E. canadensis occurs in diverse intermediate hosts including cervids, camels, cattle, small ruminants, pigs, wild boar and other species. The G8 and G10 genotypes mainly infect cervids (including semi-domesticated reindeer, Rangifer tarandus), while the G6 and G7 genotypes generally occur in other hosts; however, G6 has been described in reindeer, and G8/G10 occurred in a muskox (Ovibos moschatus) and two mountain goats (Oreamnos americanus), with possible additional reports in a pig and an American bison (Bison bison).
Lions are important definitive hosts for E. felidis. This organism also occurs in spotted hyenas (Crocuta crocuta) but an attempt to infect dogs was unsuccessful. Reported intermediate hosts include zebras (Equus quagga), wildebeest (Connochaetes spp.), warthogs, bushpigs (Potamochoerus larvatus, P. porcus), hippopotami (Hippopotamus amphibius), African buffalo (Syncerus caffer) and various species of antelope. Some cysts in warthogs and hippopotami were confirmed as E. felidis by genetic methods, and zebras were substantiated as an intermediate host by experimental infection of lions, but it is possible that some hosts were infected with other species of Echinococcus.
Foxes are the most important definitive hosts for E. multilocularis. Red foxes (Vulpes vulpes) and Arctic foxes (Alopex lagopus) are major hosts, while Tibetan foxes (V. ferrilata) and sand foxes (V. corsac) are regionally important. Other canids including wolves, coyotes (C. latrans), raccoon dogs (Nyctereutes procyonoides), golden jackals and dogs can also be infected. Dogs have a minor role in most areas, but they are important hosts in a few regions, such as parts of China and certain indigenous communities in northwestern Alaska (St. Lawrence Island). E. multilocularis has also been reported in felids such as lynx (Lynx spp.), wild cats (Felis silvestris) and house cats.
The importance of house-cats in shedding eggs is debated, but most individuals seem to have a low tapeworm burden and cats are generally said to have a minimal role. A few mature tapeworms were found in experimentally infected pine martens (Martes martes), and immature tapeworms were described in experimentally infected American black bears (Ursus americanus); however, bears and mustelids are not generally thought to be good hosts for this organism.
Echinococcus shiquicus has been described in plateau pikas (Ochotona curzoniae), which serve as the intermediate host, and Tibetan foxes (Vulpes ferrilata), which are the definitive hosts. Its tapeworms can mature in dogs.
The definitive hosts for E. oligarthrus are wild felids including the pampas cat (Leopardus colocolo), Geoffroy’s cat (L. geoffroyi), ocelot (L. pardalis), jaguarundi (Herpailurus yagouaroundi), jaguar (Panthera onca),and puma (Puma concolor). E. oligarthrus was found in a bobcat (Lynx rufus) in northern Mexico, and it can mature in experimentally infected house-cats.
Intermediate hosts for E. oligarthrus include agoutis, spiny rats (Proechimys spp.) and pacas. Infections have also been reported in opossums (Didelphis marsupialis) and eastern cottontails (Sylvilagus ﬂoridanus). Climbing rats (Tylomys panamensis), cotton rats (Sigmodon hispidus) and Mongolian gerbils (Meriones unguiculatus) have been infected experimentally.
The bush dog (Speothos venaticus) is the most important definitive host for E. vogeli. Cerdocyon thous, the crab-eating fox, has been infected experimentally, and other wild canids might be susceptible. Dogs can be infected with these tapeworms. Pacas (Cuniculus paca) seem to be the most important intermediate hosts, but larvae have also been reported in agoutis (Dasyprocta spp.), nutrias and nine-banded armadillos (Dasypus novemcinctus), as well as in captive nonhuman primates.
Veterinarians who suspect echinococcosis should follow their national and/or local guidelines for disease reporting. State regulations should be consulted in the U.S.
Intestinal carriage of Echinococcus can be prevented in cats, dogs and captive carnivores by not feeding them raw or undercooked animal tissues (e.g., sheep entrails) and by not allowing them to hunt or roam. Potentially infected tissues can be cooked or frozen before feeding to destroy the parasite.
Echinococcosis in the intermediate host is controlled by reducing its exposure to parasite eggs. Periodic deworming of farm dogs is the cornerstone of control or eradication programs for E. granulosus s.s. in sheep. Dogs that might be infected with Echinococcus should not be allowed onto pastures where livestock may graze. Stray dogs have been culled in control programs in some areas, particularly where echinococcosis is an issue in humans.
In some countries, foxes are treated with praziquantel in bait to decrease the incidence of E. multilocularis. Foxes have also been culled, but these programs are controversial, the effects on the fox population can be complex, and their effectiveness is debated.
One zoo with E. multilocularis issues in nonhuman primates set up a program to safeguard the animals’ food by washing and steaming vegetables, sourcing vegetables to be fed raw from areas where this organism is not thought to occur, and storing all food indoors. Wood chips used in the exhibit are heat treated.
A recombinant vaccine (EG95) for E. granulosus in sheep has been licensed in a few countries. It can reduce the risk of infection but is not useful against existing cysts. This vaccine may also be also effective in goats and cattle.
Definitive hosts can be treated with various anthelminthic drugs. Praziquantel, the most commonly used agent, is effective against both immature and adult tapeworms.
There is relatively little experience in treating animal intermediate hosts, especially those infected with E. multilocularis or E. vogeli. Surgery is often the treatment of choice for cysts that can be completely resected. Longterm treatment with benzimidazoles (e.g., albendazole or mebendazole) may suppress some cysts and/or prolong the animal’s life in non-surgical cases.
Drugs are also used as an adjunct to surgery, especially for E. multilocularis. Non-curative debulking of an E. multilocularis mass is no longer recommended in humans infected with this organism, and there was no evidence that it was helpful in a few dogs.