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Echinococcosis (Cystic Hydatid Disease) is the result of an infection with the larval or adult form of the tapeworm Echinococcus granulosus (E. granulosus) and occurs in humans, wildlife species, and livestock. The adult form of the parasite is present in canids, the larval form in wild cervids, livestock, and humans. The disease is potentially dangerous for humans.
There are two biologically and ecologically distinct forms of E. granulosus in North America: a northern biotype found in the holarctic tundra and boreal forests that is an indigenous sylvatic or wild form that parasitizes free-ranging wolves, bison, and cervids (moose, elk, deer, and caribou); and a southern European biotype that is a pastoral or domestic form that is generally found in domestic ungulates and dogs, but in areas may involve wild canids and other carnivores, wild ungulates, macropodial marsupials, and rarely lagomorphs. The domestic form was spread as Europeans migrated throughout the world with their livestock.
Echinococcosis (Cystic Hydatid Disease) is an emerging disease found in many parts of the world. There are at least nine strains of E. granulosus that have adapted to different hosts and in most cases occupy a wide geographical area. There are pastoral and sylvatic forms of the disease affecting domestic and wild animals, respectively. The pastoral form has been reported in sheep and dogs from the Mediterranean region, South America, Africa, the Middle East, Russia, Central Asia, Mongolia, China, and Oceania. A horse and dog cycle has been reported from Belgium, Ireland, Italy, Switzerland, the United Kingdom, Australia, and possibly the United States (Maryland). A cattle and dog cycle has been reported in Belgium, Germany, South Africa, and Switzerland; a swine and dog cycle has been reported in Poland; a reindeer and dog cycle has been reported in the subarctic regions of Finland, Norway, Sweden, and Alaska; and a camel and dog cycle has been reported in Iran. In Australia the pastoral form has spilled over into wildlife and has been reported in kangaroos, wallabies, wombats, feral dogs, dingoes, and foxes. The sylvatic form has been reported in sheep, jackals, hyenas, warthogs, bushpigs, zebra, buffalo, wildebeest, and lions in Africa and moose, elk, caribou, white-tailed deer, wolf, coyote, and feral dogs in North America and Eurasia.
In Michigan, we have observed a deer/moose and coyote/wolf cycle in the Upper Peninsula and a deer/elk and coyote cycle in the northern portion of the Lower Peninsula.
Transmission and Development
In North America the life cycle of E. granulosus requires two hosts: a definitive carnivore (wolf, coyote, or dog) and an intermediate herbivore (moose, elk, deer, caribou). Humans are a dead-end intermediate host.
The adult tapeworm is very small, usually consisting of only three proglottids and measuring 3 to 6 mm in total length and residing in the small intestine. The eggs of the tapeworm are voided via gravid (mature) segments of the tapeworm in the fecal material of the definitive host. The eggs can survive at least a year in the environment as they are highly resistant to environmental stress. The eggs are vulnerable to high temperatures and desiccation however, dying in two hours under these conditions. Egg survival time is increased in damp and cool (the eggs can survive freezing) conditions (for example near watering holes). Once passed in the feces the eggs can be transported by the wind, water, and insects (flies). Egg shedding in the definitive host may be cyclical and each worm can produce by sexual means up to 1000 eggs every 10 days for up to 2 years. Each egg contains an embryo or onchosphere that serves as the infective stage. When the eggs are voided from the canid definitive host, they contaminate vegetation and are accidentally ingested by the cervid intermediate host. Humans can be infected by ingestion of eggs acquired from contaminated food or water, from handling live canids or pelts from dead canids, or by handling canid fecal material.
In the cervid intermediate host, the eggs hatch and release tiny, hooked embryos (oncospheres or larvae) once they reach the small intestine. The embryo burrows through the wall of the intestine and enters the bloodstream, eventually lodging in an organ (liver, lungs, kidneys, brain, or bone marrow) with the lungs being the most common site. In humans the egg hatches in the duodenum, the hooked embryo penetrates the intestinal wall and is carried via the bloodstream to various organs (liver, lungs, brain, skeletal muscle, and eye) with the liver being the most common site.
In the intermediate host, once the larvae reach the organ of choice they form a metacestode or hydatid cyst. This larval cyst is unilocular, subspherical in shape and fluid-filled, lined with an inner germinal membrane that produces brood capsules. On the inner wall of the brood capsules, an asexual budding process which enhances infectivity and compensates for low sexual egg production occurs that produces thousands of larval tapeworms or protoscolices. The cysts are thick walled, fluid-filled, and range in size from 2 to 30 cm in diameter. Development of these cysts is slow as the parasite is adapted to the long-lived intermediate hosts with protoscolices developing in 1 to 2 years.
The canid definitive host is infected by eating the intermediate host organ that contains the hydatid cyst which contains the protoscolices which has the ability to grow into an adult worm. One small cyst may contain hundreds of protoscolices and one large cyst may contain tens of thousands of protoscolices. Following ingestion, the protoscolices develop into adult tapeworms which eventually produce eggs to complete the life cycle.
Infections with the adult stage of E. granulosus are generally asymptomatic and non-pathogenic to the canid host. Infections with the larval stage of E. granulosus can be pathogenic depending on the localization, size of the cyst, and intensity of the infection in the cervid or human intermediate host. Most hydatid cysts reside in the lung parenchyma, but they are also found in the liver parenchyma, just below the capsule. Displacement of lung or liver tissue and fibrosis of the area surrounding the cyst, as well as pressure placed on organs as a result of the hydatid cyst(s) increasing in size during the life of the intermediate host, results in pathological tissue changes. Occasionally larvae localize in kidney, spleen, or brain tissue where their effects are more severe and often fatal. In cervids the hydatid cysts usually develop in the lungs where they are often superficial and may protrude into the pleural cavity. In humans the hydatid cysts are large with numerous protoscolices with the cysts varying in size from 2 to 35 cm (1 to 14 inches) in diameter. Usually humans are a dead end in the life cycle of this parasite but Cystic Hydatid Disease in humans remains a serious problem in humans because the disease can cause extensive pathological damage.
Diagnosis of E. granulosus in the definitive host is accomplished by demonstrating the presence of adult cestodes (usually less than 6 mm long and possessing 2 to 6 proglottids) in the feces or in the upper one-third of the small intestine and identifying them using morphological characteristics (position of the genital pore, the uterus or the testes). Enzyme Linked Immunosorbent Assay (ELISA) tests for detecting coproantigens in the feces of canids can be used to test for E. granulosus. Coproantigens can be detected shortly after infection and prior to the release of eggs by the adult tapeworms. Serological testing can also be performed to determine the presence of oncosphere, cyst fluid, and/or protoscolex antibodies in the serum. This test however does not distinguish between current and previous infections and cross reactivity between Echinococcus sp. and Taenia sp.
Diagnosis of E. granulosus in the intermediate host is accomplished through necropsy examination of the animal and identifying the larval cyst in the organs, usually the liver or the lungs. Formalin fixed tissue positive on periodic-acid-Schiff (PAS) staining demonstrates a positive acellular laminated layer with or without an internal cellular nucleated germinal membrane (a specific characteristic of the metacestodes of Echinococcus sp.).
Diagnosis of E. granulosus in humans is accomplished through an ELISA test which uses an antigen preparation (hydatid fluid) which detects antibodies. Serological testing can also be performed to determine the presence of oncosphere, cyst fluid, and/or protoscolex antibodies in the serum. The presence of hydatid cysts can be determined on autopsy examination.
Treatment in definitive hosts can be accomplished by giving canids Praziquantel or Arecoline. Arecoline is a parasympathetic agent and increases the tonus and the mobility of smooth muscle resulting in the purgation of E. granulosus adults from the intestinal tract and passing them from the body in the mucus that follows the formed fecal material. The drug works by paralyzing the tapeworm, resulting in its relaxing its hold on the intestinal wall. Dosage with Arecoline is 1 tablet/10 kg. body weight but pregnant bitches and animals with cardiac abnormalities should not be treated.
Treatment of cervid intermediate hosts is unnecessary as this parasite causes limited pathological damage and is not a significant mortality factor.
Treatment of human intermediate hosts consists of removal of the hydatid cyst(s). Removal of the cyst(s) is recommended for pastoral infections, but cysts of sylvatic origin may allow for a more conservative treatment. If surgery is performed to remove the cyst(s), a course of drugs (the drug of choice is Albendazole) is prescribed to kill any remaining tapeworm larvae that might still be in the body. The disease may not always be cured by surgery.
Control of the parasite in wild canids is not feasible. Control in domestic canids can be accomplished by preventing the availability of hydatid-infected offal (do not feed dogs carcasses or allow them to scavenge) and a regular worming regiment with Praziquantel or Arecoline. A vaccine has not been developed for canids.
Control of the parasite in livestock is possible through the use of a vaccine that has been developed utilizing a protein contained within the parasite's egg. The vaccine has not been successful in cervids.
Prevention of E. granulosus in humans can be accomplished primarily through education
and proper hygiene. Eggs can be ingested either from handling a canid (either alive or dead) that may have eggs on its fur or by handling canid fecal material. Examination procedures of either animals or fecal material poses a risk of infection and potentially fatal disease to humans but this can be minimized by appropriate safety measures. Laboratory materials should be frozen at -80 degrees C for 48 hours. A disposable face mask, gloves, and coveralls should be worn whenever handling animals or fecal material. Contaminated material must be destroyed by heat as chemical disinfection is not reliable.
There are no precautions that need to be taken when handling tissue of the intermediate hosts as the lung cysts are not infective to humans.
Though common in both its definitive and intermediate hosts, the low virulence of E. granulosus in natural hosts reduces its potential as an important limiting factor on the population. E. granulosus is not a significant parasite in the definitive canid host. The cervid intermediate hosts are usually unaffected by an infection with E. granulosus but heavily infected animals may have reduced stamina and be predisposed to predation. Meat from infected cervids is suitable for human consumption but tissues or organs containing the cysts should not be eaten.
Cystic Hydatid Disease in humans can be a significant disease because of the mechanical and toxic effects of the cyst(s). The tremendous reproductive potential of the tapeworm as well as the sheer size of the hydatid cyst(s) can cause problems in the organs where they are lodged. If the cyst(s) bursts, the resultant toxic (anaphylactic) shock would probably be fatal. In Alaska and Canada most infections are benign, indicating humans are probably a less suitable host for the sylvatic form of E. granulosus than for the pastoral form.