Echinococcus multilocularis

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Geographic Range

Echinococcus multilocularis lives in various regions around the world. In Europe, the endemic area includes central and eastern France, Switzerland, Austria, and Germany. In Asia, E. multilocularis ranges from the zone of tundra from the White Sea to the Bering Strait and parts of the former Soviet Union. The southern distribution in Asia extends from Turkey eastwards through Afghanistan to the northern areas of Japan. In North America, this cestode appears in subarctic Alaska and Canada as well as regions in north-central and southern United States. (Gottstein 1992) (Gottstein, 1992)

Habitat

The definitive hosts of E. multilocularis are usually carnivores. Dogs and cats are good definitive hosts. Voles, muskrat, lemmings, Northern mole-vole, deer mouse, gerbils, and striped hamster are typical intermediate hosts of this species, while ground beetles may act as paratenic hosts. Humans may act as intermediate hosts as well. The habitat for this species is mainly sylvatic (forest), and rural areas, rather than urban. However, once a hydatid cyst is brought into a domestic community via dogs, humans, or farm animals, the E. multilocularis may begin a domestic cycle. Hunters and trappers tend to be the best human hosts. Prevalence and distribution of E. multilocularis may vary with seasonal fluctuations (especially temperature fluctuations). Their life span is inversely proportional to temperature. Once worms are in the host, they reside in the small intestine, and travel to various other organs through the lymphatic system. (Leiby and Nickel 1968), (Gottstein 1992), (Roberts and Janovy, Jr. 2000), (Polydorou 1992) (Gottstein, 1992; Leiby and Nickel, 1968; Polydorou, 1992; Roberts and Janovy Jr., 2000)

Physical Description

An adult E. multilocularis is 1.5 to 3.5 mm in length. It consists of a head, or scolex, a neck, and a segmented body, the strobila. This is the basic tapeworm anatomy. The rostellum is located on the frontal end surface, or apex, of the scolex. It has twenty-eight to thirty hooks each consisting of a shaft, root process, and blade. There are four suckers on the dorsolateral surface of the scolex, also called the rostellum. Like other tapeworms, this species has no mouth; they absorb nutrients through their external covering and microvilli. They also have no anus. The scolex is attached to the strobila via a long, thin neck. The segmented strobila consists of a number of reproductive sets called proglottids. Each proglottid is a complete set of reproductive organs, normally both male and female organs.

The larval stages differ from the adult. One distinct stage is the hydatid, a complex cysticercus, developing from the hexacanth. They have an alveolar structure, ovoid shaped, and make up a porous, spongy mass of daughter hydatids and protoscolices. A protoscolex is juvenile scolex budded within a hydatid metacestode. They are a grayish-white color with gelatinous contents and liters of fluids. Hydatids range from 200 to 2000 micrometers. (Polydorou 1992), (Roberts and Janovy, Jr. 2000) (Roberts and Janovy Jr., 2000; Polydorou, 1992)

  • Range length
    1.5 to 3.5 mm
    0.06 to 0.14 in

Development

The adult worm releases its proglottids filled with eggs from the definitive host via the feces. Definitive hosts are carnivorous mammals, such as canines. The eggs contain the hexacanth, the "six-hooked larva", produced through a process called embryogenesis. They require a short period of maturation time after ingestion by the first intermediate host. The first intermediate host may be an invertebrate or a vertebrate, usually rodents. Humans may also act as intermediate hosts. Once inside the intermediate host, the hexacanth migrates through the gut wall to a parenteral or extraintestinal site within it. At this time, the larva metamorphoses to a juvenile, or metacestode, with an introverted and invaginated scolex and multiplies by endogenous and exogenous budding (asexual). In E. multilocularis, the metacestode is a multilocular or alveolar hydatid. The germinal layer buds off daughter hydatids and protoscolices, the stage ingested by the definitive host. This part of the life cycle may take place in two to four months. Hydatids and protoscolices will differentiate further to reach full adulthood once inside the definitive host. The four stages involved in adult maturation are proglottisation, maturation, growth, and segmentation. Proglottisation and maturation form the reproductive units. Growth and segmentation lengthen the body. (Roberts and Janovy, Jr. 2000), (Thompson 1986), (Gottstein 1992), (Morris and Richards 1992) (Gottstein, 1992; Roberts and Janovy Jr., 2000; Morris and Richards, 1992; Thompson, 1986)

Reproduction

Adults of E. multilocularis are monoecious, they possess all parts of both male and female systems and reproduce by self-fertilization. However, cross-insemination may occur between individuals. They have both sexual and asexual life stages. Each segment has a set of male and female systems. The reproductive life cycle typically requires two hosts, an intermediate and a definitive host in which it will reach sexual maturity. The adult worm releases its proglottids filled with eggs from the definitive host via the feces. (Roberts and Janovy, Jr. 2000), (Thompson 1986), (Gottstein 1992), (Morris and Richards 1992) (Gottstein, 1992; Roberts and Janovy Jr., 2000; Morris and Richards, 1992; Thompson, 1986)

  • Key Reproductive Features
  • simultaneous hermaphrodite
  • sexual
  • asexual
  • Parental Investment
  • no parental involvement

Behavior

Echinococcosis multiocularis activity after ingestion by definitive host varies. The protoscolices tend to be quite active. This activity is a prerequisite to its establishment within the definitive host. Once E. multiocularis has found its way into the small intestine, specifically in the crypts of the Lieberkuehn, it remains there in a more sessile state. The organism inserts its rostellum into the intestinal wall using its hooks and together with the suckers attaches to the intestinal wall. A secretion from the rostellum may also anchor this species to the host. There is little information about sensory mechanisms in E. multiocularis. However, the scolex is equipped with several sensory endings on its anterior surface for both physical and chemical stimuli. Sensing its environment may allow the correct placement of the scolex and the strobila on the intestinal surface. Dipersal of this species, as mentioned above, varies depending on the definitive host. Humans acquire the infection via ingestion of eggs from dog feces, and carnivores may acquire the hydatid by consuming the intermediate host. (Roberts and Janovy, Jr. 2000), (Thompson 1986) (Roberts and Janovy Jr., 2000; Thompson, 1986)

Communication and Perception

Cestodes in general have sensory organs in the scolex, which are attached to longitudinal nerves extending down the body. The nerves are attached to organs and the cestodes can detect tactile stimulation. (Brusca and Brusca, 2003)

Food Habits

Echinococcus multilocularis feeds through the tegument, an external covering. Actual mechanisms of food absorption include active transport, mediated diffusion, and simple diffusion. During the initial stages of feeding, the adult tapeworm attaches to the epithelium of the small intestine via the rostellar hooks and the suckers adhere to the small hairs or villi of the intestine. These organs secure the parasite to the host. Microvilli called microtriches layer the scolex and strobila. These mictotriches increase the absorptive area of the tegument. Glycocalyx is found on the surface of microtriches. When glycocalyx interacts with certain molecules some reactions may occur such as the inhibition of host trypsin and the absorption of bile salts. Trypsin inhibition may protect the organism against digestion by host enzymes and maintain the integrity of the surface membrane. The rostellar glands aid nutrient absorption by releasing bioactive molecules that process nutritive components of the host. The most important nutrient molecule for energy metabolism is glucose. The only carbohydrates that can be metabolized are glucose and galactose; they are both actively transported. Specifically, glucose absorption in E. multilocularis is coupled to a sodium pump concentration. Amino acids are absorbed by active transport while purines and pyrimidines are absorbed by facilitated diffusion. Lipids are also absorbed by diffusion mechanisms. Although vitamins are necessary for the tapeworm, mechanisms of absorption are unknown. Juvenile E. multilocularis absorb glucose in a similar fashion. (Roberts and Janovy, Jr. 2000) (Roberts and Janovy Jr., 2000)

  • Animal Foods
  • body fluids

Predation

These animals are probably not preyed on directly but are ingested. Egg and larval mortality are high due to the parasite not reaching appropriate hosts

Ecosystem Roles

The definitive hosts of E. multilocularis are usually carnivores. Dogs and cats are good definitive hosts. Voles, muskrat, lemmings, Northern mole-vole, deer mouse, gerbils, and striped hamster are typical intermediate hosts of this species, while ground beetles may act as paratenic hosts. Humans may act as intermediate hosts as well.

Species Used as Host

Economic Importance for Humans: Positive

There is no known positive economic importance for this species.

Economic Importance for Humans: Negative

Humans can be parasitized by E. multilocularis. One of its own life stages, the hydatid cysts, is harmful to human populations. The adult stages are tolerated by definitive hosts. Human infection usually occurs because of close contacts with dogs. Eggs may be picked up from the dog's coat and ingestion from dog feces. The eggs may be air borne due to the wind. Cultural or social practices determine the risk to which individuals are exposed. Some of these cultural factors are: ethnicity, occupational factors, economic variables, and husbandry or farming practices. In humans, the greatest somatic site for infestation is the liver followed by pulmonary (lung) infestation. The skeletal and nervous systems may also be affected. However, the liver is almost always involved. There are no specific symptoms related to hydatid disease-they will depend on what organ is affected by the disease. Once ingested by humans, the cysts grow slowly and may continue for many years. Alveolar echinococcosis of the liver can be progressive and metastasize to other organs. In humans, mortality from toxic shock occurs if the cysts rupture. The processes by which the cestode proliferates within the human have been described above (see reproduction and development). (Uchino and Sato 1993), (Roberts and Janovy, Jr. 2000), (Thompson 1986), (Polydorou 1992) (Polydorou, 1992; Roberts and Janovy Jr., 2000; Thompson, 1986; Uchino and Sato, 1993)

Conservation Status

There is no conservation status for E. multilocularis.

Other Comments

Echinococcus multilocularis can have serious effects on animal populations, such as mortality in humans and farm animals from hydatid cyst infection. Several types of control and prevention programs have been initiated. Animal control is one preventative measure, such as reducing the number of stray or wild dogs. Legislation on dog control would also prevent the spread of hydatid disease. In some areas infested by E. multilocularis, like Cyprus, local farmers who practice animal husbandry slaughter animals for meat supplies. The potentially hydatid-infected carcasses of these animals are discarded in unmanaged waste disposal sites. Some regulations on slaughtering practices are: slaughtered animals may not be transported far and they must be inspected for worms by a veterinarian. Drugs, such as prazquantel, kills hydatid cysts in canines. Mebendazole and mitomycin C seem to inhibit hydatid cyst growth in humans. Once an infection has occurred in humans, surgical management may ensue. Hepatic (liver) resection is the only surgical therapy for E. multilocularis. This involves cutting away of hydatid lesions. Sometimes liver transplantation must occur. (Morris and Richards 1992), (Thompson 1986), (Polydorou 1992), (Uchino and Sato 1993) (Morris and Richards, 1992; Polydorou, 1992; Thompson, 1986; Uchino and Sato, 1993)

Contributors

Renee Sherman Mulcrone (editor).

Christopher Bonadio (author), University of Michigan-Ann Arbor, Teresa Friedrich (editor), University of Michigan-Ann Arbor.

Glossary

Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map

Palearctic

living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.

World Map

agricultural

living in landscapes dominated by human agriculture.

asexual

reproduction that is not sexual; that is, reproduction that does not include recombining the genotypes of two parents

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

carnivore

an animal that mainly eats meat

causes disease in humans

an animal which directly causes disease in humans. For example, diseases caused by infection of filarial nematodes (elephantiasis and river blindness).

causes or carries domestic animal disease

either directly causes, or indirectly transmits, a disease to a domestic animal

ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

forest

forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.

heterothermic

having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.

motile

having the capacity to move from one place to another.

oriental

found in the oriental region of the world. In other words, India and southeast Asia.

World Map

parasite

an organism that obtains nutrients from other organisms in a harmful way that doesn't cause immediate death

sedentary

remains in the same area

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

suburban

living in residential areas on the outskirts of large cities or towns.

tactile

uses touch to communicate

taiga

Coniferous or boreal forest, located in a band across northern North America, Europe, and Asia. This terrestrial biome also occurs at high elevations. Long, cold winters and short, wet summers. Few species of trees are present; these are primarily conifers that grow in dense stands with little undergrowth. Some deciduous trees also may be present.

temperate

that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).

terrestrial

Living on the ground.

tropical

the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.

tropical savanna and grassland

A terrestrial biome. Savannas are grasslands with scattered individual trees that do not form a closed canopy. Extensive savannas are found in parts of subtropical and tropical Africa and South America, and in Australia.

savanna

A grassland with scattered trees or scattered clumps of trees, a type of community intermediate between grassland and forest. See also Tropical savanna and grassland biome.

temperate grassland

A terrestrial biome found in temperate latitudes (>23.5° N or S latitude). Vegetation is made up mostly of grasses, the height and species diversity of which depend largely on the amount of moisture available. Fire and grazing are important in the long-term maintenance of grasslands.

urban

living in cities and large towns, landscapes dominated by human structures and activity.

References

Brusca, R., G. Brusca. 2003. Invertebrates. Sunderland, Massachusetts: Sinauer Associates, Inc..

Gottstein, B. 1992. Echinococcus multilocularis infection: immunology and immundiagnosis. Advances in Parasitology, 31: 321-380.

Kahlill, L., A. Jones, R. Bray. 1994. Keys to the cestode parasites of vertebrates. Wallingford: CAB International.

Leiby, P., P. Nickel. 1968. Studies on the Sylvatic Echinococcosis: Ground Beetle Transmisson of Echinococcus multilocularis Leuckart, 1863, to Deer Mice, Peromyscus maniculatus (Wagner). Journal of Parasitology, 54: 536-537.

Morris, D., K. Richards. 1992. Hydatid Disease: Current Medical & Surgical Management. Butterworth-Heinemann, Ltd.: Oxford.

Polydorou, K. 1992. Echinococcosis Hydatidosis: The Problem and its Control, Case Study: Cyprus. Cyprus: Polydorou.

Roberts, L., J. Janovy Jr.. 2000. Foundations of Parasitology (6th ed.). New York: McGraw-Hill.

Thompson, C. 1986. The Biology of Echinococcus and Hydatid Disease. London: George Allen & Unwin.

Uchino, J., N. Sato. 1993. Alveolar Echinococcosis of the Liver. Sapporo: Kokoku Printing Co. Ltd..