Parasitology - Flat Worms - Tapeworms

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Three major parts of adult tapeworms

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Scolex (head), neck, proglottids

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Proglottids

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Segment of adult tapeworm involved in egg formation. They grow from the neck so the most mature ones are found distally, immature ones proximal to the neck

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This part of the tapeworm features specialized structures for attachment to intestinal mucosa

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Scolex (head), which features suckers

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Features of the mature proglottid of tapeworms

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Testis, uterus, ovary, York reservoir, genital pore

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This is how adult tapeworms feed

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Absorb contents of intestinal lumen through the entire surface of the worm

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Latin binomial of beef tapeworm

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Taenia saginata

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Latin binomial of pork tapeworm

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Taenia solium

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Latin binomial of fish tapeworm

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Diphyllobothrium latum

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Treatment of tapeworms

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Praziquantel

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Definitive host of taenia saginata (beef tapeworm)

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Human

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Infective stage of taenia saginata

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Cysticercus in raw or under-cooked beef

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Cysticerci

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Encysted larvae of Taenia spp. (beef and pork tapeworms)

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Length of Taenia spp. (pork and beef tapeworms)

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12 to 30 feet, adult length reached over the course of several months

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Diagnostic stage of Taenia saginata

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Gravid proglottid (containing 50,000 to 10,000 eggs) in feces or environment (may exit when patient is not defecating). Free eggs may also be seen

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Intermediate host of Taenia saginata

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Cow

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Appearance of Taenia saginata and solium eggs

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Radial striations on yellow-brown egg shell

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Appearance of Taenia saginata proglottid

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15-20 lateral branches on each side; count branches that come off central stem (count one side only)

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Signs and symptoms of Taenia saginata

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No convincing evidence that the worm causes any marked symptoms

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Presentation of Taenia saginata

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Patient reports proglottid in feces, or patient reports feeling worm exit anal sphincter

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This giant tapeworm is highly prevalent in Africa, where cattle and extensive human fecal contamination co-exist

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Taenia saginata

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Transmission of Taenia saginata

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Human feces -> Cow ingests egg-containing human feces -> Beef -> Human ingests larvae-containing beef

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Definitive host of Taenia solium

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Human

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Intermediate host of Taenia solium

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Human, pig

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Transmission of Taenia solium

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Ingestion of raw or undercooked pork containing cysticerci (pig eats human feces containing proglottid or free eggs -> human eats pig)

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How does somebody get Cysticercosis

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Direct ingestion of Taenia solium eggs (vs. ingesting contaminated, undercooked pork)

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Pathogenesis of cysticercosis

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Cysticerci form in the tissues rather than the gut lumen--occurs when humans ingest the eggs of Taenia solium, rather than the larvae

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Cysticercosis can affect these tissues

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Lung, heart, brain, eye, connective tissue (serious, life-threatening disease)

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Diagnostic stage of Taenia solium

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Mature proglottid in human feces, free eggs may also be seen

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Appearance of Taenia solium proglottid

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8 to 13 (<15) lateral branches on each side; count branches that come off central stem (count one side only)

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How do you differentiate between T. solium and T. saginata infection?

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The eggs look the same, but the proglottids differ -- T. solium proglottid has <15 branches one each side, whereas T. saginata has >15

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Signs and symptoms of T. solium

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No convincing evidence that the worm causes any marked symptoms; but if eggs rather than larvae are ingested, cysticercosis can result

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This worm can cause epilepsy

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Taenia solium (cysticercosis can affect the brain)

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Presentation of Taenia solium

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Patient reports proglottid in feces

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T/F: Taenia solium is widespread in the US

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False. It is not endemic in the USA. It is more common in Central and South America so it is seen in US hospitals.

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Why is treatment of infection with adult T. solium mandatory?

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Cysticercosis can be caused by ingestion of eggs present in the proglottids and in fecal contamination

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Diagnosis of neurocysticercosis

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Radiological imaging showing calcified cysts (easy) or live cysts (may be more difficult to see)

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Treatment of neurocysticercosis

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Praziquantel or albendazole

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Prevention of T. solium and T. saginata infection

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For T. saginata, cook beef and protect cows from human fecal contamination. For T. solium, cook pork and protect pigs and humans both from human fecal contamination. Ingestion of eggs by humans is not a risk in T. saginata because humans are not an intermediate host, but ingestion of eggs by humans *is* a risk in T. solium because the eggs can hatch and larvae can get into tissues.

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T/F: "Intermediate host" implies that eggs of a species can hatch in that host

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True. "Definitive host" implies that the already-hatched larvae can sexually reproduce in the host

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The life cycle of this human pathogenic giant tapeworm is the most complex

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Diphyllobothrium latum

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Diagnostic stage of Diphyllobothrium latum

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Unembryonated egg in feces

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Infective stage of Diphyllobothrium latum

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Cysticercus (encysted larva, NOT egg) in fish meat

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Life cycle of Diphyllobothrium latum

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Egg hatches in fresh WATER --> free-living ciliated stage --> COPEPOD eats ciliated stage --> worm matures into later stage larva --> FISH eats copepod --> larva encysts in fish muscle --> HUMAN eats infected fish

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Appearance of Diphyllobothrium latum egg

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"Trap door", underdeveloped ciliated stage

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This worm can cause B-12 deficiency

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Diphyllobothrium latum

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This worm has great affinity for B-12

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Diphyllobothrium latum

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Signs and symptoms of Diphyllobothrium latum

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B-12 deficiency, diarrhea

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Geographic distribution of Diphyllobothrium latum

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Great Lakes region, Alaska; highly prevalent in Finland

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This tapeworm is sensitive to freezing

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Diphyllobothrium latum (fish worm)

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Human disease caused by this worm requires transmission to three different hosts

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Diphyllobothrium latum requires a copepod host, fish host, and human host

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This is a tapeworm of dogs that can cause human disease

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Echinococcus granulosus, which causes Hydatid disease

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It is this form (egg, larva, adult) of Echinococcus granulosus that causes serious cystic disease in humans

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Larval form (though the human ingests the egg, which matures - the human is an intermediate host)

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Humans are the accidental intermediate host of this tapeworm

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Echinococcus granulosus

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Life cycle of Echinococcus granulosus

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Sheep/swine/cattle ingest egg-containing dog feces --> early stage larvae hatch and enter sheep's portal circulation --> eggs mature in liver over the course of years --> sheep viscera are fed to dogs --> larva mature in dog gut

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Transmission of Echinococcus granulosus

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Humans are infected (like the sheep) by ingesting eggs from dog feces

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Site of maturation of Echinococcus granulosus

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Liver

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Maturation of Echinococcus granulosus

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Occurs over the course of years. Eggs become trapped in liver and develop into cysts with an inner germinal layer which grows, vesiculates and differentiates into larval scolices (rudimentary heads). Diameter may be >20 cm.

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This worm undergoes asexual multiplication by a larval stage worm in the intermediate host

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Echinococcus granulosus

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Definitive host of Echinococcus granulosus

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Dog

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Intermediate host of Echinococcus granulosus

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Sheep, swine, cattle

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Length of mature Echinococcus granulosus

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6 mm

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How many eggs does Echinococcus granulosus lay?

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500 eggs per gravid segment (proglottid)

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This worm causes hydatid disease

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Echinococcus granulosus

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Hydatid cysts

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Slow-growing cysts that undergo asexual multiplication to contain many larval scolices

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Hydatid sand

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Refers to the larval scolicies within hydatid cysts

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Location of hydatid cysts in hydatic disease in humans

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Liver (65%), lung (25%), other organs (10%) [similar distribution as CRC mets]

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Signs and symptoms of hydatid disease

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Usually asymptomatic. Symptoms are caused by gradual expansion. Hepatic cysts cause pain and hepatomegaly. Pulmonary cysts cause cough and dyspnea. Cysts in other organs cause sx of space-filling lesions in that organ

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Complications of hydatid disease

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Ruptured cyst causing anaphylactic shock

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This worm infection can present with anaphylactic shock

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Hydatid disease caused by Echinococcus granulosus; anaphylactic shock is caused by rupture of cyst

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Diagnosis of hydatid disease

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Calcified cysts on x-ray, specific serological tests for Ig to E. granulosus

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Geographic distribution of Echinococcus granulosus (hydatid disease)

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Sheep and cattle livestock areas of South America, USSR, South Africa, Mediterranean basin; in Alaska, dogs are infected by eating wild animals (deer, etc.)

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This disease is notable because infection could have occurred decades before symptoms begin

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Hydatid disease (cysts are slow growing). Therefore geographic history should not be just recent, but also past (eg, a trip to South America 10 years ago)

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Treatment of hydatid disease

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Surgical excision of cyst -- sterilize cyst with disinfectant (eg, ethanol) before removal to prevent initiation of new cysts by scolices that happen to be released

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Prevention of hydatid disease

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Avoid dog feces in endemic areas, protect dogs vs. infection, treat infected dogs

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Intermediate host of Echinococcus multilocularis

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Rodents

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Treatment of Echinococcus multilocularis

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Surgical excision of cyst -- more difficult than in hydatid disease because metastatic growth at distant sites can occur (lung, brain)

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Life cycle of Echinococcus multilocularis

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Same as E. granulosus

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Definitive host of Echinococcus multilocularis

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Dog, fox

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Geographic distribution of Echinococcus multilocularis

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Far northern climates (Alaska, but also seen in Minnesota and adjacent northern states)