Midterm 1: Cestodes (Tapeworms)

Cestodes: Basic anatomy of a tapeworm?

• Flat, segmented worms

• Scolex (head) attach to host with suckers, hooks, or grooves

• The neck makes new proglottids

  • Each proglottid is hermaphroditic (containing both male & female reproductive organs)

• No gut or tube-like GI tract

  • Instead, they absorb all nutrients directly from specialized outer body called tegument

Proglottids

Anatomically independent segments

How do tapeworms feed?

They have no gut. Nutrients are taken up by active transport across the tegument. Nerves link the segments to the scolex.

Tapeworm Reproduction?

Each proglottid has male and female organs. However, mating within a segment is unusual. Sperm transfer is usually between adjacent proglottids.

Generic tapeworm life cycle?

1. Eggs in the environment are ingested by an intermediate host (usually cattle and pigs)

2. The oncosphere (embryo) penetrate out of the gut & forms metacestode in the intermedate host’s muscle

3. Muscle of intermediate host (containing metacestodes) is eaten by definitive host (usually humans or dogs)

4. Metacestodes are released in the stomach, migrate to small intestine, and develop to adults

5. Since worms are hemaphroditic, the segments from adults containing eggs are released in feces

Define metacestode. What are the two metacestode types?

Last larval stage of cestodes

Metacestode types:

• Cysticercus

• Hydatid cyst

Where do tapeworm eggs hatch? How long do they survive?

• In the small intestine of intermediate host

• Stay viable for weeks to months in soil until ingested

Which cestode stage causes disease in humans?

• Adult tapeworms generally do not cause pathology in human intestine

• When humans are the intermediate host, the juvenile forms cause the morbidity

Taenia Saginata: Generic name?

Beef tapeworm

Taenia saginata: Route & Life Cycle

• Ingestion of cysticerci in raw/undercooked beef

• The cyst wall is digested, and the scolex attaches to lining of the small intestine

• The adult takes about 4 months to grow (up to 8-10m)

• Eggs or gravid proglottids are passed in feces

Taenia saginata: Definitive host & Intermediate host

Definitive host: Humans

Intermediate host: Cattle

Taenia saginata: What happens in the cow?

• Cows become infected by ingesting vegetation that are contaminated by eggs or gravid proglottids

• The eggs hatch as oncosphere in small intestine of the cow, penetrate intestinal wall, and enter the bloodstream to circulate in striated muscle.

• The oncospheres develop into metacestodes in muscle

• They live for years and calcify when dead

Taenia saginata:

Can humans get cysticercosis? Can cows get adults?

No to both. The eggs hatch in the cow digestive tract (not humans), and cattle don’t develop adult worms from eating cysticerci.

Basically:

• Cows only harbor juvenile forms (intermediate host)

• Humans only harbor adults (definitive host)

Taenia saginata: Clinical Disease

• Most infected humans harbor a single adult tapeworm

  • If >1 worm —> worms are shorter due to crowding

• Adult tapeworms live in the small intestine

• Mostly asymptomatic

• Rare symptoms:

  • Epigastric fullness (bloating)

  • Nausea & vomiting after eating

  • Bowel obstruction

  • Jejunal perforation

Taenia saginata: Diagnosis, treatment, & prevention

Diagnosis:

• Infection is detected by observing proglottids in stool, bedding, or clothing (proglottids migrate out of host overnight)

• PCR to differentiate species

Treatment:

• Praziquantel (can recover intact scolex)

• Niclosamide inhibits parasite ATPase & host digestive enzymes destroy worm (cannot recover scolex)

Prevention/Control:

• Proper human feces disposal

• Freeze beef prior to cooking

• Thoroughly cook beef

Taenia solium: Generic name?

Pork tapeworm

True or False: Pork tapeworm is a parasitic flatworm capable of causing two distinct types of infection in humans depending on the stage ingested.

True

Taenia solium: Adult Intestinal Infection (Humans as definitive host)

How is it transmitted?

Humans ingest cysticerci (larval metacestodes) in raw/undercooked pork

Taenia solium: Adult Intestinal Infection (Humans as definitive host)

Route & Life Cycle of the Adult

• A person eat cysticerci in raw/undercooked pork

• Larval capsule is digested in stomach

• The parasite is free in the small intestine and scolex attaches to intestinal wall

• Adult takes about 3 months to develop and can live ~5 years in human GI tract

• Gravid proglottids exit in feces and pigs eat them

Taenia solium: Adult Intestinal Infection (Humans as definitive host)

Pathologies?

• Adult infection have no significant inflammatory response

• Patients are asymptomatic

• Rarely gut perforation

Taenia solium: Juvenile Tissue Infection (Cysticercosis—Humans as Intermediate Host)

How is it transmitted?

Humans ingest T. solium eggs via fecal-oral contamination or autoinfection (e.g. poor hand hygiene)

Taenia solium: Juvenile Tissue Infection (Cysticercosis—Humans as Intermediate Host)

Route & Life Cycle of the Juvenile Form

• Human ingests T. solium eggs from fecal-oral contamination or autoinfection

• Larvae hatch in small intestine, enter bloodstream, penetrate tissue (muscles, heart, brain, eye)

• Cysts form and act as space-occupying lesions. Living cysts suppress immunity

• They die in about 2 years, calcify, and release antigens, causing acute inflammation.

Taenia solium: Juvenile Tissue Infection (Cysticercosis—Humans as Intermediate Host)

What is neurocysticercosis?

Inflammation around degenerating cysticerci in the brain. Causes seizures, raised intracranial pressure, and hydrocephalus (when there is blocked circulation of cerebrospinal fluid)

Taenia solium: Juvenile Tissue Infection (Cysticercosis—Humans as Intermediate Host)
What is extraneural cysticercosis?

Inflammation in subcutaneous tissue or muscles. Painless nodules when juvenile forms are alive. When they die, the nodule becomes swollen and calcifies.

Taenia solium: Juvenile Tissue Infection (Cysticercosis—Humans as Intermediate Host)

Pathologies?

• Serious because this is the stage that normally infects pig. Now it is in human tissue

• Leading cause of epilepsy

• Intracranial hypertension

• Hydrocephalus

• Stroke (due to brain inflammation)

• Extraneural cysticercosis

Taenia solium: Diagnosis, treatment, prevention

Diagnosis: It is important to be able to distinguish between beef and pork tapeworm!

• Identification of eggs by microscopy (doesn’t distinguish)

• Identification of scolex (distinguishes)

• DNA-based tests (distinguishes)

• Detect fecal antigens

Treatment:

• Niclosamide acts in the gut and isn’t absorbed

• Praziquantel kills adults & juveniles

  • However, this triggers inflammation and CNS manifestations (occult cysticercosis)

  • Therefore, steroids are required (to control inflammation)

Prevention:

• Proper sewage disposal

• Mass treatment of humans and pigs to reduce incidence

Dipyllobothrium latum: Generic name?

Fish tapeworm

Dipyllobothrium latum: How is it transmitted?

Acquired from ingesting raw of undercooked freshwater fish harboring infective metacestodes

Dipyllobothrium latum: Key Facts

• Longest human parasite (>10 m)

• Scolex has two grooves

• Common in Scandanavia

Dipyllobothrium latum: What is its reservoir host?

Reservoir host = Bears & other carnivores

Dipyllobothrium latum: Route & Life Cycle

Eggs released via feces in freshwater —> coracidium —> eaten by copepod —> procercoid (in copepod) —> small fish eats infected copepods —> procercoid migrates into muscle tissue of fish & develops to plerocercoid metacestode —> larger predator fish (e.g. salmon) eat the infected small fish —> human (or other carnivores) eats raw fish —> larva attaches to small intestine wall with scolex and mature into adults

Dipyllobothrium latum: Definitive host? Reservoir host? Intermediate host?

Definitive = humans

Reservoir = bears (and other mammals)

Intermediate = copepods & freshwater fish

Diphyllobothrium latum: Pathologies?

• The worm has a receptor that absorbs Vitamin B12 directly from the host

• This can lead to B12 deficiency in host

• In small number of patients, severe B12 depletion —> megaloblastic anemia

Diphyllobothrium latum: What is megaloblastic anemia?

Low RBCs, immature & dystunctional cells (megaloblasts) in the bone marrow. Symptons include fatgue, weight loss, tachycardia

Diphyllobothrium latum: Diagnosis, treatment, prevention

Diagnosis:

• Observe proglottid segments in stool

• Microscopic ID of eggs

Treatment:

• Praziquantel or niclosamide

Prevention:

• Eat thoroughly cooked or freeze freshwater fish

• Dispose of feces so eggs don’t reach water

Hymenolepis nana: Generic name?

Dwarf tapeworm

Hymenolepis nana: Transmission and Life Cycle Routes

There are two routes of infection:

1. Direct cycle (fecal-oral): Swallowed eggs from contaminated food, water, or unwashed hands. Ingested eggs hatch in small intestine and develop directly into adult worms

2. Indirect Cycle (Two-host cycle): Ingestion of infected arthropod intermediate host (e.g. flour beetles, mealworms) harboring larval cysticercoids. Humans then eat the intermediate host.

Hymenolepis nana: Autoinfection?

Eggs released in the gut lumen can hatch internally before exiting in stool, releasing embryos that re-penetrate the intestinal villi to perpetuate the cycle

Hymenolepis nana: Pathologies?

• Mostly asymptomatic

• Sometimes:

  • Nausea

  • Weakness

  • Diarrhea

  • Abdominal pain

• Children may have:

  • headache

  • itchy bottom

  • trouble sleeping

Hymenolepis nana: Malignant transformation, unique pathology

• In severely immunocompromised patients (e.g HIV+), H. nana larval cells can undergo malignant-like uncontrolled proliferation.

• These parasite cells invade surrounding host tisses — mimicking metastatic cancer

Echinococcus granulosus: Can humans be infected with the juvenile form?

Yes

Echinococcus granulosus: Definitive host?

Domestic dogs & other canids (wolf, fox)

Echinococcus granulosus: Intermediate host?

Sheep & other herbivores. Infected by ingesting egg-contaminated pastures

Echinococcus granulosus: How are humans affected?

Humans get the juvenile stage by accident (aberrant; dead-end host) —> hydatid disease

Echinococcus granulosus: Path in the intermediate host?

Intermediate host: Sheep & other herbivores

• Sheep ingest eggs in dog (definitive host) feces.

• Eggs hatch in the small intestine, enter the bloodstream, and reach the liver via portal circulation

• Other organs invaded: lung, brain, kidney

• The larva forms a fluid-filled hydatid cyst

Echinococcus granulosus: Path in dogs/canids (definitive host)?

• Definitive host (dogs) ingest hydatid cyst with protoscolices when consuming infected sheep organs

• Protoscolices are released from the hydatid cysts & attach to the small intestine with suckers.

• They develop into adult tapeworms in the small intestine.

• Eggs are produced within 2 months. Eggs are released in feces to continue life cycle

• Dogs do not become ill from intestinal infections

Echinococcus granulosus: Accidental human infection by juvenile form?

• Humans accidentally ingest microscopic eggs via contact with infected dogs or consumption of contaminated food/water

• Eggs hatch in small intestine & release oncospheres (embryos)

• The oncosphere travel via the bloodstream and form hydatid cysts (in various organs)

• This leads to hydatid disease in humans

Echinococcus granulosus: Pathology

• Most cysts develop in liver or lungs

• About half of hydatid cysts asymptomatic

• Symptoms come from a space-occupying lesion

• Ruptured cysts release antigenic fluid & protoscolices —> analphylaxis and seeding new cysts

  • Liver rupture —> spread via biliary tree

  • Lung rupture —> spread via bronchial tree —> bloody sputum

• Rupture in brain is fatal; fatality ~2%

Echinococcus granulosus: Diagnosis, treatment, prevention

Diagnosis:

• Cysts can be detected via ultrasound, CT scan, or MRI

• Need to know case history (dogs, sheep farm, endemic travel)

Treatment:

• Surgical & medical intervention

Prevention:

• Avoid feeding organs from sheep to dogs

• Treat dogs periodically

• Mass slaughter of infected sheep & dogs to eradicate

Echinococcus multicularis: Can humans be infected with juvenile form?

Yes

Echinococcus multicularis: Definitive host?

Wild canids & domestic dogs

Echinococcus multicularis: Intermediate host?

Rodents, voles, field mice, squirrels

Echinococcus multicularis: Route and Life Cycle

• Intermediate host (rodents) ingest eggs (likely from feces)

• The oncosphere hatch in small intestine, enter bloodstream, & invades liver

• Oncosphere grow into cysts containing protoscolices

• Rodents containing infective cysts in organs are eaten by the definitive host (canids)

• The protoscolices develop into adult worms in small intestine

• Eggs are released in canid feces

• Humans can accidentally ingest the eggs in feces —> oncospheres are released in small intestine —> oncosphere travel via bloodstream —> hydatid cysts form in organs —> Hydatid Disease

Echinococcus multicularis: Human Infection? Which parasitic form is found in humans? Pathology in humans?

• Highly aggressive —> primarily affects liver (liver failure)

• No protoscolices are found in humans, only metastatic membranes that proliferate infefinitely

• Metastatic disease via bloodstream is common

Echinococcus multicularis: Diagnosis, treatment, prevention

Diagnosis:

• CT scan, other imaging

Treatment:

• No fully effectve therapy

• Surgery to remove masses

• Hepatic or pulmonary lobe resection (liver or lung removal)

• Long-term albendazole treatment improves survival rate

Prevention:

• Animal trappers & people in fur industry at risk

  • Extreme caution when handling carcasses & fur

• Control infection in foxes by baiting them with praziquantel

• This is a reemerging infection