Taenia Solium: Life Cycle, Pathogenesis and Lab Diagnosis

Taenia solium, also known as pork tapeworm, causes Taeniasis, an infection that is common amongst people eating raw or insufficiently cooked measly pork containing the cysticerci. Taenia solium is prevalent worldwide but this infection is more prevalent in poorer communities where humans live in close contact with pigs and eat undercooked pork. Taenia solium also causes a deadly disease, cysticercosis.

Taeniasis is also caused by Taenia saginata and Taenia asiatica. Taenia saginata (beef tapeworm) does not cause cysticercosis in humans. It is not clear if T. asiatica causes cysticercosis in humans or not.

Life Cycle

Taeniasis

Humans are definitive hosts for Taenia solium and pigs are the intermediate hosts.

  1. Eggs or gravid proglottids are passed with feces; the eggs can survive for days to months in the environment. Eggs of Taenia solium are immediately infectious and do not require a developmental period outside the host.
  2. Pigs become infected by ingesting vegetation contaminated with eggs or gravid proglottids. 
  3. In the animal’s intestine, the oncospheres hatch, invade the intestinal wall, and migrate to the striated muscles, where they develop into cysticerci. 
  4. A cysticercus can survive for several years in the animal. 
  5. Humans become infected by ingesting raw or undercooked infected meat. 
  6.  In the human intestine, the cysticercus develops over 2 months into an adult tapeworm, which can survive for years. The adult tapeworms attach to the small intestine by their scolex and reside in the small intestine. The adults produce proglottids that mature, become gravid, detach from the tapeworm, and migrate to the anus or are passed in the stool. The eggs contained in the gravid proglottids are released after the proglottids are passed with the feces.  

Cysticercosis

Taenia solium causes cysticercosis when humans serve as accidental intermediate hosts following ingestion of infectious eggs. Embroyonated eggs of Taenia solium infects humans and cysticeri (larval cyst) develop in the lung, liver, eye, and brain, a condition known as cysticercosis.

Cysticercosis is only acquired from the ingestion of eggs (fecal-oral route), not via the ingestion of cysticerci in undercooked pork, which is associated with intestinal taeniasis.

7. Humans acquire infection by ingestion of eggs/proglottids shed in the feces of a human tapeworm carrier or by autoinfection  (either through fecal contamination or, possibly, from proglottids carried into the stomach by reverse peristalsis). 

8. Once eggs are ingested, oncospheres hatch in the intestine.    

9. Oncosphere invades the intestinal wall and migrates to striated muscles, as well as the brain, liver, and other tissues, where they develop into cysticerci. 

In humans, cysts can cause serious sequelae if they migrate and localize in the brain, resulting in neurocysticercosis. 

Pork tapeworm (T. solium) is alo called “armed tapeworm” because it has rostellum armed with a double row of 25-30 alternating large and small hooklets. T.saginata is called “unarmed tapeworm” because of absence of hooks on the rostellum.

Clinical Disease

Taeniasis

Gastrointestinal symptoms are due to the presence of the tapeworm. Light infections remain asymptomatic. Heavier infections may produce abdominal discomfort, epigastric pain, vomiting and diarrhea, and occasionally appendicitis. Most striking feature of taeniasis is the passage of proglottids in the stool.

Cysticercosis

Cysticercosis symptoms are the result of inflammatory/immune responses. The symptoms of cysticercosis vary depending upon the location and number of cysticerci. Cysticerci mainly develop in skeletal and heart muscle, skin, subcutaneous tissues, lungs, and liver. In most locations, cysticerci cause few symptoms and spontaneously degenerate. Neurocysticercosis (cysticerci in the brain) is associated with serious neurological and epileptic manifestations and may be fatal.

Laboratory Diagnosis

Diagnosis of taeniasis is based on the recovery of eggs or proglottids in the stool or from the perianal area and cysticercosis is mainly confirmed by the presence of antibodies.

Take extreme care while processing the stool specimen from suspected cases of Taeniasis. Ingestion of Taenia solium eggs can result in cysticercosis.

Microscopy and Staining

In the suspected cases of Taeniasis, look for egg, scolex, or proglottids of Taenia solium in the stool of the patient.

Eggs:

  1. The eggs are about 30-35 micrometers in diameter and are bile stained
  2. The internal oncosphere contains six refractile hooks.
  3. The eggs do not float in the saturated solution of NaCl.

The diagnosis of cysticercosis usually depends on surgical removal of the parasite and microscopic examination for the presence of suckers and hooks on the scolex. Multiple larvae are frequently present, and the presence of cysticerci in the subcutaneous or muscle tissues indicates that the brain is probably involved. The calcified larvae can be readily seen on X-ray examination. Computed tomography (CT) scans or magnetic resonance imaging (MRI) techniques may reveal the presence of lesions in the brain.

Antigen Detection

An ELISA test for the detection of antigen secreted by viable T. solium metacestodes is available.

Serology

IgG is the predominant antibody detected in patients with cysticercosis; IgA, IgE, and IgM antibodies are of little value in diagnosis. Antibodies can be detected using serological tests such as complement fixation, IFA, ELISA, Immunoblotting, and radioimmunoassay. Serodiagnosis of cysticercosis is problematic as cross-reactions are seen in patients with Echinococcus, filariasis, and T. saginata infections.

References and further readings

  • Sastry A.S. & Bhat S. (2014) Essentials of Medical Parasitology. Jaypee Brothers Medical Publishers (P) Ltd
  • Gracia, L.S. (2016). Diagnostic Medical Parasitology. ASM Press.


Acharya Tankeshwar

Hello, thank you for visiting my blog. I am Tankeshwar Acharya. Blogging is my passion. As an asst. professor, I am teaching microbiology and immunology to medical and nursing students at PAHS, Nepal. I have been working as a microbiologist at Patan hospital for more than 10 years.

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