Causal Agent:The cestode (tapeworm) Taenia solium (pork tapeworm) is the main cause of human cysticercosis.  In addition, the larval stage of other Taenia species (e.g., multiceps, serialis, brauni, taeniaeformis, crassiceps) can infect humans in various sites of localization including the brain, subcutaneous tissue, eye, or liver.

Life Cycle:

Cysticercosis is an infection of both humans and pigs with the larval stages of the parasitic cestode, Taenia solium.  This infection is caused by ingestion of eggs shed in the feces of a human tapeworm carrier .  Pigs and humans become infected by ingesting eggs or gravid proglottids , .  Humans are infected either by ingestion of food contaminated with feces, or by autoinfection.  In the latter case, a human infected with adult T. solium can ingest eggs produced by that tapeworm, either through fecal contamination or, possibly, from proglottids carried into the stomach by reverse peristalsis.  Once eggs are ingested, oncospheres hatch in the intestine , invade the intestinal wall, and migrate to striated muscles, as well as the brain, liver, and other tissues, where they develop into  cysticerci .  In humans, cysts can cause serious sequellae if they localize in the brain, resulting in neurocysticercosis.  The parasite life cycle is completed, resulting in human tapeworm infection, when humans ingest undercooked pork containing cysticerci .  Cysts evaginate and attach to the small intestine by their scolex .  Adult tapeworms develop, (up to 2 to 7 m in length and produce less than 1000 proglottids, each with approximately 50,000 eggs) and reside in the small intestine for years .

Geographic Distribution:Taenia solium is found worldwide.  Because pigs are intermediate hosts of the parasite, completion of the life cycle occurs in regions where humans live in close contact with pigs and eat undercooked pork.  Taeniasis and cysticercosis are very rare in Muslim countries.  It is important to note that human cysticercosis is acquired by ingesting T. solium eggs shed in the feces of a human T. solium tapeworm carrier, and thus can occur in populations that neither eat pork nor share environments with pigs.

Clinical Features:The symptoms of cysticercosis are caused by the development of cysticerci in various sites.  Of greatest concern is cerebral cysticercosis (or neurocysticercosis), which can cause diverse manifestations including seizures, mental disturbances, focal neurologic deficits, and signs of space-occupying intracerebral lesions.  Death can occur suddenly.  Extracerebral cysticercosis can cause ocular, cardiac, or spinal lesions with associated symptoms.  Asymptomatic subcutaneous nodules and calcified intramuscular nodules can be encountered.

Laboratory Diagnosis:The definitive diagnosis consists of demonstrating the cysticercus in the tissue involved.  Demonstration of Taenia solium eggs and proglottids in the feces diagnoses taeniasis and not cysticercosis.  While suggestive, it does not necessarily prove that cysticercosis is present.  Persons who are found to have eggs or proglottids in their feces should be evaluated serologically since autoinfection, resulting in cysticercosis, can occur.

Diagnostic findings

Antibody detection provides a useful adjunct in specific diagnosis.

Treatment:Infections are generally treated with antiparasitic drugs in combination with antiinflammatory drugs.  Surgery is sometimes necessary to treat infection in the eyes, cases that are not responsive to drug treatment, or to reduce brain edema.  Not all cases of cysticercosis are treated and the use of albendazole and praziquantel is controversial.  For additional information, see the recommendations in The Medical Letter (Drugs for Parasitic Infections).

Cysticercosis: Antibody DetectionCDC's immunoblot assay with purified Taenia solium antigens has been acknowledged by the World Health Organization and the Pan American Health Organization as the immunodiagnostic test of choice for confirming a clinical and radiologic presumptive diagnosis of neurocysticercosis.  CDC's immunoblot is based on detection of antibody to one or more of 7 lentil-lectin purified structural glycoprotein antigens from the larval cysts of T. solium in an immunoblot format.  It is 100% specific and has a sensitivity superior to that of any other test yet evaluated.  Serum specimens from 97% of parasitologically confirmed cases of cysticercosis have detectable antibodies.  No serum samples from patients with other microbial infections react with any of the T. solium-specific antigens.  The most important factors identified as determining positive immunoblot reactions are the numbers and stage of development of cysticerci.  Cumulative clinical experience has confirmed that in patients with multiple (more than two) lesions, the test has more than 95% sensitivity.  Seropositivity in biopsy-confirmed patients with single, enhancing parenchymal cysts was <50%; in clinically defined patients with a single cyst but who were not biopsied, sensitivity was 70%.  Seropositivity in serum and CSF of patients with multiple but only calcified cysts was 82 and 77%, respectively.  In all patients, regardless of their clinical presentation, the immunoblot assay is slightly more sensitive in serum than in CSF specimens: consequently, there is no need to obtain CSF solely for use in the immunoblot assay.

CDC's immunoblot is both more specific and more sensitive than enzyme immunoassay (EIA) systems with which it has been compared.  Lack of specificity has been a major problem in most EIAs because of cross-reacting components in crude antigens derived from cysticerci; these components react with antibodies specific for other helminthic infections, especially echinococcosis and filariasis.  Most partially purified fractions evaluated in an EIA appear to have lower sensitivity than crude antigens and do not

necessarily achieve higher specificity.  Assays employing crude antigens for the detection of antibody are not reliable for the identification of this disease; all positives and any negative strongly suspected of cysticercosis should be confirmed by immunoblot.  Currently available antibody detection tests for cysticercosis do not distinguish between active and inactive infections and thus have not been useful in evaluating the outcomes and prognoses of medically treated patients.  Both the CDC immunoblot and an EIA are commercially available in the United States.

A

Typical antibody reactions in CDC's immunoblot for cysticercosis.  Individual sera from patients with either cysticercosis or echinococcosis were analyzed using the immunoblot for cysticercosis.

Cysticercosis-specific antibodies react with glycoproteins derived from T. solium cysts.  The positions of the seven diagnostic glycoproteins are marked and designated according to their relative mobilities in SDS-PAGE.  Sera from patients with cysticercosis react with at least one of the cysticercosis-specific proteins, whereas sera from patients with echinococcosis do not react with any of the seven diagnostic proteins.

Reference:

Tsang VCW, Wilson M. Taenia solium cysticercosis: an under-recognized but serious public health problem. Parasitol Today 1995;11:124-126

Causal Agents:The cestodes (tapeworms) Taenia saginata (beef tapeworm) and T. solium (pork tapeworm).  Taenia solium can also cause cysticercosis.

Life Cycle:

Life cycle of Taenia saginata and Taenia solium Taeniasis is the infection of humans with the adult tapeworm of Taenia saginata or Taenia solium.  Humans are the only definitive hosts for T. saginata and T. solium.  Eggs or gravid proglottids are passed with feces ; the eggs can survive for days to months in the environment.  Cattle (T. saginata) and pigs (T. solium) become infected by ingesting vegetation contaminated with eggs or gravid proglottids .  In the animal's intestine, the oncospheres hatch , invade the intestinal wall, and migrate to the striated muscles, where they develop into cysticerci.  A cysticercus can survive for several years in the animal.  Humans become infected by ingesting raw or undercooked infected meat .  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 .  Length of adult worms is usually 5 m or less for T. saginata (however it may reach up to 25 m) and 2 to 7 m for T. solium.  The adults produce proglottids which mature, become gravid, detach from the tapeworm, and migrate to the anus or are passed in the stool (approximately 6 per day).  T. saginata adults usually have 1,000 to 2,000 proglottids, while T. solium adults have an average of 1,000 proglottids.  The eggs contained in the gravid proglottids are released after the proglottids are passed with the feces.  T. saginata may produce up to 100,000 and T. solium may produce 50,000 eggs per proglottid respectively.

Geographic Distribution:Both species are worldwide in distribution.  Taenia solium is more prevalent in poorer

communities where humans live in close contact with pigs and eat undercooked pork and is very rare in Muslim countries.

Clinical Features:Taenia saginata taeniasis produces only mild abdominal symptoms.  The most striking feature consists of the passage (active and passive) of proglottids.  Occasionally, appendicitis or cholangitis can result from migrating proglottids.  Taenia solium taeniasis is less frequently symptomatic than Taenia saginata taeniasis.  The main symptom is often the passage (passive) of proglottids.  The most important feature of Taenia solium taeniasis is the risk of development of cysticercosis.

Laboratory Diagnosis:Microscopic identification of eggs and proglottids in feces is diagnostic for taeniasis, but is not possible during the first 3 months following infection, prior to development of adult tapeworms.  Repeated examination and concentration techniques will increase the likelihood of detecting light infections.  Nevertheless, speciation of Taenia is impossible if solely based on microscopic examination of eggs, because all Taenia species produce eggs that are morphologically identical.  Eggs of Taenia sp. are also indistinguishable from those produced by cestodes of the genus Echinococcus (tapeworms of dogs and other canid hosts).  Microscopic identification of gravid proglottids (or, more rarely, examination of the scolex) allows species determination.

TAKE EXTREME CARE IN PROCESSING THE SAMPLES!  INGESTION OF EGGS CAN RESULT IN CYSTICERCOSIS!

Diagnostic findings

Microscopy Antibody detection may prove useful especially in the early invasive stages, when

the eggs and proglottids are not yet apparent in the stools.

Morphologic comparison with other intestinal parasites

Treatment:Treatment is simple and very effective.  Praziquantel* is the drug of choice.  For additional information, see the recommendations in The Medical Letter (Drugs for Parasitic Infections).

* This drug is approved by the FDA, but considered investigational for this purpose.

MicroscopyTAKE EXTREME CARE IN PROCESSING THE SPECIMENS!INGESTION OF TAENIA SOLIUM EGGS CAN RESULT IN CYSTICERCOSIS!

The eggs of Taenia solium and T. saginata are indistinguishable from each other, as well as from other members of the Taeniidae.  The eggs measure 30-35 micrometers in diameter and are radially-striated.  The internal oncosphere contains six refractile hooks.

A B

A, B:  Taenia spp. eggs in unstained wet mounts.  Four hooks can clearly be seen in Figure A.

Scoleces of Taenia spp.  The scolex of T. solium contains four large suckers and a rostellum containing two rows of large and small hooks.  There are usually 13 hooks of each size.  The scolex of T. saginata has four large suckers but lacks the rostellum and rostellar hooks.

C D

C:  Scolex of T. saginata. Note the four large suckers and lack of rostellum and rostellar hooks.D:  Scolex of T. solium. Note the four large suckers and rostellum containing two rows of hooks.

Proglottids of Taenia spp.  Gravid proglottids are longer than wide and the two species, T. solium and T. saginata, differ in the number of primary lateral uterine branches: T. solium contains 7-13 lateral branches and T. saginata 12-30 lateral branches.

E F

E, F:  Mature proglottids of T. saginata.  Injection of India ink into the genital pore in Figure F helps with the visualization of the primary lateral uterine branches.  Note the number of branches (>12).

G H

G, H:  Mature proglottids of T. solium. Injection of India ink into the genital pore in Figure H helps with the visualization of the primary lateral uterine branches.  Note the number of branches (<13).

Cross-sections of proglottids of Taenia spp., stained with hematoxylin and eosin (H&E)

I J

I:  Cross-section of a proglottid of Taenia sp., stained with hematoxylin and eosin (H&E). Note the thick outer tegument and the loose parenchyma filling the body.  Calcareous corpuscles (red arrows), characteristic of the cestodes, can be seen in the parenchyma.  Eggs (blue arrows) can also be seen.  Images courtesy of the Washington State Public Health Laboratories.J:  Higher magnification of Figure I showing a close-up of the eggs.  Note the characteristic striations, typical for the taeniids. Not visible in these images are the hooks commonly seen in cestode eggs.  Hooks do not stain with H&E but are refractile and visible with fine focusing of the microscope.

K L

K:  Close-up of a cross-section of a Taenia sp. proglottid stained with hematoxylin and eosin (H&E), showing numerous calcareous corpuscles (yellow arrows).  Image courtesy of the Michael E. DeBakey V. A. Medical Center in Houston, TX.L:  Close-up of a cross-section of a Taenia sp. proglottid stained with hematoxylin and eosin (H&E), showing numerous calcareous corpuscles.  Image courtesy of Ameripath.

Adults of Taenia spp.  Adults can reach a length of 2-8 meters, but the scolex is only 1-2 millimeters in diameter.

M N

M, N:  Taenia saginata adult worms.  The adult in Figure M is approximately 4 meters in length.