Chapter 3-Cestodes

CestodesAssignmentsRead Chapter 3Do puzzle of page 57 and post-test on page 59(points for answers not as listed)Due next Tuesday

Cestodes
Clinically important cestodes pathogenic to man are:
Taenia solium (pork tapeworm)
Taenia saginata (beef tapeworm)
Diphyllobothrium lattum (fish or broad tapeworm)
Hymenolepis nana (dwarf tapeworm) Echinococcus granulosus
E. multilocularis (hydatid).

Tenia solium  T. saginata (Teniasis)
EpidemiologyThese cestodes have a worldwide distribution but incidence is higher in developing countries. Infection rate is as low as 1 per 1000 in most of North America and as high as 10% in the third world. Pork tapeworm shows a higher incidence but this is dependent on dietary habits.
 
Taenia Morphology
T. saginata can be up to 4 to 6 meters long and 12 mm broad; it has a pear-shaped head (scolex) with four suckers but no hooks or neck. It has a long flat body with several hundred segments (proglottids).
Each segment has a branched uterus (15-30 branches). The egg is 35 x 45 micrometers, roundish and yellow-brown. It has peripheral radial striations and contains an embryo with 3 hooklets.

Taenia
T. solium is slightly smaller than T. saginata. It has a globular scolex with four suckers and a circular row of hooks (rostellum) that gives it a solar appearance. There is a neck and it has a long flat body (0.1 meter in length). The proglottids are 5 x 10 mm with a 7-12 branch uterus. The eggs of T. solium and T.  saginata are indistinguishable

T. Solium
SymptomsLight infections remain asymptomatic, but heavier infections may produce abdominal discomfort, epigastric pain, vomiting and diarrhea.

T. Solium
T. solium is slightly smaller than T. saginata. It has a globular scolex with four suckers and a circular row of hooks (rostellum) that gives it a solar appearance. There is a neck and it has a long flat body (0.1 meter in length). The proglottids are 5 x 10 mm with a 7-12 branch uterus. The eggs of T. solium and T.  saginata are indistinguishable.

Taenia
DiagnosisDiagnosis is based on the recovery of eggs or proglottids in stool or from the perianal area. Cysticercosis is confirmed by the presence of antibodies.
Treatment and controlPraziquantel is the drug of choice. Expulsion of scolex must be assured to assume a satisfactory treatment. A thorough inspection of beef and pork, adequate cooking or freezing of meat are effective precautions, since cysticerci do not survive temperatures below -10o C and above 50o C.

Cysticercosis
A tapeworm larval cyst (cysticercus) is ingested with food/meat contaminated with the egg or the egg in feces; the larva escapes the cyst and passes to the small intestine where it attaches to the mucosa by the scolex suckers.
The proglottids develop as the worm matures in 3 to 4 months. The adult may live in the small intestine as long as 25 years and pass gravid proglottids with the feces.
Eggs extruded from the proglottid contaminate and persist on vegetation for several days and are consumed by cattle or pigs in which they hatch.

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

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.

Cysticercosis
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 . 

Cysticercosis
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 ~ 50,000 eggs and reside in the small intestine for years .
Cysticercosis
Pathology and ImmunologyGastrointestinal symptoms are due to the presence of the tape worm. Cysticercosis symptoms are a result of inflammatory/immune responses. Antibodies are produced in cysticercosis and are useful epidemiological tools.

Hymenolepis nana (dwarf tapeworm)
This is a small tapeworm which infects children.
Rodents are the reservoir. Infection is by the oro-fecal mode and, hence, cross infection and auto infection by eggs in feces in normal.
The worm develops from ingested eggs into an  adult  in the small intestine and resides there for several weeks Light infections produce vague abdominal disturbances but heavier infections may cause enteritis.
Diagnosis is based on finding eggs in the feces.
Nicolsamide is the drug of choice. Hygiene is the best control.

Diphyllobothrium latum
EpidemiologyFish tapeworm infection is distributed worldwide, in the subarctic and temperate regions; it is associated with eating of raw or improperly cooked fresh water fish.
MorphologyThis is the longest tapeworm found in man, ranging from 3-10 meters with more than 3000 proglottids. The scolex resembles two almond-shaped leaves and the proglottids are broader than they are long, a morphology reflected in the organism's name. Eggs are 30 x 50 micrometers in size and contain an embryo with 3 pairs of hooklets.

Diphyllobothrium latum
Life cycleMan and other animals are infected by eating uncooked fish that contains plerocercoid larvae which attach to the small intestinal wall and mature into adult worms in 3 to 5 weeks.
Eggs discharged from gravid proglottids in the small intestine are passed in the feces.
The egg hatches in fresh water to produce a ciliated coracidium which needs to be ingested by a water flea (Cyclops) where it develops into a procercoid larva.
When infected Cyclops are ingested by the freshwater fish, the procercoid larva penetrates the intestinal wall and develops into a plerocercoid larva, infectious to man.

Diphyllobothrium latum
SymptomsClinical symptoms may be mild, depending on the number of worms. They include abdominal discomfort, loss of weight, loss of appetite and some malnutrition. Anemia and neurological problems associated with vitamin B12 deficiency are seen in heavily infected individuals.

D. Latum
DiagnosisDiagnosis is based on finding many typical eggs and empty proglottids in feces. A history of raw fish consumption and residence in an endemic locality is helpful.
Treatment and controlPraziquantel is the drug of choice. Freezing for 24 hours, thorough cooking or pickling of fish kills the larvae. Fish reservoirs should be kept free of raw sewage.

Echinococcosis (hydatid)
Echinococcus granulosus and E. multilocularis are causative agents of hydatid cysts.
Echinococcus granulosus
EpidemiologyThe organism is common in Asia, Australia, Eastern Africa, southern Spain, southern parts of South America and northern parts of North America. The incidence of human infection about 1 to 2 per 1000 population and may be higher in rural areas of affected regions.
MorphologySmallest of all tapeworms-only 3 proglottids.
 
Echinococcosis (hydatid)
The adult worm lives in domestic and wild carnivorous animals.
Eggs, passed by infected animals, are ingested by the grazing farm animals or man, localize in different organs and develop into hydatid cysts containing many larvae (proto-scolices or hydatid sand).
When other animals consume infected organs of these animals, proto-scolices escape the cyst, enter the small intestine and develop into adult worms.

Echinococcus
Echinococcus eggs, when swallowed by man, produce embryos that penetrate the small intestine, enter the circulation and form cysts in liver, lung, bones, and sometimes, brain.
The cyst is round and consists of an outer nuclear hyaline cuticula and an inner nucleated germinal layer containing clear yellow fluid.
Daughter cysts attach to the germinal layer, although some cysts, known as brood cysts, may have only larvae (hydatid sand). Man is a dead end host.

Echinococcosis (hydatid)
SymptomsThe symptoms, comparable to those of a slowly growing tumor, depend upon the location of the cyst. Large abdominal cysts produce increasing discomfort. Liver cysts cause obstructive jaundice. Peribronchial cysts may produce pulmonary abscesses. Brain cysts produce intracranial pressure and Jacksonian epilepsy. Kidney cysts cause renal dysfunction. The contents of a cyst may produce anaphylactic responses.

Echinococcus
DiagnosisClinical symptoms of a slow-growing tumor accompanied by eosinophilia are suggestive. Intradermal (Casoni) test with hydatid fluid is useful. Pulmonary cysts and calcified cysts can be visualized using x-rays. Antibodies against hydatid fluid antigens have been detected in a sizable population of infected individuals by ELISA or indirect hemagglutination test.

Echinococcus
Treatment and controlTreatment involves surgical removal of cyst or inactivation of hydatid sand by injecting the cyst with 10% formalin and its removal within five minutes. It has been claimed that a high dose of Mebendazole results in some success. Preventive measures involve avoiding contact with infected dogs and cats and elimination of their
E. multilocularis
This is a tapeworm, similar to E. granulosus, that also causes hydatid in northern parts of Asia and North America. It has a very similar morphology and life cycle except that rodents are its intermediate host. Humans, when infected with this worm, also develop hydatid cysts which produce symptoms similar to those caused by E. granulosus. However, the cysts are multilocular (many chambers). The organism is resistant to praziquantel; high doses of Albendazole has some anti-parasitic effect. Surgery is the means of removing the cyst. Rodent control is the means of prevention

New Tapeworm
A major group of tapeworms that can grow to more than 30 feet long in the digestive tracks of humans, fish and other animals has been discovered by Canadian researchers. The new tapeworm group, is in an order now dubbed Rhinebothriidea. Infection with a tapeworm is rare in the United States. People are often unaware they are infected, via an animal or water, but symptoms can include abdominal pain, nausea, vomiting, loss of appetite and malnutrition. The treatment is a pill that kills the worm and helps the body expel it.