Overview of Platyhelminthes: Tapeworms and Flukes
Platyhelminthes, such as tapeworms and flukes, are multicellular organisms with a flat body and hermaphroditic reproductive systems. Tapeworms, a type of Cestode, have a unique structure with a scolex and proglottids for reproduction. They are dioecious and develop through the production of proglottids that contain eggs. Diagnosis involves identifying the scolex or proglottid. Hymenolepis nana, the Dwarf Tapeworm, is a common infection in humans that can be diagnosed by finding eggs in feces.
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The Platyhelminthes (Flatworms): Tapeworms & Flukes General Characteristics: Multicellular animals characterized by a flat, bilaterally symmetric body. It is dorsoventrally flattened and is solid (no body cavity). Most are hermaphroditic, having both male and female reproductive organs in the same individual. Overall size varies greatly, some members are as small as 1mm, and others may be 20 meters or longer. The classes Cestoda (tapeworms) and Trematoda (flukes) contain species parasitic in humans.
The Platyhelminthes (Flatworms): Tapeworms The Cestodes (tapeworms): Adult worms - Flat & ribbon- like; body consists of an anterior attachment organ (scolex) and a body (strobila) consisting of a chain of segments called proglottids. Scolex (head) - Characterized by the presence of sucking disks or lateral grooves. Some have hooks (armed). Rostellum - A small button- like structure on the scolex of armed tapeworms from which the hooks protrude. It may be retractable.
The Platyhelminthes (Flatworms): Tapeworms The Cestodes (tapeworms): Dioecious - each proglottid has both male & female reproductive organs; can fertilize itself. Reproductive organs - male: testes are spread throughout the segment; sperm is collected in the seminal vesical; delivered to female organ via copulatory spicules. Female: ovaries produce eggs, which are stored in the uterus.
The Platyhelminthes (Flatworms): Tapeworms The Cestodes (tapeworms): Development of proglottids - New proglottids bud from behind the scolex. As they are pushed back, they mature & eggs are produced. When filled with eggs, they are gravid proglottids. Eggs are sometime released in feces, but often are retained within the segment. Hexacanth embryos (onchospheres) develop within the eggs. Proglottids - a few are usually shed every 2-3 days.
The Platyhelminthes (Flatworms): Tapeworms The Cestodes (tapeworms): Diagnosis - the scolex or proglottid can be speciated; eggs of some tapeworms can be speciated, but not the Taenia spp. eggs. Anal swabs (cellophane tape preparations such as used for pinworm diagnosis) are often helpful due to squeezing of segments (causing release of eggs) on the outside of the stool as it passes through the anus.
The Platyhelminthes (Flatworms): Tapeworms Hymenolepis nana The Dwarf Tapeworm Frequency - most common tapeworm infection in humans. Requires no intermediate host - can use insects such as fleas or beetles, but these are not required. Definitive hosts - man, rodents. Autoinfection - infected humans can ingest eggs from their own feces. Mode of Infection - Ingestion of eggs or insects containing larvae. Diagnosis - most often by finding eggs in a feces specimen.
The Platyhelminthes (Flatworms): Tapeworms Hymenolepis nana - The Dwarf Tapeworm Morphology - small, 2 to 4 cm in length. Scolex - 4 sucking disks & short rostellum with hooks. Proglottid - are broader than long; rarely seen in feces specimens (usually disintegrate in intestine). Eggs - most often seen stage in specimens, measuring 45 to 50 microns in diameter and exhibiting polar filaments lying between the egg shell and the hexacanth embryo.
The Platyhelminthes (Flatworms): Tapeworms Hymenolepsis diminuta The Rat Tapeworm Must be able to differentiate from H. nana. A common parasite of rats, mice and other rodents. Mode of infection - ingestion of intermediate host containing larval stage(s). Grain weevils are the most important intermediate host (other intermediate hosts include earwigs, larval fleas, various beetles). Infection is often due to ingesting bugs in precooked cereals.
The Platyhelminthes (Flatworms): Tapeworms Hymenolepsis diminuta The Rat Tapeworm Morphology - considerably larger than H. nana (adult as well as egg). Size - adults measure 20 to 60 cm in length. Scolex - exhibits no hooks; is larger than that of H. nana. Eggs - measure 60 to 80 microns in diameter and do not exhibit polar filaments.
The Platyhelminthes (Flatworms): Tapeworms Taenia spp. - two species that infect humans. Life cycle: Infective eggs (from human feces) are ingested by the intermediate host (cow or pig). The onchosphere hatches and penetrates the intestinal mucosa of the intermediate host. It is delivered to various parts of body via the circulatory or lymphatic systems. Most localize & encyst in muscle. Infective stage - the encysted larva, called a cysticercus develops within 2 months. Human infections take place when uncooked or undercooked meat containing larvae is ingested.
The Platyhelminthes (Flatworms): Tapeworms Taenia spp. (continued) General: Scolex - attaches to human intestine. Larva - cysticercus, a fluid filled bladder with an invaginated scolex. Eggs of both species are identical. Cysticercosis results when humans become the intermediate host. More serious than infections with the adult stage alone. This condition only occurs with the pork tapeworm, Taenia solium.
The Platyhelminthes (Flatworms): Tapeworms Taenia saginata The Beef Tapeworm General: Man is the only definitive host. Infection - ingestion of cysticercus in flesh of cow. Size - up to 10 meters in length (although 70' worms have been reported). life expectancy - 25 years or more.
The Platyhelminthes (Flatworms): Tapeworms Taenia saginata The Beef Tapeworm Morphology: Scolex - unarmed (no hooks); 4 sucking disks. Gravid proglottids - one inch or slightly longer; uterus contains 15 to 20 primary branches. The uterine pore is located in a lateral position. Eggs are identical to T. solium, measuring about 40 microns in diameter.
The Platyhelminthes (Flatworms): Tapeworms Taenia solium The Pork Tapeworm General: Man is the only intermediate host. Infection - ingestion of cysticercus in flesh of swine. Size - up to 7 meters in length. Life expectancy - 25 years or more. Autoinfection called cysticercosis, due to ingestion of eggs from feces if infected with adult worm. Racemose form of infection may develop in the brain. A Racemose is a larva which is branching, spreading throughout tissue.
The Platyhelminthes (Flatworms): Tapeworms Taenia solium - The Pork Tapeworm Morphology: Scolex - armed, with 4 sucking disks. Gravid proglottid has 7-13 primary uterine branches. Eggs are identical to T. saginata, measuring about 40 microns in diameter.
The Platyhelminthes (Flatworms): Tapeworms Diphyllobothrium latum The Broad Fish Tapeworm General: Two intermediate hosts are required, copepods (water flea) and fish. Several fish-eating mammals serve as definitive hosts: bears, cats, dogs and humans. The infective stage for mammals is the plerocercoid larva, which is ingested in raw or undercooked fish. Larvae attach to the mucosa lining of the small intestine and develop to adults. Diagnostic stage - eggs or proglottids in feces.
The Platyhelminthes (Flatworms): Tapeworms Diphyllobothrium latum - The Broad Fish Tapeworm Morphology: Adult worms are up to 15 meters in length. Scolex - almond shaped with a pair of suctorial grooves, known as bothria. Proglottids rosette shaped uterus; the uterine pore is centrally located, not lateral as with Taenia spp. Eggs - measure 30 x 60 microns, and exhibit an operculum (this is the only cestode with an operculated egg).
The Platyhelminthes (Flatworms): Tapeworms Diphyllobothrium latum - The Broad Fish Tapeworm Major pathology: Intestinal obstruction. Many people exhibit vague diagnostic symptoms such as weight loss and weakness. May cause a macrocytic anemia and eventual nervous system disturbances due to vitamin B12 deficiency. The worm has affinity for vitamin B-12, and can selectively absorb it.
The Platyhelminthes (Flatworms): Tapeworms Echinococcus granulosis The Hydatid Tapeworm General: Adults are only in canines. Eggs are shed in the feces of infected animals. Humans accidentally ingest eggs from close contact with an infected animal or from canine feces. Hexacanth embryo penetrates the intestinal mucosa and migrates to tissues. The hexacanth develops into the larval stage (hydatid cyst) in the tissue (liver, lung or brain, most often). Viscera containing hydatid cysts are eaten by canines. Adult worms develop.
The Platyhelminthes (Flatworms): Tapeworms Echinococcus granulosis - The Hydatid Tapeworm Morphology: Adult worm - Small, consists of only 3 segments. Hydatid cyst - a thin walled larva containing many thousands of invaginated scoleces.
The Platyhelminthes (Flatworms): Tapeworms Echinococcus granulosis - The Hydatid Tapeworm Major pathology - hydatid cysts in the brain. Can grow to size of a coconut and contain as much as two liters of fluid. Pressure necrosis of tissue can result from larger cysts. Most commonly found in liver, but can develop anywhere, including in bone.
The Platyhelminthes (Flatworms): Tapeworms Dipylidium caninum - The Double-Pored Tapeworm General: Rare in humans, occurring primarily in infants and small children closely associated with pets. Eggs are ingested by fleas (dog or cat fleas are intermediate hosts); larvae develop within fleas. The definitive host (dog, cat, human) is infected by ingestion of larvae within fleas. The worm develops in the small intestine of the definitive host. Diagnosis - Usually based on identification of gravid proglottids since free eggs not usually found.
The Platyhelminthes (Flatworms): Tapeworms Dipylidium caninum - The Double-Pored Tapeworm Morphology: Adult - the adult is up to 70 centimeters in length. Scolex - armed rostellum;4 sucking disks. Gravid proglottids - are the size of rice grains, and exhibit a genital pore on each side. Eggs - contained within packets (up to 12 eggs/packet).
Common Tapeworm Infections LAB. CLINICAL PICTURE DIAGNOSIS TRANSMISSION OF INFECTION LOCATION OF ADULT IN HUMANS LOCATION OF LARVA IN HUMANS DISEASE Taenia saginata eggs or proglottids in stools ingestion of larva in undercooked beef Small Intestine not present taeniasis vague digestive disturbance s vague digestive disturbance s depending on locality: from none to epilepsy Taenia solium- eggs or proglottids in stools ingestion of larva in undercooked pork Small Intestine not present taeniasis ADULT Taenia solium- LARVA (cysticercus cellulosae) X - ray,CT,MR I Serology not present (except in autoinfection: ,small intestine) sub- cutaneous muscles brain,eyes ingestion of egg Cysticercosis Hymenolepis nana eggs in stools ingestion of egg Small Intestine Intestinal Villi hymenolepiais Enteritis diarrhoea X-ray,CT,US Serology Hydatid sand Echinochoccus granulosus ingestion of egg not present Liver, lungs, Bones etc hydatid disease depending on locality
