Cestodes (tapeworms) Pro. Dr. Mohammed Sabri Characters (general):- Tape worms consist of a round head, called a scolex, and a flat body of multiple segments called proglottides. The scolex has specialized means of attaching to the intestinal wall, namely suckers, hooks, or sucking grooves. The worms grows by adding new proglottids from its germinal center next to the scolex. The oldest proglottids at the distal end are gravid and produce many eggs, which are excreted in the feces and transmitted to various intermediate hosts such as cattle, pigs and fish. Human usually acquire the infection when undercooked flesh of the animals containing the larvae is ingested. In certain instances, e.g. cysticercosis and hydatid disease, the eggs are ingested and the resulting the larvae cause the disease. There are 4 medically important cestodes:- Taenia solium, Taenia saginata, Diphyllobothrium latum, and Echinococcus granulosus. Two cestodes of lesser importance are Echinococcus multilocularis and Hymenolepis nana. Adult tape worms:- The mature tape worms live attached to the mucosa of the small intestine. All adults tape worms consist of the following parts:- a- Scolex or head:- which is the organ of attachment, and serves to orient rather than to support the rest of the worm. b- Neck:- immediately behind the scolex, the region of the growth and proliferation from which the more distal portion of the worm is derived. c- Segments or proglottids:- beginning with immature worms, followed by the mature with fully developed reproductive organs, and last by gravid ones that contain mature eggs. The entire series of proglottids and neck is a strobili. The no. of proglottids varies: example 3-4 proglottids in Echinococcus, 1000 or more Taenia saginata (beef tape worm), 3000 or 4000 in Diphyllobothrium latum (fish tape worm). All parts are mobile, and the strobili may form several loops in the intestine. The worm is typically flattened dorsoventrally, creamy to chalky white in color.
Intestinal cestodes Taenia solium (pork tape worm):- 1- Caused taeniasis solium. 2- The larval stage (infective stage) of the pork tapeworm is cysticercus cellulosae 3- Pork tapeworm infection occurs wherever raw or lightly cooked pork is eaten. 4- In most respects taenia solium resembles T. saginata, but its shorter, usually having a length of less than 3 meters due to a smaller number of proglottids (fewer than 1000) and smaller gravid proglottids. Anterior to the 4 sucker on the scolex is a restellum with a double circle of alternating large and small hooks. The mature proglottid of T. solium closely resembles that of T. saginata but is usually readily differentiated because it contains approximately one half the number (usually 9 or 10) of main lateral uterine arms on each side of the longitudinal uterine stem. Eggs are indistinguishable from those of T. saginata. Gravid proglottids actively migrate from the anus or are passed in the feces. Eggs are discharged by migrating proglottids or are freed when they disintegrated on the ground. To develop, the eggs must be ingested by a pig or by man himself. The hexacant embryos hatch in the duodenum, migrate through the intestinal wall, and reach the blood and lymphatic channels, which carry them to skeletal muscle and myocardium. Here the embryos transform (in 2 to 3 months) into cysticerci (cysticercus cellulosae). When people eat pork containing cysticerci, the larvae are digested out from the meat of the pork and become adult in small intestine. Human can serve as intermediate host for T. solium. Clinical disease:- The presence of the adult worm usually causes no problem other than slight irritation at the site of attachment or vague abdominal symptoms (hunger pains, indigestion, diarrhea, and or constipation). There may be low-grade eosinophilia, usually under 15%. Cysticercosis: The larval forms of T. solium have been called cysticercus cellulosae. These cysticerci have been recovered from all areas of the body, and symptoms depend on the particular body site involved. Cysticerci in the brain represents the most frequent parasitic infection of the human nervous system. Neurocysticercosis usually affects males and females of all ages, with a peak incidence between the 30 & 50 years of age.
In most endemic areas, its reported that the CNS is the most frequent location for cysticerci to develop, followed by the muscle and subcutaneous tissues, the eye, lungs, heart, liver and other visceral location. If there are many visible or palpable cysticerci in the subcutaneous tissues or superficial muscles, one or more are likely to have matured in the brain and other internal organs. Cysticerci develop in the subcutaneous and muscle tissues cause essentially no pain. CNS symptoms resemble those caused cerebral tumors, meningitis, and encephalitis, and include intracranial hypertension, and behavioral changes. Neurocysticercosis usually develops years after infection, when the brain cyst die and induce an intense inflammatory reaction. Briefly:- cysticercosis is a more serious condition it results from dissemination of T. solium larvae, which encyst in the muscle and other tissues. The most common types of cysticercosis:- - Cerebral cysticercosis - Subcutaneous cysticercosis - Ocular cysticercosis Lab diagnosis:- 1- The clinical diagnosis is supported by the detection of gravid proglottids in the feces and is confirmed by finding eggs (round, approm. 4µm diameter and containing an onchosphere with hook) in the feces. 2- Radiograph and serology, and sometime a biopsy to diagnose cysticercosis (CT scan and MRI to see the larva). CSF if its done, eosinophils can be seen but not the cyst). Treatment:- a- Drug of choice praziquantal for treating taenia inf. Adult. b- In patients with cysticercosis, higher doses of praziquantal are recommended and also albendazole. c- Niclosamide is also highly effective. d- Killed larvae may evoke inflammatory reactions, so dexamethazome is recommended. Notes:- 1- Cysticerci in the eye can be found in various area, including in the eyelid and under the conjunctiva (inflammation of eye and may effect on vision are common). 2- Extraintestinal form of the infection is far more serious than the presence of the adult worm in the intestine.
Taenia saginata (beef tapeworm) (taeniasis saginata; beef tapeworm infection) 1- Taenia saginata causes taeniasis. T. saginata larvae donot cause cysticercosis. 2- T. saginata has a scolex with 4 suckers but, in contrast to T. solium, no hooklets. Its gravid proglottids have 15-25 primary uterine branches. In contrast to T. solium proglottids, which have 5-10. The eggs are morphologically indistinguishable from these of T. solium. 3- T. saginata is called the beef tapeworm because cattle are the intermediate host. 4- The adult worm typically develops in the middle third of the small intestine (the average length 5meters). The adult has 1000 to 2000 proglottids. The terminal proglottids become separated from the strobili and actively migrate out of the bowel or are evacuated in the stool with only partial loss of eggs. Cattle grazing on infected ground pick up the eggs, which hatch in the duodenum. The emerging embryos penetrate into the mesenteric venules or lymphatics and reach skeletal muscles or the heart, where in about 2 months they transform into typical cysticercus stage (cysticercus bovis), which measures roughly 5 by 10 mm and has a head like that of the adult worm, invaginated into the fluid-filled bladder. Thereafter, for a period of more than a year, a person who eats the raw infected beef is subject to infection. Pathogenesis: Infection with T. saginata is ordinarily asymptomatic except for the discomfort, embarrassement resulting from the gravid proglottids crawling out of the anus. In this respect, the infection resembles that of Enterobius vermicularis. However, toward the end of the prepatent period, diarrhea and abdominal cramps may occur. Intestinal obstruction is rarely occurred bythis worm. Diagnosis: Since the eggs of T. saginata and T. solium look identical, identification to the species level is normally based on the recovery and examination of gravid proglottids (often the gravid proglottids of T. saginata are somewhat larger than those of T. solium). Diagnosis is more likely to be made by identifying proglottids or eggs from the anus than by examination of feces.
Taeniasis which is caused by T. saginata occurs worldwide but is endemic in Africa, South America and Eastern Europe. Treatment: The use of praziquantel or niclosamide has been recommended.
Diphyllobothrium latum (Broad fish tapewom) 1- This worm causes diphyllobothriasis. 2- The fully developed strobili of D. latum is up to 10 meters or more in length and has up to 4000 proglottids. Spatulated scolex measures about 2.5 mm in length by 1mm in breadth and is provided with a median ventral and a median dorsal grooved sucker. 3- The eggs differ from the cestodes, that they are not comprised of the typical hexacanth embryo. Rather, the eggs consists of a coracidium surrounded by a smooth yellow to brown shell. The typical size range is from 55 to 75 µm by 40 to 55 µm with an average of 65 µm by 48 µm. An operculum consumes one end of the eggs. A small distinct terminal knob (also known as an abopercular knob) extends from the opposite end of the eggs. The eggs within fully developed proglottid are continuously discharged through the uterine proglottid. The eggs are unembryonated when passed in the host's feces. Embryonation of eggs that reach cool fresh water requires 11-15 days. Then the ciliated oncospher, coracidium, escapes through the opened operculum and swims about in the water. In order to proceed with development, it must be eaten by an appropriate copepod. Once ingested by a copepod, the embryo burrows into the hemocoel and transform into procercoid larva. If the infected copepod is then eaten by a fresh water fish, the larva migratesinto the flesh or connective tissue and transform into a plerocercoid larva (sparganum). When the infected fish is eaten raw by definitive host, the worm develops to maturity and begins laying eggs in about 4 weeks. Clinical features: 1- Asymptomatic: it's estimated that the majority of infected persons harboring adult D. latum worms exhibit no clinical symptoms. 2- Fish tape worm infection (Diphyllobothriasis): person infected with D. latum often experience symptoms consistent with digestive discomfort. Overall weakness, weight loss and abdominal pain may also develop. When the adult D. latum worm takes up residence in and attaches itself to the proximal part of the jejunum, the patient is at risk for developing vitamin B 12 deficiency.
When the procercoid larva is ingested by man, sparganum will develop in anywhere in the body and the disease sparanosis will occur. The most common type is subcutaneous sparganosis where the large migrates under the skin. It forms nodules which are itchy, inflamed and painful. The condition can be serious if the tape worm larva migrates to the eye or brain however this is rare. Breifly, the most common type of sparganosis is subcutaneous followed by cerebral and ocular (rare cases). Notes: 1- Humans have been known to suffer from sparganosis. Persons become infected by ingesting the procercoid larva of D. latum, as well as that of other Diphyllobothrium species or spirometra species. There are two primary route by which human may contract the parasite: a. Water contaminated with infected copepods. b. Medicines contaminated with infected animal byproducts. The formation of sparganum typically results. Surgical removal of the sparganum is the treatment of choice. Praziquantel has also proven effective against sparganum. 2- Infective stage of human by D. latum is pleurocercoid (sparganum). Laboratory diagnosis: Stool specimens should be examined to detect eggs or less frequently the proglottids. Treatment: Niclosamide or praziquantel is the drug of choice.
Dipylidium caninum (Dog / cat tape worm- pumpkin seedtape worm) D. caninum is commonly found throughout the world in dogs and cats. Human accidently infected with this worm and most reported infection have been in children. 1- The average D. caninum egg may range 30 to 60 µm in diameter, it consists of the typical six-hooked oncosphere. Unlike the eggs of the other cestodes, which appear individually, those of D. caninum form membrane-enclosed packets. Each packets may contain 5-30 eggs. 2- The D. caninum scolex is equipped with 4 suckers and armed with a rostellum that resemble a club in shape. D. caninum scolex contains from 1-6(7) circlets of spines that reside on rostellum. 3- Mature and gravid proglottids resemble pumpkin seeds in their shape. Each mature segment contains two sets of both male and female reproductive organs. Following self-fertilization, the resulting gravid proglottids is full of eggs enclosed in an embryonic membrane. 4- The adult worms are medium-sized, measuring from 10-70 cm in length and consist of several hundred proglottids. The adult worms are found in the dog or cat intestine, and gravid proglottids separate from the strobili and may migrate singly or in short chains out of the anus. Egg packets may be found in the stool. The egg are ingested by the dog, cat or human flea, where they develop into cysticercoid larvae. Where these fleas are then ingested by the definitive host (dogs, cats, humans), the adult worm develops within 3-4weeks. Notes: 1- The fleas may be transmitted by the lick of an infected dog or cat, or by handto-mouth contamination. 2- Egg packets or gravid proglottids is the diagnostic in the feces samples. Clinical symptoms: A) The majority of infected persons experience no symptoms because of light infection. B) Dipylidiasis /or cat-dog tapeworm disease: Patients with a heavy worm burden may develop appetite loss, diarrhea, abdominal discomfort and ingestion. Such patients may also experience and pruritus caused by gravid proglottids migrating out of the anus. Treatment: Niclosamide is the drug of choice, Praziquantel is alternative.