Parasitology

TAENIASIS (caused by Taenia species)

Written by MicroDok

Taeniasis is a parasitic infection of the small intestines of humans, and it is caused by a range of parasites generally known as tapeworms. The three groups of tapeworms known to cause infections in human beings include: Taenia species, and tapeworms in the genus Hymenolepidae and Diphyllobothrium. Parasitic diseases caused by tapeworms are often prevalent in regions where cattle and pigs are both reared and eaten raw or undercooked. Taenia infection is widely distributed; and poor hygiene and poor sewage management contributes to the spread and distribution of the disease. However, Taenia saginata is more widely distributed than Taenia solium which occurs mainly in places where human feacal matter reach pigs that eventually eats it and transfers the parasite to human hosts especially when pork meat is undercooked or eaten raw. Only tapeworm infections caused by T. saginata and T. solium shall be expanded here.

Taeniasis is specifically caused by T. solium (pork tapeworm), T. saginata (beef tapeworm), D. latum (broad fish tapeworm) and Hymenolepsis nana (rodent and human tapeworm). H. nana is known as dwarf tapeworm because it is the smallest species of tapeworm that cause infection in humans. Echinococcus granulosus (dog tapeworm that is only found in the intestines of carnivores including dogs) and E. multilocularis (intestinal tapeworms of foxes) are other examples of intestinal and tissue tapeworms.

Type and morphology of tapeworms

Tapeworms are generally known as cestodes, and they are in the class of parasites known as Cestoda. They are usually segmented in form and band-like. They are phenomenal in their reproductive structures. Thus, tapeworms are hermaphrodites (i.e. they do not have separate sexes). Tapeworms are intestinal cestodes because they predominantly dwell in the intestine of their human host. Morphologically, adult tapeworms consist of three different parts including the scolex (head), proglottids/strobila (segments), and the neck (which is usually unsegmented). The scolex contains hooks (lacking in T. saginata but present in T. solium) and four muscular sucking disks (known as suckers) that enable the parasite to attach firmly to the intestines of its human host. The male and female reproductive organs of tapeworms are borne on the proglottids. Tapeworms lack respiratory and vascular systems, and they also lack body cavity and a functional alimentary canal. All tapeworms are distinguished based on the morphological differences that exist in their scolex, neck and segments. Specifically, the strobila of T. saginata is longer than that of T. solium.

Vector, reservoir and habitat of tapeworms

Tapeworms have no insect vectors as is the case for other parasitic diseases. Human beings are the natural definitive hosts of T. saginata and T. solium. Tapeworms predominantly dwells in the intestine of their human hosts for up to 20 years and above, and their larval forms (known as cysticerci) can contaminate the food or water of the parasites intermediate host (cattle or pig) including that of humans from which they go on to infect human hosts. Cysticerci (singular: cysticercus) which can also be called bladderworm is very infectious to humans, and they are predominantly found in the muscles of their intermediate hosts. Metacestodes are specifically the larval forms of T. solium tapeworms. However, both cysticerci and metacestodes can sometimes be used synonymously to describe the larval stages of tapeworms. Both cysticerci and metacestodes cause even more dangerous pathological damage in the human hosts than the intestinal cestodes (the adult worms). While the adult tapeworms can only be found in the small intestine of humans, the larval forms of tapeworms (including cysticerci and metacestodes) can develop in the tissues of various intermediate hosts that may be invertebrates (insects) or vertebrates (pigs and cow).

Clinical signs and symptoms of taeniasis

Clinically, T. saginata infection is asymptomatic in infected patients, and symptoms may include abdominal pains, diarrhea, vomiting, appetite loss or gain, and loss of weight. Many human infections with beef tapeworm may go clinically undetected, and patients only become aware of T. saginata infection following the natural passage of proglottids from the patient’s anus or the presence of segments in the stool. Proglottids of T. saginata may occasionally block the duct of the appendix, and this may subject the patient to an appendectomy. Though symptoms of pork tapeworm infection may be comparable to those of beef tapeworm infection, cysticercosis however, is the most significant clinical symptom of T. solium infection in humans. T. solium occasionally cause a series of neurological damage known as neurocysticercosis (epilepsy) in infected patients. Aside the muscles and the nervous system, T. solium may also affect the eyes of infected patients including other vital organs such as the heart and liver. Fever, muscular pains and other inflammatory reactions may ensue in patients following the death of T. solium metacestodes. It is noteworthy that the metacestodes of T. solium are notorious in colonizing various vital organs and tissues of human beings following the invasion of the parasite into the body.

Pathogenesis of taeniasis

Tapeworms (including T. solium and T. saginata) are ingested by humans by eating undercooked or raw beef (for T. saginata) or pork meet (for T. solium) containing any of the larval stages of the worms (cysticerci and metacestodes). Infection with pork tapeworm can also occur through the ingestion of viable eggs of T. solium in water, contaminated fingers or food infected with the parasites larva (Figure 1). After ingestion, the larval forms of the parasite (with an invaginated or inward turning head) eviginates (turns outwards) in the small intestine of the human host, and then attaches firmly to the intestinal walls using the hooks on its head. The growth of the tapeworm then proceeds from the neck to the proglottids (which is usually very long and contains thousands of eggs). At maturity, the proglottids (segments) of tapeworms detaches and start releasing infective eggs into the humans stool that may eventually be ingested by cow, swine or other intermediate hosts through water or food. Adult tapeworms are again produced when humans ingest viable tapeworm larvae from undercooked or raw meat. Neurocysticercosis and cysticercosis are often the most severe forms of T. solium infections in humans.

Cysticercosis is a clinical condition that occurs during tapeworm invasion of the human gut when the hatched eggs of the worm releases tapeworm embryo (known as hexacanth or oncosphere) that eventually migrates to the brain, eye and other vital tissues of the body from the gut. However, cysticercosis (whose clinical manifestation depends greatly on the site or location of the parasite in the affected host body) is more common with T. solium infection since T. saginata rarely causes any significant clinical disease in humans. For example, ocular cysticercosis is cysticercosis of the eye (i.e. when the tapeworm invades the eye of the affected patient). When the tapeworm affects the central nervous system (CNS), it is called neurocysticercosis. Thus, pork tapeworm (T. solium) is the most important parasitic tapeworm in human hosts since it is both responsible for cysticercosis in pigs and humans. Hexacanths are the well-developed six-hooked embryo of the eggs of all Taenia species, and they are made of a hard shell that protects the embryo of the tapeworms. Mature eggs of T. saginata and T. solium are morphologically similar.

Figure 1: Life cycle of Taenia species. 1. Eggs or gravid proglottids are passed with feces into the environment. The eggs of Taenia species can survive for days to months in the environment. 2. Cattle (T. saginata) and pigs (T. solium) become infected by ingesting vegetation contaminated with eggs or gravid proglottids of the parasite. 3. In the animal’s intestine, the oncospheres hatch, and 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. Humans become infected by ingesting raw or undercooked infected meat harbouring the cysticercus. 5. 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. 6. Adult worms reside in the small intestine. 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).  CDC

Laboratory diagnosis of taeniasis

The diagnosis of taeniasis is usually straight-forward and this involves the detection and identification of the parasites egg and proglottids (segments) in the feaces of infected patients. Fresh feacal samples are collected from patients and examined microscopically. Concentration techniques can also be used to demonstrate the eggs of Taenia species in feacal samples. A careful examination and understanding of the patients medical history in the diagnosis of taeniasis is important as this will help to ascertain whether or not the patient was exposed to any of the source of acquiring the parasites including eating raw or undercooked beef or pork meat. PCR analysis and other serological techniques can also be used to detect specific antigens and antibodies of Taenia species in cases when the eggs and proglottids of the parasite cannot be detected in feacal samples of patients. Magnetic resonance imaging (MRI) and computed tomography (CT scan) can be employed in detecting brain cysticercosis (i.e. neurocysticercosis).

Treatment of taeniasis

Antihelminthics including praziquantel, albendazole, paramomycin, and mebendazole are often the drugs of choice for the treatment of all forms of taeniasis. However, praziquantel has been reported to be the most effective therapy of them all. Praziquantel is teratogenic and has embryotoxic effects. Thus, it should not be used during pregnancy or administered to children under the age of two years old. Neurocysticercosis can be treated with a concomitant administration of corticosteroids and praziquantel.

Control and prevention of taeniasis

Taeniasis occurs when people eat undercooked or raw meat (including pork and beef). Drinking water and eating foods containing eggs of the parasite can also facilitate the transmission of the protozoa responsible for the disease. Thus, strict meat inspection in abattoirs and other slaughter houses including the practice of good food, water, and personal hygiene is critical in the control and prevention of tapeworm infection across the world. Untreated human or animal feaces should not be used as fertilizers in farming practices as the crops from such farms (e.g. vegetables) can serve as route via which the parasites eggs enters the body. Beef and pork meat should not be eaten raw or undercooked, but should be properly cooked before consumption. Breaking the lifecycle of the parasite by reducing human contacts with the parasites intermediate hosts (cattle’s and pigs) and also ensuring adequate public enlightenment so that people will take appropriate measures in preventing any possible infection is critical. Improvements in sanitation and water supply services in most developed countries have helped in containing the disease in these regions. However, taeniasis still persists in most underdeveloped and endemic regions where sanitation and water supply is still poor. Pigs and cattle’s should be kept away from feeding on human feaces, and sanitary facilities such as toilets should be provided in endemic regions as a way of containing the eggs and proglottids of Taenia species.

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