Taenia solium (Pathogen – Intestinal Cestode)
T. solium infection may have been recognized since biblical times, with the life cycle being delineated in the mid‑1850s. The tapeworm is found in many parts of the world and is considered an important human parasite where raw or poorly cooked pork is eaten.
|Taenia solium gravid proglottid||T. solium scolex||Taenia spp. egg|
|Cysticercus in arm||Cysticercus in brain||Cysticerci in brain|
The life cycle is very similar to that of T. saginata. Infection with the adult worm is initiated by the ingestion of raw or poorly cooked pork containing encysted T. solium larvae. The larva is digested out of the meat in the stomach, and the tapeworm head evaginates in the upper small intestine, attaches to the intestinal mucosa, and grows to the adult worm within 5 to 12 weeks. Although usually a single worm is present, there may be multiple worms in the intestine. The adult worm reaches a length of 2 to 7 m and may survive 25 years or more. On ingestion by hogs or humans, the eggs hatch in the duodenum or jejunum after exposure to gastric juice in the stomach. The released oncospheres penetrate the intestinal wall, are carried via the mesenteric venules throughout the body, and are filtered out in the subcutaneous and intramuscular tissues, the eye, the brain, and other body sites.
Infection in humans is acquired through ingestion of cysticerci within raw or rare pork. Cysticercosis is acquired from the accidental ingestion of infective eggs from proglottids of T. solium.
Worldwide, primarily human to human transmission
Few symptoms are associated with the presence of the adult worm in the intestine. The presence of cysticerci in the brain represents the most frequent parasitic infection of the human nervous system and the most common cause of adult‑onset epilepsy throughout the world. In Latin America it is unusual for both brain and muscle cysticercosis to occur in the same patient, with fewer than 6% of patients having cysticerci in both sites. However, elsewhere, subcutaneous involvement by T. solium cysticerci has been reported in as many as 78.5% of patients with cerebral cysticercosis. Possible reasons for such differences may include (i) the immune status of the patient, (ii) the human leukocyte antigen type, (iii) the nutritional status of the patient, (iv) the burden of eggs infecting the patient, and (v) a difference in the strains of T. solium.
Stool: The standard O&P examination is the recommended procedure for recovery and identification of Taenia spp.eggs in stool specimens, primarily from the wet preparation examination of the concentration sediment. 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.
Adult worms: Gravid proglottids may be recovered in stool; often they can be seen lying on the top or bottom of the stool specimen submitted as a fresh specimen. Occasionally, only the proglottid may be submitted fresh or in preservative. In the case of T. solium, two or three proglottids may be seen attached to each other, while in infections with T. saginata, the proglottids are passed singly.
Stool: The standard O&P examination is the recommended procedure for recovery and identification of Taenia spp.eggs in stool specimens, primarily from the wet preparation examination of the concentration sediment. The eggs are most easily seen on a direct wet smear or a wet preparation of the concentration sediment.
Adult worms: Identification to the species level is normally based on the recovery and examination of gravid proglottids, in which the main lateral branches are counted (count on one side only; 15 to 20 for T. saginata and 7 to 13 for T. solium). Often the gravid proglottids of T. saginata are somewhat larger than those of T. solium; however, this difference may be minimal or impossible to detect. If the scolex is recovered after therapy (which may require purgation), there will be four suckers and a rounded rostellum with a double set of hooklets (see image above). Preliminary examination of the gravid proglottid may not allow identification without clearing or injection of the uterine branches with India ink.
Note Since there is always the possible danger that the proglottid is T. solium, with the inherent problem of egg ingestion and cysticercosis, all specimens should be handled with extreme caution.
Egg: The eggs are usually spheroidal and are yellow-brown in color. They are thick-shelled eggs, measuring 31-43 µm; they will contain a six-hooked oncosphere (embryo). The eggs are routinely found in the stool, even if gravid proglottids are not found in the specimen.
Adult worm: The adult tapeworm is comprised of the attachment organ (scolex), to which is attached a chain of segments or proglottids called the strobila. Each proglottid contains a male and female reproductive system. The proglottids are classified as immature, mature, or gravid. The gravid proglottids are found at the end of the strobila and contain the fully developed uterus full of eggs. The branched uterine structure in the gravid proglottids is often used as the main criterion for identification of the organism to the species level. The scolex and eggs can also be used to identify a cestode to the species level. Often the adult worms can reach about 15 – 20 ft in length and may survive for up to 25 years. The scolex of T. solium is quadrate shaped, has four suckers, and a rounded rostellum with a double set of hooklets (see image above). The gravid proglottids can be found in feces and are longer than wide (19 x 17 mm) with 7-13 lateral branches on each side of the central uterine stem. They may pass as several gravid proglottids attached to each other.
Proficiency Testing: Note: In Proficiency Testing specimens, Taenia spp. eggs may be harvested from gravid or mature proglottids. In some cases the eggs may contain a gelatinous coating that may be somewhat confusing. This coating is generally found on eggs that have been recovered from the mature proglottids and not from the gravid proglottids at the end of the strobila. Don’t let this extra “coating” confuse you; the six-hooked embryo can still be seen within the egg shell. The true tapeworm egg will have a radially striated shell that is relatively thick.
Garcia, L.S. 2007. Diagnostic Medical Parasitology, 5th ed., ASM Press, Washington, D.C.
Pigs should not be allowed to graze on ground contaminated by human sewage.