Taenia saginata (Pathogen – Intestinal Cestode)
Taenia saginata (Pathogen – Intestinal Cestode)
T. saginata was apparently differentiated from T. solium in the late 1700s; however, cattle were not identified as the intermediate host until 1863. This infection has a worldwide distribution and is generally much more common than T. solium infection, particularly in the United States. The overall impact on human health is much less than that seen with T. solium, since cysticercosis with T. saginata is apparently quite rare.
|Taenia saginata gravid proglottid||T. saginata scolex||Taenia spp. egg|
The life cycle is very similar to that of T. solium. Infection with the adult worm is initiated by the ingestion of raw or poorly cooked beef containing encysted T. saginata larvae. As with T. solium, the larva is digested out of the meat in the stomach, and the tapeworm evaginates in the upper small intestine and attaches to the intestinal mucosa, where the adult worm matures within 5 to 12 weeks. The adult worm can reach a length of 25 m but often measures only about half this length. The scolex is “unarmed” and has four suckers with no hooks. The proglottids usually number 1,000 to 2,000, with the mature proglottids being broader than long and the gravid proglottids being narrower and longer. Although a single worm is usually found, there can be multiple worms present (personal observation).
Infection in humans is acquired through ingestion of cysticerci within raw or rare beef. Other animals found to harbor cysticerci include buffalo, giraffe, llama, and possibly reindeer. Actual cases of human cysticercosis with T. saginata are rare in the literature, and there is speculation that some reported cases have been inaccurately diagnosed.
Worldwide, primarily human to human transmission
Few symptoms are associated with the presence of the adult worm in the intestine. Although rare symptoms (obstruction, diarrhea, hunger pains, weight loss, or appendicitis) have been reported, the most common complaint is the discomfort and embarrassment caused by the proglottids crawling from the anus. This occurrence may be the first clue that the patient has a tapeworm infection. Occasionally, the proglottids are also seen on the surface of the stool after it is passed.
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.
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 no hooks. 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. saginata is quadrate shaped, has four suckers, and no rostellum or hooklets. The gravid proglottids can be found in feces and are longer than wide (19 x 17 mm) with 15-20 lateral branches on each side of the central uterine stem. They usually appear singly and can actively crawl like an “inchworm” – they may actually migrate under a fresh stool specimen.
Proficiency Testing: Note: In Proficiency Testing specimens, the 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.
Cattle should not be allowed to graze on ground contaminated by human sewage.