The life cycle of the beef tapeworm, Taenia saginata, is very similar to that of the pork tapeworm, T. solium. Human 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 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. Although a single worm is usually found, there can be multiple worms present (personal observation).

The scolex, the attachment organism of the tapeworm, is "unarmed" and has four suckers with no hooks. The proglottids, a body segment of the tapeworm, usually number 1,000 to 2,000. Identification to the species level is usually based on the number of main lateral uterine branches, which are counted on one side of the gravid proglottid (Fig. 2). There are between 15 and 20 branches, with an average of 18. Gravid proglottids often crawl from the anus during the day (Fig. 2), when the host is most active.

The eggs cannot be distinguished from those of T. solium (Fig. 1). They are round to slightly oval, measure 31 to 43 µm, have a thick, striated shell, and contain the six-hooked embryo (oncosphere). These eggs can remain viable in the soil for days to weeks. Upon ingestion by cattle, the larvae hatch in the duodenum, penetrate the intestinal wall, and are carried via the lymphatics or bloodstream, where they are filtered out in the striated muscle. They then develop into the cyst larval form (cysticercus) within approximately 70 days. The mature cysticercus measures 7.5 to 10 mm wide by 4 to 6 mm long and contains the immature scolex, which has no hooks (unarmed; Fig. 3). Other animals found to harbor this cyst form of the tapeworm include buffalo, giraffe, llama, and possibly reindeer.

There are usually few symptoms associated with the presence of the adult worm in the intestine. Although rare symptoms such as obstruction, diarrhea, hunger pains, weight loss, and 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 may also be seen on the surface of the stool after it is passed.

Figure 1. The helminth egg is that of Taenia spp., photographed using the high, dry 40X objective. From the egg morphology, it is impossible to identify the cestode to the species level, either T. saginata (beef tapeworm) or T. solium (pork tapeworm).

Figure 2. This structure recovered from the patient's stool specimen is a T. saginata proglottid. The structure was stained with a Congo-red-based dye.

Figure 3. T. saginata scolex; note four suckers and no hooklets.

Additional resources:

1. Garcia, L. S. 2001. Diagnostic medical parasitology, 4th ed. ASM Press, Washington, D.C.
2. Garcia, L. S. 1999. Practical guide to diagnostic parasitology. ASM Press, Washington, D.C.
3. http://www.med-chem.com/ - PARA-SITE Online