Taenia saginata and Cysticercus bovis — beef tapeworm of people

The adult cestode Taenia saginata (beef tapeworm) and its larval stage, Cysticercus bovis, occur in people and cattle, respectively, in many parts of the world where beef is part of the human diet.


Summary

The adult cestode Taenia saginata (beef tapeworm) and its larval stage, Cysticercus bovis, occur in people and cattle, respectively,  in many parts of the world where beef is part of the human diet.  The parasite occurs occasionally in Canada, and is most often detected in cattle at slaughter.  The life cycle is indirect. The human definitive host becomes infected by ingesting the cysticercus infective larval stage in skeletal muscle and rarely other tissues of infected cattle.  The cattle acquire the parasite by ingesting eggs from human faeces.  In people and cattle this parasite is rarely of any clinical significance, but in Canada all commercially slaughtered cattle are checked for infection, initially by visual inspection of prescribed cuts into muscle tissues and offal.  Even when followed by histology, this testing procedure has problems with sensitivity and specificity.  An immunohistochemical test for parasite antigen developed recently by the CFIA greatly improves the accuracy of detection of C. bovis in cattle at slaughter.  Regardless of the test method used, a positive animal is usually condemned and removed from the human food chain.  There is also trace back and quarantine of the farm of origin, and an investigation of the extent of the problem on the farm and attempts to identify the source of infection, which is often very difficult.

Taxonomy

Phylum: Platyhelminthes
Class: Cestoidea
Order: Cyclophyllidea
Family: Taeniidae

The Order Cyclophyllidea includes several families that contain virtually all tapeworms of domestic animals and birds. Adults of most genera within the family Taeniidae are parasites of carnivores and omnivores, and have many morphological and biological similarities.  Taenia saginata is one of three species of the genus Taenia infecting people; the others are Taenia solium, the pork tapeworm, which occurs around the world where pigs are a source of meat, and Taenia asiatica, which is widespread in the countries of southern Asia and is acquired from pigs, and rarely from cattle. 

The larval stage of Taenia saginata is Cysticercus bovis, the cause of bovine cysticercosis.

The larval stage of T. saginata has a  name different from the adult because when the adults and larvae were first discovered it was not realized that they were different life cycle stages of the same species of tapeworm.

Note: Our understanding of the taxonomy of helminth, arthropod, and particularly protozoan parasites is constantly evolving. The taxonomy described in wcvmlearnaboutparasites is based on that in the seventh edition of Foundations of Parasitology by Larry S Roberts and John Janovy Jr., McGraw Hill Higher Education, Boston, 2005.

Morphology

Adults of Taenia saginata infecting people are large (up to several metres in length) and have an anterior scolex (attachment or holdfast organ), behind which is the ribbon-shaped body composed of segments. Typically, anterior immature segments are smaller than the mature or gravid segments towards the posterior. At the anterior tip of the scolex is a rostellum, which in T. saginata has no hooks. Behind the rostellum are four circular, muscular suckers.

Each mature segment of Taenia species contains a single set of reproductive organs, with a lateral genital pore at about the mid-point of the segment. In mature segments, details of the reproductive structures can be seen in fixed and stained, but not fresh, specimens. In gravid segments, however, the laterally-branched uterus (with the structure of a “Christmas tree”) is usually visible, even in fresh specimens.  In the absence of the scolex (which is armed with rostellar hooks in Taenia solium - the pork tapeworm), adults of the two species of Taenia infecting people cannot be distinguished morphologically without careful microscopic examination of the reproductive organs in stained, MATURE segments. 

Eggs of T. saginata are round to oval, measure approximately 30 to 35 µm in diameter and have a thick, radially striated shell. Each egg contains a hexacanth larva with six hooks, not all of which are visible in every egg. The eggss of T. saginata cannot be distinguished microscopically from those of T. solium.

Infective cysticerci of T. saginata (Cysticercus bovis) are typically oval and measure approximately 10 mm by 6 mm.  Initially they appear semi-translucent but become more opaque as the surrounding host-origin capsule develops.  Each cysticercus contains a single protoscolex.  The protoscolex in C. bovis is unarmed (no hooks on the rostellum) and immediately behind the rostellum are four approximately circular, muscular suckers.  The scolex structures should be visible microscopically in sufficiently developed but non-degenerate specimens.  Staining, although difficult, may be helpful.

Host range and geographic distribution

The only natural defintive hosts for Taenia saginata are people, and the only natural intermediate hosts are cattle, in which the larval stage - Cysticercus bovis - develops.

Taenia saginata occurs in people many countries of the world where beef is a source of food for people.  Prevalence tends to be higher where beef is eaten without appropriate cooking and/or where cattle can ingest material contaminated with human feces containing the eggs of the parasite.

The prevalence of Taenia saginata in the human population in Canada is unknown, but there are occasional occurrences of cycticercosis in cattle in Canada, most often in animals in feedlots.

Bovine cysticercosis is a NOTIFIABLE DISEASE  in Canada under the Health of Animals Act and Regulations.

Life cycle - indirect

Adult Taenia saginata live in the small intestine of the human host.  Gravid segments and eggs are passed in the feces, and the segments can leave the host through the anus without defecation.  The eggs are immediately infective for the bovine definitive hosts.  Cattle ingest the segments and/or the eggs.  In the GI system of the cattle, the eggs hatch, each releasing a hexacanth larva.  These larvae penetrate into the wall of the intestine, enter blood vessels and lymphatics are are transported to a variety of sites in the body.  Those located in the skeletal and cardiac muscles have the best opportunity to develop to infective cycticerci.  Each cysticercus contains a single protoscolex.  People are infected when they ingest beef containing viable cysticerci.  The protoscolex is released from the cysticercus, attaches to the small intestinal mucosa and produces segments.

Life Cycle: Taenia saginata

Epidemiology

People acquire Taenia saginata by ingestion of viable, infective cysticerci in fresh beef, eaten raw or inadequately cooked.  Dishes containing raw beef (e.g., steak tartare), sometimes associated with particular cultures, increase the risk of human infection.

Cattle acquire Cysticercus bovis by ingestion of materials containing viable eggs of T. saginata from the stools of infected people.  Adult T. saginata produce large numbers of eggs that can survive in a suitable environment for at least months.  Additionally, gravid segments of T. saginata can migrate from the anus without defecation.  Cattle can access the parasite eggs following direct contamination of pens, feed bunks or water troughs ("promiscuous defecation") by farm workers and others, by indirect contamination of feed (as occurred in Oregon when cattle were fed potato waste transported in a tanker previously used for human sewage, or when possibly contaminated tomato plant waste was fed to cattle) or water, by grazing pastures previously fertilized with human sewage or sludge containing viable T. saginata eggs, or by ingesting materials contaminated by eggs from the feet or feces of birds that had visited a sewage treatment facility, or possibly a field recently fertilized with human sewage.

Although the possible transmission routes of T. saginata from people to cattle are known, in investigations of occurrences of the parasite in cattle, it is often very difficult, and sometimes impossible, to identify the actual source of the eggs.

Pathology and clinical signs

In people, adult Taenia saginata is sometimes associated with clinical signs, including abdominal pain, weight loss and nausea. 

In cattle it is generally assumed that infection with Cysticercus bovis is asymptomatic.

Diagnosis

In people, infection with Taenia saginata can be detected by finding eggs in the stool.  Eggs of T. saginata cannot, however, be differentiated microscopically from those of Taenia solium, the pork tapeworm.  DNA-based techniques do permit differentiation between the two species, but are not widely available in a diagnostic setting.  If a scolex or mature segments are recovered, then microscopic differentiation is possible.  Also, the local situation, particularly meat consumption patterns, can help to indicate which parasite is the more likely in a particular situation.  In many developing countries both species are present, and in Asia, T.asiatica also.

In many countries including Canada, legislation requires that the carcasses of cattle slaughtered at regulated abattoirs are inspected for Cysticercus bovis by incising designated muscles, including the heart.  In Canada, suspect lesions must be submitted to the CFIA for examination.  While this examination is relatively straightforward where the appropriate larval cestode elements are obvious on direct microscopy or histology, it is more complex where such elements are not obvious, for example when the larval cestode is degenerating - which most do eventually.  Recently, DNA-based and immuno-histochemical techniques have been developed to assist in the detection of C. bovis.  Serological tests are also available to aid in the diagnosis of cysticercosis in cattle, but while they can be helpful on a herd basis, problems with sensitivity and specificity inhibit their usefulness in individual animals.

In Canada, if C. bovis is suspected in an animal at slaughter, all animals from that premises are held pending trace back, quarantine and investigation by the CFIA.  This can be a lengthy process, particularly because it is often difficult or impossible to identify the sources of infection for the cattle.  It can also be very expensive for the abattoir operator and for the farmer, although compensation is available to the latter.

Treatment and control

Adult Taenia saginata in people is readily treated with an apppropriate anthelmintic.  Praziquantel is the current drug of choice.

Cysticercus bovis in cattle is not treated.

Successful control of T. saginata and C. bovis depends on interrupting transmission from cattle to people and from people to cattle.  Exactly how this is achieved will vary with the prevalence of the parasite, the linkages between people and cattle that allow trsnsmission, and the resources available.  In theory in Canada, beef from a regulated abattoir should be free of cysticercosis, bearing in mind the detection limit of current carcass inspection procedures could miss lightly infected animals.  Beef from other sources, however, may be infected.  Preventing transmission from people to cattle may be more complex, and despite the low perceived prevalence of T. saginata in the human population in Canada, obviously there is a sufficient level of infection to cause the periodic cases and outbreaks that are recognized at abattoirs in this country.  These may result from a locally restricted chain of events that results in access by the cattle to parasite eggs, for example promiscuous defecation by a individual working with the cattle.  Infection with T. saginata is often asymptomatic, and infected individuals may not seek medical advice or treatment.

Public health significance

In Canada, infection with Taenia saginata is rarely a major issue for human health, although the adult parasites are large and can produce clinical signs.  The greater significance of this parasite is within the beef industry.  Other than in their early developmental stages, the cysticerci are clearly visible to the naked eye, and even lightly infected meat is distinctly unattractive (and may give you a parasite).  Additionally, trade in beef would be adversely affected without carcass inspection and follow-up.  In countries with fewer financial resources than Canada and control measures, if present, are limited, beef tapeworm is much more common in both its hosts.    
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