Taenia solium and Cysticercus cellulosae

Taenia solium is a cyclophyllid cestode with adults found in the small intestine of people and the larval stages (Cysticercus cellulosae) found in muscle tissue of pigs. It occurs in people in many countries of the world especially where pork is a major source of food. Taenia solium has an indirect life-cycle and people are infected when they ingest cysticerci found in pig muscle. Prevalence tends to be higher where pork is eaten without appropriate cooking and/or where pigs can ingest material contaminated with human feces containing the T. solium eggs. If people are infected with adult worms then clinical signs are rare but may include GI disturbances. Infected pigs are generally asymptomatic as well. The major pathology associated with T. solium occurs in association with larval stages in human in human tissue. If people ingest eggs of T. solium, they can hatch in the stomach and intestine. The larvae released from the eggs then penetrate into the stomach or intestinal wall, and are distributed to a variety of organs and tissues. The nature of the clinical signs depends on the location, and to a lesser extent the age, of the cysticerci. A common site for the cysticerci is the CNS and here they can produce a range of signs, especially seizures. Many people with cysticercosis are, however, asymptomatic. In Canada, human infection is generally thought to be acquired outside the country. Cysticercus cellulosae has not been detected in pigs in Canada for many decades, and is assumed not to occur.

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 solium is one of three species of the genus Taenia, adults of which infect people; the others are Taenia saginata, the beef tapeworm, which occurs around the world where cattle 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 solium is Cysticercus cellulosae, the cause of porcine and human cysticercosis.

The larval stage of T. solium 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.

Adults of Taenia solium 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. solium is armed with two rows of 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 unarmed with rostellar hooks in Taenia saginata - the beef 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. solium 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 eggs of T. solium cannot be distinguished microscopically from those of T. saginata .

Infectivecysticerci of T. solium (Cysticercus cellulosae) in pigs 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. cellulosae is armed (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.

The only natural defintive hosts for Taenia solium are people, and almost the only natural intermediate hosts are pigs, in which the larval stage - Cysticercus cellulosae - develops.  There are published reports of C. cellulosae in other mammals, some of which are possible mis-identifications. 

The ingestion of eggs of T. solium by people, or the reflux of eggs from the small intestine into the stomach -where they hatch, can result in the development of cysticerci in the tissues, although only very rarely will the person thus infected serve as an intermediate host.  This infection of people with the larval stage of T. solium is known as human cysticercosis.

Taenia solium occurs in people in many countries of the world where pork is a source of food.  Prevalence tends to be higher where pork is eaten without appropriate cooking and/or where pigs can ingest material contaminated with human feces containing  T. solium eggs.  Human cysticercosis occurs where personal hygiene is sub-optimal, often because of limited resources and other priorities.  Other than in endemic areas, which include Central and South America, human cysticercosis - especially neurocysticercosis in which the parasite develops in the CNS - occurs in non-endemic areas, including the United States and rarely Canada.

The prevalence of Taenia solium in the human population in Canada is unknown, and C. cellulosae has not been detected in pigs in Canada for many decades. A recent review of cases of cysticercosis in people in Canada identified 60 published cases; of these fifty-five had neurocysticercosis, and in most of these seizures were the primary clinical sign. Of the 28 cases for which citizenship data were available, almost all were immigrants.

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

Adult Taenia solium 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 porcine definitive hosts.  Pigs ingest the segments and/or the eggs.  In the GI system of the pigs, 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 cysticerci.  Each cysticercus contains a single protoscolex.  People are infected when they ingest pork containing viable cysticerci.  The protoscolex is released from the cysticercus, attaches to the small intestinal mucosa and produces segments.

If people ingest eggs of T. solium, the eggs can hatch in the stomach and intestine.  The hexacanth larvae released from the eggs then penetrate into the stomach or intestinal wall, enter blood vessels or lymphatics and are distributed to a variety of organs and tissues.  The brain , eye , heart , subcutaneous tissues  and skeletal muscle are among the sites where C. cellulosae have been frequently detected in people.  In some CNS infections, the cysticercus develops unusually, forming a large cyst (up to 15 cm in length) with many branches.  This recemose form of the larval parasite (also called Cysticercus racemosus) can be more difficult to treat than the more usual cysticerci.

Life Cycle: Taenia solium

The basic transmission routes for Taenia solium and Cysticercus cellulosae are from people to pigs and to people via eggs in human stools, and from pigs to people via larval stages in infected pork.  In many areas of the world where the parasite is endemic, transmission occurs on a large scale and is essentially a result of sub-optimal attention to waste disposal, and personal and meat hygiene, which are a frequent result of inadequate resources and education.

There is a large and fascinating literature describing the ecology of T. solium in endemic areas, and an increasing number of reports of the occurrence of the parasite, and especially neurocysticercosis, in non-endemic areas, including the United States and very rarely in Canada.  The information below applies only to the latter situation.

The most recent published report of neurocysticercosis in Canada was in 1986 and described eight patients seen at McGill University.  Three patients were from South America, three from Haiti, and two from Greece.  They had lived in Canada from between 1 and 18 years.  Five of the cases had parenchymal cysts with epilepsy and three had the racemose form of cyticercus with hydrocephalus; one of these died.  All the other cases responded well to medical and/or surgical treatments.  In the 1970s there was a case of subcutaneous cycticercosis in Saskatoon in a recent immigrant from South East Asia.  Infection in this case, and in all the cases in Montreal, was assumed to have been acquired outside Canada.

In the United States, most cases of human cysticercosis are seen in immigrants from endemic countries, particularly those of Central America.  Large case series of neurocysticercosis have been reported from California, Oregon, Texas, Chicago, Houston, Los Angeles and New York.  Individual cases have been reported from several states, including Massachusetts and North and South Carolina.  While many of these patients were assumed to have acquired the parasite in endemic areas outside the United States, local (autochthonous) transmission is known to occur in New York, Oregon and  Chicago.  One intriguing report described symptomatic and asymptomatic neurocysticercosis in orthodox Jews in New York City, apparently acquired from Central American servants.

Adult T. solium in people is often asymptomatic, and any symptoms involve primarily the GI system and are non-specific.  Infection with Cysticercus cellulosae in pigs is also usually asymptomatic.

The major pathology associated with T. solium occurs in association with the larval stages in human cysticercosis.  The nature of the clinical signs depends on the location, and to a lesser extent the age, of the cysticerci.  A common site for the cysticerci is the CNS and here they can produce a range of signs, especially seizures.  Many people with cysticercosis are, however, asymptomatic.

In people, infection with Taenia solium can be detected by finding eggs in the stool.  Eggs of T.  solium cannot, however, be differentiated microscopically from those of Taenia saginata , the beef 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 os 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 coutries, both species are present, and in Asia, also T. asiatica.

In Canada, Cysticercus cellulosae has not been detected in pigs for many decades, and is assumed not to occur.  The parasite is notroutinely sought in pigs at abattoirs.  The situation is similar in the United States, where less than ten infected pigs are detected annually (among 80 million slaughtered), and the last published report of the C. cellulosae in pigs in the US was in 1978, from Colorado.

Adult Taenia solium in people is readily treated with an apppropriate anthelmintic.  Praziquantel is the current drug of choice.  Cysticercus cellulosae in pigs is not treated.

Successful control of T. solium and C. cellulosae depends on interrupting transmission from pigs to people and from people to pigs.  In Canada this is not a real concern because the parasite is assumed not to occur in pigs.  A much greater concern is the control of human cysticercosis, particularly neurocysticercosis.  This control is centred on individuals who are at particular risk of harbouring the adult parasites (which can easily be treated) and on maintaining high levels of personal hygiene among these individuals and among others who may be at particular risk of contact with the infected people.  In the United States, most cases of neurocysticercosis are associated with individuals with some contact with Central or South America, and the few cases reported from Canada reflect a similar situation.

In Canada, the major issue related to human infection with Taenia solium is the potential for cysticercosis, particularly neurocysticercosis, acquired by ingesting the T. solium eggs from human feces.  Fortunately this condition is very rare in this country. 

Del Brutto OH (2012). A review of cases of cysticercosis in Canada. Canadian Journal of Neurological Sciences 39: 319-322.