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This week at infection landscapes we enter into the second group of flatworms, and the last group of helminths that we will be considering in this extended series. These are the cestodes or, as they are more commonly known, tapeworms. The tapeworm species that most commonly infect humans are Taenia solium (pork tapeworm), T. saginata (beef tapeworm), Diphyllobothrium latum (fish tapeworm), and Echinococcus granulosus (dog tapeworm). Because only T. solium causes a significant public health burden, this species will be the focus of this discussion.
The Worm. Taenia solium is the pork tapeworm.The Taenia genus is comprised of over 100 species, some of which cause human infection and disease, including T. solium and T. saginata. "Pork" and "beef" tapeworms are actually misnomers, as both T. solium and T. saginata are human tapeworms whose intermediate hosts are pigs and cows, respectively. The adult tapeworm of both species only occurs in the definitive host, which is humans for both helminths.
The Taenia genus is a member of the Taeniidae family, which is a member of the Cestoda class. Cestodes have a unique structure that is quite distinct in many ways from the trematode flatworms. The length of their body is composed of segments, called proglottids. At the anterior portion of the worm is the scolex, which the helminth uses to attach to its host's intestinal epithelium. These helminths do not have a gut tract.
Notice the individual proglottid segments in the picture above, as well as the scolex at the anterior tip, which appears as a small knob.
Let's consider the life cycle of T. solium. Embryonated eggs pass in gravid terminal proglottids from infected humans. When the gravid proglottids, or the eggs they contain, are ingested by pigs, porcine infection ensues. The pig is the intermediate host for this helminth.The eggs hatch in the small intestine of the pig, releasing oncospheres into the gut. These oncospheres are hooked larvae, which subsequently penetrate the epithelium of the intestine and migrate to various tissues, usually skeletal muscle tissue, by way of the blood circulation. These oncospheres encyst in the muscle tissue and develop into cysticerci (which is the stage infectious to humans), where they can remain viable for several years before calcifying. It is important to note that, while muscle tissue is the most common site for oncospheres to locate and encyst, they can potentially locate in many other tissues including liver and brain.Human infection occurs following consumption of undercooked pork products from pigs infected with cysticerci. Having reached their human definitive host, the encapsulated cysticerci lose their capsules in the stomach and subsequently enter the small intestine. In the small intestine, the juvenile cestode everts its scolex, which attaches to the intestinal epithelium using four suckers and a crown of small hooks known as the rostellum:
The juvenile grows into an adult as developing proglottids extend down the length of the small intestine. As described above these proglottids are the individual segments of the helminth:
Cestodes do not have a digestive tract, so T. solium transport nutrients obtained from the host's meals across the surface of the tegument. As they progress toward the terminal end of the helminth, the proglottids mature. The terminal proglottids are gravid with hundreds of embryonated eggs, which are infectious to the intermediate host (i.e. pig) and, in aberrant infections, they are also infectious to the definitive host (i.e. humans). These gravid proglottids can pass from the human host as single segments or as segment chains. Moreover, they can be expunged passively with feces or they can actively migrate out of the host:
Control and Prevention. Given that there are multiple modes of transmission of T. solium to humans, there are also multiple potential points to block transmission. Let's discuss these in the context of the two types of human infection, 1) as definitive host, and 2) as accidental intermediate host.
1) As described above, human infection with the adult tapeworm is essential to the tapeworm's life cycle since humans are the sole definitive host. Thorough cooking or freezing of pork products destroys cysticerci and prevents infection with the adult tapeworm in humans.
This intervention, while fairly simple, is the only one which can be implemented on an individual basis at the level of the household. To block other transmission pathways, larger community-wide strategies are required.
2) As described above, humans are also capable of being infected as intermediate hosts when they ingest the eggs of T. solium (or also possibly by autoinfection following infection as the definitive host). Because only humans harbor the adult tapeworm, only humans pass eggs in their feces, which means human feces are the essential source of infectious embryonated eggs. As such, good sanitation and water infrastructure are important in blocking the transmission of the eggs to both humans and pigs, and would thus have wide impact on reducing intermediate host infection in both pigs and humans.
This week at infection landscapes we enter into the second group of flatworms, and the last group of helminths that we will be considering in this extended series. These are the cestodes or, as they are more commonly known, tapeworms. The tapeworm species that most commonly infect humans are Taenia solium (pork tapeworm), T. saginata (beef tapeworm), Diphyllobothrium latum (fish tapeworm), and Echinococcus granulosus (dog tapeworm). Because only T. solium causes a significant public health burden, this species will be the focus of this discussion.
The Worm. Taenia solium is the pork tapeworm.The Taenia genus is comprised of over 100 species, some of which cause human infection and disease, including T. solium and T. saginata. "Pork" and "beef" tapeworms are actually misnomers, as both T. solium and T. saginata are human tapeworms whose intermediate hosts are pigs and cows, respectively. The adult tapeworm of both species only occurs in the definitive host, which is humans for both helminths.
The Taenia genus is a member of the Taeniidae family, which is a member of the Cestoda class. Cestodes have a unique structure that is quite distinct in many ways from the trematode flatworms. The length of their body is composed of segments, called proglottids. At the anterior portion of the worm is the scolex, which the helminth uses to attach to its host's intestinal epithelium. These helminths do not have a gut tract.
Adult Taenia solium
Notice the individual proglottid segments in the picture above, as well as the scolex at the anterior tip, which appears as a small knob.
Let's consider the life cycle of T. solium. Embryonated eggs pass in gravid terminal proglottids from infected humans. When the gravid proglottids, or the eggs they contain, are ingested by pigs, porcine infection ensues. The pig is the intermediate host for this helminth.The eggs hatch in the small intestine of the pig, releasing oncospheres into the gut. These oncospheres are hooked larvae, which subsequently penetrate the epithelium of the intestine and migrate to various tissues, usually skeletal muscle tissue, by way of the blood circulation. These oncospheres encyst in the muscle tissue and develop into cysticerci (which is the stage infectious to humans), where they can remain viable for several years before calcifying. It is important to note that, while muscle tissue is the most common site for oncospheres to locate and encyst, they can potentially locate in many other tissues including liver and brain.Human infection occurs following consumption of undercooked pork products from pigs infected with cysticerci. Having reached their human definitive host, the encapsulated cysticerci lose their capsules in the stomach and subsequently enter the small intestine. In the small intestine, the juvenile cestode everts its scolex, which attaches to the intestinal epithelium using four suckers and a crown of small hooks known as the rostellum:
The juvenile grows into an adult as developing proglottids extend down the length of the small intestine. As described above these proglottids are the individual segments of the helminth:
Cestodes do not have a digestive tract, so T. solium transport nutrients obtained from the host's meals across the surface of the tegument. As they progress toward the terminal end of the helminth, the proglottids mature. The terminal proglottids are gravid with hundreds of embryonated eggs, which are infectious to the intermediate host (i.e. pig) and, in aberrant infections, they are also infectious to the definitive host (i.e. humans). These gravid proglottids can pass from the human host as single segments or as segment chains. Moreover, they can be expunged passively with feces or they can actively migrate out of the host:
The adult tapeworm will only develop in humans, not pigs. As described, humans are the definitive hosts, for whom normal transmission of infection occurs following consumption of infectious cysticerci in infected pigs. The adult tapeworms then develop in their definitive hosts following the standard life cycle of normal infections.
However, humans can acquire aberrant infections outside the normal life cycle. While only the intermediate developmental pathway can proceed in the intermediate pig host:.
eggs → oncospheres → cysticerci
both the definitive and intermediate developmental pathways can proceed in humans:
cysticerci → juvenile → adult → eggs,
and
eggs → oncospheres → cysticerci
Humans acquire intermediate host infection by consuming water or food that has been contaminated by embryonated eggs from another infected human, or by autoinfection from the adult tapeworm already harbored in the definitive host. Ordinarily, the ingested eggs would simply pass out of the gut of the definitive host and not cause infection, as is the case when humans consume the eggs of the beef tapeworm, T. saginata. However, the oncospheres of T. solium can frequently mistake the human gut for the pig gut because of their similarity, and subsequently these larvae penetrate the gut and migrate to peripheral tissues as if the definitive host were the intermediate host. These aberrant infectious are the source of the major clinical disease and public health burden associated with pork tapeworms. The graphic below developed by the Centers for Disease Control and Prevention (CDC) nicely depicts the life cycle of T. solium:
For comparison, another graphic developed by the CDC depicts the life cycle of both T. solium and T. saginata under normal infection cycles:
The Disease. Most taeniasis is asymptomatic. In normal definitive host infections, in which the adult tapeworm inhabits the small intestine, no symptomatic disease occurs, though there is histologic evidence of minor cellular damage at the site of attachment of the scolex. Taeniasis refers specifically to infection with the adult tapeworm in the gut of the definitive host. As such, taeniasis only occurs in humans. The quantity of nutrients the adult helminth takes from the definitive host is negligible. The same is true for T. saginata (beef tape worm), but not D. latum (fish tapeworm), which can deplete the host of vitamin B12 and eventually lead to megaloblastic anemia over a period of years. So, contrary to popular belief, infection with the adult tapeworm does not make you excessively hungry or cause you to lose weight.
Cysticercosis, on the other hand, is infection with the larval stage tapeworm in peripheral tissues in the intermediate host. Pigs are the normal intermediate hosts and, therefore, the hosts normally affected by cysticercosis. However, as described above, humans can be accidental intermediate hosts if they ingest eggs deposited from another human. Cysticercosis generally presents in muscle tissue, subcutaneously, or in the brain. Subcutaneous and muscle cysticercosis may present with fever and/or pain in nodules under the skin or in muscle tissue, respectively, but these infections are generally mild or asymptomatic. Below is a nice chart that summarizes the transmission cycles and differentiates between taeniasis and cysticercosis (Published in: Sumit Sinha, B.S. Sharma, Neurocysticercosis: A review of current status and management, Journal of Clinical Neuroscience, Volume 16, Issue 7, July 2009, Pages 867-876):
Cysticercosis in the brain is referred to as neurocysticercosis and constitutes the most important clinical manifestation of this infection in humans. Oncospheres can locate and encyst in brain parenchyma, in the subarachnoid space, or in the ventricles. Here is an example of extensive infection with intraparenchymal cysts:
The inflammatory response to intraparenchymal cysts, or later edema surrounding calcified lesions, is the primary mediator of symptoms. The most common clinical presentation of neurocysticercosis is seizure. When cysts grow in the subarachnoid space or the ventricles, cerebral spinal fluid can be blocked leading to hydrocephalus and/or intracranial hypertension, which are associated with greater mortality. Psychiatric disorders and neurologic deficits can also present.
The Epidemiology and the Landscape. There are an estimated 50 million people infected with neurocysticercosis across the world, with approximately 50,000 deaths each year, though these are both likely to be underestimated, particularly the prevalence. Globally, T. solium, and cysticercosis in particular, is widely distributed across Latin America, South, Southeast, and East Asia, most of sub-Saharan Africa, and some islands of the Pacific, wherever pigs are part of the agricultural landscape and raised for food. The map below produced by the World Health Organization displays the global distribution of cysticercosis risk based on country-specific prevalence:
The landscape of Taenia infections is fundamentally one of animal husbandry. Perhaps more than any other helminth infection, human tapeworms are both culturally and evolutionarily derived from the relationship between humans and their livestock. This is the case for both human tapeworms, T. solium and T. saginata. This unique relationship between definitive and intermediate hosts, wherein the former consumes the latter, is necessary for the helminth to complete its life cycle. Infection derives from two fundamental points of intersection in the landscape: 1) the move to agricultural economies with the domestication of subsistence animals in the course of human evolution, and 2) the contamination of the human environment with human feces due to a lack of sanitation and good water infrastructure.
1) As described above, human infection with the adult tapeworm is essential to the tapeworm's life cycle since humans are the sole definitive host. Thorough cooking or freezing of pork products destroys cysticerci and prevents infection with the adult tapeworm in humans.
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This intervention, while fairly simple, is the only one which can be implemented on an individual basis at the level of the household. To block other transmission pathways, larger community-wide strategies are required.
2) As described above, humans are also capable of being infected as intermediate hosts when they ingest the eggs of T. solium (or also possibly by autoinfection following infection as the definitive host). Because only humans harbor the adult tapeworm, only humans pass eggs in their feces, which means human feces are the essential source of infectious embryonated eggs. As such, good sanitation and water infrastructure are important in blocking the transmission of the eggs to both humans and pigs, and would thus have wide impact on reducing intermediate host infection in both pigs and humans.
Western-style flush toilet
Squat toilet
Of course, maintaining vigilance in personal hygiene is also critical to stop human to human transmission of aberrant infections at the level of the household. However, while good personal hygiene will block aberrant human infections and thus prevent human cysticercosis, it will not block normal infections with the adult tapeworm, and thus cannot be expected by itself to contribute substantively to reducing the global burden of pork tapeworm infections.