Taenia solium- Life history, development and adaptations
Taenia solium is a parasitic tapeworm that belongs to the family Taeniidae. It has a complex life cycle that involves two hosts: humans and pigs. Humans can become infected with T. solium in two ways: by eating undercooked pork that contains the larval stage of the parasite (cysticercus), or by ingesting the eggs of the adult worm that are shed in the feces of another human (oncosphere). The former infection leads to taeniasis, which is a mild intestinal disease characterized by the presence of a long, segmented worm in the gut. The latter infection leads to cysticercosis, which is a serious tissue disease caused by the development of cysts in various organs, especially the brain and muscles. Cysticercosis can cause neurological symptoms such as seizures, headaches, blindness and even death. Neurocysticercosis is the most common preventable cause of epilepsy in many endemic areas .
T. solium is found throughout the world, but it is most common in countries where pork is eaten and where sanitation and hygiene are poor . It is estimated that about 50 million people are infected with T. solium worldwide, and that about 2.5 million people suffer from neurocysticercosis. The disease burden of T. solium is mainly borne by poor and marginalized communities, where people and pigs live in close proximity and where access to health care and diagnosis is limited. T. solium also causes economic losses for pig farmers, who have to discard infected meat or sell it at lower prices.
T. solium can be controlled and eliminated by a combination of interventions, such as mass drug administration for human taeniasis, pig vaccination and treatment for porcine cysticercosis, meat inspection and proper cooking of pork, and improvement of sanitation and hygiene practices . The World Health Organization (WHO) has issued guidelines for preventive chemotherapy for the control of T. solium taeniasis/cysticercosis in endemic areas. The goal is to reduce the morbidity and mortality associated with this neglected tropical disease and to improve the quality of life of affected populations.
Taenia solium is a hermaphroditic parasite, meaning that each mature proglottid has a complete set of male and female reproductive organs. The male organs include testes, vas deferens, cirrus, and cirrus pouch. The female organs include ovary, oviduct, ootype, vagina, seminal receptacle, vitelline glands, and uterus.
The fertilization process in Taenia solium involves both self-fertilization and cross-fertilization. Self-fertilization occurs when the cirrus of a proglottid inserts into its own vagina and releases spermatozoa that are stored in the seminal receptacle until the ovary produces ova. Cross-fertilization occurs when the cirrus of a proglottid inserts into the vagina of another proglottid, either on the same or a different tapeworm. Cross-fertilization is more common than self-fertilization and enhances genetic variation among the offspring.
Taenia solium is also protandrous, meaning that the male organs mature earlier than the female organs. This ensures that spermatozoa are available for fertilization when ova are released from the ovary. Fertilization takes place in the oviduct, where the zygotes are formed by the fusion of sperm and egg nuclei. The zygotes then move to the ootype, where they are surrounded by yolk cells from the vitelline glands and a thin shell or chorionic membrane from the yolk cell secretion. The resulting structures are called capsules or eggs, which then pass into the uterus for further development. The uterus expands and develops lateral branches to accommodate the large number of capsules produced by each proglottid. Each capsule contains a single embryo or onchosphere that will undergo further development in the secondary host.
The onchosphere is the first larval stage of Taenia solium, which develops inside the egg capsule in the uterus of the gravid proglottid. The onchosphere has a spherical shape and is surrounded by several protective layers that help it survive in the external environment and infect the intermediate host.
The formation of the onchosphere involves the following steps:
- Cleavage: The zygote undergoes holoblastic and unequal cleavage, producing a large megamere and a smaller embryonic cell. The megamere divides further to form several similar megameres, while the embryonic cell divides repeatedly to produce two types of cells: larger mesomeres and smaller micromeres. The micromeres form a rounded mass called the morula, which is surrounded by an inner envelope of mesomeres and an outer envelope of megameres .
- Morula: The morula is the solid mass of cells that will give rise to the embryo. The yolk cell transfers its yolk to the megameres and gradually disappears. The megameres fuse to form the outer embryonic membrane, which eventually degenerates. The mesomeres form the inner embryonic membrane or embryophore, which is a hard, thick, cuticularized and radially striated shell that protects the morula .
- Hexacanth: The morula differentiates into an embryo that develops three pairs of chitinous hooks at its posterior end. The hooks are secreted by specialized cells called onchoblasts. This six-hooked embryo is called the hexacanth, and it has a pair of large penetration glands that help it invade the intestinal wall of the intermediate host. The hexacanth is surrounded by two hexacanth membranes .
- Onchosphere: The onchosphere is the term used to refer to the hexacanth together with all the membranes surrounding it. It is also known as the activated embryo, since it is ready to hatch from the egg capsule and infect the intermediate host . The onchosphere has a diameter of about 30-40 micrometers and can survive for days to months in the environment.
The onchosphere is released from the gravid proglottid when it passes out from the definitive host`s body with feces. The onchosphere can then be ingested by an intermediate host (usually a pig) or by another human (accidental host). Once inside the host`s stomach, the onchosphere hatches from the egg capsule and loses its embryophore and basement membrane by the action of acidic juice. It then passes into the small intestine, where it loses its hexacanth membranes by the action of alkaline juices. The activated hexacanth then bores through the intestinal wall with its hooks and penetration glands and reaches a blood or lymph vessel that carries it to various tissues and organs where it develops into a cysticercus or bladderworm .
- The secondary host of Taenia solium is the pig, which acquires the infection by ingesting the oncospheres (embryos) of the parasite that are shed in the feces of a human tapeworm carrier .
- Sometimes, other animals such as dogs, monkeys, and sheep can also serve as secondary hosts for Taenia solium.
- The oncospheres hatch in the stomach of the pig and penetrate the intestinal wall with the help of their hooks and penetration glands.
- The oncospheres then enter the blood or lymph vessels and are carried to various organs and tissues, especially the striated muscles of the tongue, shoulder, neck, thigh, and heart .
- In these sites, the oncospheres develop into cysticerci or bladderworms, which are fluid-filled sacs containing an inverted scolex (head) of the tapeworm .
- The cysticerci of Taenia solium are also called cysticercus cellulosae. They can reach a diameter of about 18 mm and take about 10 weeks to mature.
- The cysticerci can survive for several years in the muscles of the pig, causing a condition known as cysticercosis .
- The infected pork is called measly pork because it appears white-spotted due to the presence of cysticerci.
- Humans can become infected with Taenia solium by eating undercooked or raw measly pork containing viable cysticerci .
Migration of Taenia solium within secondary host
After ingesting the oncospheres, the secondary host (usually a pig) becomes infected with the larval stage of Taenia solium.
- The oncospheres lose their outer membranes in the stomach and small intestine of the pig and release the hexacanths, which have six hooks and penetration glands.
- The hexacanths use their hooks and glands to bore through the intestinal wall and enter the blood or lymph vessels .
- The hexacanths are carried by the circulatory system to various organs and tissues of the pig, such as the liver, heart, lungs, brain, eyes, and skeletal muscles .
- In these sites, the hexacanths develop into cysticerci or bladderworms, which are fluid-filled sacs with an inverted scolex inside .
- The cysticerci grow in size and mature over 60 to 70 days. They can survive for several years in the pig`s body.
- The cysticerci cause damage to the host tissues by compressing them or inducing inflammation and fibrosis. They may also cause neurological symptoms if they affect the brain or eyes .
The cysticerci are infective to humans who consume undercooked pork containing them . They are also called cysticercus cellulosae.
Infection of primary host (Man) by Taenia solium
Man is the primary or definitive host of Taenia solium, meaning that the adult tapeworm lives and reproduces in the human intestine. Man acquires infection by eating undercooked pork containing cysticerci or measly pork. Cysticercus is the larval stage of Taenia solium that develops in the muscles of pigs.
When a person ingests pork with viable cysticerci, the bladder of the cysticercus is digested in the stomach and the scolex (head) evaginates and attaches to the intestinal wall by its suckers and hooks. The scolex then develops a neck and a series of proglottids (segments) that form the strobila (body) of the adult tapeworm. The adult tapeworm can grow up to 3 meters long and live for several years in the human intestine.
The proglottids mature as they move away from the scolex and become gravid (full of eggs) at the posterior end of the strobila. Each gravid proglottid contains about 30,000 to 40,000 eggs. The gravid proglottids detach from the strobila and are passed out with feces. The eggs can also be released from the proglottids and contaminate the soil in areas where sanitation is poor. The eggs can survive for days to months in the environment.
Most people with taeniasis (tapeworm infection) have no symptoms or mild symptoms, such as abdominal pain, loss of appetite, weight loss, or upset stomach. The most visible symptom of taeniasis is the active passing of proglottids through the anus and in the feces. In rare cases, tapeworm segments can become lodged in the appendix, or the bile and pancreatic ducts.
Infection with Taenia solium tapeworms can also result in cysticercosis, which is a serious disease that occurs when humans ingest eggs from feces of a person with taeniasis. The eggs hatch in the intestine and migrate to various organs, such as the brain, eyes, muscles, or heart, where they form cysts that can cause seizures, muscle or eye damage, or even death. More on: cysticercosis
Taenia saginata does not cause cysticercosis in humans. It is not clear if Taenia asiatica causes cysticercosis in humans or not.
The tapeworm shows several adaptative features to its internal parasitic life, in comparison with the free-living animals. Some of these are as follows:
- Taenia solium has a flattened leaf or ribbon-like body so that it can fit in the spaces where it has its habitat.
- The tegument or the outer covering of Taenia is freely permeable to water and nutrients, but it protects against digestion by the host’s alkaline digestive juice. It also has microvilli on its surface to increase the area for absorption.
- It has well-developed four suckers and hooks to anchor with the intestinal wall of the host, by which it is not dislodged from the host.
- There are no cilia and organs of locomotion since they are not needed, the host transporting the parasite.
- As the alimentary canal is absent, they absorb the digested food from the host through the general body surface.
- They lack the special circulatory, respiratory, and sense organs and have a not well-developed nervous system as these parasites do not need these systems.
- Their reproductive system is well-developed and has the capability to produce a huge number of eggs (40,000 per gravid proglottid). Each mature proglottid has one complete set of male and female genitalia. The reason for the production of such a huge number of eggs is that these parasites face many challenges for survival.
- Hermaphroditism and proglottization ensure self-fertilization or cross-fertilization within another proglottid in the same worm or within the same proglottid.
- The resistant covering, shell, or capsule around eggs and embryo protects them from unfavorable conditions.
- The internal osmotic pressure is higher than that of the surrounding host’s fluid or tissue, and pH tolerance is high, 4 to 11, that helps the parasite to reside conveniently in the host body.
- They live in an oxygen-free environment, as such they possess a very low metabolic rate which requires a very little amount of oxygen. Respiration is anaerobic as free oxygen is not available. The energy is obtained by the fermentation of glycogen in an oxygen-free environment, where carbon dioxide and fatty acids are produced.
Taenia solium has several morphological features that enable it to survive and thrive as an internal parasite of humans and pigs. Some of these adaptations are:
- Flattened body: The body of Taenia solium is flattened like a ribbon or a leaf, which allows it to fit in the narrow spaces of the intestinal lumen of the host. This also increases the surface area for absorption of nutrients and water from the host`s digestive juices.
- Tegument: The outer covering of Taenia solium is a syncytial layer of cells called the tegument, which is freely permeable to water and nutrients, but protects the parasite from digestion by the host`s alkaline digestive juice. The tegument also has microvilli that increase the absorptive surface area and secrete substances that modulate the host`s immune response.
- Suckers and hooks: The head or scolex of Taenia solium has four circular suckers and a rostellum armed with a double row of hooks. These structures help the parasite to anchor firmly to the intestinal wall of the host and prevent dislodgment by peristalsis or other factors. The hooks also help the larval stage (hexacanth) to penetrate the intestinal wall and reach the blood or lymph vessels.
- Absence of alimentary canal: Taenia solium lacks an alimentary canal or a digestive system, as it absorbs the digested food from the host through its general body surface. This eliminates the need for a mouth, pharynx, esophagus, stomach, intestine, anus, and associated glands and muscles. This also reduces the metabolic waste production and energy expenditure of the parasite.
- Hermaphroditism and proglottization: Taenia solium is a hermaphrodite, meaning that each mature segment or proglottid has a complete set of male and female reproductive organs. This enables self-fertilization or cross-fertilization within the same or different proglottids of the same worm. Proglottization is the process of formation of new proglottids from the neck region of the worm, which gradually mature and become gravid (filled with eggs) as they move towards the posterior end. This ensures continuous production of eggs and transmission to new hosts.
- Resistant eggs and embryos: The eggs of Taenia solium are enclosed in a thin outer shell and a thick inner shell called the embryophore, which protects them from unfavorable conditions in the environment. The embryos inside the eggs have six hooks (hexacanth) and two penetration glands that help them to infect the secondary host (pig) by boring through its intestinal wall. The eggs can survive for days to months outside the host and are infective to both pigs and humans.
These are some of the morphological adaptations of Taenia solium that enable it to live as an internal parasite in humans and pigs.
Taenia solium has evolved several physiological adaptations to survive and thrive in its parasitic lifestyle. Some of these adaptations are:
- Osmoregulation: Taenia solium maintains a higher internal osmotic pressure than that of the surrounding host`s fluid or tissue, which helps the parasite to avoid water loss and dehydration. The parasite also has a high pH tolerance, ranging from 4 to 11, which enables it to cope with the acidic environment of the stomach and the alkaline environment of the intestine .
- Respiration: Taenia solium lives in an oxygen-free environment, as such it possesses a very low metabolic rate which requires a very little amount of oxygen. Respiration is anaerobic as free oxygen is not available. The energy is obtained by the fermentation of glycogen in an oxygen-free environment, where carbon dioxide and fatty acids are produced .
- Detoxification: Taenia solium has to deal with various toxic substances produced by the host`s immune system or ingested with the host`s food. The parasite has developed several enzymes that can neutralize or eliminate these toxins. For example, the parasite has a cystosolic Cu,Zn superoxide dismutase that can scavenge reactive oxygen species generated by the host`s immune cells. The parasite also has a 2-Cys peroxiredoxin that can regulate the level of hydrogen peroxide, which is essential for several physiological functions. Moreover, the parasite has two isoforms of glutathione transferases that can conjugate glutathione with various electrophilic compounds and facilitate their excretion.
- Antigenic variation: Taenia solium has to evade the host`s immune system, which can recognize and destroy foreign antigens. The parasite has developed a strategy of antigenic variation, which means changing its surface antigens periodically to avoid recognition by the host`s antibodies. The parasite achieves this by expressing different variants of glycoproteins on its tegument, which are encoded by a multigene family. This way, the parasite can escape the host`s immune response and prolong its survival.
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