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What is Taenia Solium? - Life Cycle & Cytology

Instructor: Artem Cheprasov
This lesson provides a very detailed overview of the life cycle, morphology, and cytology of ''Taenia solium''. You'll learn exactly how it involves man and swine.

Taenia Solium

Taenia solium, where Taenia means 'tape' and solium refers to 'alone', is better known as the pork tapeworm. It is a parasite that is responsible for two distinct infections in people known as taeniasis and cysticercosis. The latter can lead to significant morbidity in individuals affected by it while the former usually does not. These conditions occur due to poor hygiene and sanitation, which includes improperly cleaning and cooking our food.

But this lesson isn't about the details of the disorders Taenia solium leads to. Rather, it's about this parasite's morphology, cytology, and life cycle.

In the Human Intestine

Adult Worms


The Lifecycle of Taenia solium.
The Lifecycle of Taenia Solium


Despite the name, pork tapeworm, the hermaphroditic adult tapeworm of this species actually lives inside of the human intestinal tract, commonly in the jejunum. In fact, the only definitive host of this tapeworm is actually man, not swine. The adult tapeworm can be between 2-7 meters in length, takes about 2-4 months to fully mature, and can live for decades!


An adult pork tapeworm.
An adult pork tapeworm


Ironically, this tapeworm doesn't have a digestive tract despite being located in one. It gets its nutrients across the tegument, a skin-like covering, which has many microvilli (hair-like projections) that increase the surface area for nutrient absorption. Its excretory system is composed of a specialized cell, called a flame cell, which is basically a kidney and anus rolled into one.

It also has a 1-2 mm sized head, called the scolex, with a very simple brain, called the cephalic ganglion. The scolex has four unpigmented suckers (0.5 mm in diameter), called acetabula, and a hooked rostrum (beak/snout-like portion). The rostrum contains a double row of 20-50 dagger-shaped hooks that alternate in size from small to large. All of this helps the tapeworm firmly latch onto the inside of the small intestinal wall.

The neck of the tapeworm is short and thin and attaches to the strobila (body) of the worm. The body is a long band, white in color, which is segmented into a chain-like fashion. Each link in the chain, so to speak, is called a proglottid and there can be hundreds, if not a couple thousand, of these in each worm. They measure about 5 x 10 mm.

Near the neck, the proglottids contain the male sexual organs (many spherical testes). In the middle of the worm's length, the proglottids contain the female organs. In the terminal (end) portions of the worm, the proglottids contain a branched uterus (approx. 5-10 branches), which is filled with upwards of 80,000 eggs! Each gravid (egg-containing) proglottid is about 12 mm x 6 mm in size.

Eggs

Each egg (aka ovum) is roughly 30-45 micrometers in diameter. The eggs originally have a thin, outer, hyaline membrane (like an eggshell) that sloughs off by the time the eggs are passed in the feces. This leaves a thick protective layer, called an embryophore, surrounding the fully-developed embryo of the egg, called the oncosphere. The embryophore is yellow-brown in color under the microscope due to staining from a person's bile.


Taenia ovum (egg).
Taenia ovum (egg).


The oncosphere contains 3 pairs of hooklets (6 total hooks) and is thus called a hexacanth embryo as well, where 'hexa-' means 'six' and '-canth' means 'hook'.

As you just learned, the eggs are expelled in a person's feces as they defecate. Additionally, small sections of proglottids (usually the last 4-10) may be expelled during defecation in the form of short (sometimes moving) chains. These proglottids may be gravid. In other words, they may contain eggs. The eggs are able to escape out of these gravid segments (out of the uterus within the segment, to be specific).

Once the eggs or gravid proglottids have been passed into the environment, they can stay alive for several days. And so, one of two things can happen during this timeframe. We'll start with the easy one first.

Undercooked Pork

One possibility is that swine (pig), the intermediate host in this part of the lifecycle, will eat the eggs or the gravid proglottids. Once the pig eats the eggs in either form, the eggs pass into the pig's duodenum, the first section of the small intestine. Here, the egg's embryophore ruptures and the oncosphere is released.

The oncosphere penetrates the wall (mucosa) of the intestine, enters a venule (small vein), and thus general circulation. It can end up in numerous organs of the pig, but the key one for human disease is skeletal muscle. Here, the oncosphere develops into cysticercus cellulosae, the infective form of this worm. The cysticercus is the larval (baby) form of the adult tapeworm. It is enclosed in a protective, bladder-like, sac known as a cyst. Hence the reason why it's also called the 'bladder worm'. The cysticercus is about 7-10 mm wide, 5 mm long, ovoid, milky-white, opaque, and has a functional scolex. It becomes infective after about 2 months of development and can live for several months.

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