William S. Burroughs once philosophically pondered, “Which came first, the intestine or the tapeworm?” While philosophers are off somewhere hotly debating the topic, one thing is for certain; they were born in the same household, and they certainly grew up together.
Tapeworms have certainly held an interesting place in the American landscape, viewed with fear, disgust, and a general lack of understanding. Tapeworms have been referenced in negative advertising campaigns, popular culture as seen in The Office, and in legendary, word-of-mouth stories depicting the horror of someone finding what the ancients called cucurbitini, because they resembled cucumber seeds, within their feces. Ever wondered, among others reasons, why it is important to wash your hands after using the restroom? One word, auto-infection.
I’m here to provide you with a quick and dirty guide to general tapeworm knowledge, and as my Parasitology professor once told me, “Your dinner conversations will forever be enriched.”
Despite the immense diversity in tapeworm morphology, there are three anatomical features that most often persist; a scolex (head), neck, and strobila. The scolex, as clearly seen in these images, is used not as a mouth, but as a holdfast in the gut, for it would be quite a shame to expend all that energy getting to the gut just to pass through it in a jiffy. This Trypanorhyncha that you’re staring at in terror possess four hooked tentacles for anchoring purposes and four bothridia, or suckers, that you seen stained dark red, used for additional stability. Next is the neck, which in this case, houses the internal sheaths of the tentacles and provides the “duh” moment, connecting the head to the strobila, or body. The body is composed of multiple segments known as proglottids, that once mature, break off and pass during defecation. Using the term body is misleading though, because tapeworms do not need the strobila to survive, they do however, need it to reach sexual maturity and release their developed eggs or shelled embryos. This tapeworm clearly hasn’t been living in the gut a long time, as it hasn’t received enough nutrients to start producing proglottids, or segments, that compose the strobila. Cestodes also lack a digestive tract, and absorb their necessary materials through microtriches, that somewhat resemble the microvilli found in, you guessed it, gut mucosal cells. Talk about efficiency.
Life cycles of most tapeworms include at least two hosts; the primary host, where sexual maturity is reached, and the intermediate host, which is often the means used by the parasite to get to the primary host. NOTE: YOU DO NOT WANT TO BE AN INTERMEDIATE HOST. Most pathogenic, or harmful, effects felt by parasites are caused by being the intermediary. It would be senseless to kill the host that you reproduce in, they’re purely living rent-free in a 24-hour apartment/buffet. Once the egg has been passed into the open environment, it develops into a free-swimming oncosphere, where it penetrates the body cavity, via hooks, of nearby animals. These intermediate hosts are most often a step or two down the food chain from the primary host that possesses the mature tapeworm. Next, as the oncosphere develops into a procercoid and ultimately a plerocercoid, the intermediate host it resides within will get eaten by the primary host, and as digestion proceeds, the beautiful circle of parasitic life continues restarts all over again.
Most tapeworm life cycles are unknown and has become a growing field in research, and it quite fun to employ the basic principles I just described to you in order to lay the framework for an guided start. As many of my loved ones are surfers, let us imagine a tapeworm whose primary host is a shark. Now as that shark is constantly doing its business, it is releasing eggs into the aquatic environment. It would be a safe guess to assume that fish are being infected as an intermediate host, and if it is a three host lifecycle, perhaps copepods are as well. Identifying the chain of command in a food web can provide you with a strong lead on parasite activity, and in fact much research is currently being done on constructing parasitic food webs and comparing them to the presently accepted food webs in our ecosystems.
Oh, and rest easy my friends, the seemingly malicious parasite you’re seeing on these slides does not infect humans. It is actually of the same Order as the hypothetical tapeworm described above that resides within the digestive tract of sharks. However, we aren’t out of the woods quite yet. But alas, I suppose that is a conversation best saved for another day.
-photos courtesy of Andrew.