Diphyllobothrium latum, A Very Macroscopic Parasite

Introduction to Diphyllobothrium latum:

Diphyllobothrium latum is a tapeworm that is spread to humans via raw or undercooked fish. This tapeworm is the largest that parasitizes humans. It can also parasitize other mammals. It can be spread through salmon, trout, perch, walleyed pike and other fish commonly consumed by humans. These fish become infected by eating a smaller infected fish who becomes infected by ingesting an infected crustacean, t

he first host in the life cycle of D. latum. The majority of infections occur in the Northern Hemisphere, Europe, North America, and Asia being major targets for this parasite. But the parasite is also found in other locations. The parasite does not cause any fatal diseases and infection by D. latum is not fatal. It can, however, cause diarrhea, vomiting, weight loss, discomfort, and a deficiency of vitamin B12. And because D. latum become so large, it may become an obstruction in the intestines, leading to complications. [1]

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Description of Diphyllobothrium latum:

D. latum is an organism that grows throughout its life, reaching lengths of up to 30 feet. They are slightly yellow to white in color and have flat bodies. Lacking a mouth, nutrition is obtained by osmotrophy. D. latum are hermaphrodites and are constantly producing eggs inside the host. Upon entering a host, the parasite travels to the intestines where it matures and reproduces. Eggs leave the host via the host's fecal matter. D. latum has basic sensory organs located at the scolex (anterior end of the parasite where attachment to the host occurs) that can detect tactile (touch) stimuli. [4]

Life Cycle of Diphyllobothrium latum:

Eggs of D. latum develop into an embryo in fresh water. There, they are ingested by a crustacean and develop into larva. Small freshwater fish prey on the infected crustacean and the parasite moves to a further stage in its development. These small fish are then eaten by larger freshwater fish, fish that are a food source for mammals, including humans. The parasite will die if it is cooked properly, but if ingested raw or undercooked, the parasite will infect its final host, a mammal (often humans.) Upon ingestion, the immature tapeworm will mature into an adult and migrate to the small intestines to grow and reproduce. It is here that the parasite will live out the rest of its adult life, producing up to 1,000,000 eggs a day. [2]

Ecology of Diphyllobothrium latum:

D. latum inhabits freshwater lakes and streams. At each stage in its development, the parasite has a different habitat. From water they inhabit crustaceans, then fish, then a mammal. [4] This parasite does not pose a great medical risk to humans, though it may cause symptoms such as the ones mentioned above. It may also cause anemia due to vitamin B12 deficiency. There are medicines that can treat the parasite and the symptoms. There are also preventative methods such as proper preparation of fish. [1]

Diphyllobothrium latum, An Example of a "Stage 3" Parasite:

In The Art of Being a Parasite, Claude Combs proposes that there are three stages of parasitism. First stage parasitism means a parasite is not entirely inside its host and is not completely dependent on it for survival/reproduction. Second stage parasitism is where the parasite is to some degree in its host yet maintains a lot of contact with the environment. [5] And stage three parasitism can be exemplified by D. latum. At this stage, the parasite no longer has direct contact with the environment (outside of the host) and relies on the host for the expulsion of its eggs and for nutrition. An advantage that wasn't mentioned in class about being a stage three parasite (related to protection) is "hiddenness." The guinea worm we mentioned in class creates sores on the lower extremities of its host and breaches the skin when it senses water (making it a stage 2 parasite). In a human host, this communication with the environment prevents a threat to the success of the parasite because the human is now aware of the parasite. For a stage three parasite like D. latum, its entire adult life is carried out inside the host. On top of that, being infected with the worm is primarily asymptomatic. This acts as a "camouflage" for the parasite (in a human host with access to medical treatment.) It seems to me that it is an evolutionary advantage for a parasite to be a stage three parasite and remain hidden from its host. The longer it remains unknown to the host or the host's immune system, etc., the more eggs and therefore the more of its own genetic material it is able to pass to a new generation.

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References:

[1] http://www.cdc.gov/parasites/diphyllobothrium/index.html

[2] http://www.parasitesinhumans.org/diphyllobothrium-latum-fish-tapeworm.html

[3] http://cal.vet.upenn.edu/projects/dxendopar/parasitepages/cestodes/d_latum.html

[4] http://nsdl.org/resource/2200/20061002151437262T

[5] The Art of Being a Parasite, Claude Combs