Unfortunately, as disgusting as it is, it is actually possible to poop out a parasite. However, it is quite more frequent to poop out parasite’s eggs or cysts rather than actual parasite. It is important to note that these structures are usually invisible to the naked eye, meaning that the person would never know that they were pooped out. These structures are nevertheless distinguishable using a microscope. The ingestion of food or water contaminated by faeces containing parasite’s eggs or cysts is a major mode of transmission of many parasitic diseases. The majority of the known intestinal parasites used the excretion of eggs or cysts in faeces in order to fulfill their life cycle by infecting other people or animals. As such, detection of parasite’s eggs or cysts in faeces is one of the most commonly used tools to diagnose intestinal parasite.
Even though it is theoretically possible to poop out an intestinal protozoan parasite, you would not notice it if it happens to you. In fact, it is important to remember that protozoan parasites are microscopic and then not visible with the naked eye. However, intestinal helminths are much bigger and longer. As an example, Diphyllobothrium, the longest tapeworm able to infect human, can reach an impressive length of up to 10 meters. It is then quite easy to visualise helminths or parts of helminth in the faeces. It is interesting (but quite disgusting) to note that it is also possible to poop out worms that are still alive. Furthermore, in case of massive parasite infestation, it is possible that many worms come out at the same time.
The presence of visible worms in the faeces is an excellent way to diagnose easily parasitic diseases. Furthermore, well-trained medical laboratory workers are usually able to determine by eyes or with the help of a microscope the species of worm involved. Worm’s DNA sequencing methods can also help to confirm this diagnosis in order to be 100 % sure of the species involved. This diagnosis step is important, as it will determine the right treatment for the right parasite.
However, even though it might be quite difficult, it is very important to remember to stay calm if you found something that looks like worms within your faeces. In fact, it is possible that some not digested or only partially digested food look exactly like worms in the faeces. It is the case for example of germinated bean sprouts, a common ingredient, especially in Asian cuisine. If you ate this kind of food the day before, it is probably just that. However, it could be better to consult a doctor to be sure, especially if you are still anxious about it.
Sources: Centers for Disease Control and Prevention; Leung, F. H., & Watson, W. (2011). The parasite that wasn’t: A case of mistaken identity. Canadian Family Physician, 57(10), 1145-1147.