Shana Weidner, Paige Peck, Sarah Hubbard, and Guilli Dia
What are flatworms?
Flatworms, making up the phylum Platyhelminthes, are a group of invertebrates that are soft-bodied and flat. They are often, but not always, parasitic to both humans and animals. Their detriment spans the globe, but countries with improved and accessible healthcare and more frequent sanitation have much lower rates of infection. Widespread infections occur in underdeveloped countries with limited access to sanitation supplies and methods.
There are four types, or classes, of flatworms: Trematoda (flukes), Cestoda (tapeworms), Turbellaria (planarians), and Monogenea. There are upwards of twenty thousand species of flatworms that have been identified. Most of the flatworms that are parasitic are a part of the fluke and tapeworm classes.
How do flatworms affect humans?
An estimated 25-35% of all humans are currently infected with at least one parasitic worm species, and there are over 200 million infections each year. Many cases originate in poverty-stricken areas in Africa, South America, and east Asia but flatworms pose a threat around the globe. With travel opening up and climate change and global warming worsening, flatworms are expanding their reach even more. Two main classes of flatworms cause most of human infections: tapeworms and flukes.
Tapeworms
Taeniasis
Within tapeworms there are two main contributors to human disease, T. saginata (beef tapeworm) and T. solium (pork tapeworm). Both tapeworms can cause a disease called taeniasis through ingestion of undercooked, infected meat. The life cycle of these tapeworms begins when the eggs are ingested by cows or pigs, hatching and circulating to muscle tissue causing cysts. The juvenile forms of the tapeworm can be transferred to humans through ingestion of raw or undercooked meat of the infected animal. The adult worms travel to the small intestine where they lay eggs that pass through the feces and may be ingested, starting the cycle over.
Many people may be unaware they have the disease as often there are no short-term symptoms. Because beef tapeworms, on average, are larger than pork tapeworms, symptoms are more likely to occur in those infected with beef tapeworms. Potential symptoms include loss of appetite, nausea, vomiting, diarrhea and in rare, long-term cases include malnutrition and intestinal blockage.
Cysticercosis
The pork tapeworm can have a secondary branch in their life cycle, causing a more devastating human disease called cysticercosis. As mentioned, tapeworm eggs are passed through the feces. If a human ingests feces with eggs directly, cysticercosis occurs as opposed to taeniasis caused by ingesting juvenile tapeworms in an animal host. These eggs hatch inside the human host and travel to the muscles where they can form cysts in various locations including muscles, brain, eyes, spinal cord and in rare cases can form in the liver, lungs, heart, thyroid, or pancreas.
Due to their life cycle, it is possible for a person with taeniasis to accidentally ingest their own infected feces and contract cysticercosis which is much more serious. Depending on size and location of cysts, symptoms may not be present or can include soreness, headaches, confusion, balance issues, brain swelling, seizures, stroke, or death.
Flukes
Schistosomiasis
Caused by blood
flukes of the genus Schistosoma, schistosomiasis is a widespread, devastating disease. There are other diseases caused by flukes, including those caused by tissue flukes, but schistosomiasis is the most prevalent and in need of attention. Second only to malaria in terms of impact, schistosomiasis is considered one of the neglected tropical diseases (NTDs). It is contracted through skin penetration by juvenile flukes when in waters populated by the flukes’ host snails. Infections may be asymptomatic and if present, symptoms are not directly caused by the flukes but rather the body’s reaction to the eggs.
Common symptoms include rash, fever, chills, cough, muscle aches, while chronic symptoms include abdominal pain, enlarged liver, blood in stool/urine, and increased risk of liver fibrosis and bladder cancer. In rare cases, schistosomiasis can cause seizures, paralysis, spinal cord inflammation, or death. School-aged children are more at risk for contraction due to more exposure and time spent swimming/bathing in unsafe waters. Repeated infections can also cause malnutrition, anemia, and learning disabilities. Flukes can survive in their hosts for many years, possibly not showing symptoms short-term. This adds to the reason deaths caused by schistosomiasis are difficult to estimate in addition to its hidden pathologies.
How can we fight the detriment caused by flatworms?
Treatments are av
ailable to fight flatworm-related diseases. Antiparasitic, particularly anthelmintic, drugs are used for parasitic worm infections and diseases. Common antiparasitics used include praziquantel, albendazole, and triclabendazole and in some cases may be used alongside corticosteroids and/or antiepileptics. In rare tapeworm cases, surgery may be necessary for removal. Drugs are generally single-dose or a few doses administered over the span of a couple days, so they do not require long-term treatment. More research is currently taking place to search for alternative treatments, particularly for schistosomiasis, as praziquantel is heavily relied on and there need to be back-ups in case resistance is ever developed.
In general, flatworm infections can be prevented by regularly washing hands with soap and water, especially before handling or eating food. Teaching children about proper hand washing habits is also extremely beneficial. Drinking and bathing in safe water is also important and there are methods to purify potentially unsafe water like boiling, filters, and iodine tablets. Properly disinfecting surfaces/tools when cooking and thoroughly cooking meats all help prevent infection. In broader terms, ensuring safe access to water and healthcare including the necessary antiparasitics is needed to truly protect people from these devastating diseases. Many people end up in cycles of poverty causing disease, making them lose money to treatment or family deaths or disabilities not allowing them to work, which intensifies their poverty.
Do flatworms do us any good?
Flatworms provide new insight to organ regeneration by studying protonephridia of flatworms. Protonephridia are hollow cells in the excretory system of some invertebrates, containing a tuft of rapidly beating cilia that serve to propel waste products into excretory tubules. Humans can also use planarians as a model to begin how an animal maintains their form and function over a very long time. Scientists have also discovered that flatworms are good for toxicity testing. Since flatworms have unique features such as a brain of intermediate complexity, short generation times, and the possibility of studying adult and developing worms in parallel, they are a good alternative system to lab mammals. Flatworms can help model the human nervous system’s response to chemicals, and by providing this information, flatworms may be able to reduce the need to use mammals for this type of testing.
Flatworms are also beneficial to their ecosystem by regulating the dynamics of zooplankton in ponds. They also consume protozoans, rotifers, and algae, and help to regulate populations of these organisms too. Researchers found that flatworms living in estuaries can indicate the health of the ecosystem. Some free-living flatworms also help the environment by acting as decomposers. While there are many parasitic species of flatworms, there are also some free-living and nondestructive species too. Research using flatworms is underway, and evidence has shown that continuing research on flatworms can provide new insights to human health.
References
Centers for Disease Control and Prevention. (2020, September 22). CDC – Cysticercosis. Centers for Disease Control and Prevention. Retrieved April 10, 2022, from https://www.cdc.gov/parasites/cysticercosis/index.html
Centers for Disease Control and Prevention. (2018, April 11). CDC – Schistosomiasis. Centers for Disease Control and Prevention. Retrieved April 10, 2022, from https://www.cdc.gov/parasites/schistosomiasis/
Centers for Disease Control and Prevention. (2020, September 29). CDC – Taeniasis. Centers for Disease Control and Prevention. Retrieved April 10, 2022, from https://www.cdc.gov/parasites/taeniasis/index.html
CK-12 Foundation. (2016, August 17). Flatworm Diseases – Advanced. CK-12. Retrieved April 10, 2022, from https://www.ck12.org/book/ck-12-biology-advanced-concepts/section/15.14/
Flatworms could replace mammals for some toxicology tests. Flatworms Could Replace Mammals for Some Toxicology Tests. (2015, June 29). Retrieved April 11, 2022, from https://ucsdnews.ucsd.edu/pressrelease/flatworms_could_replace_mammals_for_some_toxicology_tests
Hoffmann, K. (2018, August 31). Parasitic flatworms affect millions in developing countries, but new research offers hope. The Conversation. Retrieved April 10, 2022, from https://theconversation.com/parasitic-flatworms-affect-millions-in-developing-countries-but-new-research-offers-hope-98780
Myers, P. (2002). Platyhelminthes (flatworms). Animal Diversity Web. Retrieved April 9, 2022, from https://animaldiversity.org/accounts/Platyhelminthes/
Pietrangelo, A. (2021, December 21). What you need to know about parasitic worm infection in humans. Healthline. Retrieved April 10, 2022, from https://www.healthline.com/health/worms-in-humans
ScienceDaily. (2011, August 12). Flatworms provide new insight into organ regeneration and the evolution of mammalian kidneys. ScienceDaily. Retrieved April 11, 2022, from https://www.sciencedaily.com/releases/2011/08/110810085544.htm
World Health Organization. (2021, May 17). Foodborne trematode infections. World Health Organization. Retrieved April 10, 2022, from https://www.who.int/news-room/fact-sheets/detail/foodborne-trematode-infections