The Common Culprit Behind The Parasite That Causes Profuse Diarrhea Every Summer

Sometimes, what you can't see can hurt you. Microscopic parasites need a host to survive, and when you are that host, they can wreak havoc on your gastrointestinal system and cause profuse diarrhea. For instance, Giardia is a parasite that science reveals can turn your poop green, while Cyclospora on shredded iceberg lettuce from Central Mexico may have been the everyday kitchen culprit behind the recent parasitic outbreak causing explosive diarrhea. Another parasite that drives a seasonal outbreak every summer is Cryptosporidium — known as Crypto for short (not the digital currency) — and the most common culprit behind it is water.

According to a 2019 Morbidity and Mortality Weekly Report from the Centers for Disease Control and Prevention (CDC), Crypto is the leading cause of waterborne diarrhea outbreaks in the United States. There were 444 outbreaks between 2009 and 2017 with an average 13% increase in reports every year, and 41.2% of those were traced to contaminated water sources — 19.8% were person-to-person contact, 19.4% were animal contact, and the rest were from food, environmental contamination, and never determined.

Among the contaminated water culprits, though, 85.2% were linked to swimming pools, water playgrounds, and other treated water sources. More than 4,200 cases of cryptosporidiosis (the resulting disease that causes diarrhea) were reported as a result. Part of the reason could be because the parasite can survive for at least a week in chlorine-treated water. However, it's worth noting that another 7.7% of waterborne outbreaks during the same period were linked to untreated recreational water sources like lakes, while 4.4% were in drinking water and 2.7% were other water sources.

The most common species of Cryptosporidium (about 40 have been identified) to infect humans are C. parvum and C. hominis, but at least 15 others and four genotypes can also infect humans. Most of the time, humans get infected by ingesting something (typically water) that has been contaminated with fecal matter that contains sporulated oocysts of this protozoan parasite. The sporozoites emerge from the oocyst and enter the host's gastrointestinal epithelial cells, where they reproduce. After oocysts develop and sporulate, they're excreted through the feces and are already infectious.

Most people who have been infected with Crypto usually begin to experience symptoms about one week later, but the onset may happen anywhere from one to 12 days after ingestion. While fever, nausea, and vomiting could occur with cryptosporidiosis, the main symptoms are stomach cramping and profuse, watery diarrhea — the latter of which may last as long as three weeks. Anyone who goes swimming at a public pool or in a lake while they're having these symptoms can easily pass the oocysts into the treated or untreated water, which may be accidentally swallowed by others. That's why the CDC recommends that people with cryptosporidiosis wait for two weeks after resolved diarrhea to swim again.

Crypto can also spread when infected people don't practice strict hygiene. As a result, contaminated hands can leave the oocysts to food, drinking water, their own mouths, and anything else they touch. That's why it's so important to wash your hands with soap and water to prevent spreading the infection to other surfaces.