Cryptosporidiosis: Causes & Symptoms

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  • 0:58 Cryptosporidium
  • 1:28 Life Cycle
  • 3:06 Infection and Prevention
  • 5:33 Cryptosporidiosis
  • 6:50 Diagnosis and Treatment
  • 8:08 Lesson Summary
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Lesson Transcript
Instructor: Angela Hartsock

Angela has taught college Microbiology and has a doctoral degree in Microbiology.

Cryptosporidium is a protozoan parasite that can cause serious gastrointestinal distress. In this lesson, we will investigate the general characteristics of Crypto and what symptoms to expect if you are ever infected.

Waterborne Illness in Milwaukee

In April 1993, the emergency rooms in Milwaukee, Wisconsin, were flooded with people all exhibiting the same symptoms: mild fever, stomach cramps, and watery diarrhea. Over 4,000 people sought medical help, with 69 of those dying from the mystery illness. This was just the tip of the iceberg. More than 400,000 people showed some variation of the same symptoms, with the only other common thread being that they all were served by the same water treatment facility. Experts determined that spring rains combined with snow melt runoff had overwhelmed the filters at the treatment plant, resulting in billions of parasites being released into the drinking water. The name of the parasite responsible for the largest outbreak of a waterborne infection in United States history is Cryptosporidium parvum.


Cryptosporidium, or Crypto for short, is a protozoan parasite that infects the gastrointestinal lining of most vertebrates. There are many individual species of Crypto that infect specific host species. The two species that usually cause disease in humans are C. parvum and C. hominis. In the United States alone, there are an estimated 748,000 cases of cryptosporidiosis every year.

Life Cycle

Let's take a closer look at the life cycle of Cryptosporidium. Infection begins with the ingestion of an oocyst, which houses the immature, developing parasite zygote. The oocysts have thick walls and are very resistant to environmental stress, allowing the parasite to survive outside of the host.

Once in the digestive tract, the oocyst will excyst and release sporozoites into the stomach or small intestine. A sporozoite is the cell stage capable of infecting a new host. Crypto is an intracellular parasite, meaning it lives inside cells, so the sporozoites invade the cells of the stomach and intestinal lining.

Inside the host cell, the sporozoite matures into a trophozoite. This is the mature, feeding stage of the parasite. The trophozoites first reproduce asexually inside the cell. These parasite clones then burst from the host cell and begin reproducing sexually. Two types of oocysts result from the sexual reproduction: thick-walled and thin-walled.

The thin-walled oocysts remain in the host and reinfect another epithelial cell. The thick-walled oocysts are excreted in the feces and can infect a new host. These oocysts are shed in very high numbers, often in the millions.

There is also some evidence that Crypto can infect the respiratory tract as well. Inhaled oocysts can invade the lining of the lungs and complete their lifecycle, with infectious oocysts exhaled back into the environment.

Infection and Prevention

We started the lifecycle description by ingesting an oocyst. But where did that oocyst come from? Humans acquire Cryptosporidium by the fecal-oral route, usually by drinking water contaminated with feces. Officials suspect that the outbreak in Milwaukee occurred because the heavy runoff that overwhelmed the treatment plant was contaminated with farm runoff. Cattle, especially young calves, are notorious for shedding Crypto oocysts.

Accidental swallowing of contaminated recreational waters from pools and lakes is a major contributor to the spread of Cryptosporidium. Normal chemical sterilization techniques have absolutely no effect on the oocysts. Cryptosporidium is particularly resistant to chlorine and is able to survive for several hours in undiluted chlorine bleach.

Also, since the spread of Crypto is fecal-oral, daycare centers are an outbreak hotspot due to the close quarters and absence of hygiene awareness in young children. Any infected individual can contaminate surfaces with oocysts, creating sources of new infections.

The best methods of preventing Crypto infection involve sedimentation and filtration of water. The oocysts will settle out of solution, sort of like the snowflakes falling to the bottom of a snow globe. Water taken from the surface should be clear and the sunken oocysts can be easily removed from the bottom of the tank. One-micron water filters are also effective at removing oocysts. For campers, boiling water will kill all oocysts, but this is not a viable solution in large-scale municipal water treatment.

Proper hygiene is usually enough to prevent infection from fecal contamination. Regular hand-washing and bathing will strip most oocysts from people's bodies. Surface disinfection can be a bit trickier. I already mentioned that bleach is ineffective. Most commercial cleaners won't kill the oocysts regardless of contact time. The most effective surface treatment is 3% hydrogen peroxide for at least 20 minutes. This won't kill all the oocysts, but it should reduce the number to a less dangerous amount. Ammonia is also somewhat effective, but the risks of working with this toxic chemical probably outweigh the benefits.

Any individuals that have or are suspected to have Crypto should avoid swimming for several weeks to avoid spreading the illness to other swimmers.

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