Back To CourseUExcel Microbiology: Study Guide & Test Prep
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Angela has taught college Microbiology and has a doctoral degree in Microbiology.
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.
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.
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.
If you are unlucky enough to get cryptosporidiosis, you probably should know what you're in for. Following exposure to the oocysts, there is a two-to-nine day incubation where you won't exhibit any symptoms. After the incubation, most otherwise healthy people will have abdominal pain and watery, bloodless diarrhea for about two weeks. During that two-week period, your body will build up a natural defense to the Cryptosporidium and succeed in eliminating the active infection. Oocysts can still be shed for up to a month, with an occasional return of mild diarrhea.
In immunocompromised individuals, like those with AIDS or those undergoing chemotherapy, the disease can be much worse. The diarrhea can be severe and profuse. Patients often become dehydrated and emaciated from water and nutrient loss, requiring IV support. Often the disease is irreversible, leading to the death of the patient. If the patient can't mount a successful immune response, the thin-walled oocysts can continue to infect and kill epithelial cells, destroying the intestinal lining of the patient. AIDS patients that get cryptosporidiosis have a very grave prognosis.
But how do you know that your diarrhea is cryptosporidiosis and not some other parasite or disease? Currently, the most common method of identification is to use the acid-fast staining technique to look for oocysts in a fecal sample. The stain will turn the oocysts red, making them easier to see under the microscope.
Even without the stain, the oocysts are very small and easy to miss. Detecting antibodies to Cryptosporidium in the blood of the host is much more accurate and specific to Crypto. There are also genetic techniques available that will not only positively identify a Cryptosporidium infection but will also identify the exact species.
The diagnosis of Cryptosporidium is most likely to be only for peace of mind. Most people get better in a couple of weeks by doing nothing more than drinking plenty of water and having patience. The drug nitazoxanide has been shown to help reduce the impact of the illness on healthy individuals but is most likely unnecessary. For patients with compromised immune systems, there is no treatment. Nitazoxanide is not proven to be effective in these patients. IV nutrition and fluids can help with the symptoms, but the disease is most likely going to be chronic and possibly fatal.
Let's take a quick look back. Cryptosporidium is a protozoan parasite that invades the gastrointestinal tract of vertebrates. Humans usually acquire cryptosporidiosis by ingesting oocysts in fecal-contaminated water. The oocysts release sporozoites, which invade the host cells. The sporozoites become trophozoites, which reproduce, leading to more oocysts being shed in the feces of the host.
The oocysts are very difficult to kill. They are resistant to nearly all common chemical cleaners, like bleach, but 3% hydrogen peroxide for 20 minutes is somewhat effective. Contaminated water should be filtered to remove the oocysts. Boiling is also effective for small-scale drinking water, like when camping.
Otherwise-healthy individuals infected with Crypto usually get watery diarrhea for a couple of weeks before successfully fighting off the parasite. Immunocompromised patients often develop severe diarrhea that can lead to death. Currently, there is no widely used treatment other than simply letting the disease run its course.
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Back To CourseUExcel Microbiology: Study Guide & Test Prep
28 chapters | 238 lessons