Teachable moment in classrooms:
- cellular basis of life chapter – linker proteins are part of the plasma membrane
- digestive system chapter – location and function of duodenum and jejunum
- digestive system – the enterocytes are the individual cells of the epithelium of the small intestine
- digestive system – normally about 2.5 gallons of water is absorbed from small intestine daily
- digestive system – the large intestine can absorb only 0.25 gallons of water daily
The news item: Recently this news item appeared online:
Diarrhea-causing parasite that can contaminate raw produce causing misery across several states
A parasite called Cyclospora, which can contaminate raw produce and causes ferocious and long-lasting bouts of diarrheal illness, is making people sick across several states.
The article reports that several US states recorded an outbreak of a disease caused by Cyclospora parasite. The article states that water or Chlorox does not kill or remove the parasite, and that the hallmark of the disease is severe diarrhea.
So, Why Do I Care?? Around 2000 people per year in USA show infection with each outbreak. Adults with normal immune system can fight off the disease in a few weeks. Older, or immunosuppressed individuals or infants may suffer several months of diarrhea, dehydration and poor absorption of nutrients. While this disease is rarely fatal, the general weakness and decreased quality of life make it important to learn about it.
Plain English, Please!!! First, let’s talk about how Cyclospora survives in the human digestive system. Cyclospora is not a bacterium, not a virus, not a fungus, but a single-cell organism, like the Paramecium you probably observed in your school’s biology lab. On lettuces and raspberries Cyclospora is in a dormant stage where the parasite is protected by a shell, and the shell resists acids and other chemicals, such as chlorine, explaining why Cyclospora is not destroyed by stomach acid, or by disinfectants on produce. The shell dissolves at slightly alkaline pH of the duodenum and jejunum, and infectious sporozoites are released. It is these sporozoites that bind to cell surface linker-type proteins of the enterocytes, and through that linker protein bridge the sporozoites move inside the enterocytes.
Second, let’s talk about how Cyclospora damages the enterocytes. Once inside the enterocytes, the sporozoites multiply, and they use up nutrient resources of the enterocyte. In addition, the mass of the Cyclospora organisms physically destroys the enterocytes. Normally, enterocytes carry out the absorption of nutrients like glucose, amino acids, electrolytes and water, thus, Cyclospora infection slows the speed of absorption of those materials because of the destruction of the absorbing cells.
Third, let’s talk about how Cyclospora infection causes diarrhea. During a normal day, from a total of 2.85 gallons of water in the digestive system, 2.5 gallons is absorbed through the enterocytes, 0.25 gallons through the large intestine, and 0.1 gallons is eliminated with fecal matter. Because the infected small intestine cannot absorb such a large volume, the unabsorbed water moves into the large intestine. Picture the Cyclospora infected digestive system as a street with two drain holes. The first hole (small intestine) can now absorb only 1 gallon of water, and the remaining 1.85 gallons are moving to the second drain hole (large intestine). Because the second drain hole can absorb only 0.25 gallons, the unabsorbed 1.6 gallons of water is forced to leave with the fecal matter. Elimination of such watery fecal matter is called diarrhea.