Protozoa - Apicomplexa: Gastrointestinal Parasites
Apicomplexa
Generalized Apicomplexa Life Cycle:
May include one or multiple hosts.
Some of the following processes and stages have different names in different species.
1. Apicomplexa enter the host as sporozoites.
2. Within the epithelial cells of the intestines, the sporozoites undergo a process of asexual reproduction called merogony, which produces merozoites.
3. Some of these merozoites will undergo additional rounds of merogony, while others will enter gametogony, which is a sexual form of reproduction that ultimately produces male and female gametes.
4. The gametes fuse and produce a zygote.
5. The zygote passes through another stage of asexual reproduction called sporogony, which produces the sporozoites that will enter a new host.
Some apicomplexa sporozoites are released in their infective state, whereas others are released in a non-infective state.
Clinical implications: the non-infective sporozoites require additional time outside of the host to transition to the infective state, and, therefore, are not typically transmitted directly from person to person.
Key Pathogens: Cryptosporidium parvum and Cryptosporidium hominis
Gregarines
Associated with waterborne outbreaks, specifically in drinking water and swimming pools. Cryptosporidium is highly resistant to most forms of disinfection, including chlorination.
Oocysts are shed in large quantities, and can survive for several days, especially in water, which facilitates their transmission to new hosts. Read more about Cryptosporidium life cycle.
Cryptosporidium is characterized by an interesting form of intracellular infection that involves the formation of parasitophorous vacuoles in the brush border of the intestinal villi.
Signs/Symptoms:
In healthy individuals, symptomatic cryptosporidiosis is characterized by mild enterocolitis with watery diarrhea (no blood), vomiting, and abdominal cramping.
Typically resolves on its own after a couple of weeks.
In immune-compromised patients, illness is associated with severe diarrhea that can lead to dehydration; dissemination to the biliary ducts, stomach, and lungs has been reported.
Coccidian species
Csytoisospora belli (formerly known as Isospora belli), is especially common in tropical and subtropical areas.
It is associated with watery diarrhea, and, like giardiasis, can lead to nutritional malabsorption.
TMX is often prescribed as treatment.
Sarcocystis species are associated with livestock, especially pigs and cows.
Infection can lead to intestinal and/or muscular illnesses.
Cyclospora cayetanensis is associated with contaminated water, as well as fruits and vegetables; it is not associated with animal reservoirs. Infection can cause watery diarrhea, with possible blood or mucus.
In healthy individuals, illness tends to be self-limiting; in immune compromised, it is more severe and prolonged.
Images:
“Cryptosporidiosis.” Wikipedia, November 7, 2018. https://en.wikipedia.org/w/index.php?title=Cryptosporidiosis&oldid=867660277
“Isosporiasis.” Wikipedia, September 9, 2017. https://en.wikipedia.org/w/index.php?title=Isosporiasis&oldid=799736433.