Amebae & Flagellates: Gastrointestinal & Urogenital Infections

Separately, we'll learn about the ciliates and apicomplexa protozoans that cause similar illnesses. Amebae

• Entamoeba, which causes amebiasis (gastrointestinal illness).

Flagellates

• Giardia causes giardiasis (gastrointestinal illness)
• Trichomonas causes trichomoniasis (urogenital infection).

Transmission The gastrointestinal parasites are transmitted via the fecal-oral route. They are especially common in crowded institutionalized settings, such as daycares and prisons; they are often endemic in areas with poor sanitation where water sources are contaminated. Trichomonas vaginalis, a urogenital parasite, is transmitted via sexual contact. L Learn more about STIs Diagnosis Diagnosis of the gastrointestinal parasites relies findings of trophozoite or cyst forms in the stool, fecal antigen tests, and serology. Trichomonas vaginalis can be observed in urogenital secretions; culture is more sensitive, but takes a few days. Most infections can be treated with metronidazole. The most important causative agent of amebiasis is Entamoeba histolytica. Many individuals are asymptomatic reservoirs. Intestinal amebiasis, sometimes called amebic dysentery. is characterized by abdominal pain, colitis, and diarrhea; blood may be present in the stools. Extra-intestinal amebiasis occurs when the parasites reach the bloodstream, and the parasites travel to other organs. Because the trophozoites can get "caught" in the liver as they move through the hepatic portal system, liver involvement is the most common extra-intestinal manifestation. Liver abscesses and pain, especially in the right upper quadrant, as well as hepatomegaly and leukocytosis are associated with hepatic amebiasis. Entamoeba histolytica in the human host First, cysts are ingested in contaminated water. The cyst has multiple nuclei and elongated chromatoid body. Then, in the small intestine, the cyst releases trophozoites by a process called excystation. The trophozoites migrate to the large intestine and multiply. Some transform to the inactive cyst form via a process called encystation, which prepares them for survival in the external environment. Trophozoites release cytotoxins that destroy host tissues; they also ingest red blood cells. We show an example of the characteristic flask-shaped lesion produced by invasive amebiasis. This helps us make sense of the species name "histolytica," which combines Greek words for "tissue" and "dissolving." Trophozoites can invade deeper tissues; once they reach the bloodstream, they can be disseminated and cause extra-intestinal disease. The liver is most commonly affected; other important targets include the lungs and brain. Trophozoites and cysts are excreted in the feces; recall that trophozoites cannot survive in the external environment, but the cysts, with their protective coverings, are adapted to survive until ingested by the next host. In fecal samples, one must be careful not to confuse Entamoeba histolytica with a morphologically similar species, Entamoeba dispar, which is a non-pathological commensal. A helpful distinguisher is that only Entamoeba histolytica engulfs red blood cells (though these may not always be visible). Carriers of Entamoeba histolytica can be prescribed paramomycin or iodoquinol; as we denoted in our table, treatment for active amebiasis typically involves metronidazole. Caused by Giardia duodenalis (aka, Giardia lamblia, aka, Giardia intestinalis). Approximately half of infected people are asymptomatic. When symptomatic, intestinal illness ranges from mild diarrhea to severe malabsorption Giardiasis produces watery or fatty foul-smelling diarrhea with bloating and flatulence; however, unlike amebiasis, bloody stools are rare because the causative parasites are not invasive. Extra-intestinal spread is rare (unlike amebiasis). Post-giardiasis complications are reported, including lactose intolerance and irritable bowel syndrome. Morphology & pathogenesis: Giardia duodenalis trophozoites have a pear-shaped body, with flagella. Two nuclei, and, surrounding them, the large adhesive disk (aka, ventral disk). The disk facilitates firm adhesion to the villi of the small intestine. Giardiasis pathogenesis is similar to amebiasis; we'll highlight some key exceptions. The parasites damage the enterocytes, leading to diarrhea and inhibition of nutrient absorption. Because the parasites are rarely invasive, giardiasis is not typically associated with bloody stools. Chronic giardiasis can lead to developmental impairment; this a major concern in developing countries, for example, where the effects of giardiasis exacerbate existing malnutrition. Caused by Trichomonas vaginalis; this protozoa has no cyst form. Thus, its trophozoites, which can't survive outside the host, are transmitted during sexual contact in urogenital secretions. Trichomoniasis is the most common non-viral STI in the United States. However, most infected individuals, especially males, are asymptomatic carriers. When symptomatic, trichomoniasis produces vaginitis or urethritis, and sometimes prostatitis. Vaginitis and urethritis are characterized by a burning, itchy sensation, and may be accompanied by a foul-smelling "frothy" discharge. Trichomonas vaginalis morphology Oblong shape with an axostyle that runs the length of the body and extends posteriorly. They have four anterior flagella, and the undulating membrane, which is a modified fifth flagellum.