Protozoal Infections

GENERAL PROTOZOA FEATURES (Micro made easy 231)

A protozoa is a single celled organism, with organelles and nuclei – just like other cells. They injest solid pieces of food through its mouth called cytostome. For example: Entamoeba histolytica can ingest RBCs. They can reproduce asexually, simply by replicating their DNA and splitting up into two cells, or sexually – by two cells fusing together and exchanging DNA before splitting into two cells. When exposed to a changed environment, the protozoa can shrink into a protective form called a cyst, and this is the infectious state. Humans ingest cysts and the cyst converts to a trophozoite (active, motile feeding form of a parasite).

Parasitic infections of the GI tract cause one of the following: dysentery, watery diarrhoea, malabsorption diarrhoea.

GIARDIASIS (Micro made easy pp 231)

• Giardia duodenalis (a.k.a: Lamblia, intestinalis): Giardia exists in two forms, as a kite shaped motile (has flagellum) trophozoite or as a cyst (transmission). Transmitted by drinking sewage contaminated water – campers often get it. It is also harboured asymptomatically. Once the cysts are ingested, it converts to the motile trophozoite form and with its suckers, hooks onto the small intestinal epithelium. Fat not absorbed; therefore you get malabsorption diarrhoea≫ fatty, frothy, smelly stools. You DO NOT get bloody stools because Giardia does not invade epithelium.
• Diagnosis: 1) Examination of fresh stools for trophozoites or cysts, 2) Immunoassay kit to detect Giardia antigens in stool.
  
• Treatment: metronidazole + albendazole (best). Treat all in family if one affected, due to common water supply.

AMOEBIASIS (Micro made easy pp 231)

• Entamoeba histolytica: Infection causes amoebiasis. Most infections are asymptomatic but can be symptomatic. It is common in tropical areas. It is transmitted by faecal-oral route. It exists in three forms: the motile trophozoite, the binucleated precyst, and tetranucleated mature cyst. The mature cyst is ingested ≫ converts to trophozoite form and travels to LI eating all in its path such as other protozoa, bacteria and RBC (asymptomatic luminal form). Sometimes, it becomes invasive ≫ erodes the mucosa ≫ causes bloody diarrhoea, abdominal pain. It can cause: non-dysenteric colitis, amoebic dysentery, amoebic appendicitis, amoeboma (tumour like, but no tumour cells). From here it can penetrate the portal circulation and travel to liver, lungs forming abscesses there. Diagnosis: 1) Examination of stool (3 times) for trophozoites or cysts, 2) serology
• Treatment: 1) Luminal: Diloxanide, tinidazole, metronidazole 2) Invasive: same + emetine, dihydroemetine.

BALANTIDIASIS (Micro made easy pp 245)

• Balantidium coli: This causes balantidiasis – balanitidial dysentery. Cysts exist in pigs faeces ≫ ingested by humans ≫ converts to trophozoite (LI) ≫ attaches to intestinal epithelium and causes diarrhoea (notice a trend among these protozoa and how they cause disease!).
  
• Diagnosis: 1) Examination of stool sample for trophozoites + cysts.
  
• Treatment: tetracycline

CRYPTOSPORIDIOSIS (Micro made easy pp 232)

• Cryptosporidium parvum: Transmitted by contaminated water supplies. It is ingested as oocysts containing 4 motile sporozoites ≫ these enter intestinal cells and impair function. Cardinal diarrhoeal symptoms. Self limiting in immunocompetent patients but in immunocompromised patients (i.e.: AIDS, organ transplants, cancer) it causes life threatening diarrhoea.
• Diagnosis: Giemsa or ZN stain of stool smear.
  
• Treatment: azithromycin.

ISOSPORIASIS (Micro made easy pp 232)

• Isospora belli: Oocysts found in faeces ≫ ingested ≫ severe diarrhoea in immunocompromised patients and children.
  
• Diagnosis: 1) Examination of stool for oocysts, 2) Jejunal biopsy.
  
• Treatment: cotrimoxazole, pyrimethamine.

Category: Microbiology Notes