Protozoa are an exceedingly diverse group of single celled, eukaryotic organisms. Pathogenic protozoa are referred to as parasites and cause a wide array of clinical diseases. They are broadly divided into: Intestinal protozoa e.g. Entamoeba histolytica, Giardia lamblia and Cryptosporidium parvum. Urogenital protozoa e.g. Trichomonas vaginalis, and Blood and Tissue protozoa e.g. Toxoplasma gondii and Plasmodium falciparum.

Intestinal Protozoa

In the developing world it is “usual” to harbor several protozoa at any given time due to sanitation inadequacies. In North America, the finding of any protozoan in the GI tract is not “usual” and implies contact with fecally contaminated material. The distinction is important because there are several non-pathogenic intestinal protozoa that do not cause disease but, if they are found in a patient in North America, are still noteworthy as a marker of breakdown in sanitation. The finding of non-pathogenic protozoa in a person in the developing world is completely inconsequential and “usual”. Diagnosis of the presences of intestinal protozoa is done by microscopically examining fresh or fixed stool and distinguishing between organisms morphologically. See DPDx Diagnostic Procedures

Non-Pathogenic Intestinal Protozoa

  • Entamoeba coli
  • Entamoeba hartmani
  • Iodamoeba buetschlii
  • Endolimax nana
  • Chilomastix mesnili

These organisms may be found in stool and identified by the microbiology laboratory but are not the cause of diarrhea. The reason that their presence is reported is that it implies contact with feces and should prompt a look for other known pathogens and assessment of sanitation. In Canada (and other jurisdictions with higher order sanitation) intestinal protozoa are not found in the stool under normal circumstances.

Entamoeba histolytica

A worldwide pathogen of large import, E. histolytica is said to have decided many more wars than bullets have. It causes amebic dysentery manifest by frequent, bloody, mucousy stools and epidemics during wartime were frequent. In North America disease is most prevalent in chronic care institutions and in homosexual men. It is a frequent cause of illness in travelers to endemic areas such as Mexico. It is also capable of causing liver abscesses.

Further reading DPDx – Amebiasis


By a considerable margin, G. lamblia (sometimes referred to as G. intestinalis)is the most important and frequently identified protozoan pathogen in North America. The organism is acquired from untreated or inadequately treated water and illness is often referred to as beaver fever, though beavers have nothing to do with the epidemiology and it very rarely causes fever! Infection occurs principally in the upper small intestine and, while often asymptomatic, is also associated with a wide variety of intestinal symptoms including diarrhea, nausea, bloating, cramps, flatulence and pale greasy stools that float. Symptoms can be of long duration or intermittent. Young children have a high incidence rate and daycares are a particular problem because of increased fecal-oral transmission.

Microbiologic diagnosis is often difficult as shedding of organisms in stool is intermittent. Sampling of upper small intestine contents via endoscopy is more sensitive but, obviously, more invasive.

Further Reading – DPDx Giardiasis


These coccidian protozoa are very important human and veterinary pathogens. The principal manifestation in humans is watery diarrhea that may be quite severe but is almost always self limiting within 30 days in immunocompetent hosts. Large waterborne outbreaks have been described and daycares are a particular problem with transmission. Immunodeficient people, in particular AIDS patients, may be unable to clear the parasite and have a prolonged and even fatal course.

Further Reading – DPDx Cryptosporidiosis

Urogenital Protozoa

Trichomonas vaginalis

This is a flagellate protozoan that is the cause of a common and often persistent infection of the genito-urinary tract. In women it is characterized by vaginitis that is often painful and acutely inflammatory and associated with profuse, thin, greenish, foul smelling discharge. It may cause urethritis in both sexes but also may be completely asymptomatic.

It is transmitted by sexual intercourse and its finding should prompt a complete workup for STDs. Diagnosis is made by examining fresh vaginal secretions by microscope and identifying the very typical actively mobile flagellated organism. Antigen detection and nucleic acid amplification tests are becoming more available.

Further Reading – DPDx Trichomoniasis

Blood and Tissue Protozoa

Invasive protozoa are responsible for a very large amount of morbidity and mortality worldwide. In developing countries diseases such as Leishmaniasis and Trypanosomiasis are major public health problems. Here, they are mainly medical curiosities. This brief account will focus on only two protozoa, Toxoplasma gondii– a common cause of intrauterine infection in North America and Plasmodium spp. – the cause of malaria, which is of considerable import to travelers.

Toxoplasma gondii

Two important clinical syndromes are caused by T. gondii in North America: congenital disease similar to disease caused by Rubella and Cytomegalovirus and intracerebral lesions in AIDS patients that present as space occupying masses. The natural terminal hosts for T. gondii are cats. Humans are infected as accidental intermediate hosts by ingestion of oocysts excreted by infected cats, especially kittens or by ingestion of infected undercooked meat. This is the reason that expectant mothers are advised to not empty litter boxes.

Congenital Disease
Results from acute infection in mother during pregnancy. If mom was infected before pregnancy (IgG positive) she is at no risk for transmission. Demonstrating the presence of IgM in mom’s serum makes diagnosis in pregnancy. Routine screening of moms is advocated by many but is uncommon in North America at present.

Clinical disease may be severe and have many manifestations however infected babies are often asymptomatic. They may go on to have significant pathology in later life including learning and behavioral problems. Chorioretinitis usually presents in the 2nd or 3rd decade of life and is usually a result of congenital infection. Disease in AIDS patients

Reactivation of latent infection with T. gondii is responsible for a common disease of late HIV infection. Patients present with CNS symptoms and multiple space occupying lesions that are revealed by CT scanning or MRI. Diagnosis is based on typical radiological findings and clinical presentation and therapy is lifelong. Serology for indication of prior toxoplasma infection is indicated for any newly diagnosed HIV patient in an attempt to define who is at risk and increase suspicion later on in their course.

Further Reading DPDx – Toxoplasmosis

Plasmodium spp. (Malaria)

Malaria is one of the most common diseases on the planet and causes much morbidity and mortality for the 2 billion inhabitants of malarious areas and for the ever-increasing numbers of travelers to those areas. Though many massive attempts at control or eradication of malaria have been attempted, all have been failures. Incidence of malaria is increasing and resistance to antimalarial drugs is a bigger and bigger problem every day.

There are four main species of Plasmodia that cause disease: Plasmodium vivax, P. ovale, P. malariae and P. falciparum. P. falciparum is, by far, the most important species, both because of the severity of disease it causes and the amount of clinically important antimalarial drug resistance to this species.

Anopheline mosquitoes are the vector of malaria and therefore prevention of mosquito bites is central to the effort. Nets, repellents, habitat control, pesticides all play a role. Prophylactic antimalarial drugs for travelers have been very successful, however there are now many areas where reliable prophylaxis is not available due to resistance.

Disease is manifest by fever and constitutional symptoms. Fever is intermittent corresponding to waves of parasitemia after red blood cells are lysed by the parasites invading them. Cerebral malaria caused by P. falciparum is the most feared manifestation with very high mortality.

Diagnosis is made by visualizing parasites invading red blood cells. Travelers returning from malarious areas that do not have symptoms i.e. fever do not need to have blood smears sent to the lab to look for malaria. Travelers originating from non-malarious areas do not have asymptomatic parasitemia.

Further Reading DPDx – Malaria

Category: Microorganisms

Other Notes