With the exception of Entamoeba gingivalis (found in the mouth), non-intestinal amebae are pathogenic, free-living organisms that may be associated with warm, freshwater environments. They have been found in the central nervous system, the eyes, and other body sites. Amebae that invade the central nervous system (Naegleria fowleri) can cause severe, life-threatening infection that often ends in death within a few days. Other amebae in this group can cause more chronic central nervous system disease (Acanthamoeba spp., Balamuthia mandrillaris, and Sappinia diploidea, particularly in the immunocompromised patient). Acanthamoeba can also cause keratitis; untreated cases can result in blindness.
Flagellates
Trichomonas vaginalis is found in the genitourinary system and is usually acquired by sexual transmission. Trichomonas tenax can be found in the mouth and is considered to be nonpathogenic.
Coccidia
Coccidian parasites are particularly important in the compromised patient and can cause life-threatening disease. These organisms can disseminate from the intestinal tract to other body sites. They may also infect many individuals who have relatively few symptoms. In the immunocompetent patient, symptoms may be minimal or absent; however, in the compromised patient sequelae may be very serious and even life-threatening.
Microsporidia
As mentioned above, microsporidia are the most difficult protozoa to diagnose (size range, 1 to 2.5 μm). Dissemination from the intestine to other body sites has been well documented. Modified trichrome stains have been developed specifically for detection of these organisms, since routine parasitology stains for fecal specimens are not very effective for microsporidial spores. Optical brightening agents such as Calcofluor are also recommended; although they are very sensitive, they are nonspecific. Currently, at least 14 species have been identified as human parasites. Genera include Brachiola, Pleistophora, Trachipleistophora, Vittaforma, Microsporidium, Nosema, Encephalitozoon, and Enterocytozoon.
Protozoa (Blood and Tissue)
Sporozoa
All sporozoa are arthropod borne. The genus Plasmodium includes parasites that undergo exoerythrocytic and pigment-producing erythrocytic schizogony in vertebrates and a sexual stage followed by sporogony in mosquitoes. Babesia spp. are tick borne and can cause severe disease in patients who have been splenectomized or otherwise immunologically compromised. Diagnosis may be somewhat more difficult than for the intestinal protozoa, particularly if automated blood differential systems are used; the microscopic examination of both thick and thin blood films is recommended. Note: examination of blood for these parasites is considered a STAT test request. Representatives in the genus Plasmodium include P. vivax, P. ovale, P. malariae, P. falciparum, and P. knowlesi. Organisms in the genus Babesia include B. microti, B. divergens, and additional organisms from the West Coast of the United States that are not yet classified to the species level.
Flagellates
Leishmaniae
The leishmaniae have undergone extensive classification revisions. However, from a clinical perspective, recovery and identification of the organisms are still related to body sites such as the macrophages of the skin (cutaneous), the skin and mucous membranes (mucocutaneous), and the whole reticuloendothelial system (visceral—bone marrow, spleen, liver). Recovery of leishmanial amastigotes is limited to the site of the lesion in infections other than those caused by the Leishmania donovani complex (visceral leishmaniasis). These protozoa have both amastigote (mammalian host) and promastigote (sand fly) stages in the life cycle. Reproduction in both forms occurs by binary longitudinal division. Their primary hosts are vertebrates; Leishmania commonly infects hyraxes, canids, rodents, and humans. Representative organisms include L. tropica, L. major, L. mexicana, L. braziliensis, and L. donovani.
Trypanosomes
The trypanosomes are normally identified to the species level based on geographic exposure history and clinical symptoms. These protozoa are characterized by having, at some time in the life cycle, the trypomastigote form with the typical undulating membrane and free flagellum at the anterior end. Unfortunately, the longer the duration of the infection, the more difficult it may be to confirm the diagnosis. The organisms that cause African sleeping sickness (T. b. gambiense and T. b. rhodesiense) generally cause different disease entities; one tends to be chronic and more typically the patient appears to have sleeping sickness (T. b. gambiense), and the other causes a more fulminant disease often leading to death before typical sleeping sickness symptoms can develop (T. b. rhodesiense).
The etiologic agent of American trypanosomiasis (formerly South American trypanosomiasis) is T. cruzi, which contains amastigote and trypomastigote stages (in the mammalian host) and the epimastigote form (in the arthropod host). Human American trypanosomiasis, or Chagas’ disease, is a potentially fatal disease of humans. It has two forms, a trypomastigote found in human blood and an amastigote found in tissues. The acute form usually goes unnoticed and may present as a localized swelling at the site of entry of the parasites in the skin. The chronic form may develop 10 to 20 years after infection. This form affects internal organs (e.g., the heart, esophagus, colon, and peripheral nervous system), and patients may die from heart failure. In 2007, the Red Cross began screening donor blood units for antibody to this parasite. Also, the geographic range of T. cruzi within the United States continues to expand and now includes all of Texas. Infected triatomid bug vectors are also present in other states such as California.
Nematodes (Intestinal)
The largest number of helminthic parasites of humans belongs to the roundworm group. Nematodes are elongate-cylindrical and bilaterally symmetrical with a triradiate symmetry at the anterior end. They have an outer cuticle layer, no circular muscles, and a pseudocoelom containing all systems (digestive, excretory, nervous, and reproductive). These organisms are normally acquired by ingestion of their eggs or skin penetration by larval forms from the soil. Nematodes commonly parasitic in humans include Trichuris trichiura, Necator americanus, Ancylostoma duodenale, Enterobius vermicularis, Ascaris lumbricoides, Strongyloides stercoralis, and Trichostrongylus spp.
Nematodes (Tissue)
Some tissue nematodes are rarely seen within the United States; however, some are more important and are found worldwide. Diagnosis may be difficult if the only specimens are obtained through biopsy and/or autopsy, and interpretation must be based on examination of histologic preparations. Examples include infections caused by Trichinella spp., Toxocara spp., Baylisascaris procyonis (raccoon roundworm), Ancylostoma spp., Angiostrongylus spp., and Gnathostoma spp.
Nematodes (Blood and Tissue)
Blood and tissue nematodes (filarial worms) are arthropod borne. The adult worms tend to live in the tissues or lymphatics
