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Chapter 31 : Protozoan Parasites
Category: Applied and Industrial Microbiology; Food Microbiology
Protozoan parasites have long been associated with foodborne and waterborne outbreaks of disease in humans. A major characteristic of apicomplexan parasites is that a vertebrate host is required to complete the complex life cycle and produce infectious cysts. Of this group, Cryptosporidium species, Cyclospora cayetanensis, and Isospora belli inhabit the intestinal mucosa and produce diarrheal illnesses in humans. Microsporidia, once considered to be protozoan parasites, have been reclassified as fungi based on recent phylogenetic analyses. Cryptosporidium was first isolated in 1910 from the intestines of mice. Various species of Cryptosporidium capable of infecting animals have since been described. Genotype differentiation is based on the sequencing of the thrombospondin-related adhesive protein (TRAP-C2). Cyclospora was probably first reported to occur in humans in 1979 by Ashford, who described it as an Isospora-like coccidian affecting humans in Papua New Guinea. In Nepal, the prevalence of Cyclospora infection is highest in adult expatriates, whereas in areas of Peru, where infection is endemic, children under 10 years of age are the most susceptible to infection, though most are asymptomatic. Isospora is a coccidian parasite which infects humans. Isospora can be acquired by ingestion of contaminated food or water. Toxoplasmosis can be acquired by ingestion of lamb, poultry, horse, and wild game animals. Giardia infection can be diagnosed by finding cysts or, less commonly, trophozoites in fecal specimens. Fulminant colitis is characterized by severe bloody diarrhea, fever, and abdominal tenderness due to transmural necrosis of the bowel.
Scanning electron micrographs of oocysts and excysting sporozoites of C. parvum. (A) Intact oocyst prior to excystation (×11,200). (B) Three sporozoites (Sp) excysting from oocyst simultaneously via the cleaved suture (Su) (×11,200). (C) Empty oocyst (×11,200). (D) Excysted sporozoite; Ae, apical end (×9,800). From Fayer et al. ( 61a ).
C. cayetanensis oocysts. (A) Phase-contrast microscopy of unsporulated oocysts. (B) Oocysts (OO) in the process of excystation. Note the two sporozoites (SP) free of the sporocyst (SC). (C) Transmission electron microscopy of human small intestine showing Cyclospora intracellular stages. ME, merozoite.
Bright-field photomicrograph of I. belli oocysts unsporulated (A) and sporulated (B).
Transmission electron micrographs of T. gondii. (A) Sporozoite in parasitophorous vacuole (Pv) of host cell (Hc) at 24 h after inoculation; Am, amylopectin granule; Co, conoid; Mn, microneme; Nu, nucleus of sporozoite; Rh, rhoptry. (B) Final stage of endodyogeny to form two daughter tachyzoites that are still attached to the posterior ends (arrowheads); DG, dense granules; HC, host cell; IT, intravacular tubules; M, mitochondrion; MC, micro-neme; MI, micropore; N, nucleus; RO, rhoptry. From Dubey and Beattie ( 55 ).
Scanning electron micrograph of G. lamblia trophozoites. One trophozoite shows the dorsal surface and the other shows the ventral surface with sucking disk and flagella.
Hematoxylin and eosin staining. (A) G. lamblia cyst (a) with two nuclei and trophozoite (b) showing two nuclei and median body visible at one pole. (B) B. coli cyst (a) showing large macronucleus and cilia beneath cyst wall and trophozoite (b) showing oval macronucleus. (C) E. histolytica trophozoite (a) showing a nucleus and few red blood cells in the cytoplasm and cyst (b) showing two of four nuclei and rod-shaped inclusion bodies with rounded ends. (Courtesy of Lynne S. Garcia.)
Protozoa of medical importance acquired from food and water