Tags: Metronidazole


Nematodes (roundworms) are nonsegmented, tapered, bilaterally symmetrical, cylindrical organisms that have complete digestive tracts and reproduce sexually. Although > 500,000 species of nematodes have been described, only a small number are commonly encountered as human parasites. Most nematodes have complex life cycles, sometimes involving several larval forms and intermediate hosts or free-living stages. The pathogenic nematodes may be categorized as primarily intestinal or extraintestinal tissue parasites (Box 1).


Giardia, a genus of primitive eukaryotes, is a flagellated enteric protozoan of the class Zoomastigophorea. Giardia lamblia, also known as Giardia intestinalis or Giardia duodenalis, is the species known to infect humans. Its name comes from Vilem Lambl, who first reported the organism in 1859.

Giardia: Clinical Syndromes

After ingestion of G lamblia cysts, 5-15% of patients will have asymptomatic cyst passage, and 25-50% of patients will have diarrhea. From 35% to 70% of these patients will have no evidence of infection. The three manifestations of infection include asymptomatic cyst passage, self-limited diarrhea, and chronic diarrhea with associated malabsorption and weight loss. Factors related to each of these manifestations are unknown but are believed to be related to specific host factors, parasite load, and virulence variation among G lamblia isolates.

Cryptosporidium, Cyclospora, & Isospora Species & Microsporidia

Within the last decade, the AIDS epidemic has heightened awareness of several gastrointestinal spore-forming protozoan pathogens. The genera Cryptosporidium, Isospora, and Cyclospora are members of the subclass Coccidia and phylum Apicomplexa; the microsporidia are a group of organisms belonging to the phylum Microspora.


Isospora infection is endemic in several tropical and subtropical climates in areas of South America, Africa, and southwest Asia. In the United States, Isospora belli infection occurs primarily in patients with AIDS but is still quite rare in this population, accounting for = 0.2% of AIDS-defining illnesses. Isospora infection is more common in patients with AIDS from developing countries in which the prevalence of spore passage is 15% compared with 5% in industrialized nations.

Pathogenic Amebas

There are numerous distinct species of ameba within the genus Entamoeba, and the majority of these do not cause disease in humans. E histolytica is a pathogenic species that is capable of causing disease, such as colitis or liver abscess, in humans. E dispar is prevalent and is indistinguishable from E histolytica by conventional laboratory methods. E dispar exists in humans in only an asymptomatic carrier state and does not cause colitis.

Chlamydia Trachomatis Infections

C trachomatis is associated with urethritis, proctitis, conjunctivitis, and arthritis in women and men; epididymitis in men; and mucopurulent cervicitis (MPC), acute salpingitis, bartholinitis, and the Fitz-Hugh and Curtis syndrome in women (Box 1). C trachomatis and Neisseria gonorrhoeae (see site) coinfections are common in women with MPC and men with urethritis. In men, C trachomatis is the most common etiologic agent of the nongonococcal (NGU) and postgonococcal urethritis (PGU) syndromes.


Disease occurs when mechanical insult disrupts the mucosal barrier or organisms gain access to privileged sites. For example, actinomycosis commonly occurs after dental procedures, trauma, surgery, or aspiration. Actinomyces israelii causes the majority of human disease owing to this genus, but other species, including Actinomyces naeslundii, Actinomyces viscosus, Actinomyces enksonii, Actinomyces odontolyticus, and Actinomyces meyeri have also been implicated. Actinomycosis is threefold more common in men than women.