Intestinal, Urogenital & CNS Protozoa

Giardia and Entamoeba

Giardia lamblia. Cysts in untreated stream or well water, in daycare centers, and on travel are the usual sources, and as few as ~10 cysts can infect. In the duodenum each cyst releases pear-shaped, binucleate, flagellated trophozoites that cling to duodenal and jejunal epithelium by a ventral sucking disk; on microscopy they look like a face and move with "falling-leaf" motility. They do not invade, so the result is malabsorption rather than dysentery: foul-smelling, greasy, non-bloody stools, bloating, flatulence, sulfurous belching, and weight loss, without fever. Secretory antibody normally clears the parasite, so chronic or severe giardiasis points to selective IgA deficiency, common variable immunodeficiency, or X-linked agammaglobulinemia.

Giardia diagnosis and treatment. Stool antigen tests (EIA or DFA) and PCR panels are more sensitive than ova-and-parasite microscopy. Single-dose tinidazole, metronidazole, or nitazoxanide (a liquid, convenient for children) is effective, and boiling or filtering water prevents infection. Secondary lactose intolerance may persist for weeks and should not be mistaken for treatment failure.

Entamoeba histolytica. Cysts with up to four nuclei are swallowed in fecally contaminated food or water. Trophozoites invade the colonic mucosa and undermine it, producing flask-shaped ulcers and bloody dysentery with abdominal pain and tenesmus. Trophozoites containing ingested red cells (erythrophagocytosis) are diagnostic; stool antigen or PCR distinguishes E. histolytica from the look-alike, nonpathogenic E. dispar.

Amebic liver abscess. Trophozoites reach the liver through the portal vein and usually form a single abscess in the right lobe, which receives most of the portal blood from the right colon. Fever, right upper quadrant pain, and tender hepatomegaly are typical, often in young men and frequently without current diarrhea. Aspirated material is odorless, reddish-brown "anchovy paste" — liquefied liver rather than true pus. Serology is positive in most patients, and metronidazole cures most abscesses without drainage.

Metronidazole or tinidazole kills trophozoites in tissue but not the cysts in the bowel lumen, so every course is followed by a luminal agent — paromomycin (a nonabsorbed aminoglycoside) or iodoquinol. Asymptomatic cyst passers receive the luminal agent alone.

Acid-fast coccidia

Cryptosporidium. Small acid-fast oocysts (modified acid-fast stain) resist chlorine, so outbreaks follow swimming pools, water parks, and contaminated municipal water. In immunocompetent hosts the watery diarrhea resolves in 1–2 weeks. In AIDS with CD4 counts below 100, it becomes chronic, profuse, and wasting, and it can involve the biliary tree. Nitazoxanide is the specific drug, but in AIDS the essential treatment is antiretroviral therapy to restore immunity.

Cyclospora and Cystoisospora. Cyclospora cayetanensis arrives on imported fresh produce such as raspberries, basil, cilantro, and lettuce and causes prolonged, relapsing watery diarrhea with fatigue; its oocysts are variably acid-fast and autofluoresce under ultraviolet light. Cystoisospora belli causes chronic watery diarrhea in AIDS, sometimes with peripheral eosinophilia. Unlike Cryptosporidium, both respond to TMP-SMX.

Trichomonas vaginalis

Organism and presentation. Trichomonas is a pear-shaped, flagellated, motile trophozoite with no cyst stage; it dies quickly outside the body, so it spreads almost exclusively by sexual contact. Women develop a frothy, yellow-green, malodorous discharge with vulvar itching and dysuria; punctate hemorrhages give the cervix a strawberry appearance, and vaginal pH is above 4.5. Most men are asymptomatic or have mild urethritis.

Diagnosis and treatment. NAAT is the most sensitive test; a wet mount showing jerky, motile trichomonads is quick but misses many infections. Treatment is oral metronidazole (a 7-day course for women; a single 2 g dose is an option for men) or tinidazole, and all sex partners must be treated, with abstinence until both are cured. Women are retested within 3 months. Metronidazole is safe in pregnancy; it can cause a metallic taste and a disulfiram-like reaction with alcohol.

Toxoplasma gondii

Transmission. Cats are the definitive host and shed oocysts in their feces, contaminating litter, soil, and garden produce; people also swallow tissue cysts in undercooked pork, lamb, or venison. Primary infection during pregnancy can cross the placenta, so pregnant women avoid litter boxes and undercooked meat (congenital disease is covered with TORCH infections).

Immunocompetent hosts. Most infections are silent; some cause a heterophile-negative mononucleosis-like illness with cervical lymphadenopathy. Toxoplasma is also the leading infectious cause of posterior uveitis (retinochoroiditis).

AIDS. Latent cysts reactivate when CD4 falls below 100, causing headache, fever, confusion, focal deficits, and seizures, sometimes with chorioretinitis. MRI shows multiple ring-enhancing lesions with edema at the basal ganglia and gray-white junction. Patients are treated empirically, and failure to improve after about 2 weeks prompts brain biopsy.

Treatment and prevention. Pyrimethamine (a DHFR inhibitor) plus sulfadiazine (a dihydropteroate synthase inhibitor) blocks folate synthesis at two steps; leucovorin (folinic acid) is added because it bypasses DHFR in human cells and prevents megaloblastic marrow suppression. Patients with CD4 below 100 and positive Toxoplasma IgG receive TMP-SMX prophylaxis, which also prevents Pneumocystis pneumonia.

Free-living amebae

Naegleria fowleri. This ameba thrives in warm fresh water — lakes, hot springs, and poorly chlorinated pools — and in tap water used for neti pots. It enters the nose during swimming or diving and follows the olfactory nerves through the cribriform plate to the frontal lobes, causing primary amebic meningoencephalitis. Altered smell or taste, headache, fever, and a stiff neck progress within days to seizures, coma, and death in more than 95% of cases. CSF resembles bacterial meningitis (neutrophils, low glucose, high protein), but a wet mount shows motile amebae. Treatment combines amphotericin B and miltefosine.

Acanthamoeba. In contact lens wearers exposed to tap water, homemade saline, or swimming, it causes keratitis with pain out of proportion to the exam, photophobia, and a ring-shaped stromal infiltrate. It is often mistaken for HSV keratitis until antivirals fail. Corneal scrapings or confocal microscopy show double-walled cysts, and treatment uses topical biguanides or chlorhexidine. In immunocompromised hosts, blood-borne spread causes a subacute granulomatous amebic encephalitis.