Microbiology

Cholera is caused by Vibrio cholerae serogroups O1 (classic and El Tor biotypes) and O139-highly motile, facultatively anaerobic, curved gram-negative rods. The natural habitat of V. cholerae is coastal salt water and brackish estuaries. Toxin production causes disease manifestations.

Epidemiology

Currently, Africa, Asia, and the Americas are the source of >40%, 35%, and 20% of cases reported to the World Health Organization (WHO), respectively; however, an estimated 90% of cases go unreported or do not have a specific bacterial etiology identified.

• It is estimated that there are >2-3 million cases annually, with >50,000-100,000 deaths.
• Spread takes place by fecal contamination of water and food sources. Infection requires ingestion of a relatively large inoculum (compared with that required for other pathogens) of 10⁵ -10⁸ organisms.

Clinical Manifestations

After an incubation period of 24-48 h, pts develop painless watery diarrhea and vomiting that can cause profound, rapidly progressive dehydration and death within hours.

• Volume loss can be >250 mL/kg in the first day.
• Stool has a characteristic “rice-water” appearance: gray cloudy fluid with flecks of mucus; no blood; and a fishy, inoffensive odor.

Diagnosis

Stool cultures on selective medium (e.g., thiosulfate-citrate-bile salts-sucrose [TCBS] agar) can isolate the organism. A point-of-care antigen-detection assay is available for field use.

Microbiology and Epidemiology

These single-stranded RNA viruses are common causes of traveler's diarrhea and of viral gastroenteritis in pts of all ages as well as of epidemics worldwide, with a higher prevalence in cold-weather months. In the United States, ∼50% of outbreaks of nonbacterial gastroenteritis are caused by noroviruses. Very small inocula are required for infection. Thus, although the fecal-oral route is the primary mode of transmission, aerosolization, fomite contact, and person-to-person contact can also result in infection.

Clinical Manifestations

After a 24-h incubation period (range, 12-72 h), pts experience the sudden onset of nausea, vomiting, diarrhea, and/or abdominal cramps with constitutional symptoms (e.g., fever, headache, chills). Stools are loose, watery, and without blood, mucus, or leukocytes. Disease lasts 12-60 h.

Diagnosis

PCR assays have been developed to detect these viruses in stool and other body fluids. Because of poor sensitivity, enzyme immunoassays (EIAs) have limited clinical utility outside of outbreaks.

Microbiology and Epidemiology

Rotavirus is a segmented, double-stranded RNA virus that infects nearly all children worldwide by 3-5 years of age; adults can become infected if exposed.

• Reinfections are progressively less severe.
• Large quantities of virus are shed in the stool during the first week of infection, and transmission takes place both via the fecal-oral route and from person to person.
• Disease incidence peaks in the cooler fall and winter months.

Clinical Manifestations

After an incubation period of 1-3 days, disease onset is abrupt. Vomiting often precedes diarrhea (loose, watery stools without blood or fecal leukocytes), and about one-third of pts have temperatures >39°C (>102.2°F). Symptoms resolve within 3-7 days.

Diagnosis

EIAs or viral RNA detection techniques, such as PCR, can identify rotavirus in stool samples.

Microbiology and Epidemiology

Giardia lamblia (also known as G. intestinalis or G. duodenalis) is a protozoal parasite that inhabits the small intestines of humans and other mammals.

• Cysts are ingested from the environment, excyst in the small intestine, and release flagellated trophozoites that remain in the proximal small intestine. Some trophozoites encyst, with the resulting cysts excreted in feces.
• Transmission occurs via the fecal-oral route, by ingestion of contaminated food and water, or from person to person in settings with poor fecal hygiene (e.g., day-care centers, institutional settings). Infection results from as few as 10 cysts.
• Viable cysts can be eradicated from water by either boiling or filtration. Standard chlorination techniques used to control bacteria do not destroy cysts.
• Young pts, newly exposed pts, and pts with hypogammaglobulinemia are at increased risk-a pattern suggesting a role for humoral immunity in resistance.

Clinical Manifestations

After an incubation period of 5 days to 3 weeks, the manifestations of infection range from asymptomatic carriage (most common) to fulminant diarrhea and malabsorption.

• Prominent early symptoms include diarrhea, abdominal pain, bloating, belching, flatus, nausea, and vomiting and usually last >1 week. Fever is rare, as is blood or mucus in stool.
• Chronic giardiasis can be continual or episodic; diarrhea may not be prominent, but increased flatulence, sulfurous belching, and weight loss can occur.
• In some cases, disease can be severe, with malabsorption, growth retardation, dehydration, and/or extraintestinal manifestations (e.g., anterior uveitis, arthritis).

Diagnosis

Giardiasis can be diagnosed by parasite antigen detection in feces, by identification of cysts (oval, with four nuclei) or trophozoites (pear-shaped, flattened parasites with two nuclei and four pairs of flagella) in stool specimens, or by nucleic acid amplification tests. Given variability in cyst excretion, multiple samples may need to be examined.

Microbiology and Epidemiology

Cryptosporidial infections are caused by Cryptosporidium hominis and C. parvum.

• Oocysts are ingested and subsequently excyst, enter intestinal cells, and generate oocysts that are excreted in feces. The 50% infectious dose in immunocompetent individuals is ∼132 oocysts.
• Person-to-person transmission of infectious oocysts can occur among close contacts and in day-care settings. Waterborne transmission is common. Oocysts are not killed by routine chlorination.

Clinical Manifestations

After an incubation period of ∼1 week, pts may remain asymptomatic or develop watery, nonbloody diarrhea, occasionally with abdominal pain, nausea, anorexia, fever, and/or weight loss lasting 1-2 weeks. In immunocompromised hosts (particularly those with CD4⁺ T cell counts <100/µL), diarrhea can be profuse and chronic, resulting in severe dehydration, weight loss, and wasting; the biliary tract can be involved.

Diagnosis

On multiple days, fecal samples should be examined for oocysts (4-5 µm in diameter, smaller than most parasites). Although conventional stool examination for ova and parasites does not detect Cryptosporidium, modified acid-fast staining, direct immunofluorescent techniques, and EIAs can facilitate diagnosis.