コレラ

Vibrio cholerae, a Gram-negative bacterium, adheres to epithelial cells in the small intestine and releases cholera toxin. This toxin enters host cells and triggers excessive cyclic AMP production, opening ion channels that force water and electrolytes into the intestinal lumen. The resulting osmotic gradient causes profuse watery diarrhea and severe dehydration.

Cholera toxin consists of one A subunit and five B subunits. The B subunits bind to GM1 ganglioside receptors on epithelial cells, enabling toxin entry. The A subunit then ADP-ribosylates the Gs protein, locking it in an active state and causing adenylate cyclase overstimulation, which continuously produces cyclic AMP.

Cholera spreads primarily through the fecal-oral route via ingestion of contaminated water or food. Large outbreaks occur in overcrowded areas, after natural disasters, or during displacement where fecal contamination of drinking water is prevalent. Regions with poor sanitation and inadequate infrastructure face the highest disease burden.

Although over 200 serogroups of Vibrio cholerae exist, only O1 and O139 produce cholera toxin and sustain transmission in human populations. The O1 serogroup, particularly the El Tor biotype, has caused global pandemics since the 1960s. The O139 serogroup emerged in South Asia in 1992, marking a significant epidemiological shift.

Cholera causes painless, voluminous diarrhea without blood or leukocytes. Severe cases result in fluid losses exceeding 20 liters daily, leading to hypovolemic shock and electrolyte imbalance. Untreated mortality reaches 50%, but oral or intravenous rehydration therapy significantly improves outcomes. The disease typically resolves within 5 to 7 days.

The infectious dose of Vibrio cholerae is normally high due to the bacterium's vulnerability to acidic gastric conditions. However, reduced stomach acidity from medications, surgery, or co-ingested food allows lower inocula to initiate infection. Young children, older adults, and individuals with underlying health conditions face higher mortality risk.

Unlike bacterial gastroenteritis, cholera produces painless, watery diarrhea without blood or inflammatory cells. The disease mechanism involves toxin-mediated ion channel activation rather than tissue invasion. Cholera's rapid fluid loss and potential for hypovolemic shock distinguish it as a severe waterborne gastrointestinal emergency requiring immediate rehydration.