Typhoid Fever (Salmonella Typhi)
Salmonella Typhi is an uncommon but important enteric and systemic bacterial pathogen in the United States (US), causing an estimated 400 cases per year. While uncommon in the US, typhoid fever is highly endemic in developing countries in Africa, Asia (especially Southeast Asia and the Indian subcontinent), and Central and South America. Most cases in the US are travelers returning from endemic areas. S. Typhi infection is restricted to humans, and food or water contaminated by the feces or urine of typhoid fever cases or carriers are the leading sources of exposure.
Acute illness, usually gastroenteritis, occurs after an incubation that varies from 3 to over 60 days depending on size of the inoculum and host factors. Onset is often insidious. A carrier state may follow acute illness or mild or even subclinical infections. About 1.0 to 4.0 percent of untreated cases will become carriers and the chronic carrier state is more common among persons infected during middle age. S. Typhi resistant to first-line drugs became so common by the 1990s, that fluoroquinolones became the drugs of choice for treatment. However, nalidixic acid-resistant S. Typhi, with decreased susceptibility to fluoroquinolones, and fluoroquinolone-resistance have now been reported in South and Southeast Asia. Two typhoid vaccines are currently available in the US. Both vaccines confer about 70 percent protection in older children and adults but neither is licensed for use in young children. Notably, vaccine induced immunity provides little protection against large challenge doses.