Abstract
Typhoid fever is an illness caused by Salmonella enterica serotype Typhi. In developing regions, it affects an estimated 20 million people annually, causing 200 000 deaths. Although uncommon, cases occur in the USA each year, predominantly due to international travel. During February 2015, the Oklahoma State Department of Health (OSDH) detected an outbreak of typhoid fever among residents of northwestern Oklahoma. OSDH conducted case-patient interviews to identify the source and symptomatic contacts. Whole genome sequencing (WGS) was performed to characterise the genetic relatedness of isolates among the four outbreak-associated pulsed-field gel electrophoresis (PFGE) patterns. We identified 38 cases, 25 confirmed and 13 probable, in two states. WGS revealed a 0-10 single-nucleotide polymorphism variation between isolates. Although we were unable to determine the source, almost all case-patients were members of the Marshallese community that attended a common event in Oklahoma, or were contacts to a confirmed case. This is the largest outbreak of typhoid fever in the USA since 1989, and first to apply WGS to complement interpretation of PFGE results during a typhoid fever outbreak investigation. This investigation illustrates the potential risk of outbreaks among communities comprised of international populations from regions where typhoid fever remains endemic.
Highlights
Typhoid fever is caused by Salmonella enterica serotype Typhi
300 cases of typhoid fever occur in the USA each year, with more than 75% due to international travel [6,7,8]
In 21 (39%) of these outbreaks, an asymptomatic carrier was identified as the source; 13 (62%) involved a carrier that was either foreign born, reported international travel within the last year, or had a history of typhoid fever [9]
Summary
Typhoid fever is caused by Salmonella enterica serotype Typhi. 300 cases of typhoid fever occur in the USA each year, with more than 75% due to international travel [6,7,8]. Despite the low incidence of typhoid fever in the USA, outbreaks continue to occur due to contamination of food and water sources by infected individuals, or importation of contaminated products [9,10,11,12,13,14,15,16]. In 21 (39%) of these outbreaks, an asymptomatic carrier was identified as the source; 13 (62%) involved a carrier that was either foreign born, reported international travel within the last year, or had a history of typhoid fever [9]. Of the 28 outbreaks detected from 2000 to 2010, 16 (57%) were attributed to a confirmed or suspected carrier [10]
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