How is Shigellosis Diagnosed?
Shigella infections are diagnosed with a laboratory test (stool culture) on a person’s stool specimen. Prompt processing of specimens and use of appropriate culture media increases the likelihood of isolating the bacteria. The laboratory can also do special tests to tell which species of Shigella the person has and which antibiotics would be best to treat it; antibiotic sensitivity tests are important since Shigella is often resistant to multiple antibiotics.
Advanced testing methods, such as plasmid profiling and chromosomal fingerprinting using pulsed-field gel electrophoresis (PFGE) are two molecular techniques that can help to characterize Shigella isolates in food and human samples. These tests can assist in determining whether cases are isolated or associated with outbreaks.
The number of shigellosis cases reported annually to the Centers for Disease Control and Prevention (CDC) has varied over the past several years, reaching all-time low of 14,000 in 2004, to almost 20,000 in 2007 (CDC, 2009b). Many cases go undiagnosed and/or unreported, however. The CDC estimates that 450,000 total cases of shigellosis occur in the U.S. every year (Baer et al., 1999; CDC, 2009a).
Shigellosis is characterized by seasonality, with the largest percentage of reported cases occurring between July and October and the smallest proportion occurring in January, February, and March (Gupta et al., 2004).