ATLANTA A multidrug-resistant strain of Salmonella serotype typhimurium known as Definitive Type 104 (DT104), which originally emerged in the United Kingdom, is now emerging in the United States, according to the Centers for Disease Control and Prevention (CDC).
During July and August 1996, the Public Health Laboratory Information System indicated that in 29 states, the number of Salmonella syphimurium isolates had substantially increased when compared with five years ago.
Although it is unknown whether the increases were associated with the emergence of DT104, the Salmonella strain resistant to ampicillin, chloramphenicol, streptomycin, sulfonamides and tetracycline was present in 90 (32%) of the 282 human Salmonella typhimurium isolates tested at CDC in 1996.
A Salmonella outbreak occurred among elementary school children in Cass County, Nebraska, during October 1996. In one week, 19 (59%) of 32 children attending the elementary school developed diarrhea (100%), fever (89%), headache (89%), nausea (89%) and vomiting (58%); three reported bloody diarrhea. None required hospitalization and all recovered, the CDC reported.
Culture stool samples from seven children all yielded Salmonella typhimurium R-type ACSSuT. Phage-typing at the CDC confirmed the isolates as DT104.
During school lunch on Oct. 10, the children were served cold chocolate milk poured from cartons. Of the 22 children who drank the milk, 18 (82%) developed diarrhea.
Inspection of the school refrigerator revealed numerous milk cartons with expiration dates predating Oct. 10, but cultures of samples from these remaining cartons were negative for enteropathogens. In addition, some children handled a turtle brought in for "show and tell" and a reportedly ill kitten during Oct. 7-9. However, neither the turtle nor the kitten were available for testing.
Reservoirs for DT104 infection in the United States are not known, but the Nebraska outbreak suggests possible associations with animal reservoirs. The CDC is conducting a national case-control study of Salmonella typhimurium infections through the Emerging Infections Diseases Program's Foodborne Diseases Active Surveillance Network (FoodNet).
Resistance to chloramphenicol in Salmonella typhimurium isolates is a highly specific marker for DT104 and state health departments investigating clusters of infections can measure chloramphenicol resistance and, if present, contact the CDC's Foodborne and Diarrheal Diseases Branch, Division of Bacterial and Mycotic Diseases, National Center for Infectious Diseases.
DT104 was first identified in the United Kingdom in 1984 and is now the second most prevalent strain of Salmonella isolated from humans after Salmonella serotype enteritidis phage type 4. DT104 isolates in the United Kingdom are highly resistant to ampicillin, chloramphenicol, streptomycin and tetracycline.
Contact with ill farm animals and consumption of chicken, pork sausages and meat paste were identified as risk factors for DT104 infection in England and Wales, and an outbreak investigation in 1995 implicated beef consumption. The organism has been isolated from several species (poultry, sheep, pigs, cats, wild birds, rodents, foxes and badgers) and has been transmitted from cattle and sheep to humans, but the ecology of the organism in the United States has not been well characterized. Efforts similar to those taken in the United Kingdom are needed to elucidate its distribution in the environment and human food chain, according to the CDC.
However, transmission of Salmonella typhimurium through food probably occurs through a complex route that may include wild animals, animal feed, farm animals, slaughterhouses, processing and distribution networks, retail outlets and the consumer.
Although R-type ACSSuT is the most common antimicrobial resistance pattern of DT104 isolates, resistance to trimethoprim and fluoroquinolones is emerging in the United Kingdom, the CDC stated. From 1993 to 1995, trimethoprim-resistant DT104 (R-type ACSSuTTm) increased from 1% to 27% of isolates and ciprofloxacin-resistant DT104 (R-type ACSSuTCp) increased from 0% to 6% of isolates.
According to the CDC, use of trimethoprim to treat DT104 R-type ACSSuT infections in cattle may be responsible for the resistance. In addition, the emergence of fluoroquinolone resistance may be related to veterinary use enrofloxacin was licensed for veterinary use in the United Kingdom in 1994, and the observed increased resistance of human isolates of DT104 to fluoroquinolones followed this introduction.
The Food and Drug Administration (FDA) in 1995 approved the fluoroquinolone sarafloxacin for treatment of Escherichia coli infections in poultry flocks, but fluoroquinolone resistance has not been detected in DT104 isolates from humans in the United States. However, ciprofloxacin is currently a treatment of choice for Salmonella infections in adults.
For more information:
- CDC. Multidrug-resistant Salmonella serotype typhimurium - United States, 1996. MMWR 1997;14:308-10.
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