S. Thompson¶

At a Glance


Antigenic Formula	`6,7,14:k:1,5`
Serogroup	O:7 (C1)
NCBI Pathogen Detection	View isolates

Background Information¶

Salmonella enterica subsp. enterica serovar Thompson (antigenic formula 6,7,14:k:1,5) is part of Group O:7 (C1). The first isolation of serovar Thompson was reported by Scott (1926) during a foodborne illness outbreak in Yorkshire, suspected to be linked to the consumption of rabbit and beef pie. Surveillance data indicate that serovar Thompson has remained one of the top ten Salmonella serovars causing diarrhea in both China and US over the last ten years. Zhang et al. suggested that serovar Thompson can be transmitted between poultry farms and humans. Serovar Thompson has been found to form biofilms on eggshells, leading to potential contamination risks.

Genetic Characteristics¶

Serovar Thompson has been found to be polyphyletic with four lineages identified and one stand-alone singleton that does not cluster with any other Salmonella Thompson isolates. Zhang et al. revealed that all serovar Thompson strain in their study carried aac(6′)-Iaa and the polymyxin resistance gene mcr-9, though susceptibility to polymyxin B confirmed that mcr-9 did not produce a resistant phenotype. Antimicrobial sensitivity testing against 18 antibiotics showed that serovar Thompson isolates were resistant to streptomycin (100%), ampicillin (35.7%), and doxycycline (14.3%). Additionally, all serovar Thompson isolates harbored well-characterized virulence genes, including those from Salmonella pathogenicity islands (SPI1-5), fimbriae (fimA), flagellar assembly (bcfC, flhD, fliA, fliC, fljB, flgK, lpfC), and other virulence factors (iroN, pagC, cigR). Pathogenicity was further confirmed through yolk sac inoculation in 1-day-old chicks, suggesting that clinical serovar Thompson strains are highly pathogenic to newborn chicks.

Another Chinese study analyzed 141 serovar Thompson ST26 isolates from diverse sources and found that 29 strains—25 from diarrhea patients and four from animal-derived foods—exhibited co-resistance to ciprofloxacin, cefotaxime, and azithromycin (CIP^RCTX^RAZI^R), key frontline antimicrobials for treating human Salmonella infections. The predominant mobile genetic element in these CIP^RCTX^RAZI^R isolates was the IncC plasmid, which carried four resistance genes (qnrS1, qepA4, bla_CMY-2, and mph(A)) conferring resistance to these priority antibiotics. Closely related IncC-harboring isolates (0–23 SNPs) suggested clonal dissemination across multiple Chinese provinces. Notably, the presence of CIP^RCTX^RAZI^R strains in aquatic food products implied the potential role of aquaculture in driving the emergence and spread of high-risk antimicrobial resistance. Sheng et al. stated that, in China, serovar Thompson is the dominant Salmonella serovar carrying the mcr-9 gene. They found that chicken juice substantially increases the proportion of mcr-9-positive serovar Thompson transconjugants. The dissemination of mcr-9 is primarily linked to the IncHI2-IncHI2A plasmid.

Animal Reservoir¶

Unknown, chicken may be the potential reservoir for serovar Thompson. According to the metadata from NCBI PD, serovar Thompson was isolated from other animals such as cattle, horses, and turtles.

Geographical Distribution¶

Serovar Thompson has been reported worldwide.

Human/Animal Outbreaks¶

Year	Location	Associated source	Number of cases
2024	US: WA	Food or drink from IHOP in Bellevue	32
2023	US: multistate	Fresh diced onions	80
2021	US: multistate	Seafood	115
2019	US: multistate	Chicken¹	34
2018	US: multistate	Kratom²	199
2018	Korea	Chocolate cake	1,111
2017	US: multistate	Imported maradol papayas³	220
2006	US: SC	Peanuts	100

¹ The outbreak was associated with consumption of chicken purchased from an El Pollo Loco restaurant located in Barstow, California.

² The outbreak involved six serovars including Salmonella I 4,[5],12:b:-, Heidelberg, Javiana, Okatie, Weltevreden, and Thompson. The case number here (n=199) represents the total cases in this outbreak, not just those infected with serovar Thompson.

³ Across 23 states, 220 infections were linked to five Salmonella serovars involved in the outbreak—specifically Salmonella Thompson (144 cases), Kiambu (54 cases), Agona (12 cases), Gaminara (7 cases), and Senftenberg (3 cases).

Border Rejections¶

There have been no recent border rejections linked to this serovar.

Recalls¶

Year	Location	Recalled food	Type
2023	US: multistate	Fresh diced onions¹	Fruits and vegetables
2021	US: multistate	Seafood²	Fish and fish products
2017	US: multistate	Imported maradol papayas³	Fruits and vegetables

¹ This recall was caused by a multistate outbreak described above. On October 23, 2023, Gills Onions issued a recall of their fresh diced onion products.

² This recall was caused by a multistate outbreak described above. On October 8, 2021, Northeast Seafood Products, Inc. issued a recall of their multiple products.

³ This recall was caused by a multistate outbreak described above. Five Salmonella serovars, Salmonella Thompson, Kiambu, Agona, Gaminara, and Senftenberg, were involved. Grande Produce, Agroson’s LLC, and Freshtex Produce, LLC issued recalls of their papayas.