S. Virchow¶

At a Glance


Antigenic Formula	`6,7,14:r:1,2`
Serogroup	O:7 (C1)
NCBI Pathogen Detection	View isolates

Background Information¶

Salmonella enterica subsp. enterica serovar Virchow (antigenic formula 6,7,14:r:1,2) is a serovar of the O:7 (C₁) serogroup. Serovar Virchow was first isolated from a patient with a typhoid-like illness in 1930 by Kaufmann. This serovar has been reported to infect a infant that was transmitted by the household dogs. The ranking of serovar Virchow compared to other Salmonella serovars ranged from 4th to 8th place in Switzerland. Annual incidence rates demonstrated minor change during this period, declining from 0.45 per 100,000 people in 2004 to 0.40 per 100,000 in 2009. Children under 5 years of age showed a seven times higher Salmonella Virchow infection rate compared to those above 5 years. In South-East Asia, Europe, and Oceania, serovar Virchow is consistently found within the top 20 Salmonella serovars. Most often, serovar Virchow infection causes gastrointestinal illness, however, in Israel and other countries including US, human infections occur infrequently but demonstrate a high propensity for invasive manifestations.

Genetic Characteristics¶

Serovar Virchow has been found to be polyphyletic with two lineages identified. Weinberger et al. revealed that only 6% of serovar Virchow isolates in their study were sensitive to all ten antibiotics evaluated. Resistance patterns varied, with 34% resistant to one antibiotic, 54% resistant to one to three agents, and 40% resistant to four to six agents. Notably, a substantial proportion of isolates showed resistance to nalidixic acid (89%), streptomycin (56%), tetracycline (43%), trimethoprim-sulfamethoxazole (38%), and chloramphenicol (28%). However, none exhibited resistance to ciprofloxacin or ceftriaxone. Similarly, a Swiss study found that 48 serovar Virchow isolates (32%) showed resistance to 1-3 antibiotics, with a higher proportion (54 isolates, 36%) displaying resistance to more than three drugs. Resistance patterns remained stable throughout the study period (2004-2009). Nalidixic acid resistance was prevalent (62%), strongly suggesting concurrent decreased fluoroquinolone susceptibility. Ciprofloxacin showed intermediate resistance, and ESBL production was rare (2 isolates).

In an in-vivo study, after oral infection of 7-day-old chicks, Salmonella Virchow colonized both the gut and spleen. This infection triggered increased specific IgA, IgG, and IgM antibodies, along with changes in leukocyte populations (CD3, CD4, CD8α, CD8β, MHC II, KuL01, γδ TCR positive cells) locally and systemically. Elevated expression of the pro-inflammatory cytokines IL-1β and IL-6 and the chemokine CXCLi2 was also observed. While primary infection of S. Virchow provided limited systemic protection against re-infection with itself or S. Typhimurium, it failed to protect against cecal colonization.

Wang et al. used whole-genome sequencing and identified the emergence of monophasic Salmonella Virchow variant (serotype 6,7,14:r:-). This phenotype resulted from replacement of the fljAB operon with a 4.8-kb E. coli cassette, abolishing phase-2 flagellar antigen expression. Compared to 16 published serovar Virchow genomes, these two isolates possess a unique CRISPR type (VCT12) and show closest genetic relatedness to serovar Virchow strains BCW_2814 (Denmark) and BCW_2815 (China). Additionally, acquisition of Salmonella Genomic Island 2 (SGI2) harboring an antimicrobial resistance gene cassette conferred multidrug resistance to chloramphenicol, tetracycline, trimethoprim, and sulfamethoxazole.

Animal Reservoir¶

Poultry is the main reservoir for serovar Virchow.

Geographical Distribution¶

According to the metadata from NCBI PD, serovar Virchow has been reported in worldwide. In Scotland, serovar Virchow represented almost 25% of confirmed human salmonellosis cases, while in England and Wales, it was the third most common Salmonella serotype in humans, comprising 6% of isolates. However, its prevalence has significantly decreased, now accounting for less than 2% of non-typhoidal Salmonella (NTS) cases in the UK. In Australia, especially Queensland, serovar Virchow has been endemic since the mid-1980s.

Human/Animal Outbreaks¶

Year	Location	Associated source	Number of cases
2017-2023	Multiple countries	Kebab meat containing chicken	210¹
2016	US: multistate	Organic meal & shake powder	33
2012	US: WA	Undetermined	At least 120²
1994	Spain	Infant formula	48
1994	UK	Turkey sandwiches	11³
1968	UK	Spit-roasted chicken	Over 160

¹ Salmonella Virchow ST16 is the outbreak strain. In total, 210 cases have been reported across several countries, including Denmark (2), France (111), Germany (26), Ireland (4), the Netherlands (34), the UK (32), and the US (1).

² Among these cases, 43 were laboratory-confirmed, while 77 others showed symptoms and had a matching exposure history at the restaurant.

³ These cases were confirmed to get infected by Salmonella Virchow, phage type 26.

Border Rejections¶

Year	Exporting country	Importing country	Associated source	Product category
2021	India	Germany	Dog chews (dried tripes)	Pet food
2020	Poland	Italy	Frozen marinated chicken fillets¹	Poultry meat and poultry meat products

¹ Salmonella Enteritidis, Newport, and Virchow were found.

Recalls¶

Recalls have been associated with S. Virchow.

Year	Location	Recalled Product	Type
2023	Europe: multi-country¹	Frozen lime leaves²	Fruits and vegetables
2016	US: multistate	Meal replacement³	Other food product / mixed

¹ Ireland, Croatia, and Hungary issued recalls.

² Frozen lime leaves were from Thailand via Netherlands.

³ This recall was caused by a multistate outbreak described above. Garden of Life, LLC extended its recall of RAW Meal Organic Shake & Meal chocolate, original, vanilla, and vanilla chai products on February 12, 2016, adding more lots to the list.