S. Paratyphi A¶

At a Glance


Antigenic Formula	`1,2,12:a:[1,5]`
Serogroup	O:2 (A)
NCBI Pathogen Detection	View isolates

Background Information¶

Salmonella enterica subsp. enterica serovar Paratyphi A (antigenic formula: 1,2,12,a:[1,5]) is a serovar of the O:2 (A) serogroup. Serovar Paratyphi A, is one of the typhodial Salmonella serovars that leads to approximately 3.4 million cases and 19,100 deaths annually worldwide. This serovar mainly causes paratyphoid fever whose symptoms closely resemble those of typhoid fever, which is caused by Salmonella Typhi. Similar to typhoid, paratyphoid is endemic in low- and middle-income regions, particularly South Asia, accounting for 5–25% of enteric fever cases, primarily due to fecal contaminated water, food, and poor sanitation. Globally, paratyphoid fever is rarer than typhoid fever, with exceptions such as China and Myanmar. A study analyzing serovar Paratyphi A incidences across China, Indonesia, India, and Pakistan using standardized population-based methods found that Pakistan had the highest incidence (72 cases per 100,000 people annually), while Indonesia reported the lowest (13.7 cases per 100,000 per year). In 2019, prior to the COVID-19 pandemic, US recorded around 130 culture-confirmed cases of paratyphoid fever caused by Salmonella Paratyphi A. During 2020–2021, when travel restrictions were in place due to the pandemic, reported cases dropped to 30–50 annually. Infections from serovar Paratyphi B or C are exceedingly rare. About 80% of US paratyphoid cases are travel-related, with over 70% of these linked to travel to South Asia—particularly Bangladesh, India, and Pakistan. Other reported cases involve travelers returning from Africa, Latin America, Southeast Asia, and, less frequently, East Asia and the Caribbean.

Genetic Characteristics¶

Serovar Paratyphi A has been found to be polyphyletic with three lineages identified. Tanmoy et al. developed a genotyping tool, Paratype, based on single nucleotide polymorphisms (SNPs), which classifies Salmonella Paratyphi A into three main clades, nine sub-clades, and 18 genotypes. Each genotype is identified by a unique allele marker situated on an essential gene. Pereira-Dias et al. included a total of 117 serovar Paratyphi A isolates collected from an outbreak of enteric fever in Vadodara, India in their study, with the majority (72.6%) belonging to genotype 2.4.2, the predominant genotype worldwide. The remaining isolates were primarily genotype 2.3 (25.6%), while only two were identified as genotype 2.4.1. All outbreak isolates carried a single gyrA mutation linked to reduced fluoroquinolone susceptibility—74.35% had an S83F substitution, and the rest had an S83Y substitution. Rahman et al. studied serovar Paratyphi A isolated from enteric fever patients in Bangladesh from 2008 to 2018 and revealed that most serovar Paratyphi A isolates from Bangladesh (67.2%) belonged to the globally dominant lineage A, while the remaining isolates were distributed between lineages C (19.4%) and F (13.4%). The population structure exhibited minimal variation across different regions of the country. All Bangladeshi isolates carried point mutations in gyrA—either at codon 83 or 87—which are linked to reduced fluoroquinolone susceptibility. Additionally, they found a pHCM2-like cryptic plasmid closely related to plasmids found in Salmonella Typhi strains circulating in Bangladesh.

Jacob et al. suggested that genome degradation, gene acquisition, and loss are key drivers in the evolution of new serovar Paratyphi A lineages, with 10 pseudogene-forming mutations identified as potentially linked to lineage emergence. Pan-genome analysis highlights the insertion of P2/PSP3 phage in genotypes 2.3.2/2.3.3 and the acquisition of an IncX1 plasmid in genotype 1.2.2 as notable evolutionary events. Meanwhile, they identified six missense mutations in LPS biosynthesis genes, though structural predictions suggest these have minimal impact on lipopolysaccharide structure and likely do not affect vaccine effectiveness.

A review paper mentioned that unlike serovar Typhi, serovar Paratyphi A lacks the viaB locus and does not express the Vi capsule, instead evading antibody-mediated immunity through extended O-antigen chains. While SPI-1 expression and epithelial invasion are markedly reduced under aerobic conditions compared to microaerobic growth, serovar Paratyphi A uniquely exhibits enhanced SPI-2-dependent intracellular replication—a trait not observed in serovar Typhimurium. Genomic comparisons reveal distinct virulence gene profiles: SopE2 and CigR are exclusive to serovar Paratyphi A, whereas SteC, SifB, and SspH2 are specific to serovar Typhi.

Animal Reservoir¶

Serovar Paratyphi A is host-restricted to humans.

Geographical Distribution¶

Serovar Paratyphi A has been found worldwide, primarily in middle- and low-income countries like Pakistan, Nepal, India, and Bangladesh.

Human/Animal Outbreaks¶

Year	Location	Associated source	Number of cases
2014	India	Not identified¹	43
2010-2011	China	Uncooked vegetables irrigated with contaminated water	601
2009	Israel	Kosher food venue²	37
2004	China	Unboiled water³	394 (suspected)
1995	India	Vegetarian food⁴	33

¹ An unusual Salmonella Paratyphi A variety durazzo (2,12:a:-) was associated with this outbreak.

² The outbreak involved 37 Israeli travelers to Nepal. All of them developed bacteremia.

³ Among 394 suspected cases, 95.5% of them were students.

⁴ An unusual Salmonella Paratyphi A variety durazzo (2,12:a:-) was associated with this outbreak.

S. Paratyphi A¶

Background Information¶

Genetic Characteristics¶

Animal Reservoir¶

Geographical Distribution¶

Human/Animal Outbreaks¶

Border Rejections¶

Recalls¶

References¶