S. Napoli¶

At a Glance


Antigenic Formula	`1,9,12:l,z13:e,n,x`
Serogroup	O:9 (D1)
NCBI Pathogen Detection	View isolates

Background Information¶

Salmonella enterica subsp. enterica serovar Napoli (antigenic formula 1,9,12:l,z₁₃:e,n,x) is a serovar of the O:9 (D₁) serogroup. In 2006, serovar Napoli ranked 22nd in the Enter-net Salmonella database, accounting for 295 cases (0.28%) out of 105,635 reported across 29 European countries. Between 2000 and 2006, Europe saw a 140% rise in serovar Napoli infections, with over 87% of cases originating in France, Italy, and Switzerland. This serovar has been an emerging serovar in these three countries, particularly in Italy, where it is the fourth most frequent Salmonella serovar, responsible for 2-4% of human infections. Additionally, from 2010 to 2014, serovar Napoli was the leading cause of invasive salmonellosis, affecting 40 out of 687 cases in Italy—a notably higher invasive rate compared to other nontyphoidal serotypes. Although serovar Napoli is a nontyphodial Salmonella serovar some of its symptoms, including a prolonged incubation period (7–14 days), high fever, bacteremia, and elevated rates of hospitalizations, are similar to those caused by typhoidal Salmonella. Serovar Napoli has also been frequently isolated from rucola (rocket salad).

Genetic Characteristics¶

Serovar Napoli has been found to be monophyletic. An Italian study used genomic and phylogenetic approaches and found that an invasive strain of serovar Napoli was part of the Typhi subclade within clade A, with Paratyphi A as its closest serovar. This isolate was found to have the typhoid-associated genes hlyE and taiA (SPI-18), cdtB, pltA, and pltB (CdtB-Islet), as well as the GICT18/1 islet. Another Italian study analyzed 112 serovar Napoli isolates and revealed that all tested isolates were susceptible to the examined antimicrobials, with SPI-associated genes (ssaQ, mgtC, spi_4D, and sopB) present in 75–100% of isolates, while avrA was rare (detected in only one human and four environmental isolates). The phage-related sopE1 appeared in 93% of strains, whereas sodC1 and gipA were limited to four and two environmental isolates, respectively. The fimbrial gene bcfC was universal in animal/food strains but less frequent in environmental (71.4%) and human (46.8%) isolates. Ten virulence profiles (VPs) were identified, with VP1–3 encompassing 99 of 112 isolates. PFGE analysis grouped 103 of 111 isolates into four major and three minor clusters (two isolates were unrelated), with dominant clusters strongly correlating with VP1–3.

Mastrorilli et al. further corroborated the genomic relatedness of serovar Napoli to typhoidal serovars (Salmonella Typhi and Paratyphi A), with two distinct subclades evident, including the biphyletically distributed ST-474. All sequence types (STs) were distributed across isolation sources and years. Compared to other non-typhoidal serovars, serovar Napoli harbors fewer SPIs and ARGs and rarely acquires plasmids. Notably, their study documented the second known instance of an extended-spectrum β-lactamase (ESBL)-producing serovar Napoli isolate and the first reported cases of MDR serovar Napoli isolates, all of human origin.

Animal Reservoir¶

Unknown, poultry and wild boars may be potential reservoirs for serovar Napoli. It has also been isolated from wild birds but uncommonly from cattle.

Geographical Distribution¶

Serovar Napoli has been reported in US, Canada, Australia, and Europe, mainly in Italy.

Human/Animal Outbreaks¶

Year	Location	Associated source	Number of cases
2023	Norway	Not identified	7
2014	Italy	Ham	47
2011	Italy	Water	10
1982	England and Wales	Chocolate bars	245

Animal outbreaks:

Year	Location	Breed	Number of cases
2022-2023	Italy	Holstein Friesian	4¹

¹ The herd consisted of 310 cows. There were four animals infected: two calves and two milking cows.

S. Napoli¶

Background Information¶

Genetic Characteristics¶

Animal Reservoir¶

Geographical Distribution¶

Human/Animal Outbreaks¶

Border Rejections¶

Recalls¶

References¶