Salmonella is one of the most important bacterial pathogens affecting the poultry industry worldwide. Poultry, especially chickens, are a major reservoir of this bacterium and can carry it without showing clinical signs. In Europe, Regulation (EU) No 2160/2003 stipulates that for breeders, the 5 most common serovars (S. Enteritidis, S. Infantis, S. Hadar, S. Virchow and S. Typhimurium, including monophasic variants) (REGULATION (EU) No 200/2010) should be targeted.

Salmonella bacteria are classified into serotypes according to the Kauffman–White scheme, which uses surface antigens to differentiate strains. The O-antigen represents the somatic lipopolysaccharide, and the H-antigen represents flagellar proteins. Based upon reaction with specific antisera (against the above-mentioned antigens), Salmonella species are classified into so-called serogroups (Table 1). It has been shown that vaccines can give cross-protection within the same sero group, which is relevant for vaccination strategies, since there is no specific vaccine for each Salmonella serotype.

Salmonella Infantis

The increase of S. Infantis infections has been attributed to the presence of the megaplasmid pESI (plasmid for Emerging Salmonella Infantis). It appears that pESI confers a clear advantage in a poultry-host or poultry-related farm environment. Advantageous genes incorporated into the plasmid include expression of unique fimbriae, a siderophore-iron transport system, heavy-metal resistance genes and antibiotic resistance genes. Moreover, superior capabilities for biofilm formation and adhesion, invasion into avian and mammalian host cells and bacteriostatic/bactericidal properties of disinfectants have been described.

Epidemiology and distribution

S. Infantis is ranked as the fourth leading Salmonella serovar causing human salmonellosis in Europe. It became dominant in broiler flocks, accounting for 57% of isolates from broiler meat, according to the European Union One Health 2024 Zoonoses Report. Vertical transmission seems to be less important than for some other serovars. The pESI plasmid contributes to its spread and survival in the broiler house and in the environment. Longer-lived birds, such as layers, seem to be less susceptible, which is probably related to a more mature gut morphology and microbiome. Biosecurity measures are of the utmost importance for control, but once it has been shown to be present in a broiler flock, it is very hard to eradicate it from the house and the surrounding environment, leading to consecutive positive flocks and increasing the risk of further spread to other broiler flocks.

S. Infantis in stealth mode

A major concern is the under-detection of these bacteria due to the presence of atypical strains that lack O-antigen expression. Identifying atypical strains would pose a diagnostic challenge because they would not be recognised as Salmonella Infantis by traditional serotyping, which remains by far the most common method used by laboratories responsible for Salmonella surveillance. This difficulty in identifying such atypical strains could compromise the recognition of infected poultry flocks.

Vaccination: AviGate S. Infantis

Today, there is only one live attenuated vaccine for S. Infantis (AviGate S. Infantis), which is registered for active immunisation of healthy chickens to reduce faecal excretion and colonisation of internal organs with Salmonella Infantis. The vaccine strain was constructed by deleting 3 genes (cpxR, cpxA, and lon1), resulting in a strain with decreased environ mental survival, reduced replication and systemic infection in host cells, allowing clearance from host cells without causing side effects. S. Infantis belongs to the serogroup C, opening up perspectives for combatting Salmonella spp. of serogroup C.