Characteristics of Salmonella spp. in Pigs

Salmonellosis is an important infectious disease caused by Salmonella spp., a group of Gram-negative bacteria widely distributed among warm-blooded animals, including livestock and humans. Due to its zoonotic nature, Salmonella represents a major concern for animal health, public health, and food safety in pig production systems. 

A defining characteristic of Salmonella spp. is their high environmental resistance. The bacterium survives across a wide range of conditions, tolerating pH values from 4 to 9 and temperatures between 6 and 42 °C (Ricke et al., 2013; Adley & Ryan, 2016). Survival is particularly prolonged in organic material; under freezing or drying conditions, Salmonella may persist for up to 50 months in slurry or dusty environments (Van der Wolf et al., 2021). This persistence contributes to the endemic nature of salmonellosis on pig farms. 

Currently, 2,659 serotypes of Salmonella have been officially described (Yang et al., 2022). In pigs, there are two serotypes which have primary importance: Salmonella choleraesuis and Salmonella typhimurium. 

Salmonella choleraesuis is associated with the septicemic form of the disease and is considered pig-adapted, although it can infect humans. Clinical signs include fever, depression, anorexia, dyspnea, wet cough, cyanosis, and bluish-red discoloration of the ears, chest, and ventral abdomen. Diarrhea may be absent. In the necropsy, it can be found commonly splenomegaly, gastritis with eroded gastric mucosa, enlarged mesenteric and gastrohepatic lymph nodes, pulmonary congestion, pneumonia, and occasional enterocolitis. The main way of transmission may occur via feces and milk, particularly from asymptomatic sows (Schwartz, 2021). 

Salmonella typhimurium is linked to the enteric form of salmonellosis and it is frequently subclinical. Infected pigs may shed the bacterium intermittently for long periods without clinical signs. When the disease occurs, it is characterized by yellow-orange diarrhea, dehydration, reduced feed intake, and fever. The mortality is increased in the presence of immunosuppressive diseases such as influenza, PRRS, or PCV2. The recovered pigs may remain carriers for at least five months. The yypical lesions include enlarged lymph nodes and necrotizing enterocolitis with “button ulcers”, mainly affecting the ileum, cecum, and colon. Clinical outbreaks are often associated with stress events. 

The transmission occurs throughout the pig production via contaminated environments, feed, water, vehicles, aerosols (Proux et al., 2001), poor hygiene, and high stocking density. It has been described the vertical transmission from sows to piglets(Campos et al., 2019). Salmonella primarily colonizes the gastrointestinal tract but it is also found in lymph nodes, tonsils, and feces (Fashenpour et al., 2024). The infective dose is low 10²–10³ CFU .(Rostagno & Callaway, 2012). 

The clinical form is more common in weaned pigs, whereas fattening pigs are often asymptomatic, although they can spread large amounts of Salmonella for weeks (Rostagno & Callaway, 2012). The pigs ,which are positive in mesenteric lymph nodes, are more likely to shed under stress (Casanova-Higes et al., 2016). Higher prevalences have been reported in spring (Fashenpour et al., 2024), in gilts compared with sows during peripartum (Hollmann et al., 2022), and early in gestation (Larivière-Gauthier et al., 2017), confirming sows as key reservoirs (EFSA, 2011).