What is Salmonella?
Salmonella microorganisms are gram-negative, motile, non-sporeforming, facultatively anaerobic bacilli with peritrichous flagella (Griffith et al., 2006). Salmonella may be found almost everywhere in the environment given that they can survive under very difficult conditions. They can survive freezing and persist in the environment from days to weeks, depending on the conditions, while they can multiply at temperatures ranging from 7oC to 45oC. The main reservoir where they are reproduced in large numbers is considered the intestine of the animals.
Salmonella are categorised depending on their antigenic differences in different serotypes. Depending on their somatic antigens Salmonellae have been categorised in serogroups, which are used to be tested. Indeed, rapid methods have been developed such as a mix-ELISA (Nielsen et al., 1995), which has been used extensively in national monitoring programs for Salmonella in a number of European countries for farm characterisation and monitoring (Alban et al., 2002b; EFSA, 2008b; EFSA and ECDC, 2009a; Hill et al., 2003; NRLSalmonella, 2010).
There are about 2400 serotypes of Salmonella. Some of them they can be found or create health issues in one only host species, e.g. S. Typhi for humans, while can be found and create health issues in more species, such as S. Typhimurium (humans, pigs, bovine). When a serotype that is adapted to one species is found in another one it is considered as an "exotic" serotype.
Table 1. Serogroups of selected Salmonella serotypes and ranking of frequency of isolation from diseased pigs, swine sources and humans. Adapted from Griffith et al. (2006)
|Serogroup||Serotype||Isolation from Diseased Pigs||Isolation from Swine Sources||Isolation from Humans|
|A||S. Paratyphi A|
|S. Typhimurium var copenhagen||
|S. Cholerasuis var kunzerdof||1||2|
Two are the major serotypes that can be found in pigs (Table 1). The one is S. Cholerasuis, which is well adapted to pigs and causes health issues, among which gastroenteritis and in some cases septicaemia. The other one is S. Typhimurium, which may cause enterocolitis when inoculated to pigs in high doses. Usually in everyday farming it causes only subclinical infections with no overt symptoms, making it difficult for people working in a farm to understand the problem. However, S. Typhimurium can be introduced from the harvested pigs into the food production chain and may pose a risk of foodborne zoonoses to the human population.
Once pigs get infected with Salmonella, regardless of the presence of overt symptoms, they shed the pathogen to the environment for a period of time that could be around two weeks (depending on the serotype that is involved), while at the same time a reaction from the immune system is happening. By the end of that period, pigs develop antibodies and stop shedding the organism, while they still carry it in their body, especially in their lymph nodes, for a further period of time. However, given the antibody protection pigs cannot become re-infected or start shedding for this period of time.
Except if the environment is very contaminated, not all the pigs become infected at the same time. In this way at different time points there would be some pigs shedding and keeping the infection alive in the farm. On top of this, given the long time of survival of Salmonella, under usual farming conditions, Salmonella infections in pig farms are well established and difficult to be eradicated.