Salmonella infection in pigs, as in many other species, either give rise to clinical disease or the pig becomes a carrier with no clinical signs of infection although it sheds salmonella bacteria. Some types of salmonella are adapted to one or a few hosts, while others have a very wide host range. Pigs can become infected with salmonella of all types.
In Sweden, clinical disease caused by salmonella is very unusual in pigs. Therefore, salmonella infection in pigs in Sweden is mainly important as a source of infection in humans through contaminated pork and pork products.
Cause and prevalence
Salmonella Cholerasuis is especially adapted to pigs and is rarely detected in other species. It can also cause serious illness in humans. S. Cholerasuis was diagnosed in Sweden in the late 1970´s, but has not been detected since. Internationally, it is still common in pigs. Apart from S. Cholerasuis, Salmonella Typhimurium and Salmonella Derby are the most common types of salmonella in pigs worldwide.
In Sweden, S. Typhimurium has been the most commonly occurring salmonella type in pigs during recent years, with the exception of 2003 when Salmonella Cubana dominated due to a feed-borne outbreak.
S. Cholerasuis is an invasive type of salmonella that has the ability not only to colonize the intestine but also cause systemic disease. Affected animals get high fever, purple discolorations of the skin and later on diarrhoea. The mortality rate is high. The disease usually affects animals younger than 4 months. Animals who survive become carriers.
Other types of salmonella
Other types of salmonella generally do not cause clinical disease in pigs, but can cause enteritis with diarrhoea. The diarrhoea is often watery and yellow and can at a later stage in the disease contain blood. In cases of clinical disease with diarrhoea, S. Typhimurium is the most frequent cause. Animals that recover become carriers.
Spread of the infection
Salmonella is a faecal-oral infection, i.e. it is spread by faeces-contaminated feed or food or through direct contact with the infected faeces.
Animals recovering from clinical salmonella infection, as well as carriers, excrete salmonella bacteria intermittently as long as the bacteria remain in the intestine. The pattern of excretion and if it differs depending on serotypes is in many cases not known, but research on this matter is being carried out. When salmonella is spread between and within herds, birds and rodents may also play a role.
Control of the disease is based on minimizing the risk that animals are exposed to salmonella in their feed or through contact with animals carrying salmonella, and by all-in-all-out production with careful hygiene. Since the trade of live pigs is not extensive in Sweden the risk of this being a potential source of infection is not regarded as high, whereas salmonella exposure from feed is regarded as a more important risk factor. Salmonella-free feed is therefore a prerequisite for the successful salmonella control in the Swedish pig industry.
The salmonella control program for pigs in Sweden includes sampling in breeding herds, lymph node samples taken at slaughter, clinical surveillance, and autopsies.
In each case of salmonella suspicion in a pig herd, an investigation is made to determine if the herd is infected. The reason for suspicion may be findings of salmonella in samples taken in control programs, findings of salmonella at the autopsy of dead animals from a herd, findings in feed or confirmed cases of salmonella in humans. If contamination is verified in the herd, further tracing of the infection is made to determine the possible source of infection and possible spread of infection from the herd.
Sweden has a good health status when it comes to salmonella in pigs. Since 1980, the number of verified salmonella infected herds has been below ten, and most years below five per year, except in 2003 when 30 herds were infected in a feed-borne outbreak.