As for all animal species, salmonella infection in cattle is regulated by the Act on Zoonoses. This means that any veterinarian who suspects a salmonella infection is required to take samples for analysis. When infection in a cattle herd is confirmed, the herd is put under restrictions to prevent spread of the disease to other animals or food. This means that no animals may enter or leave the herd and that restrictions on the handling of manure will be issued, since animals and manure are the two main vehicles for transmission. Tracing is always performed in order to establish where the infection has come from and whether it may have spread to other herds. In the restricted herd cleaning and disinfection is performed in order to, if possible, eliminate the infection. The restrictions are lifted after samples from all animals have been negative at two occasions, and when cleaning and disinfection of the buildings and other relevant areas have been completed.
Since 2000, the number of cattle herds with salmonella infection has varied between 4 and 13 per year. In 2008, 21 herds were detected with salmonella, and in 2009 19 herds were detected with salmonella, which was more than expected. Since then, the number of detected herds has been lower again. The temporary peak may be the result of increased monitoring. The most common serotype is Salmonella Dublin, which is a serotype adapted to cattle. This serotype is verified in approximately half of the cattle herds where salmonella has been detected. The second most common type is Salmonella Typhimurium but other serotypes are also found although in small numbers.
How to discover infected herds
Below is a description of situations prompting a salmonella investigation in cattle.
• When clinical symptoms are such that there is reason to suspect salmonella infection, there is a legal requirement for the veterinarian to take samples for a salmonella investigation.
• At autopsy of cattle, sampling for salmonella is always made if the animal is younger than 6 months. At autopsy of older animals, sampling is made if the medical history and/or the macroscopic findings give reason to suspect salmonella infection.
• At slaughter, random sampling of lymph nodes in carcasses is made. How these random samples are structured is described under “Control/Surveillance.”
• Tracing of infection. Samples can be collected from infected herds, infected individual animals or humans, in feed establishments, domestic food, waters, etc. If salmonella is discovered during routine sampling in feed establishments, it may lead to a sampling of herds. This applies to situations when the results give reason to suspect that salmonella infected feed has been delivered from a feed plant.
When salmonella infection is diagnosed, the herd is put under restriction and a follow-up sampling of all animals in the herd is made. Further handling depends on where and to what extent salmonella is detected in the herd. The restriction is lifted after samples from all animals have been negative at two occasions.
Symptoms of salmonella infection in cattle vary widely, and in many cases infected animals show no symptoms at all. This depends on serotype, infection dose, and immune status of the animals that become infected. Symptoms that can be seen are diarrhoea, abortions, fever, poor general condition, decreased appetite, pneumonia, arthritis, blood poisoning, and death. It can be difficult to notice if the illness in a herd has increased, so it is useful to review production results and different rates such as calf mortality. These figures may reflect changes that might otherwise be difficult to percieve.
Current serotypes of cattle
S. Dublin is the most common type of salmonella in cattle; it contributes to approximately half of the salmonella cases in Swedish cattle herds. The infection often causes clinical disease and production loss. International studies have shown that S. Dublin infection primarily causes disease and increased mortality in calves. However, all the symptoms described under the heading “Clinical symptoms” may occur. Danish studies have shown that S. Dublin causes decreased milk production, resulting in significant loss of income for affected dairy companies.
Introduction of infected animals to uninfected herds is the major route of transmission of S. Dublin, but other indirect routes of transmission can occur as well, such as spreading of manure, sharing farm equipment or by humans. The presence of S. Dublin varies between different regions in Sweden. Approximately half of the S. Dublin infected herds found during the period 1993-2009 were located in Kalmar County. Since 2012 there is an ongoing outbreak of S. Dublin in an area in Skåne County in southern Sweden. Screening of bulk milk in 2013 showed that about two thirds of the dairy herds which showed evidence of S. Dublin, is located on Öland Island.
Studies in Denmark have shown that S. Dublin is a relatively rare diagnosis in humans, but can nevertheless cause serious disease in a higher proportion of cases than other types of salmonella.
S. Typhimurium is the second most common serotype in cattle. In contrast to S. Dublin, S. Typhimurium is not adapted to cattle. It is also one of the most common serotypes in other species. Symptoms in infected cattle can vary, and are described under the heading “Clinical symptoms.” Infection can be spread through animal transports, feed contamination and probably also by vectors such as birds and rodents as well as indirect transmitted by spread of manure, by people, tools, vehicles and environment.
This serotype was detected for the first time in Swedish animals in a cattle herd in the County of Skåne in 2006. The serotype has subsequently been detected in several herds, in samples from the environment – particularly from waterways, in wild birds and in humans. In October 2006, S. Reading was detected in a batch of raw material in a feed factory. This batch had no connection to the first verified infected farm, but gave reason to suspect that the infection was initially introduced through feed. In many cases, the affected cattle herds initially had severe clinical symptoms of the type described under “Clinical symptoms.”
The Swedish salmonella control is based on bacteriology, i.e. the detection of salmonella bacteria by cultivation. By culture, all types of salmonella can be diagnosed and the risk for false positive results is considered low. Evaluation of serological methods for detecting antibodies to S. Dublin, S. Typhimurium and some other serotypes in cattle is currently in progress at SVA. Antibody tests may be a cost-effective complement to bacteriological tests. The method may be used for example in surveillance, tracing of infection, when purchasing animals, or as a tool for a more efficient reduction of the infection in an affected herd. There is more information about salmonella serology below the heading “Research.”