As for all other animal species, salmonella infection in sheep is regulated by the Act on Zoonoses. This means that a veterinarian that suspects salmonella infection is required to take samples for salmonella analysis. When infection in a sheep 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 put in place, 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.
Salmonella enterica subsp diarizonae is in some respects exempt from these measures (see below Serotypes in sheep).
How to discover infected herds
Below is a description of when sampling of salmonella in sheep occurs.
• 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 examination.
• In tracing of infection. Samples can be collected from infected herds, infected individual animals or humans, in feed establishments, domestic food, watercourses, etc.
• In sheep, there is no herd based sampling for active surveillance purposes.
Symptoms of salmonella infection in sheep vary greatly depending on serotype, infection dose, and immune status of the animals that become infected. Symptoms described in sheep are similar to other species: such as diarrhoea, abortions, fever, depression, decreased appetite, blood poisoning, and death. Often there are no clinical symptoms.
Serotypes in sheep
There are over 2.500 different types of salmonella bacteria, many of which can infect sheep. One type of salmonella, Salmonella enterica subsp diarizonae, is considered to be particularly adapted to sheep. In a study conducted by SVA in 2012, approximately 17 per cent of the sampled herds were infected with this type of salmonella. Studies in Norway also show that this salmonella type is common in sheep. Infected sheep rarely show clinical symptoms but described symptoms include diarrhea, abortion and single cases of inflammation of the nasal cavity and testes. In Sweden, only one case in humans has been diagnosed over the past 25 years.
The Swedish Board of Agriculture and the National Food Agency have decided that this particular type of salmonella shall be handled in a special way. When a salmonella infection is suspected, there is a legal requirement for the veterinarian to take samples for a salmonella examination, and submitted samples are tested for antibiotic resistance. However, the herd will not be put under restrictions and there will be no tracing of the infection, or cleaning or disinfection in the infected herd. In special cases, for instance if the bacterium starts to develop resistance or if a connection to serious health problems of the herd is suspected, actions may be taken to control the infected herd.