Salmonella is a notifiable disease under the Act on Zoonoses and veterinarians should report cases to the Board of Agriculture and to the concerned county veterinary officer.
Infections caused by Salmonella spp are common in dogs worldwide, with the exception of the Nordic countries (Sweden, Norway, and Finland) where the incidence is low.
Transmission of the disease
Salmonella is a group of gram-negative bacteria of the family Enterobacteriaceae. Salmonella is mainly an intestinal pathogen, but can also cause systemic disease and can then be isolated from blood and various organs. The bacteria may also be carried by dogs without symptoms.
Infection can be transmitted directly between infected animals and between animals and humans. Salmonella spreads mainly through faeces from infected individuals and infects new individuals via the mouth. Dogs can become infected through contaminated food and contaminated water. International veterinary publications indicate the feeding of raw meat as the most common route of transmission in dogs. Particularly North America has reported salmonella infections in competing greyhounds and working sled dogs fed with raw animal products.
Cats usually become infected during late winter and early spring, after catching and eating birds (such as passerines) infected with salmonella. Dead or live birds as well as bird droppings may be a source of infection both for cats and dogs.
Clinical salmonella infection in dogs is rare. Only a few cases in dogs are reported annually in Sweden.
Pathogenesis and clinical symptoms
Several different virulence mechanisms have been identified in salmonella bacteria and the pathogenicity may vary between different serotypes. Pathogenesis studies performed in dogs are missing, but studies in other animals have shown that Salmonella spp. can attach to and invade intestinal epithelial cells and thereby cause inflammatory enteritis with subsequent secretory diarrhoea and/or systemic infection. In most cases, the infection is localized exclusively to the intestine, but sometimes a translocation of bacteria occurs and a bacteraemia or sepsis develops. Systemic infection may also develop without previous gastrointestinal symptoms.
Salmonella may persist for a long time after the initial infection, in intestinal epithelia and lymph nodes. Faecal excretion may be continuous during the first week and then become intermittent. The period of excretion is usually 3-6 weeks but there are occasional reports of longer excretion time. Up to 117 days have been reported after experimental infections in dogs.
The bacterium can also persist in phagocytic cells in intestinal lymph nodes, liver and spleen. This may explain why reactivation of infection can be seen in some individuals in, for instance, immunosuppression or different states of stress.
The severity of clinical symptoms in infected dogs vary depending on virulence factors of the infecting salmonella strain, the infection dose, the animal´s immune status and any coexisting conditions. Subclinical infections without symptoms occur.
Acute gastroenteritis is the most common symptom in clinical salmonella infection in dogs. Fever, nausea, anorexia followed by vomiting, abdominal pain, and diarrhoea can be seen. Diarrhoea may vary in degree and consistency; sometimes it can also contain blood. Weight loss is most likely a result of fluid loss. Infections that are more serious may lead to sepsis, shock, and possibly death. Subsequent bacteraemia can cause infection in other organs such as pneumonia. Systemic infection can occur in animals with no gastrointestinal symptoms.
Infection in the uterus during pregnancy may lead to abortion or the birth of weak or dead puppies.
Diagnosis is made by bacterial culture from faeces. In case of sepsis, blood culture may need to be performed.
Currently there are no strict guidelines or rules for follow-up samples to demonstrate freedom of infection in dogs. Multiple samplings may be needed to verify freedom.
There is always a risk of false negative results since salmonella can be excreted intermittently, particularly in the later stages of infection. The highest probability of finding salmonella in faeces is by sampling in the acute stage of the disease.
Supportive treatment is usually sufficient in acute uncomplicated salmonella infection. Rehydration with adequate supply of fluids and continuous compensation of fluid losses due to vomiting and diarrhoea is important. Antibiotics are not indicated except as a supportive treatment during a short time for life threatening symptoms, shock, and in case of sepsis. Antibiotic treatment may prolong the period of faecal excretion. The risk of selecting for antibiotic resistant strains should also be considered.
The prognosis for dogs with uncomplicated salmonella infection is generally good. In cases of sepsis the outcome could be less favourable.
Although infection with salmonella is uncommon in dogs in Sweden, one should bear in mind that dogs, like humans, can become infected through contaminated food. To feed dogs raw or inadequately heated animal products may pose a risk of infection. Like cats, dogs may also become infected from infected birds and bird tables and such exposure should be avoided.
Advice when handling salmonella infected dogs
Good hand hygiene is always important in order to avoid salmonella infection, especially when cooking and eating, and people often become infected by contaminated food. Hands should be washed thoroughly after handling the sick dog, and cleaning up any vomit or diarrhoea. When walking the dog, faeces should be picked up to minimize contamination of the environment. Always wash your hands afterwards. Wash the dog´s dishes separately with a dish brush that is not used for other things.
Children, and perople with impaired immune defence may be more susceptible to infection and it may be inappropriate for an infected dog to sleep in the same bed as a child. You should also try to avoid being licked by the dog.
If there are other pets in the household extra careful hygiene should prevail for all as long as there is an infected dog in the household. This applies even if the other animals do not show any symptoms.