Many strains of Salmonella infect calves, and many calves carry Salmonella in their guts, but disease does not develop until the dose of infection becomes overwhelming, or other stress factors e.g. lack of colostrum, poor weather or housing, allow infection to take hold.
Means of spread of salmonella in calves:
1. Infected calves – contact with other infected calves or their faeces is the main means of spread in an outbreak.
2. Adult carrier animals who start shedding due to stress e.g. calving.
3. Carrier calves – these are of particular importance especially bought in calves that have passed through sale yards, and met salmonella strains that they do not have colostral immunity to, and had potentially stressful journeys.
4. Wildlife – many species of wild birds are infected with Salmonella, and the main route of spread is through calf meal, which wild birds can have access to.
5. Sewage – uncontrolled sewage or contaminated water is a potential though rare source of infection for calves.
6. Human carriers – transmission of Salmonella from animals to humans is common but human workers are also regularly implicated in spreading disease between calves.
7. Fomites – Salmonella can be carried on a range of inanimate objects from boots, tyres, and feedstuffs to old bedding and buildings, as well as contaminated milk and colostrum.
Infection is by the oral route and can occur at any stage, however peak incidence is at 7-10 days old. The incubation period is 24-48 hours and calves become rapidly sick, and will often die from Septicaemia without showing much in the way of signs apart from scours. When diarrhoea is a feature of the disease, there is often blood and mucus in the scour and calves will frequently fail to respond to normal scour therapy. Salmonella can rapidly develop antibiotic resistance and it is important the correct antibiotic at the correct dose is used to treat the disease. Vaccine is available and if instigated in the early stages of an outbreak, will control the disease from around a week after vaccination. In subsequent seasons, vaccination should be given prior to the period of challenge, due to the difficulty of removing Salmonella completely from the environment.