Some species of Salmonella may cause abortion. A notable feature of Salmonella bacteria is that they will often invade a bird or animal, which then becomes a carrier of the disease. Carrier animals appear normal or to have recovered from disease but shed the bacteria into their surroundings through manure, saliva and discharges. Carrier animals cannot be cured by treatment with antibiotics or other drugs. Fortunately, Salmonella bacteria are usually opportunistic, rarely causing disease in healthy, vigorous animals.
Animals develop salmonella infections when their immune defenses are lowered.
A common predisposing factor among calves is a lack of immunity due to a failure to receive adequate colostrum. An animal infected with Salmonella may shed bacteria in large numbers. Other animals become infected from direct contact with infected manure, discharges and filthy udders. Contamination of the environment occurs and other animals can pick up the bacteria from pens, feedstuffs, pasture or bedding. Carrier animals shed the bacteria in their manure, especially at times of stress such as calving.