Salmonellosis is caused by bacteria from the Salmonella group. There are more than 2,000 different strains of Salmonella that have been found in reptiles, birds, and mammals. Though salmonellosis can affect wild mammals, this disease description will focus on the disease in wild birds.
Salmonellosis is one of the most widely distributed diseases of humans and animals. It is a particularly significant disease of songbirds that congregate at bird feeders. Humans can become ill from this disease, though human infection is rarely linked to direct contact with infected wild birds. People can contract salmonellosis by handling contaminated bird feeders, so people who handle feeders should wear gloves and practice good personal hygiene. Pets and birds of prey are also at risk if they prey on infected birds.
All bird species are likely susceptible to salmonellosis and large-scale mortality events are common at feeding stations. Songbirds, European starlings, blackbirds, common grackles, and mourning doves are the most common species affected. House sparrows, pine siskins, American goldfinches, and common redpolls are also highly susceptible. Salmonellosis has caused varying degrees of mortality in ducks, mute swans, gulls, terns, American coots, double-crested cormorants, eared grebes, egrets, and herons, but these species usually experience smaller-scale mortalities compared to songbirds and colonial nesting birds. Salmonellosis also commonly infects domestic poultry and captive birds. Humans, house cats, and birds of prey that are exposed to sick and dying songbirds at feeders can become infected with salmonellosis. As with many infectious diseases, those with suppressed immune systems are at greater risk.
Salmonellosis occurs worldwide. In wild birds, this disease is associated with large concentrations of people and livestock where untreated sewage and manure contaminate the environment. Salmonellosis has even been introduced to wild birds in remote regions such as Antarctica.
Salmonella bacteria are shed in the feces of infected animals. The bacteria are transmitted by direct contact with infected birds or by ingestion of food or water contaminated with feces from an infected bird or mammal. Many strains of Salmonella bacteria can persist for long periods of time in the environment (up to 9 months in soil and in stagnant water). Songbirds can become infected when they congregate at high densities at feeding sites and come in contact with infected birds or contaminated food or water. Some species, such as gulls, commonly acquire salmonellosis from human sewage and domestic animal waste. When birds are confined in close quarters, the bacteria can be transmitted via inhalation. In poultry, eggs can become infected with the bacteria, though this mode of trans-mission is not known to be significant in wild birds.
Several factors such as age, stress level, species of the host bird, and strain of the bacteria influence the severity of clinical illness. In general, young birds are more susceptible to salmonellosis. Infection with Salmonella bacteria can lead to rapidly fatal illness, chronic infections that may or may not have clinical signs, or an asymptomatic carrier state. Non-specific clinical signs of salmonellosis include fluffed or ruffled feathers, lethargy, rapid breathing, diarrhea, shivering, weakness, and eventually coma and death. Neurological signs such as loss of coordination and tremors may also be observed. Along with the aforementioned signs, some birds experience a gradual onset of depression accompanied by loss of appetite, weight loss, and feathers matted with feces. In these birds, the eyelids often swell and stick together. In some cases, birds will recover and will continue to shed the bacteria without showing clinical signs. Visible gross lesions can vary greatly. In some birds, necropsy will reveal swollen and crumbly livers with red or pale spots. Other birds may exhibit small tan to white nodules or plaques on the liver, breast muscle tissue, and other organs. Songbirds often have yellow, cheese-like nodules on the esophagus, crop, and gizzard. The intestinal lining may be reddened or coated in pale material.
To diagnose salmonellosis, Salmonella bacteria must be isolated from infected tissues.
There is no treatment for salmonellosis in wild birds.
To prevent large-scale outbreaks of salmonellosis in songbirds, feeders must be kept clean and sanitary. Feeders should be cleaned regularly with soap and water followed by a solution of 10% bleach and water (9 parts water: 1 part bleach). Feces and discarded seed should be removed from the ground. Feeders should be spaced to prevent crowding. Since rodents are often a source of the bacteria, seed should be stored in insect and rodent proof containers. If an outbreak occurs, bird feeding should be discontinued for a period of 2-3 weeks to prevent further transmission. Outbreaks of salmonellosis in wild birds are also often associated with environmental contamination from human sewage, agricultural waste, and trash dumps. Addressing these sources of contamination will help to control outbreaks and reduce the number of carrier birds that can further spread the bacteria.