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Conservation > Fish & Wildlife Diseases Salmonellosis

For questions/concerns about this disease in humans, please call your doctor or the Montana Department of Public Health and Human Services  (DPHHS).

For questions about this disease/parasite in wildlife, please call the FWP Wildlife Health Lab at (406) 577-7882.

Cause

Salmonellosis is a bacterial disease caused by members of the genus Salmonella. This genus consists of over 1,100 species of antigenically related bacterial organisms which are gram negative, rod-shaped, 0.4 to 0.6 microns wide and 1 to 3 microns long. They do not form spores and are usually motile. Relatively small numbers of bacteria are required to cause disease (Miller et al, 2003). S. enterica sv Typhimurium, the most ubiquitous and common of all Salmonellae, is the most common species isolated in wild birds. Occurrence among songbirds appears to have increased significantly in the last 20 years (Western, 2009). In the winter of 2020-21, a large outbreak of salmonellosis in occurred in wild birds in the United States.  Salmonellosis in warm-blooded vertebrates is most commonly caused by Salmonella enterica. Many infected animals, particularly reptiles, carry the bacteria without showing any clinical signs Cornell). 

Distribution

Salmonellosis has been seen in many avian species throughout the world. The disease can occur throughout the year. In passerine birds, salmonellosis outbreaks are more common at bird feeders during the late winter and early spring, and during hot periods in the summer when birds are stressed and must congregate for food and water. In colonial nesting water-birds such as gulls, terns and cormorants, outbreaks of salmonellosis are more common early in the summer when the young of the year hatch (Miller et al, 2003).

Species affected

Salmonellae have a wide variety of carrier hosts ranging from humans and domestic animals to wild birds, mammals, reptiles, amphibians, and fish. Salmonellae are most commonly isolated from birds. Among wild birds, salmonellosis outbreaks are most frequently reported in association with passerine birds at feeders and in colonial nesting water-birds (Miller et al, 2003). In the United State, Pine Siskins and American Goldfinches have most commonly been affected.

Transmission

Infected animals may shed Salmonella bacteria in their feces for weeks or months.  Salmonella bacteria are tolerant of a wide-range of temperatures and survive for months in soil and water.

In mammals, salmonella infections are typically acquired from eating food contaminated with feces of carrier animals.

Infected birds may also shed Salmonella bacteria in their feces for weeks or months. Crowding, stress, contamination of feed with feces, prolonged stays, and the presence of both susceptible birds and carriers of Salmonella bacteria promote the regular occurrence of salmonellosis at bird feeders. (Miller et al, 2003). Due to their feeding habits of crowding onto the feeding area and remaining there until the food supply is exhausted, some bird species including house sparrows, pine siskins, American goldfinch and common redpolls are exposed for long periods to carriers and contaminated ground. This greatly increases the number of bacteria a bird comes in contact with, and therefore the threshold number of bacteria needed to cause an infection in a susceptible bird is more easily met. It also appears that some species of birds are inherently more susceptible to the Salmonellae bacteria than other wild birds. Salmonellosis outbreaks around feeders generally subside with the milder weather of spring. During the spring and summer, if food is no longer offered at feeders, susceptible species of birds tend not to flock but are forced to forage individually. This dispersal of flocks separates carrier and healthy birds, helping to limit the number of bacteria with which a susceptible bird will come into contact.

Signs

Some infected mammals remain healthy and continue to shed the bacteria, while others may develop severe disease and death. The extent and severity of lesions will depend on the species, age, and health status of the host as well as the type of Salmonella. The bacteria primarily invade the wall of the intestines causing inflammation and damage. Symptomatic mammals may have diarrhea (possibly with blood or mucous), fever, vomiting, and lethargy. Infection can spread in the body through the bloodstream to other organs such as the liver, spleen, lung, joints, placenta or fetus, and the membranes surrounding the brain. Toxic substances produced by the bacteria can be released and affect the rest of the body. Animals that die from Salmonellosis may have inflamed or bloody intestines, enlargement of the spleen and lymph nodes, accumulation of fluid and blood in organs such as the lungs, and damage to the liver.

The extent and severity of lesions in birds also depends on the species, age and health status of the host as well as the type of Salmonella. Young birds are more severely affected than older birds. Infected birds may suddenly die or gradually show signs of disease. Sick birds are depressed and may huddle together with ruffled feathers, unsteadiness, shivering, loss of appetite, increased or decreased thirst, rapid weight loss, accelerated breathing, watery yellow, green, or blood-tinged droppings, and closing of the eyes with swollen and pasted eyelids shortly before death. Immediately before death, infected birds may show signs of an affected nervous system such as blindness, lack of coordination, staggering, tremors, and convulsions. Feathers around the vent (cloaca) may become matted with feces. In birds that die from Salmonellosis, the liver and spleen are often enlarged, and the intestinal tract may show inflammation and hemorrhage. The inner surface of the crop may thicken into a yellow, cheese-like membrane (Michigan, 2010; Miller et al, 2003).

Public Health Concerns

Salmonellosis is of public health and veterinary significance because all members of the genus are potentially pathogenic for man and animals. It appears that wild birds acquire the infection primarily from their environment, which is contaminated by carriers, and that infected birds play a relatively small role in the transmission of the disease to domestic animals and man . Many domestic cats are reported to become ill after consuming infected birds during salmonellosis outbreaks in songbirds. The probability of humans contracting Salmonella bacteria directly from wildlife is low; however, if pet cats are affected, the risk to their owners increases. Contamination of the environment from sewage, manure or effluent from abattoirs contributes to the occurrence of salmonellosis in wildlife. Efforts to disinfect bird feeders weekly in all parts of a neighborhood with 1 part bleach to 10 parts water and remove wet or spilled bird seed should reduce the recurrence of salmonellosis outbreaks at feeders. If a Salmonella die-off has occurred at a feeder, feed should be removed for 1 month to prevent further concentration of birds at the site. Salmonella bacteria are well-documented disease-causing agents in humans, causing “food poisoning”, which is characterized by acute intestinal pain and diarrhea. Extra care with personal hygiene is warranted for people who maintain bird feeders, and for wildlife rehabilitators and biologists who handle birds or materials soiled by bird feces, even when the disease is not apparent (Miller et al, 2003).

Is it safe to eat the meat?

The meat of wildlife believed to be suffering from salmonellosis should NOT be consumed NOR fed to domestic dogs or cats. (Miller et al, 2003). All game meat should be well cooked to a temperature of at least 165° F. If you harvest an animal and are unsure whether it is safe to eat, contact your local FWP staff for guidance soon after the animal is harvested.

Citations

Cornell Wildlife Health Lab website. https://cwhl.vet.cornell.edu/disease/salmonella

Michigan Wildlife Disease Manual. 2010. Michigan Department of Natural Resources.Website

Miller, M.J.R., R.D. Dawson, and H. Schwantje. 2003. Manual of Common Diseases and Parasites of Wildlife in Northern British Columbia. University of Northern British Columbia.

Western Wildlife Disease Workshop Notebook. 2009. Southeastern Cooperative Wildlife Disease Study. College of Veterinary Medicine, University of Georgia.