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. Salmonellae have a wide variety of carrier hosts ranging from humans and domestic animals to wild birds, mammals, reptiles, amphibians and fish. Some of the common diseases caused by the genus Salmonella include fowl typhoid and pullorum disease (Salmonella enerica sv. gallinarum), mouse typhoid (S. enterica sv. typhimurium) and paratyphoid fever (Salmonella spp.).
The following discussion will be limited to salmonellosis in wild birds. S. enterica sv Typhimurium, the most ubiquitous and common of all salmonellae, is the most common species isolated in wild birds.
Salmonellosis has been seen in many avian species throughout the world, including North America. In Michigan, Salmonella bacteria have been isolated from the following species: American goldfinch, bald eagle, black-capped chickadee, blue jay, brown creeper, brown-headed cowbird, chipping sparrow, common grackle, common redpoll, Cooper's hawk, dark-eyed junco, double-crested cormorant, evening grosbeak, great blue heron, house finch, house sparrow, herring gull, mallard, northern cardinal, pine grosbeak, pine siskin, purple finch, red-breasted grosbeak, ring-billed gull, tree sparrow and white-winged crossbill. Since 1970 when salmonellosis was first diagnosed in Michigan, die-offs during the winter in house sparrows and other avian species around bird feeders has been common. There have been two large die-offs in herring gulls in the Sault Ste. Marie vicinity, one occurring in 1970 and the other in 1977.
Transmission and Development
Salmonellosis is transmitted directly through fecal contaminated food products. Outbreaks of salmonellosis in wild birds in Michigan occur mostly in passerine birds concentrated in winter around feeders. Historically, house sparrows have accounted for 95% of the mortality around feeders. In recent years it has become more prevalent in common redpolls, American goldfinches, and pine siskins. Survivors of outbreaks may become healthy carriers and remain so for long periods.
The combination of carrier and susceptible birds concentrated at feeders, and transmission of the disease through fecal contaminated feed result in outbreaks of salmonellosis. House sparrows, pine siskins, American goldfinch and common redpolls, due to their feeding habits of crowding onto the feeding area and remaining there until the food supply is exhausted, 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 these four species of birds are inherently more susceptible to the salmonellae bacteria than other wild birds.
Salmonellosis outbreaks around feeders subside with the milder weather of spring. During the spring and summer when food is usually no longer offered, susceptible species of birds, while remaining in the same area, tend not to flock but are forced to forage individually. This dispersal of flocks separates carrier and healthy birds, decreasing greatly the number of bacteria a susceptible bird will come into contact with. Also winter stress such as lower temperatures is not a factor at this time of the year.
Clinical Signs and Pathology
Signs range from sudden death to gradual onset of depression over 1 to 3 days, accompanied by huddling of the birds, fluffed-up feathers, unsteadiness, shivering, loss of appetite, markedly increased or absence of thirst, rapid loss of weight, accelerated respiration and watery yellow, green or blood-tinged droppings. The vent feathers become matted with excreta, the eyes begin to close and, immediately before death, some birds show apparent blindness, incoordination, staggering, tremors, convulsions or other nervous signs. However, most people calling about die-offs report finding dead birds around their feeder and an occasional "sick-acting" bird.
Gross internal lesions may include an enlargement of the liver and spleen and inflammation of the intestinal tract with hemorrhaging into the lumen. A lesion not frequently reported in the literature but observed commonly by DNR pathologists is a thickening of the esophagus and/or crop mucosa into a yellow-tan caseo-necrotic membrane or mass.
The finding at necropsy of a yellow-tan caseo-necrotic membrane in the esophagus and/or crop is highly suggestive of salmonellosis. However, for a definitive diagnosis, the Salmonella microorganism must be isolated from the liver.
Treatment and Control
No drugs or antibiotics have proven to be entirely effective for treating salmonellosis in any wild birds. Sulfamerizine, nitrofurane and broad-spectrum antibiotics in the feed or water may reduce losses in domestic species.
To help reduce transmission, feeders may be disinfected weekly with a 1:10 (10%) bleach to water mixture. The feeder should be thoroughly dried before refilling with feed.
The best control method to use in Michigan for die-offs around bird feeders is to disinfect all feeders with a 10% bleach solution and clean up all spilled seed. Feed should be removed for 2 to 4 weeks. With the food supply removed, birds will be dispersed, and carrier and susceptible birds separated.
Salmonellosis causes death in wild birds, especially house sparrows, pine siskins, American goldfinch, and common redpolls around bird feeders. It has also been responsible for losses of herring gulls in two Michigan outbreaks. However, it is not an important cause of population decline in any wild bird species. The disease is of interest to individuals feeding birds and is sometimes mistakenly thought to be a poison induced die-off. Therefore, wildlife biologists should be informed and knowledgeable about the disease.
Salmonellosis is of public health and veterinary significance because all members of the genus are potentially pathogenic for man and animals. Dogs and cats are rarely infected. It appears that wild birds acquire the infection primarily from their environment (carriers) and that these infected birds play a relatively small role in the transmission of the disease to domestic animals and man.
For questions about wildlife diseases, please contact the Michigan DNR Wildlife Disease Laboratory.