Salmonellosis in songbirds in winter
Feeding birds in winter is extremely popular in North America, and each winter, bird feeders attract a variety of small birds to people’s yards. Some years, to the distress of the bird watchers, sick and/or dead birds are observed. There are several diseases – Mycoplasma conjunctivitis of house finches, Trichomoniasis affecting doves and finches, and Salmonellosis – that have come to be associated with bird feeders. Of these, the most common and widespread is Salmonellosis.
Salmonellosis is caused by a bacterium, Salmonella enterica, which exists in many strains, all of which may cause disease in humans and animals. Salmonella enterica Typhimurium is the subspecies associated with disease in wild songbirds. This bacterium may be more finely identified by phage type, which allows for the tracking of different strains of the bacterium as it occurs in different species in time and space. Specific phage types tend to be associated with different species of wild birds and occur repeatedly over broad geographic areas.
In wild birds, Salmonellosis is first and foremost a disease of the digestive system. The organism localizes in the digestive tract, usually in the lower esophagus or crop, and is shed in feces. From the crop, it may spread to other tissues and in birds dying of Salmonellosis, evidence of infection may be found in many tissues, including heart, muscle, liver, kidney, spleen and brain. However, it is its occurrence in the digestive tract that is critical for its spread to other birds. The bacteria are shed in feces and may infect the seed, perches and ground below a feeder. Other birds feeding at the same site may inadvertently pick up the bacteria and become infected.
Birds that are infected with Salmonella tend to show non-specific signs of being ill. They become listless and reluctant to move, and sit with their feathers fluffed up. People often observe a bird sick in this manner one day, and find it dead the next. When a post-mortem is done, the birds are usually found to be in emaciated body condition, a fact that is hard to appreciate visually in the live bird, since the bird’s feathers tend to disguise the loss of muscle. The crop is usually markedly thickened by a layer of dead cells, bacteria and inflammatory debris and may be so inflamed and damaged that it blocks the passage of feed material. There may be no other lesions, but Salmonella are usually readily cultured from samples of the crop and other organs.
Different strains of Salmonella tend to be associated with different species of birds. House sparrows are year round residents of much of Canada and Salmonellosis occurs in this species with some regularity. Some years are worse than others, but a few cases are usually seen here in most winters. Phage type (PT) 160 is the strain associated with this species and it may occur wherever house sparrows are found. Cases were reported a number of years ago from house sparrows in New Zealand. It may also be transmitted to other birds that share the feeder with the house sparrows, and may also occur in animals that prey on house sparrows, such as hawks (e.g. Cooper’s Hawk) and house cats.
The more noticeable epidemics of Salmonellosis tend to occur in species of northern birds, particularly the Common Redpoll, that periodically appear in more southern regions during the winter months. These birds breed in the boreal forest and feed primarily on conifer seeds. In years in which the seed crop is not sufficient, the redpolls and other associated species of birds (e.g. siskins, crossbills) move south in large numbers and are seen at bird feeders. In some of these eruption years, there is significant mortality at bird feeders due to Salmonellosis. A number of different phage types have been found in common redpolls. For a number of years PT 40 was the predominant strain. This same phage type has been seen in related species of birds in Great Britain and Scandinavia. More recently, phage types, including U284 and PT51 have been found. The latter is the phage type that has predominated in the years 2009-2013. These bacteria are highly adapted to their hosts and it is quite likely that there are individuals in the populations of redpolls and house sparrows that are carriers and periodic shedders of the bacteria.
This winter has been a year in which Salmonellosis has been identified in Common Redpolls from a number of locations in southern Ontario. The occurrence of this disease may be reduced if people who maintain winter bird feeders take measures to periodically clean and disinfect their feeders (see article on The Importance of Cleaning Bird Feeders for more details). In the case of northern birds, the epidemic usually comes to an end when the birds migrate back north to begin breeding. As the population disperses across the breeding territory, transmission of the bacteria is disrupted. Since house sparrows are resident in southern parts of the country and many people continue to feed into the warmer months, the occurrence of disease in this species is less predictable, and cases have been seen into the early summer in some years.