Chapter 21 Salmonellosis in Songbirds (Order Passeriformes)

Salmonellosis has been recognized as a cause of disease and mortality in captive and free-living songbirds (order Passeriformes) for many years.^5,^13,^17,^19,²⁰ Salmonella typhimurium is one of the species often found in birds, but a wide range of other Salmonella spp. have been isolated as well, some associated with disease and others from birds showing no signs. The tremendous growth in garden and backyard provisioning of wild birds worldwide has led to increased public observation and reporting of disease and mortality incidents in songbirds.^1,⁷ Salmonellosis has been found to be one of the most common causes of these disease and mortality cases.^3,^4,^5,^12,¹⁵

Approaches to the prevention and treatment of salmonellosis in captive songbirds have received considerable attention over the years.^7,^8,¹⁶ Recently, various efforts have been initiated to explore the epidemiology of the disease in free-living songbirds, some with a view to developing preventive measures when anthropogenic factors are implicated.^7,^12,^14,¹⁵ However, the significance of the disease to the dynamics and viabilities of free-living populations has received little investigation. Outbreaks of salmonellosis may involve large numbers of birds and clearly have an impact on local population numbers at the time of their occurrence. At the individual level, it is reasonable to conclude from the nature of the clinical and pathologic findings that the disease has a severe impact on the welfare of affected birds.

ETIOLOGY

There are more than 2000 species of Salmonella. Some species, notably S. typhimurium, are frequently isolated from birds. Specific strains appear to be particularly pathogenic for various species of passerine birds. For example, mortality incidents in free-living greenfinches (Carduelis chloris) in the United Kingdom (U.K.) have been found to be commonly associated with bacteriophage (phage) type 40. This strain has been diagnosed as the cause of mortality incidents in wild birds elsewhere in Europe and also in the United States and Canada. Other phage types isolated from songbird mortality incidents in Europe include 1, 41, 56 (variant), 121, 129, and 160.^12,¹⁵ It is possible, and perhaps likely, that some strains that cause disease in some species of songbirds may be carried without causing signs in others. However, knowledge of which species of songbirds carry, and are susceptible to disease caused by various species and strains of Salmonella remains scant.

EPIDEMIOLOGY

Salmonellosis has been seen in a wide range of passerine species, and it is reasonable to expect that all are susceptible, although susceptibility may vary among species and with the strain of Salmonella. Among free-living garden birds, seed-eating passerines appear to be particularly at risk.^6,^12,¹⁴ In typical salmonellosis incidents in Northern Europe, most deaths occur in greenfinches, with chaffinches (Fringilla coelobs), house sparrows (Passer domesticus), and other species being affected to a lesser extent. Where they occur, it appears that bullfinches (Pyrrhula pyrrhula), tree sparrows (Passer montanus), and siskins (Carduelis spinus) are also often affected.¹²

No structured surveys of the occurrence of outbreaks of salmonellosis in free-living wild birds have been undertaken, although one has recently been initiated in the U.K. (see www.ufaw.org.uk/gbhi.htm). However, this disease appears to be a common cause of epidemics in garden/backyard birds, especially during the colder months: December through to April in the Northern Hemisphere.¹² In one survey, S. typhimurium DT56 was isolated from 48%, 39%, and 13% of pooled feces samples collected from a bird table in 2001, 2002, and 2003, respectively, suggesting that that a potentially pathogenic strain is often carried and excreted.¹⁵

I found some evidence that infectious diseases (notably salmonellosis) caused a greater proportion of total mortality in gardens where songbirds were fed on a large scale (>1 kg of feed per day) than in those where less food was provided.¹⁰ This preliminary finding was consistent with a greater risk of infectious disease where large numbers of birds gather in relatively small areas to feed day after day. Whether outbreaks are typically caused by spread of infection from asymptomatic carriers or by exposure to strains from other sources (e.g., rodents, food) remains uncertain, but evidence suggests that some of the strains usually involved in garden/backyard bird mortality incidents (e.g., S. typhimurium phage types 40 and 56) are primarily associated with wild birds. Phage type 40 has also been isolated from game birds, horses, calves, and other domestic animals and can also cause disease in humans.

The main route of spread among birds is fecal-oral, and the potential for transmission clearly exists through fecal contamination at feeding and drinking sites. Salmonella spp. may survive and, in suitable conditions (e.g., in moist, uneaten food on a bird table on a warm day), multiply in the environment. The seasonal pattern of prevalence in free-living birds is probably attributable to the greater risk of spread of infection in the winter, when large numbers of birds gather at feeding sites, but other factors may also be involved in this seasonal pattern.

The incubation period (time from oral infection to onset of signs of disease) depends, among other factors, on the strain of Salmonella, the species and other features of bird infected, and the route and intensity of infection. The clinical signs can range from peracute to chronic, or infection may remain subclinical. The incubation period is typically 2 to 5 days in the acute disease.

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