Epidemiology of a Salmonella enterica subsp. enterica serovar Typhimurium strain associated with a songbird outbreak

Abstract

Salmonella enterica subsp. enterica serovar Typhimurium is responsible for the majority of salmonellosis cases worldwide. This Salmonella serovar is also responsible for die-offs in songbird populations. In 2009, there was an S. Typhimurium epizootic reported in pine siskins in the eastern United States. At the time, there was also a human outbreak with this serovar that was associated with contaminated peanuts. As peanuts are also used in wild-bird food, it was hypothesized that the pine siskin epizootic was related to this human outbreak. A comparison of songbird and human S. Typhimurium pulsed-field gel electrophoresis (PFGE) patterns revealed that the epizootic was attributed not to the peanut-associated strain but, rather, to a songbird strain first characterized from an American goldfinch in 1998. This same S. Typhimurium strain (PFGE type A3) was also identified in the PulseNet USA database, accounting for 137 of 77,941 total S. Typhimurium PFGE entries. A second molecular typing method, multiple-locus variable-number tandem-repeat analysis (MLVA), confirmed that the same strain was responsible for the pine siskin epizootic in the eastern United States but was distinct from a genetically related strain isolated from pine siskins in Minnesota. The pine siskin A3 strain was first encountered in May 2008 in an American goldfinch and later in a northern cardinal at the start of the pine siskin epizootic. MLVA also confirmed the clonal nature of S. Typhimurium in songbirds and established that the pine siskin epizootic strain was unique to the finch family. For 2009, the distribution of PFGE type A3 in passerines and humans mirrored the highest population density of pine siskins for the East Coast.

Fig 1

Cluster analysis of avian S. Typhimurium pulsed-field gel electrophoresis (PFGE) patterns generated with the restriction enzyme XbaI. Tiff images of S. Typhimurium PFGE patterns were compared using DNA pattern recognition software (BioNumerics; Applied Maths, Austin, TX). Levels of similarity were calculated using the band-based Dice similarity coefficient, and clustering of samples was performed using the unweighted-pair group method using average linkages (UPGMA). Six major PFGE clusters, A to F (>85% similarity), were identified among 162 avian S. Typhimurium isolates examined. Within PFGE clusters A, B, and D, additional PFGE subtypes, in which patterns differed by only 1 to 4 DNA fragments, were identified. The majority of songbird S. Typhimurium isolates produced PFGE A (subtypes 1 to 12) patterns with the restriction enzyme XbaI.

Fig 2

Cluster analysis of avian S. Typhimurium isolates by MLVA. The dendrogram was generated in BioNumerics (Applied Maths) using the categorical coefficient and unweighted-pair group method using average linkages (UPGMA). Avian S. Typhimurium isolates subtyped by MLVA belong to one of three major clusters (I to III). Of the songbird isolates typed by MLVA, those belonging to the bird families Cardinalidae (cardinals), Fringillidae (American goldfinch, common redpoll, pine siskin, and purple finch), Icteridae (brown-headed cowbird and red-winged blackbird), and Passeridae (English sparrow and house sparrow) grouped into cluster III. Within cluster III, two additional branches were identified. All S. Typhimurium isolates typed by MLVA that belonged to cluster IIIb were isolated from birds belonging to the finch family Fringillidae.

Fig 3

Temporal distribution of S. Typhimurium strains with the PFGE pattern A3 in birds and humans for the years 2008 and 2009. Light gray, birds; dark gray, humans.

Fig 4

Spatial distribution of S. Typhimurium strains with the PFGE pattern A3 in the eastern United States. Human (yellow) and bird (light blue) isolates of PFGE type A3 that were obtained between May 2008 and November 2009 were mapped to the county level. Counties from which both bird and human isolates were obtained are colored green. Major urban centers on the Atlantic coast are depicted along with their population size, shown in gray. The scale for the human population is shown in the upper left, ranging from 100,000 to 10 million. (Map used by permission. Copyright © 2012 Esri and its data providers. All rights reserved.)