An outbreak in week-old broiler chicks caused by a highly virulent and multidrug-resistant strain of <i>Salmonella</i> Gallinarum

<i>Salmonella</i> Gallinarum is a bacterium that causes significant economic losses in poultry farming and usually infects adult birds. This study aimed to characterize the unusual histological findings and genetic profile of a <i>Salmonella</i> Gallinarum (SG) strain isolated from a natural outbreak that affected 1500 broiler chicks in their first week of life. To confirm the diagnosis, necropsy, histopathology, and immunohistochemistry were performed on 19 chicks, with bacterial isolation in two of them, followed by confirmation through PCR, genetic sequencing, and genomic analysis. The outbreak revealed unusual gross and histological lesions: in the intestines, there was macroscopic haemorrhage of varying severity and, microscopically, the lesions were characterized by histiocytic and heterophilic transmural typhlitis; in the lungs, there was histiocytic interstitial pneumonia of varying intensities. Additional lesions included splenitis, myocarditis, and hepatitis. <i>Salmonella</i> was <i>in situ</i>-labelled in all mentioned organs using immunohistochemistry. Following the virulome and resistome analysis, which compared the genetic profile of the isolate with a reference SG genome, it was found that the isolate from this outbreak contained 36 exclusive virulence genes and six exclusive antimicrobial resistance genes. These genetic alterations may explain the severe systemic lesions observed in these chicks.<b>RESEARCH HIGHLIGHTS</b><i>Salmonella</i> Gallinarum infection in chicks resulted in a mortality rate of 42%.Chicks had unusual lesions such as haemorrhagic enteritis and interstitial pneumonia.Whole genome analysis of the SG isolate revealed exclusive antimicrobial resistance genes.Higher number of exclusive virulence genes were related to severe fowl typhoid in chicks.