Background information: Salmonella enterica subsp. enterica serovar Gallinarum (antigenic formula 1,9,12:–:–) is a serovar of the O:9 (D₁) serogroup. This serovar has two biovars: Gallinarum, and Pullorum. Biovar Pullorum has been found to exhibit host specificity for aquatic birds, and poultry and is mainly associated with Pullorum disease (PD), which, in its acute form, is almost exclusively a septicemic disease affecting young chickens. PD is mainly egg-transmitted and can result in high death loss in the young birds. Transmission of PD can also occur through direct or indirect contact with infected birds, as well as through contaminated feed, water, litter, or respiratory exposure.

Relevant links: 

1. https://pmc.ncbi.nlm.nih.gov/articles/PMC10812584/#:~:text=Salmonella%20enterica%20subsp.,disease%20(PD)%2C%20respectively
2. https://www.woah.org/en/disease/pullorum-disease/#:~:text=Pullorum%20disease%20of%20chickens%20is,septicaemic%20disease%20of%20young%20chickens.
3. https://www.merckvetmanual.com/poultry/salmonelloses-in-poultry/pullorum-disease-in-poultry#Epidemiology_v3342829