Two outbreaks of pullorum disease were diagnosed in backyard
adult brown chicken flocks and ducks in Iowa in 2004. Typhlitis, as
observed in one brown laying hen, has been associated with neonates
and has not previously been reported in adult birds (22). White
Leghorn chickens have been found to be more resistant to PD than
are heavier breeds, including Rhode Island Reds, Barred Plymouth
Rocks, and White Wyandottes (10). In some countries, PD occurs
almost exclusively in brown chickens when white and brown laying
hens are housed in close proximity (2). An increased genetic
susceptibility to infection by Salmonella Pullorum may explain why
only Production Brown chickens were affected clinically (4). Ducks
on the contact farm were probably infected by exposure to
Salmonella Pullorum shed in feces of carrier chickens. Ducks are
relatively resistant to PD (3); however, reports of infection and
positive serological responses in ducks have been published (5).