Chickens can be infected by Salmonella from a number of routes; feed, water, rodent droppings, fecal material from penmates, fecal material from wild birds, fecal material from wild animals, fecal material carried on the feet or hands of humans, contaminated flies, contaminated equipment brought into the house, etc. Vaccination is but one intervention (an important one) used to prevent Salmonellosis. Treatment of feed, rodent control, fly control, biosecurity (preventing all the physical carriers of disease agents into a flock), cleaning and disinfection of houses between flock placements (normally, flocks are taken in and out as a group of one age), and the use of preventative feed additives (non-antibiotics) that aid intestinal health and improve the resistence of the intestinal tract to Salmonella invasion. Salmonella enteritidis (SE) gained the ability gain access to the ovary and become incorporated into the egg itself, sequestered from egg shell washing and disinfection. Other Salmonellae are gut-borne and contaminate the shell and are washed off during the egg washing process. Refrigeration is a very important intervention to prevent SE from growing during storage to numbers that can infect a human.
One question I failed to answer was if other eggs laid by a chicken would be contaminated. Yes, a chicken that is infected with Salmonella enteritidis (SE) will lay contaminated eggs on a regular basis until she gains sufficient immunity to the disease to cease laying contaminated eggs. Todays modern laying hen lays about one egg a day.
