When salmonella moves sideways between birds in a flock, such movement is known as lateral spread or transmission. When salmonella goes from the mother to the offspring it is known as vertical transmission or spread. As the parent hen never sees her progeny chicks, vertical transmission of salmonella in poultry must involve transmission in or on the egg and this is what we will now consider. If we look at the structure of the poultry industry and the way a breeder flock supplies a hatchery or hatcheries, which then in turn supply chicks to many commercial broiler or table egg farms, it should be apparent that the chicks from one breeder flock is can be placed on, and hence, infect many commercial farms.
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