There are two routes whereby the egg may be contaminated. First is the trans-ovarian route. In this case, the salmonella have infected the layer's blood stream, migrated to the ovary, and are then shed with each egg yolk. Methods to control this type of infection are quite different from those required for the second route of infection, called trans-shell route. Here, the salmonella are present on the shell surface, either because they were present in the birds' feces, or in the environment. Trans-ovarian transmission is largely prevented by using salmonella negative pullets, and consistent testing programs to ensure the environment is clean.

Once on the shell, salmonella can enter either through the shell pores in intact eggs, or through cracks. The egg has natural defense mechanisms to limit such entry. First is the cuticle, the outermost layer of the eggshell. Second is the structure of the shell itself, which largely (but not completely) prevents bacterial invasion.

Salmonella can enter the egg through the pores in the shell. The thicker the shell, the less likely they are to succeed. Thus eggs with thinner shells, often from older hens, are more vulnerable.

If salmonella succeed in penetrating the shell, and the shell membranes, they encounter the next defense system, which is the egg white (albumen). Egg albumen contains a range of antibacterial proteins, of which the best known is lysozyme. This protein is highly active against bacteria such as salmonella.

Europeans keep large numbers of hens in non-cage systems, ranging from high-density aviaries to free-range systems. Additionally, conventional cages are being phased out, and replaced with cages containing perches, nests and dust baths, usually with 10 or more birds/ cage.

Salmonella contamination is highly influenced by the level of egg cleanliness. In general, more dirty eggs are produced from the furnished cages and from non-cage systems. Improved cage designs and better management are believed to be capable of achieving acceptable levels of egg cleanliness in these systems.

Cracked eggs are also more susceptible to salmonella invasion. In these studies, there were no differences in the incidence of cracked eggs between the different cage systems.

Studies also showed that progress in identifying eggshell matrix proteins may assist breeders to produce hens that lay eggs with better shells. (Note that this work is partly conducted by Dr. Max Hincke at the University of Ottawa, partly funded by the Ontario Egg Producers, through the Poultry Industry Council).

Storage of eggs also influenced salmonella growth. Reducing storage temperature was shown to delay growth of salmonella in artificially contaminated eggs.

Take-home messages

Dr. Peter Hunton is a poultry consultant. This article is drawn from one that appeared in the Ontario Egg Producers' The Cackler newsletter