| The avian yolk sac is a well vascularised membrane that surrounds the yolk of an embryonated egg and functions as a placenta-like structure transferring yolk nutrients including maternal antibodies, to the embryo. The absorption of the yolk sac content during the first days post-hatching is essential for chick growth and development. However, the infection of the yolk sac is the main cause of chick mortality accounting for large economic losses to the poultry industry. The overall goal of this thesis was to study the epidemiological triad of yolk sac infections: the pathogen: Escherichia coli, the susceptible host: the chick, and the environment: the chicken barn. In the first experiment a strain of avian pathogenic E. coli was transformed with a plasmid carrying a green fluorescent protein. Using fluorescence microscopy it was observed that E. coli entered the yolk sac via the chick navel. In the second experiment, the effects of breeder flock age on the total IgY content of egg yolk and yolk sac was determined. It was concluded that IgY increased with breeder flock age in eggs and yolk sacs. The consequences of these results on chick health are unknown. In the third experiment, the effects of cleaning and disinfection methods of the chicken barn on microbial counts were analyzed in barn swabs and in pooled organ and yolk sac samples. It was surprising to observe that 4 days after placing chicks in the barn, samples of chicks from "cleaner" pens had more bacteria than those of chicks from "dirtier" pens. In the fourth experiment, all E. coli isolated from barn and chick samples from the previous experiment were typified using the RAPD method. It was determined that "cleaner" pens had greater E. coli variability than "dirtier" pens. Also, more E. coli types were shared between chicks and the environment in "cleaner" pens suggesting that chicks that are placed in very clean environments acquired more environmental E. coli than chicks placed in environments with greater bacterial loads. The long term consequences of environmental sanitation on chick growth and development, disease susceptibility, and broiler performance should be studied. |
