Investigating Vaccination Failure in Poultry Flocks1

Protecting poultry against disease involves more than simply administering preventive vaccines. Disease preven-tion is a complex and multifaceted process. Vaccine quality is commonly blamed when a disease occurs; however, there are usually other factors responsible. A comprehensive investigation is often called for to identify the cause(s) and to resolve the problem. Commercial poultry farms have continued to increase in size and more birds and farms are located in a geographic area; this has resulted in a dynamic disease situation. In recent years, several new infectious diseases have infected poultry and spread widely. However, existing diseases have undergone considerable changes. Many of these were well controlled in the past by existing health and management practices. Due to selection pressures; however, these agent