Respiratory disease complex: Under field conditions, pathogens often interact with not only the host (bird) and its environment, but also one another. For example, day-old chicks arriving infect- ed from the hatchery (vertical transmission) and remaining chroni- cally infected for life are susceptible to other respiratory diseases such as infectious bronchitis or Newcastle disease. Fine dust parti- cles in the poultry house air can then combine with superinfection by Escherichia coli bacteria contribute to additional respiratory in- sults, which will produce the (multiple) lesions that are seen at autopsy for complex respiratory disease. Field disease interactions often also involve common immunosuppressive agents, such as.
