Field vaccination against ILT in broiler chickens: lack of consistency

When vaccination against infectious laryngotracheitis (ILT) is necessary in broiler flocks, mass administration techniques are required due to the sheer numbers of birds involved. This is usually done by drinking water administration even though ILT vaccines are generally registered for individual eye drop application. Often the results are variable, with occasional outbreaks occurring in vaccinated flocks or rolling vaccine "reactions" occurring, sometimes involving clinical signs and even mortality. A field evaluation study was performed during a field outbreak using collection of tracheal swabs from birds in vaccinated sheds. DNA extracted from swab samples was submitted for quantitative real time PCR (qPCR) to detect and quantify IL T virus (IL TV) at the University of New England (UNE). The first shed examined used 70 individually identified birds which were retrieved at 7 post vaccination time points. A further 7 sheds were then followed with 40-45 randomly selected birds at 4, 7-8, 12- 13 and 25-26 days post vaccination. Vaccination procedures and timings of administration were recorded. Patterns of vaccine "take" were compared across factors recorded at vaccination day to look for putative risk factors for poor or better vaccination success. IL T virus recovery varied markedly between sheds but there were patterns observed between poor and somewhat better results. Factors showing statistical association with these patterns included use of a dye product to stabilize water as a risk factor and longer time of water stabilization with a skim milk product prior to vaccine addition as a protective factor. There was also an almost significant (P=0.07) association of chick source with vaccine take pattern. Results have implications for vaccine application and suggest a possible monitoring process for vaccination success.