Evaluation of ELISA titers to infectious laryngotracheitis

Infectious laryngotracheitis (ILT), a highly infectious upper respiratory disease of chickens, can cause serious economic loss in areas where the poultry industry is concentrated. To determine the antibody levels associated with vaccine administration, field challenge, and protection, six groups of 20 specific-pathogen-free leghorn chickens were housed in biosecured isolation units. Individual groups served as either negative controls, vaccinated (one full dose per bird of chicken embryo origin [CEO] administered by the eyedrop method) and challenged (intratracheal administration with USDA strain ILT virus at 10(4.1) 50% embryo infective dose [EID50]), or unvaccinated and challenged with USDA strain ILT virus at various dose levels (10(2.1), 10(5.1), or 10(4.1) EID50). Chickens in each group were bled weekly, and their sera were tested for antibody using a commercially available enzyme-linked immunosorbent assay test kit. The antibody response using CEO vaccine resulted in a 400-600 geometric mean titer that appeared to be protective against severe field challenge. Negative controls had no titers, whereas vaccinated and/or challenged chickens had detectable titers within 2 wk of exposure, and these titers remained high for the next 4-7 wk. Mortality in nonvaccinated controls began at 3 days post-challenge and continued for up to 10 days.