Duration of Immunity to Infectious Laryngotracheitis

THE duration of immunity to infectious laryngotracheitis (LT) of chickens has not been established conclusively. Beach et al. (1934) demonstrated that immunity to intratracheal challenge lasted for 364 days, the longest period tested. Hofstad (1959) stated: “The duration of immunity has not been demonstrated, although results indicate that birds are protected for at least a year.” Hitchner and Winterfield (1960) found that 36 percent of birds vaccinated at 8 weeks of age and 100 percent at one week of age became susceptible when challenged at 16 weeks of age. In Australia, Hunt (1962) vaccinated day-old broiler chickens by the feather follicle method and challenged 9 weeks later by contact exposure or by the intratracheal route. Thirteen of 20 birds challenged by contact exposure were susceptible, where as 9 of 20 were susceptible by the intra-tracheal route. Sinkovic (1962) noted a significant reduction in immunity from 12½ to 14½ weeks after…