The epizootiology of infectious bursal disease and prevention of it by immunization.

Transport of leftover feed from infectious disease (IBD) farms to susceptible flocks on clean premises is believed to have played an important role in the transmission of this disease to new farms and to new areas. Servicemen, caretakers, contaminated equipment and air were suspected in the spread of the disease, but these modes were not demonstrated. Litter, drinking water and samples of feed from feeders of IBD flocks were found to be infectious when added to rations of susceptible chickens but were not when added to drinking water. Infectious bursal agent (IBA) remained viable for at least 6 months in dry litter and in unused dry chicken houses for more than 1 year. IBA was found in circulating blood of infected chickens 36 hours after inoculation, but not after 72 hours. Liver, spleen, thymus, kidney, pancreas and intestines were infectious at 72 hours. Bursa and feces were infectious from 48 hours through 7 days after inoculation. Tests on 4 1/2 to 6-week-old birds from 10-12 farms of each of several broiler complexes in three southeastern states of the U.S. revealed IBA present on a high percentage of farms even though none had used vaccine. Vaccination with modified or unmodified IBA administered to millions of 4-10-day-old chicks gave good protection for the life of broilers and laying birds. Effect of IBD on development of immunity to other diseases will be discussed.