Significance of bursa of Fabricius as target organ in infectious bursal disease of chickens.

The highly virulent strain Cu-1 of infectious bursal disease virus caused 100% mortality in 4-week-old specific pathogen-free chickens. In contrast, chickens infected after bursectomy did not become sick and only showed some discrete and transient necrosis in lymphatic tissues. However, these chickens contained infectious virus and, subsequently, produced specific antibodies. The virus concentrations in the organs studied reached their maximum 2 days postinfection, but were about 1,000 times lower in non-bursectomized animals. It may be assumed that in bursectomized chickens the early events of infection are the same as in non-bursectomized ones. Virus is spread in varius organs, but due to the absence of a sufficient number of susceptible cells, virus multiplication is moderate and can be kept in check by the host defense mechanism. With the occurrence of circulating specific antibodies the virus can be rapidly eliminated. The studies particularly stress that the availability of a large number of highly susceptible cells is a crucial point in acute viral infections.