The immune response and protection from challenge afforded to adult pigeons by four different vaccination schedules were assessed. Intravenous challenge with a field pigeon isolate was done four weeks after the second of two doses of vaccine given four weeks apart. Little difference in protection was seen between two 0.25 ml and two 0.5 ml doses of oil emulsion vaccine, although the latter produced a slightly higher immune response. In both cases one of 10 challenged pigeons became sick and died. One dose of Newcastle disease virus B1 live vaccine followed four weeks later by 0.5 ml oil emulsion vaccine gave a comparable immune response to two 0.25 ml doses of oil emulsion but only six birds survived challenge. Two doses of Newcastle disease virus B1 vaccine gave a poor immune response and little protection from challenge; all 10 birds became sick and eight died. Assessment of the onset of protection following one dose of either 0.5 ml oil emulsion vaccine or Newcastle disease virus B1 indicated some partial protection in the latter group as early as five days after vaccination. Both groups showed protection at 10 days but by 21 days, although protection was sustained in the oil emulsion group, birds receiving live vaccine were fully susceptible. Measurement of the duration of protection in pigeons given two 0.5 ml doses of oil emulsion vaccine indicated that protection had begun to wane by 40 weeks after the first dose.
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