Humoral response and protection from experimental challenge following vaccination of raccoon pups with a modified-live canine distemper virus vaccine

Eight 8-wk-old raccoon pups (Procyon lotor) with maternal canine distemper virus (CDV) neutralizing antibodies (NAb) and 24 8-wk-old seronegative pups were administered a commercial modified-live CDV vaccine (Galaxy D(R), Sol vay Animal Health, Inc., Kitchener, Ontario, Canada). All 24 seronegative r accoons had detectable serum CDV NAb titers 14 days after the initial dose. Titers rose to maximum levels 4 wk post-vaccination. Mean titers for group s of vaccinated seronegative pups were maintained between 1:256 and 1:2.048 for the remainder of the 3 mo observation period. Geometric means of the s erum CDV NAb titer of eight seronegative pups given a single vaccine dose a t 8 wk of age did not differ significantly from those of eight pups that we re given serial doses at 8, 12, and 16 wk of age, or from those of eight pu ps vaccinated once at 16 wk of age. Seven unvaccinated 8-wk-old raccoon pup s used as controls remained seronegative throughout the trial. Seven out of eight 8-wk-old pups with maternal antibodies, vaccinated at 8, 12, and 16 wk of age, failed to develop a rise in their CDV NAb titers until at least 1.8 wk of age, 2 wk after the third vaccination. Titers in eight unvaccinat ed raccoons with maternal antibodies declined steadily to undetectable leve ls at 20 wk of age. A half-life of 10.55 days was calculated for maternally -derived CDV NAb in raccoon pups. Sixteen vaccinated raccoons were protecte d from clinical disease following experimental oronasal challenge with a vi rulent raccoon strain of CDV, 13 to 23 wk after vaccination. Serum CDV NAb titers at the time of challenge ranged from 1:12 to 1:384 and increased dur ing the period of observation. Three of four unvaccinated seronegative racc oons used as controls failed to mount any detectable CDV NAb and were eutha natized after developing clinical signs of canine distemper 26, 29, and 30 days post-challenge (PC). Necropsies confirmed the diagnosis. The fourth co ntrol raccoon exhibited transient equivocal clinical signs, mounted a slugg ish humoral response, but was clinically normal when euthanatized 42 days P C. In this raccoon, there was focal non-suppurative encephalitis with intra nuclear inclusion bodies typical of CDV infection.