Vaccine type and reproductive status are associated with antibody response and canine distemper infection in shelter dogs: a retrospective cohort study

Canine distemper virus (CDV) outbreaks remain difficult to control in animal shelters, where exposure may occur soon after intake. Field evidence on early vaccine response, infection risk factors, and the value of routine revaccination is limited. Records from 227 dogs tested by PCR and/or serum neutralizing antibody titer during two outbreaks at one large southwestern United States shelter were reviewed. Single-dose seroconversion was compared between modified-live virus (MLV) and recombinant canarypox-vectored (rCV) vaccines in adults with no documented prior vaccination that received a single shelter-vaccine type. PCR positivity was modeled with Firth penalized regression, and day-14 revaccination was evaluated with time-varying Cox and landmark analyses. Of 68 dogs sampled within four days of intake, 36 (53%) had detectable antibodies (≥ 1:8). Sterilized dogs and dogs vaccinated before intake most often had a detectable titer. Among adults with no documented vaccination history, 29 of 30 (97%) MLV recipients seroconverted versus 14 of 24 (58%) rCV recipients (OR 14.2, 95% CI 2.3 to 88.1). Among responders, rCV titers were not lower than MLV titers, so the difference was in reliability rather than magnitude of response. Twenty-three dogs tested positive for wild-type CDV (nine in Outbreak 1, 14 in Outbreak 2), all intact. Within the second outbreak, when both vaccine types were in use, recombinant-vaccinated dogs were more likely to test positive (adjusted OR 8.5, 95% CI 1.5 to 48.7), although residual temporal confounding cannot be excluded. Documented pre-intake vaccination was associated with lower positivity (2% versus 13%). Estimated exposure typically occurred 12 to 19 days after intake, and a routine day-14 dose showed no measurable additional benefit after accounting for immortal time bias. In this shelter, a single MLV dose more reliably produced a detectable antibody response than a single rCV dose. The association between rCV exposure and PCR positivity cannot be interpreted causally, because rCV use was temporally clustered with the second outbreak. Documented pre-intake vaccination was associated with lower infection risk, the first-dose response appeared more relevant to early risk than routine day-14 revaccination, and intact status marked higher-risk dogs. These shelter-specific findings warrant confirmation elsewhere.