Molecular Detection of Canine Distemper Virus in Portugal: What Explains the Post-2020 Decline? A Retrospective RT-qPCR Study

Canine distemper virus (CDV), currently classified within the species Morbillivirus canis, is a major vaccine-preventable pathogen of domestic dogs and a wide range of susceptible wildlife species. Still, laboratory-confirmed epidemiological data from Portugal remain scarce. This retrospective study investigated CDV molecular detection in 637 diagnostic samples from dogs with clinical suspicion of canine distemper, received from 190 veterinary medical centres across Portugal between 2013 and 2025. Cerebrospinal fluid, EDTA-anticoagulated whole blood, rectal swabs, and conjunctival swabs were tested for CDV RNA using a reverse transcription real-time PCR (RT-qPCR) assay in a qualitative approach. Overall, 215 submissions were CDV RT-qPCR-positive, corresponding to a positivity of 33.8% (95% confidence interval: 30.2–37.5%). Positivity was not significantly associated with sex, age, or Nomenclature of Territorial Units for Statistics level 2 (NUTS 2) region, but differed significantly according to specimen type, with the highest detection observed in EDTA-anticoagulated whole blood and conjunctival swabs. Mixed-breed dogs were over-represented among submitted samples and positive cases, probably reflecting management, exposure, and vaccination-related factors rather than intrinsic breed susceptibility. The central finding was a pronounced post-2020 decline in CDV RT-qPCR positivity, with very low or absent annual detection between 2021 and 2025. This pattern indicates reduced molecular detection within a passive diagnostic population but should not be interpreted as evidence of national elimination. Continued vaccination and strengthened surveillance at the domestic dog–wildlife interface remain essential.