Investigation of post-vaccinal canine distemper involving the Rockborn-like strain in nine puppies in New Zealand.

Abstract

Case history: This report details investigations into nine cases of neurological disease and/or sudden death in 8-13-week-old puppies between 2021 and 2024. Aside from two pairs of littermates, cases were unrelated. The puppies had an onset of clinical signs 9-23 days following at least one "on-label" dose of a commercially available quadrivalent vaccine containing live attenuated canine distemper virus (CDV).

Clinical findings: Eight of the nine cases displayed signs typical of "classic distemper," including seizures, circling, tremors, hypersalivation, progressive neurological deficits, pyrexia, and/or respiratory and gastrointestinal signs. Pathological and molecular investigations were undertaken in eight cases. Mononuclear/lymphohistiocytic encephalitis or meningoencephalitis with or without neuronal intranuclear inclusion bodies was present in seven cases. Five cases had bronchopneumonia. Other lesions included poliomyelitis, necrotising enteritis and myocardial necrosis or myocarditis. PCR for CDV was positive on tissues from seven cases, and immunohistochemistry for CDV was positive on neural tissues in six cases. Whole genome sequencing of PCR amplicons demonstrated a Rockborn-like strain with 99.9% homogeneity between samples from four cases and a vial of vaccine.

Diagnosis: Based on the combination of case history, pathological findings, molecular test results and/or whole genome sequencing, a diagnosis of post-vaccinal canine distemper was confirmed in six cases and presumed in two.

Clinical relevance: Outbreaks of canine distemper have been stemmed by widespread vaccination starting in the mid-twentieth century. Consequently, confirmed cases of natural CDV have not been reported in New Zealand since an outbreak in the 1980s, and CDV is considered a "notifiable organism" as per the Biosecurity Act 1993. This is the first case series to report genomic investigation of post-vaccinal canine distemper in New Zealand puppies and highlights a rare adverse event associated with routine vaccination. Our results suggest that puppies with neurological, respiratory and/or gastrointestinal disease with an onset within 6 weeks of vaccination with live attenuated CDV should be reported and investigated accordingly.