Toxigenic Corynebacterium ulcerans in raw milk of a cow with acute mastitis: case report and historical review on milk-transmitted diphtheria.

Purpose: Historically, a considerable number of diphtheria outbreaks, mainly in the UK and the USA, have been epidemiologically or bacteriologically linked to the consumption of milk and dairy products. However, with the introduction of milk pasteurization and hygienic improvements in livestock farming and animal welfare, no cases of milk-linked human diphtheria outbreaks or bovine mastitis due to toxigenic corynebacterial have been reported in the recent decades. Here we report the first isolation of a toxigenic Corynebacterium ulcerans strain from the milk of a cow with acute mastitis within nearly 40 years and outside of UK or Finland.

Methods: The isolated strain was analysed by state-of-the-art bacteriological methods including toxigenicity testing by a novel Lateral Flow Immunoassay and Elek test, molecular typing was done by whole genome sequencing and MLST/cgMLST analysis. Due to the rarity of our finding, an extensive historical review of milk-associated diphtheria outbreaks was performed.

Results: The obtained sequence type ST-331 is also found in human isolates of cutaneous diphtheria. cgMLST analysis, however, found no close relationship to 43 human ST-331 isolates from our German strain collection or to two animal samples from a zoonic cluster of this ST.

Conclusion: The risk of milk-associated diphtheria due to C. ulcerans, although today extremely rare, should be avoided by milk pasteurization and the respective hygienic standards.