Core genome multilocus sequence typing (cgMLST) indicates herd specificity of Fusobacterium necrophorum ssp. necrophorum isolated from dairy cattle interdigital phlegmon outbreaks in Finland

Numerous outbreaks of interdigital phlegmon (IP) occurred in Finnish dairy herds country-wide in the 2010s, resulting in significant but varying morbidity and severe clinical signs. These outbreaks increased antibiotic use and caused financial losses on the affected farms. During 2012–2015, we performed a study on the epidemiology, clinical presentation and bacteriology of IP outbreaks on 19 IP-affected farms. In addition, we collected Fusobacterium necrophorum isolates from IP cows for further studies. Here, we present an analysis of 25 isolates from early-stage IP from seven herds representing both moderate and high morbidity using whole genome sequencing (WGS). Our aim was to determine whether a novel contagious F. necrophorum strain had spread between the herds, causing the outbreaks, and whether the isolates possessed antibiotic resistance or known major virulence genes in silico. The Finnish F. necrophorum ssp. necrophorum isolates clustered into seven different groups, each corresponding to the herd of origin. Therefore, the isolates from high and moderate morbidity herds did not cluster together either. Our analysis revealed no known antibiotic resistance genes in the isolates, despite the clinical response to treatment being inconsistent. All isolates harboured virulence genes for leukotoxin and haemolysin but were devoid of adhesin. Instead of only one novel contagious strain, we identified herd-specific strains of F. necrophorum ssp. necrophorum in IP cows from Finnish dairy cattle herds. Therefore, the cause of IP outbreaks in Finland remains unknown and merits further research.