Dichelobacter nodosus in sheep, cattle, goats and South American camelids in Switzerland-Assessing prevalence in potential hosts in order to design targeted disease control measures.

Footrot is a contagious foot disease caused by the bacterium Dichelobacter nodosus (D. nodosus) that affects sheep worldwide. Due to substantial economic and welfare impact, various countries have developed control programs against footrot. The aim of this cross-sectional study was to estimate the national prevalence of virulent and benign D. nodosus in Switzerland in the four domestic ruminant species sheep, cattle, goats and South American camelids (SAC) to detect potential host populations and to propose targeted disease control measures. Risk factors for infection with the virulent strain of D. nodosus, based on a survey carried out among farmers, were investigated on animal and herd level. Overall, 613 farms and 2920 animals were investigated during 2017-18 applying a two-stage cluster sampling strategy. A Real-Time PCR method for simultaneous detection of virulent and benign strains of D. nodosus was used for the first time in such a large study. On animal level, the true prevalence (TP) of virulent D. nodosus in sheep was estimated at 16.9% (95% confidence interval (CI95%): 9.5-24.3%). In cattle and goats no virulent D. nodosus was detected and in SAC an apparent prevalence (AP) of 0.2% (CI95%: 0.0-0.4%) was observed. On farm level, a TP of virulent D. nodosus of 16.2% (CI95%: 8.4-25.2%) for sheep and an AP of 1.5% (CI95%: 0.3-5.2%) for SAC herds was estimated. Since the Swiss control program only targets the virulent strains of D. nodosus, it was concluded that cattle, goats and SAC do not play a role in footrot epidemiology in Switzerland. Adult sheep were at higher risk of infection for virulent D. nodosus compared to lambs and yearlings. On herd level, risk factors for infection with virulent D. nodosus in sheep were earlier occurence of footrot, winter compared to summer and autumn, and goat contact on pasture. Liming pastures had a protective effect on D. nodosus infection. For benign D. nodosus, the TP in sheep was 6.3% (CI95%: 1.6-11.0%) and in cattle 88.4% (CI95%: 83.8-93.0%). The TP for benign D. nodosus in sheep farms was 2.8% (CI95%: 0.0-10.5%) and in cattle farms 95.9% (CI95%: 91.7-98.1%). In goat and SAC farms, the AP was 6.6% (CI95%: 3.4-11.5%) and 7.4% (CI95%: 3.8-13.1%), respectively. These findings could be relevant for wild ruminants such as Alpine ibex (Capra ibex ibex), which can develop clinical footrot after infection with benign D. nodosus. The findings of this study are crucial for assessing targeted disease control measures in Switzerland.