The impact of bluetongue serotype 3 on cattle mortality, abortions and premature births in the Netherlands in the first year of the epidemic.

In September 2023, bluetongue virus serotype 3 (BTV-3) was identified among sheep and cattle in the Netherlands. Severe clinical signs and increased mortality were reported in sheep and cattle. The aim of this study was to quantify the impact of BTV-3 on mortality, abortions and premature births in cattle in the Netherlands in 2023. Data were available from 1 January 2020-31 December 2023 and were aggregated at herd-month level. Cattle herds were assigned a BTV-3 status for each herd-month: 1) herds that notified clinical signs of BTV-3, 2) herds that did not notify clinical signs but were located in BTV-3 infected areas or 3) herds that were located in BTV-3 free areas (all areas without BTV-3 notifications). Multivariable population-averaged generalized estimating equations models were used to quantify the association between the BTV-3 epidemic and cattle mortality, abortions and premature births. The results showed that from September 2023 until the end of 2023, perinatal calf mortality increased 1.11 (95 % CI:1.06-1.16) and 1.07 (95 % CI: 1.05-1.09) times, in dairy herds with a notified BTV-3 outbreak and in other dairy herds located in outbreak areas, respectively. Premature birth may be one of the explanations of increased calf mortality, as suggested by an odds ratio of 1.39 (95 % CI: 1.26-1.54) for premature births in dairy herds with a notified BTV-3 outbreak compared to dairy herds in BTV-3 free areas. The odds of abortions in infected dairy herds was 1.1 (95 %CI: 1.02-1.20) times higher compared to non-infected herds. Mortality in cattle aged 1-2 years and older than 2 years increased 2.25 (95 %CI: 1.97-2.58) and 1.71 (95 % CI: 1.62-1.81) times respectively, in infected dairy herds. In dairy herds without notification located in BTV-3 infected areas, mortality increased by 1.17 (95 %CI: 1.07-1.27) and 1.22 (95 % CI: 1.19-1.25) times respectively, compared to herds in BTV-3 free areas. In addition, suckler cow herds, beef cattle herds and small scale non-dairy cattle herds with a notified BTV-3 outbreak showed a significantly increased mortality during the BTV-3 epidemic compared to herds in BTV-3 free areas. In conclusion, BTV-3 infections markedly increased abortions, mortality and premature births in cattle herds.