Seroprevalence of Bluetongue serotype 3 and associated risk factors in Dutch sheep: An analysis of the variation of between-and within-farm prevalences following the first epidemic year

In September 2023, bluetongue virus serotype 3 (BTV-3) emerged in the Netherlands. Thousands of sheep farms became infected and over 55 thousand sheep died between September and December 2023. This study aimed to determine the final size of the epidemic after the vector-active season of 2023. To help farmers prepare for possible resurgence in 2024, risk factors were identified that were associated with presence of BTV antibodies at farm level, and if so, the proportion of affected sheep in positive farms. Archived serum samples from adult sheep (N = 387 farms; 13 samples per farm) submitted to Royal GD for monitoring purposes between January and April 2024 (prior to the vector active season of 2024) were selected and tested for presence of BTV-specific antibodies. Farm prevalence, within-farm prevalence in seropositive farms and overall (national) animal level BTV prevalence were calculated from the test results. Additionally, farmers that agreed to participate in the study were surveyed in spring 2024 regarding their farm management in 2023. Multivariable regression analyses with appropriate distributions were conducted to quantify associations between potential risk factors and i) the probability of sheep farms being BTV-seropositive, and if so, ii) the proportion of affected sheep within the farm. For the latter, the apparent within-farm seroprevalence was combined with the mortality recorded at farm level between Sept-Dec 2023, to correct for infected sheep that died in affected farms. Farm-level seroprevalence in sheep was estimated at 47 % (95 % CI: 41–52) and the national animal-level seroprevalence was estimated to be 10 % (95 % CI: 8–13). Within BTV seropositive farms, a median percentage of 15 % (mean 23 %) of sheep tested seropositive. Flocks kept indoors to prevent infection were less frequently seropositive (OR: 0.25, 95 % CI: 0.09–0.67). Within seropositive farms, shearing in summer, between July and September, was associated with a higher proportion of affected sheep compared to shearing in winter months (OR: 2.22, 95 % CI: 1.27–3.86). Although several protective factors were found, they might not fully prevent sheep from becoming infected resulting in seropositivity. Nevertheless, these measures likely offer farmers possible measures to reduce the spread and subsequent impact of BTV-3 in their sheep farms, beyond vaccination.