# High Seroprevalence of Bluetongue Virus Serotype 3 in Belgian Cattle and Sheep After the 2024 Epidemic

## TL;DR

This study found that most cattle and sheep in Belgium had been exposed to BTV-3 after the 2024 epidemic, with no signs of other BTV serotypes spreading.

## Contribution

The study provides the first detailed seroprevalence data of BTV-3 in Belgian cattle and sheep post-epidemic.

## Key findings

- BTV-3 had a very high herd-level seroprevalence in both cattle and sheep.
- No active circulation of BTV-8 or BTV-12 was detected in the sampled herds.
- BTV RNA levels declined during winter, likely due to lack of vector activity.

## Abstract

To monitor the epidemiological situation of bluetongue virus (BTV) in Belgium, a national surveillance programme was conducted during the 2024–2025 winter season. The objective was to estimate the apparent seroprevalence of BTV-3 following the 2023–2024 epidemic and to prove the absence of active circulation of other BTV serotypes in mixed herds (cattle and sheep). A total of 2551 cattle and 1458 sheep were sampled across Belgium. Serological analyses were performed using ELISA, and molecular detection of BTV-3, BTV-8, and BTV-12 was conducted by RT-qPCR. The majority of cattle and sheep herds showed evidence of exposure to BTV-3, with a very high herd-level apparent seroprevalence (100%; 95% CI: 96.2–100% in cattle and 98.9%; 95% CI: 93.8–99.8% in sheep). Apparent within-herd seroprevalence was also high in cattle (94.6%; 95% CI: 91.8–96.5%) and sheep (85.5%; 95% CI: 80.4–89.5%). No evidence of active circulation of BTV-8 or BTV-12 was detected. A moderate significant positive correlation between Ct values and sampling date was observed both for bovine and ovine samples, consistent with a progressive decline in detectable BTV RNA during winter in the absence of vector activity.