The biting midges that transmit some of Europe’s most damaging livestock viruses do not simply vanish when winter arrives, according to one of the most extensive vector surveillance studies ever conducted in Germany. But the same research delivers a reassuring counterpoint: even during an active bluetongue outbreak, the midges carried almost no virus, suggesting that the European winter remains a genuine pause in transmission — even if it is not the vector-free period regulators once assumed.
Researchers from the Leibniz-Centre for Agricultural Landscape Research and the Friedrich-Loeffler-Institut, Germany’s federal research institute for animal health, deployed UV-light traps weekly inside cattle, sheep, goat and horse stables at between 12 and 55 locations across Germany over four consecutive winters, from November 1 to March 31 in the years 2021 through 2025. The monitoring window deliberately straddled the dramatic emergence of bluetongue virus serotype 3 (BTV-3), which swept through German ruminant flocks starting in late 2023, giving the team a rare opportunity to compare midge ecology and virus circulation under both endemic and outbreak conditions.
The scale of the trapping effort allowed the researchers to assemble an unusually detailed picture of winter biting midge activity. Collected female midges of the genus Culicoides — the principal biological vectors of bluetongue virus, Schmallenberg virus and epizootic hemorrhagic disease virus — were morphologically assigned to two major taxonomic groupings, the Obsoletus Group and the Pulicaris Complex, both of which contain proven or suspected vector species. In total, 1,581 pools comprising 46,358 female midges were screened by real-time reverse-transcription polymerase chain reaction (real-time RT-PCR) for all three viruses.
The seasonal pattern that emerged was strikingly lopsided. November and March together accounted for a remarkable 95 percent of all midges captured during the winter monitoring periods, while the true mid-winter months of December through February yielded just 5 percent of the total. January and February were the quietest of all, each contributing a mere 1 percent of captures. The researchers also noted pronounced variation between sites and months, reflecting the patchy microclimates of livestock housing — stables, with their residual warmth, Organic matter and resident animal hosts, can sustain small populations of adult midges long after outdoor conditions have become lethal.
Activity peaks in March 2024 and March 2025 coincided with mild temperatures and, in part, with the inclusion of new monitoring sites, and the authors suggest that one or both factors helped drive those surges. Temperature data recorded alongside the trapping allowed the team to relate midge activity directly to thermal conditions, an important analytical step because Culicoides development, activity and — critically — the capacity of arthropod-borne viruses to replicate within their insect hosts are all strongly temperature-dependent. Below a certain thermal threshold, even if a midge ingests virus-laden blood, the virus cannot complete its extrinsic incubation period, meaning the insect never becomes infectious.
Perhaps the most consequential finding concerns blood-feeding. Although blood-fed females were very rare during the depths of mid-winter, their abundance at other points in the cold season often exceeded the European Union threshold used to define a so-called “vector-free period” — a threshold of fewer than five parous (egg-laying-experienced) female Culicoides per trap per night. Parous females are the epidemiologically dangerous ones, because only females that have already taken a blood meal can have acquired and potentially transmitted virus. The regular presence of blood-fed midges above the regulatory threshold in livestock stables, the researchers conclude, directly challenges the concept of a true vector-free period in winter as it is currently applied.
Yet the virological results tell a very different story from the entomological ones. Across the entire four-year dataset — spanning 1,581 pools and nearly 47,000 midges, including two winters during an active BTV-3 outbreak — only a single pool of Culicoides, collected in November 2024, tested positive for BTV-3 RNA. Every other pool was negative for bluetongue virus, Schmallenberg virus and epizootic hemorrhagic disease virus alike. A single RNA-positive pool among tens of thousands of midges is, epidemiologically, a whisper rather than a signal, and the team interprets it as evidence that genuine virus circulation during the central European winter is, in their words, hardly existent, even under endemic or outbreak conditions.
Reconciling these two strands of evidence — abundant, feeding midges on one hand and an almost complete absence of detectable virus on the other — points to temperature as the decisive factor. Even when midges are active enough to bite livestock in the sheltered environment of a stable, winter temperatures in Germany generally remain too low to permit viral replication within the insect, breaking the transmission cycle regardless of vector presence. Combined with the overall scarcity of midges in mid-winter, the researchers conclude that the risk of BTV infection of ruminants via biting midges between December and March is negligible.
The findings nonetheless leave a significant biological puzzle unresolved: how does bluetongue virus survive the winter in central Europe at all? Overwintering is essential for the virus to re-emerge each season, and the new data effectively rule out sustained winter transmission by vectors. The authors propose two candidate mechanisms. One is that rare infection events in ruminants or vectors during winter — such as the single positive pool they detected in November — occasionally sustain low-level viremia in animals until spring, providing a seed for renewed transmission when temperatures rise. The other is that the virus persists by means currently unknown to science, which could include mechanisms within vectors or reservoir animals that have not yet been characterized. Distinguishing between these possibilities will be essential for improving models of BTV-3 recurrence and for designing surveillance strategies that catch the virus as it reawakens.
For veterinary authorities and farmers, the practical implications are double-edged. On one hand, the study supports treating the deep winter months as a period of negligible transmission risk, which is welcome news for the timing of animal movements, vaccination campaigns and other control measures. On the other hand, the documented presence of blood-fed female midges above the EU’s vector-free threshold means that blanket assumptions about winter safety inside livestock housing should be treated with caution, particularly in the shoulder months of November and March, when more than nine in ten winter midges were captured. Stable-dwelling midge populations, the study suggests, are a real if seasonally constrained feature of the European livestock landscape.
The research also fills a long-standing gap. Despite the central importance of winter vector ecology to seasonal risk assessment for Culicoides-borne diseases, data on winter activity of these midges have remained sparse, and information on virus circulation in winter-active vectors — under either endemic or outbreak conditions — has been almost non-existent. By combining four consecutive years of standardized weekly trapping, temperature recording and molecular virus screening, the German team has produced one of the first datasets robust enough to quantify both the insect and the virus sides of the winter equation simultaneously.
The work was supported by the German Federal Ministry of Agriculture, Food and Regional Identity through the Federal Office for Agriculture and Food, and the authors emphasize that the results are best understood as an integrated picture: vectors are present and feeding in winter, but thermal constraints keep virus replication — and therefore transmission — at levels that are functionally close to zero. As BTV-3 continues to reshape livestock disease management in northwestern Europe, understanding what the virus does between outbreaks may prove as important as understanding what it does during them.
Subject of Research: Winter activity and virus infection of Culicoides biting midges in German livestock housing before and during the 2024 BTV-3 outbreak
Subject of Research: Biology
Article Title: Winter activity and virus infection of Culicoides biting midges (Diptera: Ceratopogonidae) in German livestock housing before and during the 2024 BTV-3 outbreak
Article References: Voigt, A., Kampen, H., Beer, M., Wernike, K., Scheuch, D., Sick, F., Zeiske, S., & Werner, D. (2026). Winter activity and virus infection of Culicoides biting midges (Diptera: Ceratopogonidae) in German livestock housing before and during the 2024 BTV-3 outbreak. Parasites & Vectors, 19(1), Article 382. https://doi.org/10.1186/s13071-026-07620-w
Image Credits: AI Generated
DOI: 10.1186/s13071-026-07620-w
Keywords: Culicoides, biting midges, bluetongue virus BTV-3, Schmallenberg virus, epizootic hemorrhagic disease virus, winter activity, livestock housing, vector surveillance, vector-free period, Germany, seasonal variation, real-time RT-PCR
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William Thompson. (September 4, 2026). Winter-active biting midges infected with bluetongue virus found in German livestock barns. Scienmag. https://scienmag.com/winter-active-biting-midges-infected-with-bluetongue-virus-found-in-german-livestock-barns/
William Thompson. “Winter-active biting midges infected with bluetongue virus found in German livestock barns.” Scienmag, 4 September 2026, https://scienmag.com/winter-active-biting-midges-infected-with-bluetongue-virus-found-in-german-livestock-barns/. Accessed 4 September 2026.
William Thompson. “Winter-active biting midges infected with bluetongue virus found in German livestock barns.” Scienmag. September 4, 2026. https://scienmag.com/winter-active-biting-midges-infected-with-bluetongue-virus-found-in-german-livestock-barns/
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