2022 and 2023 saw the emergence or extension of two major ruminant vector-borne diseases: the Epizootic Haemorrhagic Disease (EHD) and a new outbreak of the blue tongue disease related to a new serotype previously unknown in Europe: BTV-3.
2023: The viruses spread
EHD arrived in Europe in 2022, in southern Spain, Sardinia and Sicily. Its evolution was minimal until the second half of 2023. During summer last year, the virus spread to northern Spain and Portugal. The Pyrenees were crossed in early September 2023 and the first infected herds in France were diagnosed in the following days. During the autumn of 2023, the disease spread along the Atlantic coast of France and the number of cases continued to rise until the end of the year. By the end of 2023, 3688 outbreaks of the disease had been officially recorded in France1. In contrast, the number of outbreaks in Spain, Portugal and Italy has changed only slightly. The last outbreak was reported on 15 November 2023 in Spain, on 19 October in Sardinia and on 13 July in Portugal1.
Blue Tongue linked to the BTV serotype 3 virus emerged further north in Europe, in the Netherlands in September 2023. The disease spread rapidly in this country, reaching Belgium, western Germany and even southern England in early November. By the end of 2023, there were 4,289 outbreaks in the Netherlands, 25 in the UK, 21 in Germany and 5 in Belgium.
A winter break
Both diseases are vector-borne. The viruses are transmitted by a small insect whose activity is strongly correlated with climatic conditions. Two distinct species, both belonging to the Culicoides genus, are known to transmit the disease: Culicoides imicola, which is more common in southern Europe (mainly Spain, Portugal, Greece, southern Italy and France), and Culicoides obsoletus, which is also found further north (the whole of France and Italy, the UK, Ireland, Belgium, the Netherlands, western Germany and even Denmark)2.
The vector's activity is linked to climatic conditions. It has been shown that, on average, Culicoides imicola activity in Spain begins around week 8 (end of February) and continues until week 52 (end of December). In the case of Culicoides obsoletus, activity began around week 8 in Spain and southern France, and around week 15 (mid-April) in northern Europe. Vector activity for both species ceases between the end of November and the end of December2. However, these data should be treated with caution, as some winters are less cold than others, so the period of vector inactivity may be shorter or even absent in some years in certain areas.
Logically, therefore, the increase in the number of reported cases of both diseases falls in winter, as shown on tables 1 and 2.
*Spain: no data published since 11.12.23
Table 1. Number of cases of EHD outbreak per country and average number of new outbreaks per week
Table 2. Number of cases of BTV-3 outbreak per country and average number of new outbreaks per week
Spring: vectors and diseases are back?
Table 1 shows that the increase in the number of cases of EHD seems to have resumed in France and Portugal at the end of February. Unfortunately, the Spanish authorities have not published any data since the end of 2023, but we can assume that the number of outbreaks is also increasing in this country. In the case of BTV-3 (Table 2), the disease appears to be spreading only slightly to date (16 April 2024), but vector activity has probably not yet started in this part of Europe. Monitoring the number of cases in the coming months will provide a better understanding of the evolution of these two diseases and their possible spread to other areas.
To date, the total number of outbreaks reported since the start of the epizootics is 4490 for EHD, mainly in the south-west of France near the Pyrenees, and 4507 for BTV3, mainly in the Netherlands.
References
1 Bulletins hebdomadaires de veille sanitaire internationale | PLATEFORME ESA (plateforme-esa.fr)
2 Versteirt, V., Balenghien, T., Tack, W., & Wint, W. (2017). A first estimation of Culicoides imicola and Culicoides obsoletus/Culicoides scoticus seasonality and abundance in Europe. EFSA Supporting Publications, 14(2), 1182E.
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