Culicoides species involved in the BTV-3 epidemic, the Netherlands, 2023-2024

Bluetongue virus (BTV) is an arthropod-borne virus that is transmitted between ruminants by Culicoides (Order Diptera, Family Ceratopogonidae) midges. In September 2023, BTV serotype 3 (BTV-3/NET2023) emerged in the Netherlands, causing a devastating epidemic in sheep and cattle. The aim of this study was to determine which midge species contributed to the spread of BTV-3 and to what extent the virus is present in local midge populations. Midges were collected using Onderstepoort UV-light suction traps on BTV-affected farms in the centre of the Netherlands, from October 2023 till March 2024. Species, sex and parity of the midges were morphologically determined. Pooled female parous and gravid midges were subjected to pan-BTV and BTV-3 real-time polymerase chain reaction (PCR) testing. Pool prevalence, minimum infection rate (MIR) and infection rate (IR) were calculated. In total, 33,093 midges were morphologically identified, all being indigenous Culicoides species. Of these, 10,835 parous or gravid female midges were selected and pooled in 383 pools (mean 28.3 midges per pool, range 1-115). A total of 155 pools (40.5%) tested BTV positive, with a mean MIR/100 of 1.4 and an IR of 2.2 (95% confidence interval: 1.9-2.6). All positive pools were from batches that were collected in October 2023 (week 40 and 41). BTV-RNA was detected in the Culicoides species C. obsoletus Meigen, C. scoticus Downes and Kettle, C. chiopterus Meigen, C. dewulfi Goetghebuer and C. punctatus Meigen. The high proportion of BTV-PCR positive midge pools is indicative of a high vector competence for BTV-3/NET2023 of Dutch indigenous midges present on farms; it could potentially partly explain the rapid spread of the virus throughout the Netherlands.