Emergence of Bluetongue Virus Serotype 3, the Netherlands, September 2023

Abstract

Since 1998, notifiable bluetongue virus (BTV) serotypes 1-4, 6, 8, 9, 11, and 16 have been reported in Europe. In August 2006, a bluetongue (BT) outbreak caused by BTV serotype 8 began in northwestern Europe. The Netherlands was declared BT-free in February 2012, and annual monitoring continued. On September 3, 2023, typical BT clinical manifestations in sheep were notified to the Netherlands Food and Product Safety Consumer Authority. On September 6, we confirmed BTV infection through laboratory diagnosis; notifications of clinical signs in cattle were also reported. We determined the virus was serotype 3 by whole-genome sequencing. Retrospective analysis did not reveal BTV circulation earlier than September. The virus source and introduction route into the Netherlands remains unknown. Continuous monitoring and molecular diagnostic testing of livestock will be needed to determine virus spread, and new prevention strategies will be required to prevent BTV circulation within the Netherlands and Europe.

Keywords: bluetongue; bluetongue virus; cattle; genotyping; serotyping; sheep; the Netherlands; vector-borne infections; viruses; zoonoses.

Figure 1

Timeline of the initial bluetongue outbreak caused by BTV-3 in the Netherlands in September 2023. BTV, bluetongue virus; BTV-3, BTV serotype 3; EU, European Union; GD, Gezondheidsdienst voor Dieren; NVWA, Netherlands Food and Consumer Product Safety Authority; WBVR, Wageningen Bioveterinary Research; WGS, whole-genome sequencing.

Figure 2

Phylogenetic analysis of BTV-3 variant found in livestock in the Netherlands, September 2023. Trees were obtained by using the maximum-likelihood method. A) Initial phylogenetic comparison of genome segment 2 sequence of the emerging BTV-3/NET2023 variant from the Netherlands with segment 2 sequences from notifiable BTV reference strain serotypes 1–24. B) Available and closely related genome segment 2 sequences from different BTV-3 strains selected for detailed phylogenetic analysis. Unrooted tree branches have bootstrap values indicated at the nodes. GenBank accession numbers are included in sequence names. Scale bars indicate nucleotide substitutions per site. BTV, bluetongue virus; BTV-3, BTV serotype 3.

Figure 3

Clinical manifestations of bluetongue caused by bluetongue virus serotype 3 (BTV-3) variant infections in sheep and cattle in study of emergence of BTV-3 in the Netherlands, September 2023. A–C) Hypersalivation (A), erosion of the oral mucous membranes (B), and bleeding of the lips (C) were observed in sheep infected with the BTV-3 variant. D–F) Ulceration of the oral mucous membrane (D), crust formation at the nostrils (E), and necrosis of the teats (F) were detected in cattle infected with the BTV-3 variant.

Figure 4

Density of sheep or cattle per 2-digit postal code and number of confirmed cases of bluetongue virus serotype (BTV-3)–positive flocks or herds in the Netherlands, September 2023. Gray shading indicates the density of animals. A–D) Distribution of sheep flocks infected with BTV-3. A) Initial 4 cases of BTV-3–infected sheep flocks notified on September 3, 2023. Confirmed sheep cases during calendar week 36 (B), calendar week 37 (C), and calendar week 38 (D). E–G) Distribution of cattle herds infected with BTV-3. Confirmed cases of infected cattle herds during calendar week 36 (E), calendar week 37 (F), and calendar week 38 (G).

Figure 5

Geographic distribution of bulk milk samples in the Netherlands tested for bluetongue virus (BTV) antibodies in study of emergence of BTV serotype 3 in September 2023. Milk samples were collected in August 2023. Purple shading indicates locations of dairy cattle herds that had animals considered potential positive or positive for BTV-specific antibodies by using ELISA. A) Herds having evidence of vaccination in the previous 5 years. B) Herds having no evidence of vaccination in the previous 5 years.