Bulk milk somatic cell counts and bovine viral diarrhoea virus (BVDV) infection in 7252 dairy herds in Brittany (western France) F. Beaudeau a, * , C. Fourichon a , A. Robert a , A. Joly b , H. Seegers a a Unit of Animal Health Management, Veterinary School-INRA, BP 40706, 44307 Nantes Cedex 03, France b UBGDS, BP110, 56000 Vannes, France Abstract Variations in test-day bulk milk somatic cell count (BMSCC) according to bovine viral diarrhoea virus (BVDV) infection status of the herd were quantified. Infection statuses were defined, based on bulk-milk antibodies measured twice 4 months apart. On each test-day, BMSCC was calculated as the weighted mean of the individual cow-level somatic cell counts. A total of 97,985 test-days in 7252 herds were considered. The effect of the BVDV-infection on BMSCC was assessed using mixed linear models, controlling for herd (random), proportion of primiparous cows and average days in milk on test- day. Considering test-days contemporary to the BVDV-status definition-period, the increase in BMSCC was 12,000 (P < 0.001), 27,000 (P < 0.001) and 6000 (P = 0.97) cells/ml in past-infected recently recovered, past-but-still-infected and recently infected herds respectively, compared with not recently infected herds. A carry-over effect (at least 1 year) of BVDV-infection on BMSCC was also evidenced in past-infected recently recovered, past-but-still-infected and recently infected herds. # 2005 Elsevier B.V. All rights reserved. Keywords: Dairy cows; Somatic cell count; Bovine viral diarrhoea virus 1. Introduction In order to assess ex-ante the profitability of control schemes for bovine viral diarrhoea virus (BVDV) infection, quantitative information on its production effects are needed. www.elsevier.com/locate/prevetmed Preventive Veterinary Medicine 72 (2005) 163–167 * Corresponding author. Tel.: +33 240 687 650; fax: +33 240 687 768. E-mail address: beaudeau@vet-nantes.fr (F. Beaudeau). 0167-5877/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.prevetmed.2005.08.017