ABSTRACT: To better understand the role of vector transmission of aquatic viruses, we established an in vivo virus−parasite challenge specifically to address (1) whether Lepeophtheirus salmonis can acquire infectious haematopoietic necrosis virus (IHNV) after water bath exposure or via parasitizing infected Atlantic salmon Salmo salar and if so, define the duration of this association and (2) whether L. salmonis can transmit IHNV to naïve Atlantic salmon and whether this transmission requires attachment to the host. Salmon lice which were water bath-exposed to 1 × 10⁵ plaque-forming units (pfu) ml⁻¹ of IHNV for 1 h acquired the virus (2.1 × 10⁴ pfu g⁻¹) and remained IHNV-positive for 24 h post exposure. After parasitizing IHNV-infected hosts (viral titer in fish mucus 3.3 × 10⁴ pfu ml⁻¹) salmon lice acquired IHNV (3.4 × 10³ pfu g⁻¹) and remained virus-positive for 12 h. IHNV-positive salmon lice generated through water bath exposure or after parasitizing infected Atlantic salmon successfully transmitted IHNV, resulting in 76.5 and 86.6% of the exposed Atlantic salmon testing positive for IHNV, respectively. In a second experiment, only salmon lice that became IHNV-positive through water bath exposure transmitted IHNV to 20% of the naïve fish, and no virus was transmitted when IHNV-infected salmon lice were cohabitated but restrained from attaching to naïve fish. Under laboratory conditions, adult L. salmonis can acquire IHNV and transmit it to naïve Atlantic salmon through parasitism. However, the ephemeral association of IHNV with L. salmonis indicates that the salmon louse act as a mechanical rather than a biological vector or reservoir.