Clinical signs and pathology

Abstract

Bluetongue (BT) occurs principally in sheep and some species of wild ruminants, and bluetongue virus (BTV) infection of cattle, goats and most wild ruminant species is typically asymptomatic or subclinical. The clinical signs of BTV infection are also highly variable even in susceptible species such as sheep, reflecting inherent differences in the susceptibility of different sheep breeds, as well as of individual animals, and external stressors such as solar irradiation that can exacerbate the clinical signs of BT. The signs of BT in sheep are the result of virus-mediated vascular injury that produces oedema, hyperaemia and vascular congestion, haemorrhage and tissue infarction. Thus, sheep with acute BT have any combination of fever, anorexia and malaise, respiratory distress, excessive salivation, serous to bloody nasal and ocular discharge that becomes increasingly mucopurulent so that crusty exudates accumulate around the nostrils, petechial and ecchymotic haemorrhages in the mucous membranes of the oral and nasal cavities, oral erosions and ulcers. A variety of animal groups other than ruminants have been implicated in the life cycle of a BTV infection. Serological evidence indicates that large African carnivores are infected with BTV, whereas smaller predators that co-habit with them are not, suggesting that large carnivores are infected through feeding on BTV-infected ruminants.