Etiology

• EHD is a vector borne viral disease of wild and domestic ruminants of the family Reoviridae, genus Orbivirus. It is closely related to other members of this family, for example Bluetongue virus (BTV) Bluetongue virus and is known to cause high morbidity and mortality in white tailed deer, the species most severely affected.
• Clinical signs are dependent upon the pathogenic serotype involved.
• In cattle mortality is normally low, with morbidity of 1-18% reported.

Predisposing factors

General

• Grazing at risk pastures, for example those associate with standing water sources in endemic disease areas.
• Most disease outbreaks occur in late summer and autumn when vector populations are at their highest.

Specific

• Disease outbreaks in cattle are often associated with an epidemic in local wild deer populations or when environmental conditions support a large population of the culicoides midge vector.

Pathophysiology

• Following infection through the feeding behavior of its vector the culicoides midge, EHDV first replicates within the endothelial cells of the lymphatics and lymph nodes. From here it disseminates via the bloodstream to other body sites where further replication occurs in the spleen and regional lymph nodes.

Timecourse

• The duration of viremia has been demonstrated for up to 50 days post-infection, but in the majority of cases, 21 days.

Epidemiology

• The period of viremia is key to disease transmission between animals.
• Infected cattle amplify the virus and serve as an infective source for the culicoides vector.