Border disease without nervous signs or fleece changes

Area of Science:

• Veterinary Virology
• Sheep Pathology

Background:

• Border disease virus (BDV) infection in sheep flocks can cause significant economic losses.
• Typical clinical signs include tremors and fleece abnormalities, but atypical presentations can occur.

Purpose of the Study:

• To investigate an unusual outbreak of border disease in a sheep flock.
• To identify the source and transmission dynamics of the BDV infection.
• To evaluate the effectiveness of natural exposure for inducing immunity.

Main Methods:

• Epidemiological investigation of a sheep flock experiencing abortions and lamb mortality.
• Detection of BVDV viraemia in lambs and purchased ewes.
• Attempted natural vaccination by co-mingling susceptible sheep with infected animals.

Main Results:

• The outbreak was linked to four persistently infected ewes introduced to the flock.
• Clinical signs were limited to abortion and birth of small, weak lambs, lacking typical neurological or fleece signs.
• Natural vaccination attempt was largely unsuccessful, with low seroconversion rates in sentinel sheep.
• Removal of infected animals led to disease cessation and improved lambing percentages in the subsequent year.
• Persistently infected ewes produced healthy but persistently infected lambs.

Conclusions:

• Persistent BDV infection in purchased sheep can introduce the virus without obvious clinical signs in adults.
• Atypical presentations of BDV, focusing on reproductive losses, can occur.
• Natural vaccination strategies by co-mingling are unreliable for controlling BDV.
• Strict biosecurity and removal of persistently infected animals are crucial for disease control.