Immunohistochemical demonstration of African horse sickness viral antigen in formalin-fixed equine tissues.

This study examines how the African horse sickness virus (AHSV) spreads within the body of infected adolescent ponies, focusing on key target cells and its resting distribution in various tissues.

Objective of the Research

• The main objective of this study was to investigate the spread and presence of African horse sickness virus (AHSV) in various tissues of three ponies, aged between 3-4 years. The ponies were infected experimentally with a particularly virulent strain of the virus, AHSV serotype 4.
• Researchers were specifically interested in the distribution of the viral antigen (the part of the virus that triggers the immune response) inside the body of the ponies and its main target cells during the terminal stage of severe or ‘peracute’ form of the disease.

Research Method and Procedure

• Researchers collected tissues from the infected ponies during the terminal stage of the disease. They also used tissues from a healthy, non-infected horse for control purposes.
• An immunohistochemical method, known as the avidin-biotin-peroxidase-complex (ABC) method, was used on the formalin-fixed, paraffin-embedded tissue sections. This method helps to detect and visualize the presence of the viral antigen in the tissues.
• A polyclonal antibody with specificity for both AHSV and nonstructural protein NS2 was used for antigen detection. Polyclonal antibodies are a mix of antibodies that recognize different parts of the same antigen.

Key Findings

• The AHSV antigen was primarily located in the endothelial cells of capillaries, arterioles, and small venous vessels, especially in the cardiopulmonary tissues (heart and lungs). Endothelial cells form the inner lining of these blood vessels.
• The antigen was also found in cells resembling macrophages (a type of white blood cell that engulfs and digests foreign substances) and in reticular cells (cells that provide structure and support to the lymphatic system) of the spleen and lymph nodes. In certain lymph nodes, the presence of the viral antigen coincided with the morphology of the cellular processes of follicular dendritic cells, suggesting an involvement of these cells in the infection.
• In some tissues, the viral antigen was also detected in circulating cells, likely monocytes (a type of white blood cell), within the larger vessels.
• The totality of findings suggests that, during the late stage of the peracute form of the disease, endothelial cells and to a lesser extent mononuclear cells (cells with a single round nucleus, such as lymphocytes and monocytes) are the primary target cells of AHSV infection.