Fish Diseases and Management

5.3.5 Viral pathogenesis

The complex series of events that involve virus replication and the host defence responses finally resulting in the induction of disease in the host are collectively known as viral pathogenesis. However, the replication of the virus at the primary site of multiplication need not always result in clinical disease.

The viral pathogenesis starts with the entry of virus in the host followed by primary replication and viral spread in the host. Viruses enter the host either through vertical transmission or by horizontal transmission. Viruses replicate at the primary site of entry and in fish it could be gills, gut or integument. Viruses spread within the host through blood or lymphatic circulation and based on the tissue tropism of the virus, further multiplication of the virus in the target tissue would result in  cellular injury. Host defence response would soon be initiated and depending on the effectiveness of the immune responses, the virus replication could be prevented and  cell injury could be minimised. An viral infection in the fish could lead to any of the following outcomes:

1. No clinical disease and elimination of the virus

2. No clinical disease but establishment of persistent infection (Carrier state)

3. Development of clinical disease and death of the fish

4. Development of clinical disease but recovery of the host and elimination of the virus

5. Development of clinical disease, recovery of the host but persistence of infection