Viral Erythrocytic Necrosis (VEN)

Volume 3, Issue 1
February 2001
Updated November 2002

Viral erythrocytic necrosis (VEN) is a pathologic condition in many cold-blooded (poikilothermic) vertebrates including many anadromous fishes, marine fishes, and reptiles. Sherburne and Bean (1979) reported one incidence of VEN affecting a landlocked rainbow smelt (Osmerus mordax) population (Figure 1).


VEN can infect many species of Maine sport fishes, including: brook trout S. fontinalis, Atlantic salmon S. salar, brown trout S. salar, Atlantic herring Clupea harengus, alewife Alosa pseudoharengus, and rainbow smelt. The reservoir of infected individuals resides in the Atlantic Ocean.

The disease is recognized by identifying the pathogen’s effect in the host’s red blood cells. Typically the red blood cells contain DNA cytoplasmic inclusion bodies, a deteriorating nucleus and some nuclear inclusion bodies (Figure 2).

Blood smear from cod

The disease is believed to be caused by an Iridovirus, however, the virus has not yet been isolated and tested in experimental infections. Accordingly, information about its effects is both limited and somewhat conjectural. It is likely that effects vary with degree of infection and with host species and host’s age, health, stress level, etc. Affected fish may become anemic, have pale gills, pale visceral organs, and have prolonged blood clotting times. The percentage of red blood cells affected in a single fish can vary from 1% to 100%, but white blood cells are not affected. The virus is not believed to kill fish directly, but rather weakens them such that other diseases become fatal.

Special points of interest:

Fish with VEN cannot transmit the disease to people.

VEN is caused by a virus.

VEN infects many species of fish and reptiles.

For more information read: K. Wolf. Fish Viruses and Viral Diseases 1988, 389-398.

Images were made possible by a grant from the Maine Outdoor Heritage Fund.