Viral Hemorrhagic Septicemia: causes, symptoms and treatment

Viral Hemorrhagic Septicemia (VHS virus (Novirhabdovirus, Rhabdoviridae)) — etiology, symptoms, diagnosis, active-substance medication, recovery and prevention; mortality without treatment: very high.

Overview

Rhabdovirus affecting many salmonid and freshwater/marine species. OIE-notifiable; major aquaculture and Great Lakes wild-fish concern. Mortality up to 100% in young fish. Causative agent: VHS virus (Novirhabdovirus, Rhabdoviridae). Transmission: water. Incubation: 5-30 days. Reported mortality without treatment: very high.

Symptoms

hemorrhages in skin gills eyes
darkening
exophthalmia
swollen abdomen
erratic spiral swimming (neurologic)
mass mortality

Causes

Outbreaks typically follow temperature stress, handling, or the introduction of carriers from non-certified sources. The virus spreads through water, fomites (nets, hands, equipment), and possibly through wild-fish vectors in pond systems. Survivors can become lifelong carriers, which is the basis for OIE-listed trade restrictions.

Diagnosis

Field diagnosis relies on clinical signs (haemorrhages, exophthalmia, mass mortality at characteristic temperatures) and epidemiology. Confirmation of VHS virus (Novirhabdovirus, Rhabdoviridae) requires PCR or virus isolation in a reference laboratory; this is mandatory for OIE-notifiable diseases.

Treatment

There is no specific antiviral therapy for fish viruses, and OIE-notifiable outbreaks are managed under regulatory rules: notify authorities, isolate the facility, support sick fish with stress reduction, and prepare for stamping out and disinfection where required.

Step 1: Quarantine

In a notifiable viral outbreak the entire facility is the quarantine zone — do not move fish, water, or equipment off-site. Stop all sales and transfers, restrict access, sanitise tools between tanks, and follow national veterinary instructions on sampling and reporting.

Step 2: Medication

No cure — depopulate. No effective antiviral. Notify authorities, depopulate facility, full disinfection with chlorine 200 ppm, restart with certified VHS-free stock. (duration: n/a)

Step 3: Recovery

Recovery is a regulated process. After stamping out (where mandated) the system is disinfected with chlorine 200 ppm or peroxygen disinfectant, dried, and re-stocked only with certified virus-free stock. Surveillance sampling continues for the period required by the national authority before the facility regains free status.

Prevention

import certified VHS-free stock
biosecurity per facility
quarantine and surveillance in cool water
destruction of carriers per regulations