Overview and Clinical Importance
Viral diseases of fish represent a significant category of aquatic animal diseases on the NAVLE examination. Two of the most economically and clinically important viral infections affecting cyprinid fish species are Spring Viremia of Carp (SVC) and Koi Herpesvirus Disease (KHVD). Both diseases are notifiable to the World Organisation for Animal Health (WOAH, formerly OIE) and can cause devastating mortality in affected populations. Understanding the epidemiology, clinical presentation, diagnostic approaches, and regulatory implications of these diseases is essential for veterinary practice.
Section 1: Spring Viremia of Carp (SVC)
Etiology and Virology
Spring Viremia of Carp (SVC) is caused by Carp sprivivirus (Rhabdovirus carpio), a bullet-shaped, enveloped, single-stranded negative-sense RNA virus belonging to the family Rhabdoviridae, genus Sprivivirus (formerly Vesiculovirus). The viral genome contains five genes encoding nucleoprotein (N), phosphoprotein (P), matrix protein (M), glycoprotein (G), and polymerase (L) in the order 3'-NPMGL-5'.
SVCV Characteristics
Epidemiology
Geographic Distribution
SVC was initially diagnosed in Yugoslavia in 1971 and has since been identified throughout Europe, Russia, the Middle East, China, Brazil, and North America. In the United States, SVC was first confirmed in 2002 in farmed koi from North Carolina and wild carp in Wisconsin. Additional US cases occurred in Washington and Missouri (2004) and Minnesota (2007, 2011). SVCV is NOT considered widespread in the US and is classified as a foreign animal disease for regulatory purposes.
Susceptible Species
Transmission
SVCV enters fish primarily through the gills, replicating in gill epithelium before spreading to internal organs. Transmission routes include:
- Horizontal transmission: Direct contact with infected fish; contaminated water; feces and urine of infected fish
- Mechanical vectors: Blood-sucking parasites including fish louse (Argulus foliaceus) and leeches (Piscicola geometra)
- Fomites: Contaminated equipment, nets, water, and mud
- Frozen fish: Virus survives freezing at -20 degrees C for 1 month
- Possible vertical transmission: Virus detected in ovarian fluids (not confirmed)
Clinical Signs
Clinical signs of SVC are often non-specific and reflect a systemic hemorrhagic disease affecting multiple organ systems.
External Clinical Signs
- Darkening of the skin
- Exophthalmia (bilateral pop-eye)
- Abdominal distension (dropsy/ascites)
- Pale gills
- Petechial hemorrhages of skin, gills, and eyes
- Protruding vent with thick mucoid (white-yellowish) fecal cast - characteristic sign
Internal/Necropsy Findings
- Hemorrhagic ascites (fluid in coelomic cavity)
- Pinpoint hemorrhages on swim bladder wall - highly indicative if present
- Severe intestinal inflammation with mucus
- Splenomegaly (enlarged spleen)
- Tissue destruction in kidney, spleen, and liver
Behavioral Signs
- Lethargy and decreased respiration rate
- Fish congregate near water inlet or pond banks
- Loss of equilibrium - fish lie on sides at bottom
- Sluggish response to stimuli
Diagnosis
Treatment and Management
Section 2: Koi Herpesvirus Disease (KHVD)
Etiology and Virology
Koi Herpesvirus (KHV), also known as Cyprinid herpesvirus 3 (CyHV-3) or carp interstitial nephritis and gill necrosis virus (CNGV), is a highly contagious double-stranded DNA virus belonging to the family Alloherpesviridae, genus Cyprinivirus. It is closely related to Cyprinid herpesvirus 1 (CyHV-1, carp pox virus) and Cyprinid herpesvirus 2 (CyHV-2, goldfish hematopoietic necrosis virus).
KHV/CyHV-3 Characteristics
Epidemiology
Geographic Distribution
KHV was first recognized in the United Kingdom in 1996 (confirmed in Israel in 1998). It has since been identified on all continents except Australia. In the United States, KHV is considered ENDEMIC and widespread. Unlike SVC, there is no mandatory depopulation for KHV-positive populations in the US.
Susceptible Species
- Common carp (Cyprinus carpio) - including all koi varieties (PRIMARY HOST)
- Goldfish (Carassius auratus) - may carry virus but clinically unaffected
- Grass carp (Ctenopharyngodon idella) - may carry virus but clinically unaffected
- Hybrid goldfish x common carp - moderately resistant
Transmission
The primary portal of entry for KHV is the SKIN (including gills). Transmission routes include:
- Direct contact with infected fish
- Contact with fluids from infected fish
- Contaminated water (virus survives up to 7 days)
- Fomites: nets, buckets, equipment
- Carrier fish (survivors of previous outbreaks)
Clinical Signs
Clinical signs of KHVD are often non-specific. Mortality begins RAPIDLY - within 24-48 hours after onset of clinical signs. In experimental studies, 82% of exposed fish died within 15 days at 22 degrees C.
Characteristic Clinical Signs - GILLS
- GILL MOTTLING with red and white patches - MOST COMMON sign
- Gill necrosis - white patches represent NECROTIC tissue
- Swollen gills
- Hemorrhagic gills
Other External Signs
- Sunken eyes (enophthalmos)
- Pale patches on skin
- Notched nose (characteristic in some fish)
- Skin ulcerations and sloughing of scales
- Increased or decreased mucus production
- Sandpaper texture to skin
- Secondary bacterial/parasitic infections (may mask primary infection)
Behavioral Signs
- Lethargy - lying at bottom or hanging head-down
- Piping/gasping at surface
- Gathering near oxygen-rich areas (waterfalls, aerators)
- Erratic swimming
- Decreased appetite/anorexia