Megalocytivirus

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Megalocytivirus is a genus of large double‑stranded DNA viruses in the Iridoviridae family that infect teleost (bony) fishes, including many wild and farmed species. These viruses can cause widespread, systemic infections and are economically important because outbreaks can lead to fish losses in aquaculture. Along with Lymphocystivirus and Ranavirus, Megalocytivirus is one of the main groups that infect fish.

What they are like and how they replicate
- Megalocytiviruses are big icosahedral viruses, about 150–250 nanometers in diameter, with a large linear dsDNA genome.
- They enter fish cells by receptor‑mediated endocytosis. After entry, the DNA is copied first in the host cell nucleus and then a second round of replication happens in the cytoplasm. New viral DNA is packaged at cytoplasmic assembly sites.
- The viral DNA is circularly permuted and has terminal redundancies, a somewhat unusual feature of these viruses.

Genetic groups and where they are found
- Megalocytiviruses fall into three main genetic groups based on the major capsid protein gene: ISKNV, RSIV, and TRBIV. RSIV and ISKNV are the best known.
- Genotype I is widely distributed across several Asian countries (found in many isolates from Korea, Japan, China, Thailand, and the South China Sea). Genotype II mainly infects freshwater fishes in Southeast Asia and Australia. Genotype III mainly infects flatfish in China and Korea.

How they spread
- Transmission occurs when a naïve fish ingests tissues from an infected fish or comes into contact with contaminated water.
- The ornamental or aquarium fish trade is a notable way these viruses can spread internationally. Large numbers of freshwater and saltwater fish are traded each year, which poses a risk for moving the virus.
- There is evidence of megalocytiviruses in ornamental fish and outbreaks linked to imported species, such as gouramis being connected to farmed Murray cod outbreaks in Australia.

Symptoms and pathology
- Infected fish often show non‑specific signs: reduced appetite, abnormal or uncoordinated swimming, lethargy, abdominal swelling, darkening of the skin, tiny hemorrhages, fin erosion, and death.
- A hallmark of infection is the presence of very large, abnormal basophilic (blue‑staining) cells in organs such as the kidney, spleen, and gastrointestinal tract. These hypertrophied cells can be perivascular and contain large granular or foamy inclusions. If these cells block blood vessels, tissue damage can occur in various organs.
- The disease is still being studied, and details of how the virus causes illness are not fully understood.