Taxonomy and Virion Structure
The family Bunyaviridae comprises more than 250 serotypes. Four genera are pathogenic to humans: Orthobunyavirus, Phlebovirus, Nairovirus, and Hantavirus. The prototype of the family is the Bunyamwera virus, isolated from mosquitoes in Uganda.
Morphologically, they are complex spherical or oval viruses. Under electron microscopy, they often resemble a "doughnut".
Genome and Core Structure: The genome consists of negative-sense single-stranded RNA divided into three segments by size:
- L (long) segment;
- M (medium) segment;
- S (short) segment.
Each RNA fragment is encapsidated by the nucleocapsid N protein, forming three independent helical ribonucleoproteins. The core also contains an RNA-dependent RNA polymerase (transcriptase, encoded by the L segment).
Envelope and Surface Antigens: The core is surrounded by a lipoprotein envelope (supercapsid). Its surface features spikes composed of Gn and Gc glycoproteins (encoded by the M segment).
- Glycoproteins determine organotropism (tissue affinity), hemagglutinating properties, and transmission efficiency via vectors. These are type-specific antigens detected by neutralization tests and hemagglutination inhibition assays.
- The internal N protein carries group-specific properties and is detected by complement fixation tests (CFT).
Replication Cycle
The life cycle of bunyaviruses takes place exclusively in the host cell cytoplasm. Viral structural synthesis is rapid: the nucleocapsid N protein can be detected inside the cell within 2 hours post-infection.
A key feature of the family is the assembly mechanism of new virions. Viral particles bud not through the outer plasma membrane, but through the smooth membranes of the Golgi apparatus. The assembled virions are then packaged into vesicles, transported to the plasma membrane, and exit the cell predominantly via exocytosis. Cell lysis occurs rarely.
Epidemiology and Ecological Groups
Bunyaviridae have a global distribution and cause zoonotic infections with natural reservoirs. Based on their transmission cycles, they are divided into two major ecological groups:
- Arboviruses (Arthropod-borne).
Encompasses most bunyavirus genera. Arthropods serve as both vectors and primary natural reservoirs. The virus is adapted to two temperature ranges: 36–40 °C (warm-blooded hosts) and 22–25 °C (arthropods). Persistence in vector populations is maintained via transovarial (from parent to offspring via eggs) and transstadial (from larva to adult) transmission.
- Orthobunyavirus (California encephalitis virus) — transmitted by mosquitoes.
- Phlebovirus (sandfly fevers, Rift Valley fever) — sandflies and mosquitoes.
- Nairovirus (Crimean-Congo hemorrhagic fever virus) — primarily hard ticks (Ixodidae).
- Rodent-borne viruses.
Includes only the genus Hantavirus (agents of hemorrhagic fever with renal syndrome — Hantaan, Puumala viruses, etc.). These lack an arthropod vector. Hantavirus evolution is tightly linked to specific rodent species (co-evolution). Unlike other genera, hantaviral virions contain disordered granular-filamentous internal structures.
Routes of human infection include vector transmission (bites) or contact/inhalation routes (exposure of broken skin or mucous membranes to excreta from infected animals or humans).
Stability, Cultivation, and Laboratory Diagnostics
Due to their lipid envelope, bunyaviruses are sensitive to lipid solvents (ether, detergents) and standard disinfectants. They are stable only within a narrow pH range (6.0–9.0). The viruses are thermolabile: inactivated at 56 °C for 30 minutes, and instantly by boiling. However, they remain infectious for long periods when deep-frozen.
Because many are classified as high-consequence pathogens, work with these agents is restricted to specialized high-containment biosafety laboratories.
Cultivation Methods:
- In vivo: newborn white mice are the standard universal model. Intracerebral inoculation leads to fatal encephalitis.
- In vitro: cell cultures (human embryonic kidney, BHK-21, chick embryo fibroblasts, Vero E-6 cells for hantaviruses). Note: the cytopathic effect (CPE) is typically mild, so the immunofluorescence assay (IFA) must be used for viral identification in cell culture.