Cytopathic effect

Cytopathic effect or cytopathogenic effect (abbreviated CPE) refers to structural changes in host cells that are caused by viral invasion. The infecting virus causes lysis of the host cell or when the cell dies without lysis due to an inability to reproduce. Both of these effects occur due to CPEs. If a virus causes these morphological changes in the host cell, it is said to be cytopathogenic. Common examples of CPE include rounding of the infected cell, fusion with adjacent cells to form syncytia, and the appearance of nuclear or cytoplasmic inclusion bodies.

CPEs and other changes in cell morphology are only a few of the many effects by cytocidal viruses. When a cytocidal virus infects a permissive cell, the viruses kill the host cell through changes in cell morphology, in cell physiology, and the biosynthetic events that follow. These changes are necessary for efficient virus replication but at the expense of the host cell.

CPEs are important aspects of a viral infection in diagnostics. Many CPEs can be seen in unfixed, unstained cells under the low power of an optical microscope, with the condenser down and the iris diaphragm partly closed. However, with some CPEs, namely inclusion bodies, the cells must be fixed and stained then viewed under light microscopy. Some viruses’ CPEs are characteristic and therefore can be an important tool for virologists in diagnosing an infected animal or human. The rate of CPE appearance is also an important characteristic that virologists may use to identify virus type. If CPE appears after 4 to 5 days in vitro at low multiplicity of infection, then the virus is considered slow. If the CPE appears after 1 to 2 days in vitro at low multiplicity of infection, then the virus is thought to be rapid. Inoculations always occur at low multiplicity of infection because at high multiplicity of infection, all CPEs occur rapidly.

Typically, the first sign of viral infections is the rounding of cells. Inclusion bodies often then appear in the cell nucleus and/or cytoplasm of the host cell. The inclusion bodies can first be identified by light microscopy in patient blood smears or stained sections of infected tissues. However, to fully characterize their composition, electron microscopy must be performed. Inclusion bodies may either be accumulation of virus replication byproducts or altered host cell organelles or structures.

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