Virus life cycle

The life cycle of viruses, such as those used in neuronal tracing, is different from cellular organisms. Viruses are parasitic in nature and cannot proliferate on their own. Therefore, they must infect another organism and effectively hijack cellular machinery to complete their life cycle. The first stage of the viral life cycle is called viral entry. This defines the manner in which a virus infects a new host cell. In nature, neurotropic viruses are usually transmitted through bites or scratches, as in the case of Rabies virus or certain strains of Herpes viruses. In tracing studies, this step occurs artificially, typically through the use of a syringe. The next stage of the viral life cycle is called viral replication. During this stage, the virus takes over the host cell's machinery to cause the cell to create more viral proteins and assemble more viruses. Once the cell has produced a sufficient number of viruses, the virus enters the viral shedding stage. During this stage, viruses leave the original host cell in search of a new host. In the case of neurotropic viruses, this transmission typically occurs at the synapse. Viruses can jump across the relatively short space from one neuron to the next. This trait is what makes viruses so useful in tracer studies. Once the virus enters the next cell, the cycle begins anew. The original host cell begins to degrade after the shedding stage. In tracer studies, this is the reason the timing must be tightly controlled. If the virus is allowed to spread too far, the original microcircuitry of interest is degraded and no useful information can be retrieved. Typically, viruses can infect only a small number of organisms, and even then only a specific cell type within the body. The specificity of a particular virus for a specific tissue is known as its tropism. Viruses in tracer studies are all neurotropic (capable of infecting neurons).