What is the mechanism of viral transneuronal tracing?

In viral transneuronal tracing, a virus, such as pseudorabies virus, which is injected into the brain or a peripheral organ, is taken up by nerve endings located in the injected area. The injected virus follows the axonal flow, navigating its way to the first-order neurons that innervate the organ. Following a reproductive infection within the nerve cell, the virus traverses the synapse to reach and infect the second-order neuron. From here the infection progresses to neurons interconnected through synaptic links. The infected neuron eventually undergoes degeneration and is engulfed by phagocytic macrophages.