Rabies

Rabies transmits through saliva of infected mammals via bites, skin scratches, or contact with mucous membranes like eyes and mouth. Transmission occurs when infectious saliva contacts abraded skin or intact mucous membranes. Common animal sources include dogs, bats, raccoons, and foxes. The virus enters through these routes and initiates infection at the bite or scratch site.

The nicotinic acetylcholine receptor (nAChR) at neuromuscular junctions serves as a critical binding site for viral entry into peripheral motor neurons. After the virus replicates in muscle cells, it buds and binds to these neuronal receptors, allowing it to enter motor neurons and begin its journey toward the central nervous system.

The rabies virus encodes a phosphoprotein that blocks type I interferon signaling, weakening early antiviral defenses. Additionally, the virus replicates slowly in muscle tissue over several days, maintaining low viral load and limiting antigen presentation. This sequestration in muscle tissue further reduces immune detection during the critical early phase of infection.

Once the virus reaches the central nervous system via retrograde axonal transport, it undergoes rapid replication and disrupts neuronal function. Infected neurons activate pronounced inflammatory responses that intensify with increasing viral load and tissue damage. This neuronal injury and inflammation lead to severe dysfunction and progressive neurological symptoms characteristic of viral meningitis and encephalitis.

Following replication in the central nervous system, rabies spreads centrifugally along peripheral nerves to highly innervated tissues, especially salivary glands. High viral titers in salivary glands facilitate transmission to new hosts through saliva. The virus also reaches skin, cornea, and visceral organs during later infection stages, enabling continued transmission.

The rabies incubation period typically ranges from one to three months but varies based on proximity of the bite to the central nervous system. Bites closer to the brain result in shorter incubation periods because the virus requires less time to travel via retrograde axonal transport. Distance traveled and viral load at the infection site are primary determinants of symptom onset timing.

Post-exposure prophylaxis works because rabies has a slow progression from the bite site to the central nervous system, allowing time for intervention. Immediate wound cleansing, rabies immunoglobulin administration, and vaccination series can prevent viral establishment before CNS involvement occurs. Once symptoms develop, rabies is invariably fatal, making early prophylaxis the only effective treatment option.