Rabies is an infectious disease caused by a virus that is transmitted from infected animals to humans, also known as a zoonotic disease.
The most commonly reported offenders for transmitting the infection are dogs, accounting for the vast majority of rabies cases via a bite. Bats, cats, raccoons, skunks and foxes have also been known to transmit the virus to humans. In addition to this, the virus may be passed on through other means, such as the transfer of saliva from an effect animal into a human eye, or through human-to-human contact, although these are much more rare.
Virus Type and Genome
The rabies virus is a member of the Rhabdoviridae family and is considered to be of the Lyssavirus genus type. There are several species of the virus, including:
- Rabies virus
- Lagos bat virus
- Mokola virus
- Duvenhage virus
- European bat lyssavirus
- Australian bat lyssavirus
The virus is a bullet-shaped particle that is approximately 180 nm in length and 80 nm wide. It is considered to have a single stranded, negative sense ssRNA genome that is encased in an envelope structure. The genome encodes five genes, nucleoprotein, phosphoprotein, matrix protein, glycoprotein and viral RNA polymerase.
All viruses in the Rhabdoviridae family consist of two major structural components of the helical core and the surrounding envelope. Along the surface of the virus, there is a tight arrangement of trimeric spikes constructed of glycoproteins. Currently, the cell surface receptors of the virus have not been established, although the phophatidyl serine phospholipid has been suggested to be present on the surface.
Pathogenesis of Virus
The rabies virus is usually transmitted from an infected animal to another animal or human via saliva or body tissue; the most common method is from an animal bite.
Shortly after the transmission of the virus, initial viral replication occurs in the peripheral neurons and muscle tissue surrounding the injury site. The viral infection then spreads to the central nervous system by way of retrograde axonal transport and continues with extensive replication within neurons.
Trans-synaptic spread then allows the virus to transport along the central nervous system and throughout the body to salivary glands, which is where replication of the virus occurs.
Throughout this process, there is minimal inflammation and cell death evident in the central nervous system. Although there is significant dysfunction in the way the neurons work, the structure of the neurons is not usually affected.
The virus initially causes symptoms such as fever and headache but there is usually a quick progression to more characteristic symptoms of the disease, such as hallucinations, paralysis and fear of air or water. Eventually, it is usually fatal and ends patients’ lives through cardiac arrest.