Varicella zoster virus

Varicella zoster virus is one of the herpesviruses and it causes two diseases - varicella or chickenpox, and herpes zoster also known as shingles.

Zoster actually refers to a type of belt used by ancient Greek warriors because of the belt like appearance of shingles.

Now, let’s first talk a bit about the nervous system - it consists of two parts.

The central nervous system which includes the brain and the spinal cord, and the peripheral nervous system includes the nerves that fan out from the central nervous system to reach the skin, muscles, and organs.

Peripheral nerves that originate from the brain are called cranial nerves, and they’re in charge of motor and sensory innervation of the head and neck.

A specific cranial nerve, cranial nerve V, is the trigeminal nerve and it’s responsible for the sensation in the face.

Its sensory neurons create a nerve cell cluster called trigeminal ganglion, located in the bones on the side of the face between the eyes and ears.

The peripheral nerves originating from the spinal cord are called the spinal nerves.

Each nerve is formed by a dorsal and a ventral root.

Ventral roots contains neurons that carry motor innervation from the spinal cord to the muscles.

Sensory information, like touch, temperature, pain, and pressure from the skin and other tissues travel through 1st order sensory neurons, in the dorsal root ganglion near the spinal cord, then through the dorsal root, and into the spinal cord, where it synapses with the 2nd order neurons.

Now, each spinal nerve is in charge of the sensation of a specific area of the skin, called a dermatome.

For example, if you step on a lego, the pain would be carried by the S1 nerve, but if you hit your big toe on a table leg, the pain would be carried by the L5 nerve.

Varicella zoster virus is a double-stranded DNA virus, protected by a protein coat called capsid, which is enveloped in a lipid membrane.

The virus initially enters respiratory epithelial cells - but eventually spreads to numerous cell types.

It gets into all of these cells by fusing its membrane with the cell membrane and releasing the capsid inside the cell.

The capsid binds to the nucleus and injects it with viral DNA, where it’s copied.

The viral genes are transcribed into RNA and go over to the ribosomes, where they are translated into capsid proteins. The capsid and viral DNA fuse together.

They go through the Golgi complex, a cell organelle in charge of packing proteins, to get their lipid membrane and finally the newly formed viruses leave the cell. Leaving behind a dead cell.

When an infected person sneezes or coughs, the viruses leaves the lungs and get released into the air.

The virus can also be transmitted through contact with the oral or skin lesions of the infected person.

When the virus comes in contact with the respiratory mucosa or the skin of a new person, it starts replicating in the epithelial cells.

It soon gets picked up by nearby immune cells and gets transported to a nearby lymph node.

The virus now causes primary infection called varicella or chickenpox, and it has two stages - primary viremia and secondary viremia.

In primary viremia the virus infects a part of the immune system in the liver and spleen called the reticuloendothelial system, which is made of phagocytic cells.

About two weeks after entering the body, the virus starts infecting immune cells themselves - specifically T cells - and that is considered the secondary viremia.

Infected T cells start expressing proteins that bind to receptors on the skin cells. It’s a bit like the virus is hitching a ride in the T cell to get to the skin.

Once the T cells reach the skin, they release the viruses, which start infecting keratinocytes.

The infection spreads through the skin, going directly from cell to cell.

Sometimes the infected keratinocytes start to fuse together and create giant multinucleated cells called Tzanck cells.