The first herpes viruses we're going to talk about are HSV1 and HSV2, and they mostly cause mucocutaneous disease, meaning that they affect mucous membranes in the skin, and specifically they usually affect the oral cavity or the genital area. And it turns out that oral herpes is usually type 1, and genital herpes is usually type 2. So how does this all happen? Well first, in primary infection, the virus enters the body through a mucosal surface or through an abraded or damaged epithelium.
And it actually replicates in the epithelial cells, and that causes the lesions that you see. We'll talk later about what they look like. But it doesn't stop there, because this is a neurotropic virus, meaning it likes neurons, and it's actually going to be taken up by sensory neurons.
And it undergoes retrograde transport back to the sensory ganglia, that's the cell body, and that's where latent infection is established. so that when the epithelial infection is stopped by the immune system, HSV is still hiding out in the sensory ganglia. Then, at some point, latent virus in some neuron reactivates, and active replication starts again, and we don't really understand the reasons why. And virions are then transported back, in an anterograde fashion, to the mucosal or cutaneous sites.
And once again, the virus infects epithelial cells, and lesions form again. and this is what we call the reactivated or the recurrent infection. Now because the virus reactivates in the neuron first, people can actually start to feel tingling or pain before the recurrent skin lesions actually appear. And that's a good thing because if someone notices that tingling, they can start treatment and hopefully prevent or mitigate the outbreak. So now the big question, why is HSV-1 usually oral and HSV-2 usually genital?
And it turns out the reason is, HSV1 is good at establishing latent infection in the trigeminal ganglia, while HSV2 prefers the sacral ganglia, which innervate the genitals. But now, if you've been really paying attention, you might have an objection. The ganglia are only involved in the recurrent infection, not the primary infection.
So does this logic apply to the primary infection? And in fact, it does not. Either virus can cause primary infection at either site.
But recurrent infection is more specific. HSV-1 usually recurs in the oral cavity and HSV-2 in the genital area. So that's the main pathogenesis of mucocutaneous disease of HSV.