Human papillomavirus, or HPV, is an incredibly common sexually transmitted infection that
affects humans of all genders worldwide. In the United States alone, CDC estimates that upwards
of 79 million Americans currently have HPV, and that around 14 million people will become newly
infected each year. Truly, HPV is so common that essentially every person who is sexually active
will likely get HPV at some point in their life, provided they don’t get the HPV vaccine.
It turns out papillomaviruses aren’t unique to humans, they’re actually exceptionally common
throughout the animal kingdom. But each virus can only infect a single species.
Depending on the host, papillomaviruses are capable of causing lytic, chronic,
latent, and transforming infections. Human papillomaviruses in particular can cause warts,
and several genotypes of these viruses are associated with certain types of cancer.
Papillomaviruses are part of the Papillomaviridae family, and they are small, non-enveloped viruses
with icosahedral capsids and double-stranded DNA genomes. The tricky thing about papillomaviruses
is that they encode proteins that promote cell growth. From a virus’ standpoint, that’s great.
The ability to promote cell growth helps the virus carry out a lytic replication cycle.
From a human or animal’s perspective, it’s not the best news for a few reasons. First,
as we recall, lytic replication essentially means the virus infiltrates a host cell,
uses all the host cell machinery to replicate many times, and then explodes its way out of the
host cell, destroying it and delivering new virions to the surrounding cells.
Second, these viruses have the potential to oncogenically transform a nonpermissive cell,
which basically means triggering uncontrolled cell growth. This is known by another name, cancer.
Over 100 types of human papillomaviruses have been identified, which are classified
based on their DNA sequence homology. HPV can be further subcategorized depending
on the type of tissue they infect, as either cutaneous HPV, which means infecting the skin,
or mucosal HPV, which means infecting mucosal membranes. Within the mucosal HPV category,
there is a group of viruses associated with cervical cancer. But more on that in a moment.
Human papillomavirus is transmitted sexually through skin-to-skin contact.
Let’s break down what happens next. The virus is able to enter the basal layer of the epidermis
through tiny breaks in the skin. Once inside, the virus triggers cell growth,
which allows the virus to make even more copies of itself, since more host cells means more virions.
All those extra cells are what cause warts, a potential symptom of HPV. Of course, not everyone
infected with HPV develops warts, and in fact, most people who get HPV have no symptoms at all.
So, why does it vary so much? HPV expresses different viral
genes depending on the type and layer of skin that the virus has broken into,
with the added variable that the different types of HPV are very tissue-specific. In particular,
two strains of the virus, HPV-16 and HPV-18, are responsible for about 70% of cervical
cancer cases. The other 30% of cancer cases are caused by a dozen or so “high risk” HPV strains.
Yet another thing that helps this virus spread so easily is the fact that a person infected with HPV
can still pass the virus to someone even if they have no visible symptoms. In most cases,
HPV goes away on its own without causing any health problems. But for those cases where HPV
sticks around, it can cause either genital warts, which can be treated with prescription medication,
or different kinds of cancer. In addition to cervical cancer and other genital-specific
cancers, HPV can also cause cancer in the back of the throat. Cancers caused by HPV can take years
or even decades to develop, so it’s always a good idea to get regular check-ups.
For women between the ages of 21 and 65, getting regular screenings can catch the very early stages
of cancer and prevent it from developing. Ultimately, there are a number of ways
that you can lower your chances of getting HPV. Of course, proper condom use can help. There’s also
an HPV vaccine that’s safe and effective against cancer-causing strains of HPV. Either way, making
sure to visit the doctor for regular check-ups can catch issues before they become more serious, just
the same as pretty much all health conditions. And that covers the basics regarding HPV.