The unassuming virus that can live inside your feet. It often shows no symptoms but left untreated can cause debilitating pain and dangerous infections

A strain of a common sexually transmitted disease may be lurking in millions of Americans’ feet without them ever knowing it, but letting it go untreated can lead to pain and major complications.

There are more than 200 strains of human papillomaviruses (HPV), from high-risk versions spread via sex that can cause cancer to low-risk strains contracted through dirty surfaces that cause persistent warts. 

When HPV infects the genitals, the sexually transmitted virus often shows no symptoms but can cause cancer later in life. The risk can be greatly reduced with a vaccine. 

While HPV is most known as an STD, when it shows up on your feet, it is called a verruca, or plantar wart.

This strain is more benign but can still lurk with no symptoms – and like the STD, left untreated, verrucae can wreck havoc.

They can embed deep into the skin, causing pain, difficulty walking, rapid spreading across the body and secondary infections. 

I see dozens of patients a week dealing with the unfortunate effects of warts, including one who was so worried about the possibility of the virus spreading across his foot that he followed an intensive treatment regimen for a month. 

And an avid biker who came to see me was so concerned about the pain and size of the verruca on his heel that he instantly opted for microwave therapy to attempt to blast the virus.

What is a verruca?

Although benign, HPV verruca is highly contagious and relies on water to survive. 

The bottom of our feet is the perfect location for this unassuming virus to penetrate small cuts, splits or wet damaged skin. 

Once the virus enters and infects the top layer of skin it creates a reaction that triggers the overproduction of hard skin. 

Pressure on this area of skin from walking causes the lesion to imbed deeper into the skin, which creates its ‘cauliflower’ appearance. 

The infection caused by HPV in the skin of the foot causes small blood vessels called capillaries to clot, creating the black dots associated with the appearance of warts. 

The virus uses this environment to survive and infect other parts of the body by entering and exiting the skin in a process called autoinoculation – the spread of an infection from one part of your body to another. 

According to recent studies, around ten percent of the US population develops warts on their feet, 20 percent of these are school-aged children. 

Recent studies have suggested that HPV infection of the foot is more common in women than men, and immunosuppressed individuals at higher risk. 

How does HPV infect the foot?

Communal areas such as locker rooms, swimming pools and gyms are some of the most frequently associated places with HPV presence. 

Small cuts and cracks in the skin are prime targets for the virus to enter undetected. 

Scratching is a habit that not only creates targets for HPV infection, but also contributes to the spread of the virus to other parts of the body. As skin sheds, so does the virus, spreading across surfaces with ease.

There are approximately 250,000 sweat glands in our feet, so a dark, warm, moist environment allows HPV to survive outside the skin and re-enter when the skin is soft and more susceptible to infection.

Physical pressure can increase the likelihood of verrucae, as walking barefoot exposes the bottom of the foot to damp surfaces that suit HPV and can cause existing verrucae to grow inward, potentially causing more pain for those affected.

Contaminated objects such as shoes, socks and towels can be a higher risk for infection. 

The key is to avoid use of garments or towels belonging to other people. Using your own shoes, socks and towels is crucial to minimizing the risk of HPV infection in the foot.

How would a verruca affect me?

Many affected individuals do not even realize they are infected until the lesion appears. It can be months before the verruca is visible, and once the lesion appears it may take months or sometimes years to eventually disappear.

Pain and swelling during standing or walking can be felt by affected individuals. 

HPV viral infection not only affects the standing and walking abilities of individuals, but can have a detrimental effect on mental health, especially when it comes to aesthetic self-consciousness in social group environments.  

Major complications from verrucae could be rapidly spreading warts to other parts of the body and secondary bacterial infections caused by picking or scratching the wart.

Worse case scenarios often include severe pain, misdiagnosis of squamous cell carcinoma (cancer) or ulceration in people with diabetes and neuropathy.

If untreated, warts can spread, increase in size and become extremely painful, affecting day-to-day activities like walking. 

How is HPV of the foot treated?

Due to its autoimmune effects, the HPV virus will only become inactive once the immune system detects it and responds by combating infected cells. 

Dermoscopy is an important means of assessment that helps podiatrists to confirm whether the lesion is a verruca or not. 

It is important to avoid self-diagnosis and self-care until the suspected verruca is assessed by a podiatrist. 

Once diagnosed, there are many treatment options for treating HPV infection of the foot. 

Zinc oxide tape is a hypoallergenic conservative treatment that covers the lesion to prevent the autoinoculation process. 

Topical agents including salicylic and lactic acid can be explored as options, however, existing evidence suggests a success rate of approximately just 30 percent. 

More potent minimally invasive treatments such as cryotherapy and microwave ablation have shown higher rates of success, particularly in children and adolescents.

Jonathan Brocklehurst, MSc, MIRL, MRCPod, is a podiatrist based in the UK.