21.3 Viral Infections of the Skin and Eyes

Viral Skin Infections

Viruses that infect the skin and eyes cause some of the most recognizable infections in microbiology. From warts to cold sores to childhood rashes, these pathogens share a common theme: they're highly transmissible, often establish latency, and can recur unpredictably. This section covers the major viral agents, how they cause disease, and what sets the childhood exanthems apart from each other.

Transmission and Treatment of Viral Skin Infections

Warts (Human Papillomavirus, HPV)

HPV spreads through direct contact with infected skin or indirect contact with contaminated surfaces like public showers and pool decks. The virus infects keratinocytes in the epidermis, causing rough, bumpy growths that most commonly appear on the hands, feet, and face.

Treatment depends on severity and persistence:

• First-line: over-the-counter salicylic acid or cryotherapy (freezing with liquid nitrogen)
• Prescription topicals: imiquimod (an immune response modifier) or fluorouracil (blocks DNA synthesis in infected cells)
• Refractory cases: surgical removal or laser therapy

Many warts resolve on their own as the immune system clears the virus, but this can take months to years.

Cold Sores (Herpes Simplex Virus Type 1, HSV-1)

HSV-1 transmits through direct contact with infected saliva or active skin lesions. Sharing utensils, towels, or lip balm can also spread the virus. The classic presentation follows a predictable sequence:

1. A tingling or burning sensation at the site (the prodrome)
2. Fluid-filled blisters form on or around the lips
3. Blisters rupture, leaving shallow ulcers that crust over
4. Healing occurs within 2–4 weeks

Antiviral medications like acyclovir and valacyclovir reduce severity and duration but do not eliminate the virus. Topical creams can alleviate local symptoms, and pain relievers or cold compresses help with discomfort. In immunocompromised patients, antiviral resistance can develop, requiring alternative agents like foscarnet.

Viral Mechanisms for Recurring Lesions

Both HSV-1 and HPV establish latent infections, which is why these conditions recur rather than resolve permanently.

• HSV-1 retreats to sensory nerve ganglia (typically the trigeminal ganglion for oral herpes) after the initial infection. The viral genome persists in the neuron's nucleus with minimal gene expression, keeping it hidden from immune surveillance.
• HPV persists in the basal layer of the epithelium, the deepest layer of the skin where stem-like cells divide. Because viral protein expression stays low during latency, the immune system has little to target.

Reactivation triggers differ between the two viruses:

• HSV-1 reactivates in response to stress, illness, UV exposure, or immunosuppression. The virus travels back along sensory nerves to the original site of infection, producing a new crop of cold sores.
• HPV reactivation is linked to immunosuppression and hormonal changes. As infected basal cells divide and differentiate, they carry the virus toward the skin surface, eventually forming new warts.

The concept of viral tropism explains why these viruses affect specific body sites. HSV-1 preferentially infects mucosal epithelium and neurons, while HPV targets stratified squamous epithelium. This tropism is determined by the receptors each virus uses to enter host cells.

Viral Exanthems in Children

Childhood viral rashes (exanthems) can look alarming, but most are self-limiting. The two you need to distinguish for this unit are roseola and fifth disease, which differ in their causative agents, age groups, and rash patterns.

Rashes of Roseola vs. Fifth Disease

Roseola (Human Herpesvirus 6 or 7)

Roseola primarily affects infants and toddlers between 6 months and 2 years old. The hallmark is the sequence of events:

1. High fever (often above 39.5°C / 103°F) lasting 3–5 days
2. Fever breaks suddenly
3. A pink, maculopapular rash appears on the trunk, then spreads to the neck, face, and limbs
4. The rash fades within 1–2 days

Febrile seizures can occur during the high-fever phase, which is the main clinical concern. Mild upper respiratory symptoms and enlarged lymph nodes may also be present.

Fifth Disease (Parvovirus B19)

Fifth disease typically affects school-aged children (5–15 years) and has a longer incubation period of 4–14 days. The rash pattern is distinctive:

• A bright red, confluent rash on both cheeks gives the classic "slapped cheek" appearance
• A lacy, reticular (net-like) rash then develops on the trunk and limbs
• This rash may be itchy and can recur over several weeks, often triggered by heat or sun exposure

Prodromal symptoms are mild: low-grade fever, headache, and malaise. By the time the rash appears, the patient is typically no longer contagious. Adults who contract parvovirus B19 may develop joint pain (arthralgia) or frank arthritis. The most serious concern is infection during pregnancy, where the virus can cross the placenta and cause fetal anemia and hydrops fetalis.

Viral Pathogenesis and Epidemiology

The general pathogenesis of viral skin infections follows a consistent sequence:

1. Entry and initial replication in susceptible host cells (often epithelial cells at the site of contact)
2. Spread to target tissues, either locally through cell-to-cell transmission or systemically via the bloodstream (viremia)
3. Cellular damage and immune response, which together produce the visible signs of infection (blisters, rashes, warts)
4. Virus shedding, allowing transmission to new hosts through skin contact, respiratory droplets, or fomites

Epidemiologically, these infections vary by virus. HPV is one of the most common viral infections worldwide, with prevalence influenced by hygiene practices and immune status. HSV-1 seroprevalence is extremely high globally, with most people acquiring the virus in childhood. The childhood exanthems show age-specific susceptibility: roseola peaks in infants who have lost maternal antibodies, while fifth disease clusters in school-aged children where close contact facilitates spread. Seasonal patterns also play a role, with parvovirus B19 outbreaks most common in late winter and spring.