Cold sores and fever blisters are caused by the herpes simplex virus type 1 (HSV-1), a virus that passes from person-to-person by direct contact with infected skin or secretions, including saliva. The sores usually appear as clusters of tiny blisters on the lip. HSV-1 can cause similar, but smaller, blisters that rapidly become ulcers inside the mouth on the gums and palate (roof of the mouth).
The first time HSV-1 invades the skin, it causes a primary infection, which usually occurs in childhood or adolescence. It may cause a fever, sore mouth and sore throat. The primary infection often is similar to other viral or bacterial throat infections, and often is not diagnosed as an HSV-1 infection.
After this primary infection subsides, the virus remains dormant (inactive) indefinitely in nerve roots near the affected skin area. In some people, the virus becomes active again from time to time and causes new active sores on the skin, usually on the lip. These sores have become known as cold sores or fever blisters because the virus can be reactivated by a cold or fever. The virus also can be triggered to become active again by other factors, including stress, trauma to the lips, exposure to the sun’s ultraviolet rays, menstruation and suppression of the immune system.
In addition to causing infections around the mouth, HSV-1 can be transferred to the eyes, the skin of the fingers, the genital area and elsewhere. Most genital herpes infections are attributed to herpes simplex type 2 (HSV-2), but HSV-1 can cause genital infections in some people. HSV-1 can cause unusually severe illness in people debilitated by other health problems (malnutrition, cancer or weakened immune systems, including AIDS) or those receiving cancer chemotherapy, corticosteroids or drugs to prevent the rejection of organ transplants.
HSV-1 infections are very common in the United States, and most of us are infected by the time we reach adulthood. Even if we do not remember having a primary HSV-1 infection during childhood, the fact that we occasionally have a cold sore or fever blister indicates that HSV-1 is present.
A primary herpes infection — the first time a person is infected — causes fever, headache, nausea, vomiting, painful swelling and open sores on the gums and inside the cheeks, and a painful throat infection that is often mistaken for “strep throat,” or another form of bacterial infection in the throat. These symptoms usually begin approximately a week after exposure to someone with HSV-1.
Cold sores and fever blisters appear when the virus is reactivated later in life. They may occur after a period of illness or stress, poor nutrition or sunlight exposure, or for no known reason. Dental procedures that stretch the lip may occasionally trigger the virus to become active. The border of the lip is by far the most common place for these sores to appear. They may occasionally erupt inside the mouth, particularly in patients with compromised immune systems or those debilitated by other medical disorders.
The first sign of a cold sore is a vague tingling, burning or itching, followed by swelling and redness. Within 24 to 48 hours, one or more tiny blisters (“fever blisters”) appear. These blisters break, leak fluid and form painful sores (“cold sores”), which are eventually covered by crusts and scabs. These crusts are shed and form again while the sore is healing.
Cold sores and fever blisters can be treated with antiviral medications. These medications are of little value if therapy is started after the blisters appear. Antiviral creams applied directly to the sores shorten the healing time by less than one day. Antiviral drugs taken by mouth can help to prevent cold sores in tropical locations where exposure to the sun can trigger an outbreak. Wind and other weather conditions also can trigger cold sores if your lips become too dry. It also helps to keep the affected area clean and to apply soothing lip balm if the lips are involved. Try not to touch the affected area and avoid kissing anyone while you have blisters and sores.