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Fever Blisters and Canker Sores

Fever blisters and canker sores are two of the most common infections of the mouth, causing discomfort and annoyance to millions of Americans. The two disorders resemble each other and are often confused. Both disorders cause small sores to develop in the mouth. These sores usually heal within two weeks. Canker sores differ from fever blisters, however, by not being preceded by a blister. Canker sores are generally larger than fever blisters and several canker sores rarely merge to form one large sore, as do fever blisters. A final distinguishing trait between the two disorders is that canker sores usually occur on the movable parts of the mouth such as the tongue and the inside linings of the cheeks and lips, whereas fever blisters generally occur on the nonmovable sections of the mouth such as the gums and the roof of the mouth, or on the outside of the lips and nostrils.

Both canker sores and fever blisters have plagued mankind for thousands of years. Scientists working for the National Institute of Dental Research, one of the Federal Government's National Institutes of Health, are seeking ways to better control and ultimately prevent these and other oral infections. Recent research has furthered an understanding of the causes of these disorders and fostered improvements in their treatments.

Fever Blisters 

HISTORY: In ancient Rome an outbreak of fever blisters prompted Emporer Tiberius to curb the spread of the epidemic by banning kissing in public ceremonies. Today, fever blisters (sometimes called cold sores) still occur in epidemic proportions. About 100 million episodes of recurrent fever blisters occur yearly in the United States alone, and 45 to 80 percent of all adults and children have experienced at least one bout with the blisters.

What Causes Fever Blisters?

Fever blisters are caused by a contagious virus called herpes simplex. There are 2 types of herpes simplex viruses. Herpes simplex 1 primarily causes blister-like sores on the mouth, lips and face, although these blisters can be caused by herpes simplex 2, the virus that usually causes genital herpes. Herpes simplex is highly contagious when fever blisters are present, and the virus is frequently transmitted by kissing. Children often are initially infected with herpes by contact with their parents, siblings or other close relatives who have fever blisters. A child can also spread the virus by rubbing his or her cold sore and then touching other children. About 10 percent of oral herpes cases in adults are acquired by oral-genital sex with a person with active genital herpes.

Most people experience their first infection with herpes simplex 1 when they are less than 10 years of age. In these primary infections, the virus usually invades the moist membrane cells of the lips, mouth, or throat. The majority of persons infected have no symptoms, but about 15 percent develop many fluid-filled blisters inside and outside the mouth three to five days after they are exposed to herpes simplex 1. The blisters may be accompanied by fever, swollen neck glands, and general aches. Fever blisters tend to merge and then collapse.

Often a yellowish crust forms over the sores, which usually heal without scarring within 2 weeks.

The herpes virus, however, stays in the body. Once a person is infected with oral herpes, the virus remains in a nerve located near the cheekbone. The virus may stay permanently inactive in this site or occasionally may travel down the nerve to the skin surface where it causes a recurrence of fever blisters. Recurring blisters generally erupt at the outside edge of the lip or the edge of the nostril, and take almost as long to heal as the initial fever blisters.

Recurrent attacks of fever blisters are normally less severe than primary attacks and the frequency of recurrence appears to decline after a person reaches age 35, one study shows. Many people who experience recurring fever blisters feel itching, tingling or burning in the lip area 1 to 3 days before the fever blister occurs.