According to MayoClinic.com, fever blisters and cold sores are two names for skin lesions produced by the herpes simplex-1 virus. Herpes simplex-1 virus infections follow five stages: prodrome, blistering, ulceration, crusting and healing. A fever blister reflects an earlier stage of the infection than a cold sore, although the terms are often used interchangably, along with other terms like "oral herpes" and "herpes labialis." Since herpes simplex-1 causes skin lesions that appear in crops, it's possible to have both fever blisters and cold sores at the same time.
Onset
In the June 9, 2008 issue of "Archives of Internal Medicine," internist Christina Cernik, M.D., explains that a prodrome, or set of warning symptoms, precedes the appearance of fever blisters by anywhere between two days and two hours. Typical symptoms include tingling, burning, itching, soreness or unusual sensitivity in the area where the blisters and sores later appear. Careful examination may reveal mild swelling or redness.
Duration
Fever blisters develop from the patches of red, swollen skin over the course of a few hours. Cernik says the blisters actually begin as small, solid, red bumps rather than fluid-filled blisters, but the fluid appears so quickly that most patients never see the bumps. The blisters persist for three to four days, according to Cernik, when they spontaneously rupture. The shallow, open, pink or red sores left behind are cold sores. The cold sores only last about one day before they begin to acquire a yellow-brown crust, signaling the second to last stage of the infection. Complete healing, Cernik notes, takes about 10 to 14 days.
Location
Cernik says 90 percent of fever blisters and cold sores develop on the lips. Lesions inside the mouth, unaccompanied by lesions on the lips, suggest canker sores or other kinds of mouth problems. Lesions that predominantly occur on the face, hands or other places on the body instead of the lips suggest chicken pox, shingles, small pox, genital herpes or the coxsackievirus infection, "hand, food and mouth disease."
Associated Symptoms
In the 2008 edition of "Harrison's Principles of Internal Medicine," University of Washington Professor Lawrence Corey, M.D., explains that fever blisters are sometimes accompanied by flu-like symptoms such as low-grade fever, headache, muscle pain, poor appetite and a generalized sense of malaise. These symptoms start to subside when the fever blister becomes a cold sore and resolve completely when the lesion crusts.
Treatment
Prescription and over-the-counter antiviral drugs can reduce the severity and duration of herpes simplex-1 symptoms when they are started soon after symptoms appear. Corey says that treatment is most effective when it's started during the prodrome or fever blister stage. Cold sores are least responsive to treatment and, according to Corey, there's no evidence that starting treatment after crusts appear has any effect at all.
