Many people recognize chicken pox for its characteristic blistering rash; however, a number of other conditions can also cause a rash like chicken pox. According to University of Alabama Professor of Pediatrics Richard J. Whitely, M.D. in the 2008 edition of "Harrison's Principles of Internal Medicine," the candidates include herpes simplex virus one or two, small pox, coxsackie virus, echo virus and Rickettsialpox. Doctors use key aspects of the rash to distinguish a rash like chicken pox from a rash actually caused by chicken pox.
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According to National Institutes of Health dermatologist Stephen E. Straus, M.D. in the 2008 edition of "Fitzpatrick's Dermatology in General Medicine," chicken pox causes 2- to 3mm elliptical, fluid-filled blisters that appear on a patch of red, raised skin and run in parallel with skin folds. Initially, the blisters are filled with clear fluid, which gives them the appearance of a "dew drop on a rose petal." Herpes simplex and small pox produce blisters that are two or three times larger, so the size of the blister is frequently used to rule out these diagnoses.
Chicken pox usually starts on the face and scalp, says Straus, and then moves outward to the trunk and extremities. Blisters on the trunk usually concentrate within the area below the shoulder blades and above the buttocks. Blisters also cluster near the midline of the body, with comparatively few lesions found towards the periphery. Several types of Coxsackie viruses cause a rash similar to chicken pox; however, Coxsackie virus distributes mainly to the hands, feet and mouth, hence the common name "hand foot and mouth disease."
Chicken pox, says Whitely, sometimes causes a mild flu-like illness that includes fever, headache, body aches, poor appetite and a generalized sense of malaise. Adults and adolescents are often hardest hit, while younger children may have little or no symptoms beyond a rash. These associated symptoms can be used to distinguish chicken pox from echovirus infections where blistering rash accompanies meningitis, virus-induced inflammation of the outer protective covering of the spinal cord. Meningitis causes symptoms such as stiff neck, sensitivity to light, mental confusion and alteration in consciousness that are rarely associated with chicken pox.
Chicken pox attacks 90 percent of people by the age of 15, says Whitely. The disease usually develops 14 to 17 days after contact with an infected person. However, Rickettsialpox should be considered in people exposed to mice because the disease is carried by ticks that primarily feed on mice. For people with suspected exposure to mice, the presence of a herald spot--a red tick bite that develops a painless black crust called an "eschar"--can confirm the diagnosis as Rickettsialpox.
Since the development of an effective chicken pox vaccine in 1995, says the CDC, the number of chicken pox cases has plummeted. However, no vaccine is 100 percent effective. Some vaccinated children still develop the disease, notes Whitely. However, the presence of a small number of blisters---less than 60, says Whitely--compared to the usual 250 to 2,000 suggests that a rash like chicken pox actually is chicken pox in a person vaccinated against the disease. The good news, continues Whitely, is that people vaccinated against chicken pox are still far less likely to develop complications of the disease, such as pneumonia or hepatitis, in addition to experiencing a less severe skin rash.