The rash that develops from poison ivy, poison oak, and poison sumac contact is commonly mistaken for hives (urticaria).
This rash is caused by contact with urushiol and results in a form of contact dermatitis called Urushiol-induced contact dermatitis.
Urushiol is spread by contact, but can be washed off with a strong grease/oil dissolving detergent and cool water and rubbing ointments.
- Acute urticaria usually show up a few minutes after contact with the allergen and can last a few hours to several weeks. Food allergic reactions often fit in this category. The most common food allergies in adults are shellfish and nuts. The most common food allergies in children are shellfish, nuts, peanuts, eggs, wheat, and soy. It is uncommon for patients to have more than 2 true food allergies. A less common cause is exposure to certain bacteria, such as streptococcus or possibly ''Helicobacter pylori''. In these cases, the hives may be exacerbated by other factors, such as those listed under Physical Urticarias below.
- Chronic urticaria refers to hives that persists for 6 weeks or more. There are no visual differences between acute and chronic urticaria. Some of the more severe chronic cases have lasted more than 20 years. A survey indicated that chronic urticaria lasted a year or more in more than 50% of sufferers and 20 years or more in 20% of them. Of course this does mean that in almost half the people it clears up within a year and in 80% it clears up within 20 years or less.
- Drug-induced urticaria has been known to result in severe cardiorespiratory failure. The anti-diabetic sulphonylurea glimepiride (trade name Amaryl), in particular, has been documented to induce allergic reactions manifesting as urticaria. Other cases include dextroamphetamine, aspirin, penicillin, clotrimazole, sulfonamides and anticonvulsants.
- Physical urticarias are often categorized into the following.
- Aquagenic: Reaction to water (exceedingly rare)
- Cholinergic: Reaction to body heat, such as when exercising or after a hot shower
- Cold (Chronic cold urticaria): Reaction to cold, such as ice, cold air or water - worse with sudden change in temperature
- Delayed Pressure: Reaction to standing for long periods, bra-straps, elastic bands on undergarments, belts
- Dermatographic: Reaction when skin is scratched (very common)
- Heat: Reaction to hot food or objects (rare)
- Solar: Reaction to direct sunlight (rare, though more common in those with fair skin)
- Vibration: Reaction to vibration (rare)
- Adrenergic: Reaction to adrenaline / noradrenaline (extremely rare)
Angioedema is similar to urticaria, but in angioedema, the swelling occurs in a lower layer of the dermis than it does in urticaria, as well as in the subcutis. This swelling can occur around the mouth, in the throat, in the abdomen, or in other locations. Urticaria and angioedema sometimes occur together in response to an allergen and is a concern in severe cases as angioedema of the throat can be fatal.
Urticaria can be a complication and symptom of a parasitic infection as Fascioliasis (Fasciola Hepatica) and Ascariasis (Ascaris Lumbricoides).
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