• Pet dander can cause hives.cat image by Andrey Tolstov from Fotolia.com
  
  Hives, also known as urticaria, refer to itchy, raised welts on the skin that range in size from a pencil eraser to a dinner plate. They appear temporarily and usually do not leave a mark after they disappear. While hives are uncomfortable, they are usually harmless unless swelling constricts the throat. Hives are usually managed with antihistamine medications or by avoiding the trigger. Many things can trigger an outbreak, and sometimes there is no identifiable trigger. According to the American Academy of Dermatology, around 10 to 20 percent of the American population will develop hives at least once in their lifetime.
  
  

Allergens

• Common allergens like pet dander, pollen, latex, parasites and insect stings can cause hives to develop. Hives caused by allergens are often accompanied by other signs of an allergic reaction, including itching, watery or irritated eyes, and nasal congestion.
  
  

Food

• Hives can occur after people eat certain foods. Common problem foods include eggs, shellfish, fish, milk, nuts, berries, tomatoes and chocolate. Fresh food is more likely to cause an outbreak of hives than cooked food. Food additives or preservatives can also cause hives. Hives may appear within minutes or hours of eating a certain food. If you suspect your hives are from a food allergy, your doctor may ask you to keep a detailed food diary.
  
  

Medication

• Penicillin, ibuprofen and aspirin commonly cause hives. Antibiotics, sedatives, diuretics, vitamins and antacids can also cause hives. Anti-inflammatory drugs like ibuprofen can exacerbate chronic hives.
  
  

Environmental Factors

• Environmental factors like heat, sunlight, cold, water, exercise, vibration, pressure or emotional stress can also cause hives or worsen an existing hives outbreak. For example, the pressure caused by a tight belt or sock band can cause an outbreak. Hot showers or cold swimming pools can cause outbreaks in some people.
  
  

Infections

• Respiratory tract infections, hepatitis, infectious mononucleosis, strep throat and fungal infections can also cause hives. Colds can cause hives in children.
  
  

Dermatographia

• Dermatographia refers to a condition in which hives appear after a person scratches or applies pressure to his skin. These hives often appear in a linear arrangement, according to the scratches. Dry skin can exacerbate dermatographia. According to the American Academy of Dermatology, dermatographia occurs in approximately 5 percent of the United States population. This condition usually persists for several months or even years.
  
  

Immune Disorders

• Immune disorders like thyroid conditions, cancer or lupus can also cause hives to develop. Immune disorders often cause chronic hives, or hives that last for more than six weeks.