Allergic Reactions to Insect Stings

By: Neil L. Kao, MD

Many of us experience pain, itching, and swelling from insect stings. These symptoms are effects from or reactions to the chemicals in insect venom that irritate the skin and muscle tissue surrounding the bite.  Remember the biologic purpose of insect stings is defending the hive by either causing pain or outright killing the attacker. Here, I’d like to distinguish between an insect sting and an insect bite. The former means the insect uses a sharp point on its tail to pierce the skin and then inject venom. That latter means the insect uses either its jaws to bite the skin or its needle-like “nose” to pierce the skin and then the insect draws up blood to eat. Many insects can sting people, but only six of these insects in the U.S. can cause allergic reactions. They are the honeybee, yellow jacket, white hornet, wasp, and in the South, fire ants. Insects that have not been shown to produce allergic reactions include spiders, mosquitoes, horseflies, and chiggers.

There are several different types of reactions to insect stings. There are two broad categories: immediate, meaning from one minute to four hours after the sting, and delayed, meaning more than four hours later. First, immediate local reactions mean pain, redness, swelling, itching, and warmth directly where the sting occurred in an area of usually less than ten centimeters in diameter. This reaction may last from hours to a few days. This is a normal reaction to being stung. Second, immediate, large local reactions mean the same as immediate local reactions except for two things. The size can be quite large, even covering the entire arm or leg, so long as the reaction involves the area stung. The symptoms can include tiredness and low grade fevers. These reactions may peak in size and symptoms in two to three days and last up to ten days. Both of these immediate two reactions are not dangerous. Most can be treated with the combination of over-the-counter antihistamines, ice packs, acetaminophen, and time. Third, the most deadly type of immediate reaction is called anaphylaxis. Two vital and internal organs, the heart and lungs, are affected. The immune system release lots of histamine, which causes the heart to beat more slowly so blood pressure falls and all of the passageways in the lungs to constrict tightly. The signs and symptoms may include lightheadedness, fainting, chest pains, shortness of breath, wheezing, and coughing. Some patients may have itchy hives all over their body, sneezing and a runny nose, red and itchy eyes, feel like their throat is swelling closed and vomiting, cramping and diarrhea. The steps for treatment are to take an antihistamine by mouth as soon as possible, give an injection of epinephrine using either the Epi-pen or Twinjet devices, and seek immediate medical care. Nationwide, more than 50 people die each year from allergic reactions to insect stings. Fourth, immediate toxic reactions mean headaches, lightheadedness or fainting, seizures, fever, muscle cramps, nausea, and diarrhea. This reaction is caused by the insect’s venom entering the blood stream and having a poisonous, not allergic, effect on the body’s organs. Spider bites are a common cause of toxic reactions, for example think of black widow bites. These reactions are difficult to distinguish from anaphylactic reactions and can be life threatening also. Seek medical care as soon as possible.

The symptoms and signs of a delayed allergic reaction can include the same symptoms as anaphylaxis described above. Up to about 25% of patients with immediate anaphylaxis experience this delayed reaction. So remember, even after the immediate reaction is treated, you may not be finished yet.

Once the reaction is over, there are some precautions you can take to avoid future serious reactions to insect stings.

  1. Try to avoid being stung. Use insect repellent. Avoid wearing perfumes, after-shave, and wearing bright colors. Keep food covered at picnics and campsites. Garbage containers and pop cans should be tightly closed. Wear gloves and long pants when gardening. If you see a hive or insect nest on your house or property, call a professional exterminator to take action. Special insecticides are available to spread in yards and gardens to kill fire ants. They are aggressive and difficult to remove from yards.
  2. Have a detailed emergency plan with instructions about when to use medications and when to seek emergency medical care. Sometimes the plan should be written out for others, because you are away from home, such as being at school, or camping, or on a trip. Remember these symptoms are particularly important and not to be ignored: trouble breathing, feeling faint or dizzy, vomiting or diarrhea.
  3. Consider venom immunotherapy. If you have an allergic reaction to an insect bite, chances are good that you will react similarly for many years. If you are stung again within a short period of time or stung multiple times, then usually the allergic reaction becomes more severe. Diagnostic evaluation of reactions to insects includes a detailed history documenting your reactions to specific insect stings, a physical examination, and skin tests for the specific insects. That’s why it is important to try to identify which insect stung you. Doctors have confirmed in studies around the world that after repeated injections of controlled doses of purified insect venom, the allergic reaction is minimized. This process is called venom immunotherapy. Studies of patients with anaphylactic reactions to insect venom show protection rates exceeding 97 percent after one year of immunotherapy. Five years of venom immunotherapy is usually recommended to ensure lifelong protection from allergic reactions to insect stings. Allergists specialize in the diagnosis and treatment of allergic reactions from insect stings.
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