The bites of most spiders and insects, including mosquitoes, fleas, flies, bedbugs, and chiggers, are similar in appearance and pose little danger. Typically, the insect's injection of salivary fluid or venom into the skin provokes a small, itchy swelling that lasts a few hours or days. The bites are seldom dangerous, although mosquitoes in certain parts of the world may transmit diseases such as West Nile virus, dengue, malaria, and encephalitis.
For people allergic to insect or spider bites, these bites can cause severe trauma and even life-threatening anaphylactic shock. Also, the bites of a few spiders, ticks, and insects are poisonous or associated with specific diseases.
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Ticks. While most tick bites are harmless, several species can cause life-threatening diseases such as Rocky Mountain spotted fever and Lyme disease. Ticks may also transmit tularemia, relapsing fever, and a potentially fatal ailment called ehrlichiosis. Rarely, a bite may trigger tick paralysis, which starts with numbness and pain in the legs and can result in respiratory failure.
Spiders. Bites are seldom fatal; infants, the elderly, and people with allergies are at greatest risk. Most dangerous is the black widow spider, found throughout the U.S. and especially in warmer areas. The bite itself can pass unnoticed, but within hours, intense pain and stiffness may begin, occasionally followed by muscle spasms, abdominal pain, chills, fever, and difficulty swallowing or breathing. No one in the U.S. has died from a black widow spider bite in over 10 years.
The bite of the brown recluse spider is painless but may cause a spreading, necrotizing (tissue-killing) wound. Occasionally, reactions to a brown recluse spider bite include fever, chills, joint pain, and convulsions; death is rare.
Scorpions. Scorpion stings cause a sharp, burning pain, followed by numbness. Rarely, scorpion venom produces shock, or even a life-threatening syndrome of rapid breathing, difficulty speaking, and muscle spasm. Fewer than 1% of stings are fatal; these are usually with very young and elderly victims.
Fire ants. Recent arrivals from Mexico, fire ants produce small, fluid-filled bites that may form an ulcer. The ants bite into the skin and then sting repeatedly in an arc around the bite. The venom is capable of causing severe reactions and even, in some cases, anaphylaxis and death.