In North America, there are very few types of insects that carry venom or poison, and none whose venom is dangerous. However, it is possible for an insect bite or sting to cause an allergic reaction that can range from a mild local reaction to something like a severe asthma attack. In extreme allergic reactions, your airways can close up and you can even stop breathing.
The insect responsible for the largest number of severe allergic reactions is the yellow jacket wasp. Considering both multiple stings and allergic reactions to single stings, insects actually harm or even kill (in rare cases) more than three times as many North Americans as snakes do.
All three have different kinds of venom, but none is likely to be dangerous in small doses unless someone is allergic to the poison.
Biting and bloodsucking insects:
None of them are actually poisonous, but some have saliva that can irritate or provoke a reaction, and others can introduce infections when they bite.
Examples of infections and reactions include the following:
West Nile virus is passed on by the bite of mosquitoes that have become infected by feeding on the blood of infected birds. The first confirmed Canadian human case of West Nile virus was reported in Ontario in September 2002. Although the risk of illness from West Nile virus is low for most Canadians, it is important to recognize the symptoms and understand ways to reduce the risk of contracting this disease. Please read the article on West Nile virus for more information.
Malaria, another serious infection transmitted by mosquitoes, is fortunately uncommon in North America. Remember that travellers to areas where malaria is common who get a fever after returning home should be seen promptly by their physician. These travellers should be considered as having malaria unless proven otherwise (with a blood test).
Lyme disease is caused by the bite of a deer tick that carries a parasite capable of causing disease in humans. People who are infected with Lyme disease may not experience symptoms right away after they are bitten. It is important that people with Lyme disease be diagnosed in a timely manner, as most people respond well to antibiotics.
Rocky Mountain spotted fever (RMSF) is caused by tick bites in certain endemic areas that are infected with the bacteria Rickettsia rickettsii. These ticks are commonly found in the southeastern and south central US. While most people who are infected by the bacteria will have mild or even unnoticeable symptoms, some can become seriously ill and require emergency medical care and antibiotics. Antibiotics are very effective against RMSF.
Other ticks may cause a temporary spreading paralysis if they go undiscovered for a few days. These creatures hang on and suck blood for as long as 4 days, becoming fatter in the process. After about 2 days, some species start producing new chemicals in their saliva, which can temporarily shut down parts of the human nervous system. This is called tick paralysis, and it's quite different from Lyme disease. It clears up after the tick is removed, but can be fatal if breathing stops.
Mites that make their living around humans, such as house dust mites, don't bite, but there are bird and animal mites that occasionally attack humans and leave larvae (chiggers) in the skin. These cause local skin reactions in both allergic and non-allergic people. Chiggers cause redness and itching, while adult mites leave a small bite that usually becomes irritated hours or days later. You're most likely to be bitten by a mite if you handle live birds or poultry, pigs, rabbits, cats, dogs (especially puppies), seeds, straw, or hay.
Bee and wasp stings are immediately obvious. A sharp pain is followed by a burning sensation that soon resolves into a major itch. A red ring or bump appears at the site of the sting. The important thing to remember is that bees' stingers are barbed and usually remain in the skin. In its haste to get away, the bee literally tears the stinger and the attached poison sac out of its abdomen, killing itself in the process. Wasps and hornets lack barbs on their stingers and can attack again and again.
The most serious immediate reactions occur from stings of the yellow-and-black flying insects. A major allergic reaction that interferes with breathing is called anaphylaxis or anaphylactic shock. Histamine, a chemical released by the body during most allergic reactions, is released into the skin after any insect bite and is responsible for the redness and itching. In anaphylaxis, histamine causes major itching and redness of the skin (hives), and may also be released in the airways, lungs, and other vital organs. It causes tissue to swell, can close the airways (causing breathing to stop), and can drop blood pressure to dangerously low levels.
Anaphylaxis can occur after a single bite, but this is rare. More typically, fatal anaphylaxis occurs when somebody gets stung many times (50 to 100), still nowhere near enough times to kill a non-allergic person.
It is possible to be killed by multiple stings. The insect most likely to do this is the infamous Africanized honeybee (killer bee), which has spread in recent years from Mexico to parts of the southern United States. Contrary to popular belief, this bee is no more poisonous than native varieties, but swarms are highly aggressive and can inflict up to hundreds of stings in only a few seconds. A human can tolerate about 22 wasp or bee stings per kilogram of body weight (about 10 stings per pound of body weight) and still survive, meaning it usually takes over 1,000 stings to seriously harm a healthy adult.
Many people infected with West Nile virus will have only mild symptoms or no symptoms at all. If symptoms do occur, they usually appear within 2 to 15 days. They may vary from flu-like symptoms that include fever, headache, and body aches (in most people) to meningitis (inflammation of the lining of the brain or spinal cord) or encephalitis (inflammation of the brain). People with weaker immune systems, for instance people with cancer, diabetes, or heart disease, are at much higher risk for the more serious symptoms of the disease. Anyone experiencing signs of severe headache combined with high fever, stiff neck, nausea, difficulty swallowing, vomiting, drowsiness, loss of consciousness, lack of coordination, muscle weakness, or paralysis should receive emergency medical attention.
Ticks cause no symptoms while they're biting. The only way to find them is to examine your skin each night. Serious complications of tick bites (Lyme disease, RMSF, and tick paralysis) normally only occur after the tick has been attached for at least 24 hours. In tick-infested areas, a nightly check is a good idea. For details about the symptoms of Lyme disease, please see the Lyme disease article. The main symptom of tick paralysis is muscle weakness, poor coordination, or paralysis spreading upwards (towards the head) from the site of a tick bite or an attached tick. Symptoms of RMSF include a fever above 38.9°C (102°F), headache, malaise, body aches, and rash.
Most people can guess at what's bitten them by looking at the site of the wound or welt. Black flies, for example, leave bites around the head, neck and ears, while fleas often bite repeatedly around the feet and lower legs. Bedbugs tend to leave lines of bites, usually on the torso. While their bites can be extremely itchy, these insects don't cause serious diseases or reactions.