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A WORD ABOUT BITES AND STINGS


Bites and stings causing clinically significant reactions in humans occur throughout the world on a regular basis. The vast majority of these are bites by parasitic arthropods, which feed upon blood and cause localized allergic reactions in bitten persons: Such bites are referred to as immunoreactive bites; bed bugs, biting midges, black flies, fleas and mosquitoes are examples of insects that produce immunoreactive bites. Venomous bites and stings are those in which the biting or stinging organism actually injects a toxic cocktail of proteins into the victim, designed in most cases to immobilize, kill and assist in the digestion of prey items. Some venomous organisms, such as the honey bee, sting rays and most venomous fish use their venom primarily as a defensive mechanism.

Immunoreactive Reactions

Two distinct types of immunoreactive reactions occur following bites by parasitic arthropods: The first, and most common type, is known as an immediate type hypersensitivity (ITH) reaction. The ITH reaction occurs in persons that develop the allergenic antibody immunoglobin E (IgE) in response to proteins in the saliva of the biter. Once IgE targeted against salivary protein is present, within a couple of weeks of the initial bite(s), the individual becomes sensitized to those proteins, and will develop an ITH reaction when those proteins are later injected into tissue. The ITH reaction occurs immediately following a bite, and is produced largely by histamine and other bioactive entities being released into the localized tissue as a result of interaction between antibody and the antigen (the original protein that the antibody is targeted against): Typically, a pale centralized wheal forms at the bite site, encircled by a reddish flare, sometimes several cm. in diameter. This "wheal and flare" may itch uncomfortably, but the primary ITH reaction disappears within a few hours and is gone altogether within about 3 days.

The second type of immunoreactive reaction that can develop following a parasitic arthropod bite is known as a delayed type hypersensitivity (DTH) reaction: The DTH reaction occurs in persons that develop lymphocytes (a type of white blood cell) targeted against specific salivary proteins. DTH reactions take 24 to 48 hours to develop: Inflammation and itching of the bite site gradually worsen as inflammatory cells (white blood cells) infiltrate the area causing induration, or hardening of the lesion. DTH reactions are the persistent, itchy lesions that result following bites by mosquitoes, etc.; They generally take two to three weeks to resorb. Some people develop both IgE antibodies and lymphocytes targeted against insect salivary proteins: These persons may suffer a combination reaction, with both the immediate features of the ITH reaction, and the persistent, delayed features of the DTH reaction.

Venom Poisoning

Poisoning from a venomous bite or sting is known as envenomation: Envenomation occurs when the biting or stinging organism successfully injects venom into a victim. Many venomous animals, including snakes, spiders and scorpions have some control over the amount of venom that they inject, and thus do not always envenomate during a defensive attack; this results in a "dry" bite or sting, which does not result in poisoning. It has been estimated that as many as 50% of bites delivered by dangerously venomous snakes and spiders result in dry bites.

Venoms are complex mixtures of enzymes, toxins, and small molecular weight molecules. As these components work in concert to produce the desired effects on the prey, they can also produce very complex poisoning in humans and domestic animals: Envenomation therefore, is a complex disease state rather than a simply defined one. While the effects of specific toxins such as neurotoxins, hemotoxins, cardiotoxins, etc. often dominate the clinical picture for a bite or sting from a particular species, victims of significant envenomation usually exhibit multiple signs and symptoms from the same bite or sting.

In the case of most venomous snakebites manifestations appear rapidly, and in serious cases can progress at a rapid pace: Venomous snakebites are almost always clinical "emergencies", and often require rapid treatment with antivenin or antivenene (in Europe and Commonweath countries). In the United States, most snakebites are considered "illegitimate", as they occur when the victim is trying to kill, capture, or otherwise manipulate the biting snake.

Bites by dangerously venomous spiders fall into two broadly defined categories; those which produce severe neurological manifestations, and those which produce necrotic arachnidism, a term meaning "spider bite which causes tissue death". Powerful neurotoxins dominate the clinical picture in bites by the black widow, while the hobo spider, recluse spiders, and to a lesser extent the yellow sac spiders, can induce necrotic arachnidism. Necrotic arachnidism is characterized by the local destruction of tissues at or near the bite site.

Allergic Reactions to Venoms

Some individuals become hypersensitive to venoms, particularly to hymenopteran (bee, wasp, hornet and yellowjacket) venoms after receiving a sensitizing sting, or after inhaling dried, air-borne venom. Such persons develop allergenic immunoglobin E (IgE) antibodies against venom proteins, rather than developing the protective immunoglobin G (IgG) antibodies. The IgE antibodies targeted against the venom become attached to a type of connective tissue known as mast tissue, which is enriched with histamine. When the same type of venom protein is later introduced into the tissue of a sensitized person via a sting, the venom protein reacts with the mast cell bound IgE antibody, and massive amounts of histamine are released into the general circulation. This phenomenon, known as anaphylaxis or anaphylactic shock is an often explosive systemic reaction, characterized by (among other things) respiratory distress and vascular collapse. The reaction tends to worsen with each subsequent sting, and untreated episodes can rapidly result in death. Persons who show signs of probable systemic hypersensitivity to insect stings should at all times (at least during the warm months) carry a sting kit containing injectable epinephrine, such as AnaKit (Hollister-Stier Laboratories) or EpiPen (Center Laboratories). These should be obtained by prescription from a physician, who can instruct the recipient on use.


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Last Updated: 04 April 2007