Spider Bite

Learn more about dangerous spiders and their bite...

A spider bite is are an injury resulting from the bite of spiders or other closely related arachnids.

Spiders are active hunters and rely heavily on their bites to paralyze and kill their prey before consuming it. They also bite in self defense. While many spiders will never attack animals larger than themselves (preferring to run away), some exhibit aggressive behavior and will stand their ground when approached by larger animals. Most spider bites, however, occur when humans unintentionally press up against spiders and receive a defensive bite. On rare occasions, spiders may make mistake you or a loved one for prey, and bite a finger or other body part as though it were a caterpillar or another insect.

Only spiders of fairly large species possess chelicera (pincer-like appendages that each hold a fang) long enough to penetrate human skin. There is nothing smaller than members of the Cheiracanthium genus (around 6 mm.) that gives a bite that is greatly troublesome to humans. The effect of a bite on humans is dependent on both the toxicity of the venom and the amount of venom. About 98% of the bites inflicted by species that are large enough that their bites are noticed will have no serious medical consequences. Of those bites that humans notice, venoms can include necrotic agents, neurotoxins, and agents such as serotonin. Only some two hundred species in twenty genera (out of over 40,000 known species) are known to have serious, potentially lethal bites.

In most cases of spider bite, the chief concern is the spider's venom, although in some cases medically non-significant spiders can transmit infectious diseases. Spiders regarded as dangerous possess venom that is sufficiently toxic to humans that a single bite can deliver a medically significant dose. Only three spider families are known to be non-venomous, i.e. lacking venom glands. They are Uloboridae, Holarchaeidae and Liphistiidae. The Liphistiidae, however, are large enough to deliver unpleasant bites. In addition, their fangs can often inflict infections spread through the skin, mostly due to their large size, which in theory could represent more danger than the bite of a non-lethal venomous spider species.

According to Virginia Tech entomology specialists, only two species of venomous spiders are known to exist in Virginia, and one of these is only rarely seen along the western fringes of the state. The Black Widow Spider is the more common of the two and easily recognizable by most people for its distinctive coloration and markings. The other poisonous spider, the Brown Recluse, is more difficult to identify. Injuries attributed to brown recluse bites are more than likely because of other factors due to the extreme rarity of this species in Virginia.

A spider bite may be misdiagnosed by both the general public and medical practitioners. Many other conditions both infectious and non-infectious can be confused with spider bite. Many of these conditions are far more common and more likely to be the source of necrotic wounds.

Pathophysiology

A primary concern of the bite of a spider is the effect of its venom. A spider envenomation occurs whenever a spider injects venom into the skin. Not all spider bites involve injection of venom into the skin, and the amount of venom injected can vary based on the type of spider and the circumstances of the encounter. With very few exceptions, such as the so-called camel spider (which is not a true spider), the mechanical injury from a spider bite is not a serious concern for humans. Some spider bites do leave a large enough wound that infection may be a concern, and other species are known to consume prey which is already dead, which also may pose a risk for transmission of infectious bacteria from a bite. However, it is generally the toxicity of spider venom that poses the most risk to human beings; several spiders are known to have venom which can be fatal to humans in the amounts that a spider will typically inject when biting.

With just a few exceptions, basically all spiders are capable of producing venom. (Other arachnids often confused with spiders, such as the harvestman and sun spiders, also do not produce venom). Nonetheless, only a small percentage of species have bites that pose a danger to people. Many spiders do not have mouth parts capable of penetrating human skin. While venoms are by definition toxic substances, most spiders do not have venom that is sufficiently toxic (in the quantities delivered) to require medical attention and, of those that do, only a few are known to produce fatalities.

Spider venom works in one of two ways; they are either neurotoxic (attacking the nervous system) or necrotic (attacking tissues surrounding the bite, and, in some cases, attacking vital organs and systems).

Neurotoxic Venom

The majority of spiders with serious bites possess a neurotoxic venom of some sort, though the specific manner in which the nervous system is attacked varies from spider to spider.

Black Widow spider venom contains components known as latrotoxins, which cause the release of the neurotransmitter acetylcholine, stimulating muscle contractions. This can affect the body in several ways, including causing painful abdominal cramps, as well as interfering with respiration, and causing other systemic effects.

Necrotic Venom

Spiders known to have necrotic venom are found in the family Sicariidae, a family which includes both the recluse spiders and the six-eyed sand spiders. Spiders in this family possess a known dermonecrotic agent sphingomyelinase D, which is otherwise found only in a few pathogenic bacteria.

Some species in this family are more venomous than others; according to one study, the venom of the chilean recluse and several species of six-eyed sand spider indigenous to southern Africa, contains an order of magnitude more of this substance than do other Sicariidae spiders such as the brown recluse. Bites by recluse spiders can produce symptoms ranging from minor localized effects, to severe dermonecrotic lesions, up to and including severe systemic reactions including renal failure, and in some cases, death. Even in the absence of systemic effects, serious bites from Sicariidae spiders may form a necrotising ulcer that destroys soft tissue and may take months and very rarely years to heal, leaving deep scars. The damaged tissue may become gangrenous and eventually slough away. Initially there may be no pain from a bite, but over time the wound may grow to 10 inches (25 cm) in extreme cases. Bites usually become painful and itchy within two to eight hours, pain and other local effects worsen 12 to 36 hours after the bite, and then necrosis will develop over the next few days.

Serious systemic effects may occur before this time, because the venom spreads throughout the body in minutes. Mild symptoms include nausea, vomiting, fever, rashes, and muscle and joint pain. Rarely more severe symptoms occur including hemolysis, thrombocytopenia, and disseminated intravascular coagulation. Debilitated patients, the elderly, and children may be more susceptible to systemic loxoscelism. Deaths have been reported for both the Brown Recluse and the related South American species L. laeta and L. intermedia.

Numerous other spiders have been associated with necrotic bites in the medical literature. Examples include the Hobo spider and the Yellow Sac spider. However, the bites from these spiders are not known to produce the severe symptoms that often follow from a recluse spider bite, and the level of danger posed by each has been called into question. So far, no known necrotoxins have been isolated from the venom of any of these spiders, and some arachnologists have disputed the accuracy of many spider identifications carried out by bite victims, family members, medical responders, and other non-experts in arachnology. There have been several studies questioning whether danger is posed by some of these spiders. In these studies, scientists examined case studies of bites in which the spider in question was positively identified by an expert, and found that the incidence of necrotic injury diminished significantly when "questionable" identifications were excluded from the sample set.

United States - Virginia

The jumping spider is probably the most common biting spider in the United States, the main species being Phidippus audax. Bites from a jumping spider are usually painful, itchy and cause redness and significant swelling.

The spiders of most concern in the United States, however, are Brown Recluse spiders and Black Widow spiders. Most recluse spider bites are minor with little or no necrosis. However, a small number of bites produce severe dermonecrotic lesions, and, sometimes, severe systemic symptoms, including organ damage. Rarely the bite may also produce the systemic condition with occasional fatalities.

While Black Widow spider bites may cause muscle cramps, deaths from Black Widow bites are rare in the United States. There is a July 2011 report of what is believed to be a death from multiple Black Widow bites: Man dies after suffering 19 Black Widow spider bites.

Widow Spiders - Black Widow

The widow spiders (genus Latrodectus), such as the Black Widow, Redback Spider, and Katipo are spiders that carry a neurotoxic venom which can cause a set of symptoms known as Latrodectism.

Like many spiders, widows have very poor vision, and they move with difficulty when not on their web. Widow spiders are large, strong-looking house spiders (but still have relatively spindly legs and deep, globular abdomens). The abdomen is dark and shiny, and has one or several red spots, either above or below. The spots may take the form of an hourglass, or two triangles, point-to-point. Male widows, like most spider species, are much smaller than the females and may have a variety of streaks and spots on a browner, less globular abdomen. The males are generally less dangerous than the females of the same species. Widows tend to be non-aggressive, but will bite if the web is disturbed and the spider feels threatened.

The venom, while seldom life-threatening, produces very painful effects including muscle spasms, 'tetanus-like' contractions and, in some cases, spinal or cerebral paralysis (which is generally temporary, but might leave permanent damage to central nervous system). A serious bite will often require a short hospital stay. Children, elderly, and ill individuals are at most risk of serious effects.

Recluse Spiders - Brown Recluse

Recluse spiders (Loxosceles spp.), such as the Brown Recluse spider, also known as "violin spiders," "fiddlers," or "fiddlebacks," from the dark violin-shaped marking on the cephalothorax, are slow-moving, retiring spiders which wander about in dim areas and under things, and so are more easily trapped against one's skin by clothing, bed sheets, etc. The spiders will often creep along at a very slow pace and then make a sudden dart for a couple of inches, then return to the previous languid pace. Recluses are extremely venomous. Most encounters with this spider occur from moving boxes or rooting about in closets or under beds. The range of the Brown Recluse, L. reclusa in the US is approximately the southern two-thirds by the eastern three-quarters of the country.

Most recluse spider bites are minor with little or no necrosis. However, a small number of bites produce severe dermonecrotic lesions, and, sometimes, severe systemic symptoms, including organ damage. Rarely the bite may also produce the systemic condition with occasional fatalities.

A minority of bites form a necrotizing ulcer that destroys soft tissue and may take months and, on very rare occasions, years to heal, leaving deep scars similar to MRSA. The damaged tissue will become gangrenous and eventually slough away. The initial bite frequently cannot be felt and there may be no pain, but over time the wound may grow to as large as 10 inches (25 cm) in extreme cases. Bites usually become painful and itchy within 2 to 8 hours, pain and other local effects worsen 12 to 36 hours after the bite with the necrosis developing over the next few days.

Serious systemic effects may occur before this time, as the venom spreads throughout the body in minutes. Mild symptoms include nausea, vomiting, fever, rashes, and muscle and joint pain. Rarely more severe symptoms occur including hemolysis, thrombocytopenia, and disseminated intravascular coagulation. Debilitated patients, the elderly, and children may be more susceptible to systemic loxoscelism. Deaths have been reported for both the Brown Recluse and the related South American recluse species.

Even more dangerous is the Chilean recluse, a species native to South America and found in many parts of the world, including in southern California and other southwestern states. Bites of this spider have been known to cause systemic reactions in 15% of reported cases, and fatalities in 3-4% of cases.

Yellow Sac Spiders

Yellow Sac spiders take shelter in silk tubes during the daytime, usually coming out to hunt at night. These pale yellow or whitish spiders are often found in houses at the top of walls, or wandering across ceilings. They are also commonly found outdoors on foliage. The draglines they leave while hunting are one of the most common "spiderwebs" that are removed with broom and vacuum cleaner. People may unintentionally make contact with them in the dark and will be bitten if the spider is irritated or provoked. However, many people will live their entire lives in close proximity to them and never suffer a bite.

Information courtesy of Wikipedia and the National Pest Management Assn.