Most spider bites are not dangerous though all spiders, with the exception of the uloboridae family, have venom. However, due to varying potency of this venom, of the thousands of species in the world relatively few present a threat to humans.
Venom, whether spider or snake venom, can be categorized into two types: neurotoxic or cytotoxic.
The black widow is an example of a spider with neurotoxic venom. This venom directly affects the nervous system, though there may not be much of a noticeable wound at the site of the bite itself. Blocking impulses to the muscles, neurotoxic venom causes cramps, rigidity, and has a general paralyzing effect. This venom kills quicker than cytotoxic venom and is considered more potent. The Australian funnel web spider is an example of a spider with neurotoxic venom.
Spiders like the brown recluse spider have cytotoxic venom. Cytotoxic venom is necrotic venom, from the word, necrosis, which refers to the breakdown of cells and tissue. The bite from a spider with cytotoxic venom will cause a welt resembling a mosquito bite and a noticeable wound. Necrosis will be present and the sore will be slow to heal and could require medical attention to prevent secondary infections. Spider bites of this nature can create severe flu-like symptoms, and in rare cases, can be deadly.
Among the best-known, highly venomous spiders you'll find:
- Australia's funnel-web spider (atrax and hadronyche species)
- Redback, katipo, or black widow spiders (latrodectus species)
- South American banana or Brazilian wandering spiders (phoneutria species)
- Brown recluse spiders (loxosceles species)
- Hobo spiders (tegenaria agrestis species)
Antivenin is available for funnel-web, redback, black widows, and South American banana spider bites. There is no antivenin for the brown recluse or hobo spiders but treatment in the form of antibiotics is commonly administered to prevent secondary infection at wound sites.
The North American banana spider (argiope) is considered harmless.
It is safe to estimate that hundreds of thousands of people every year are bitten by spiders with no ill effects. Even a bite from a spider listed above may not produce symptoms. The spider may not inject venom, or may inject very little and the wound may heal on its own. In most cases a spider bite will produce a local welt that resembles a mosquito bite, and will subside within 24 hours. However children, the elderly, or people with compromised systems are at greater risk of severe reactions. Allergic reactions can also cause problems with spider bites that are normally not considered dangerous.
If you believe you have been bitten by a spider and are experiencing symptoms that concern you, contact your doctor or your local poison center. If possible, safely bring the spider with you when you seek medical attention. Many spider bites share common symptoms and can be misdiagnosed without the spider. Dead or crushed spiders can still be evaluated for diagnostic purposes.
