Encountering spiders often raises questions about their potential threat. While nearly all spiders possess venom, the number of species capable of harming humans is remarkably small. Understanding this distinction is key to assessing the true risk spiders present.
Understanding Venom and Poison
The terms “venomous” and “poisonous” are often used interchangeably, but they describe distinct biological mechanisms of delivering toxins. An animal is poisonous if its toxins are harmful when ingested or absorbed through skin contact. Conversely, an animal is venomous if it actively injects toxins into another organism, typically through a bite or a sting. Spiders are venomous creatures that produce and inject venom using specialized fangs.
Spider venom is a complex mixture of chemical substances, including proteins, peptides, and enzymes. It is designed primarily to subdue prey. These venoms often contain neurotoxins, which affect the nervous system, or cytotoxins, which cause tissue damage.
The Vast Majority of Spiders
The vast majority of the over 47,000 described spider species worldwide pose no threat to humans. While almost all spiders are venomous, their venom is typically designed for insects and is harmless to larger mammals, including humans. Many species have fangs too small or fragile to effectively penetrate human skin.
Even if a spider’s fangs can puncture the skin, the quantity of venom injected or its specific composition may not be potent enough to cause more than localized, mild symptoms. Most spider bites are less significant than a bee sting, resulting in only minor irritation like redness, mild pain, and swelling. Spiders generally are not aggressive and bite humans only when they feel threatened, such as when accidentally trapped against the skin.
Spiders of Medical Significance
Globally, only a very small percentage of spider species are considered medically significant to humans, meaning their bites can cause more than minor, localized reactions. In North America, the black widow (genus Latrodectus) and brown recluse (genus Loxosceles reclusa) spiders are the most commonly recognized species with medically significant venom. Black widow spiders, identified by a red hourglass marking on their abdomen, produce a neurotoxic venom that can lead to severe muscle pain, cramping, nausea, and sweating, a condition known as latrodectism. Symptoms typically appear within hours of the bite and can include widespread muscle rigidity and abdominal pain.
Brown recluse spiders, often recognized by a violin-shaped marking on their back, possess a cytotoxic venom that can cause tissue damage. Their bites may initially be painless but can develop into a blister or open sore, sometimes with a “bull’s-eye” appearance, and in rare cases, lead to a larger skin ulcer. Outside of North America, the Australian funnel-web spiders (genus Atrax), particularly the male Sydney funnel-web spider (Atrax robustus), are known for their highly toxic, fast-acting neurotoxic venom that can be dangerous to humans.
What to Do After a Spider Bite
For most minor spider bites, initial first aid involves cleaning the bite area with mild soap and water. Apply a cool compress or ice pack to the site for about 15 minutes each hour to reduce pain and swelling. Elevate the affected area, if possible, to aid in reducing swelling. Over-the-counter pain relievers can manage discomfort; antihistamines or calamine lotion may help with itching.
Seek medical attention if symptoms worsen or if the biting spider is suspected to be medically significant. Indicators for medical evaluation include severe or spreading pain, a growing wound, or systemic symptoms like fever, chills, body aches, nausea, vomiting, or difficulty breathing. If the spider was identified as a black widow, brown recluse, or funnel-web spider, prompt medical care is advisable.