Despite their reputation and association with causing illness, most species of spiders are harmless to humans. Spider bites are not very frequent, and only a few groups produce venom that is toxic to humans. Read on to learn about the most hazardous species of spiders, the symptoms of their bites, and the recommended treatments.
What Are Spider Bites?
Spider bites are skin injuries caused by the jaws and fangs of spiders. Despite the general perception that spiders are aggressive and dangerous, bites to humans are relatively uncommon when compared to bites and stings by insects [1, 2].
The majority of spider species are clinically irrelevant, as they have the following traits: :
- Short fangs
- Weak jaw muscles
- Low amounts of venom
- Ineffective venom
- Small size
- Widow spiders
- Comb-footed spiders
- Recluse spiders
- Australian funnel-web spiders
- Mouse spiders
- South American armed spiders
Before treating a spider bite, the following procedure must be followed :
- Confirm that the skin injury is caused by a spider bite
- Check for clinical symptoms (local and systemic)
- Collect the spider that caused the bite
- Have the species identified by a specialist
Symptoms of Spider Bites
A meta-analysis of 134 medical spider bite case studies identified 32 different symptoms resulting from a bite. The most common symptoms out of all the species were :
- Skin reddening at the bite site (72% of cases)
- Pain in the bitten body part (49%)
- Swelling of the bite site (34%)
- Discomfort (34%)
The spiders most toxic to humans cause more specific syndromes from bites.
1) Widow Spiders
There are over 40 recognized widow spider (Latrodectus spp.) species distributed throughout Africa, the Americas, Southeast Asia, and Australasia .
Widow spiders are small (3 to 10 mm body length) and dark grey or black, with a red or orange geometric pattern on the back that can resemble an hourglass. The female is darker, larger (up to 13 mm body length), and more venomous than the male .
Red-back spider (Latrodectus hasselti)
The envenoming from widow spider bites is called latrodectism. The main symptoms may vary depending on the species, but include :
- Pain, which is more often defined as local and radiating when caused by the Australian redback spider (L. hasselti). It normally affects the back, stomach, and chest when caused by species from the Americas, Africa, and Europe [11, 12].
- Excessive sweating in unusual patterns (localized to the bite area, below the knees in both legs, and local but only in one side of the body) 
In a study on 68 people bitten by this spider, 1/3 developed non-specific, systemic symptoms including :
- High blood tension
- Prolonged erections
- Patchy paralysis
- Muscle spasm
- Pins and needles
The average duration of the effects was 48 hours, with all cases resolved within a week .
In very rare cases, envenoming by widow spiders has caused rhabdomyolysis, a condition in which damaged muscles break down rapidly .
2) Comb-Footed Spiders
Comb-footed spiders (Steatoda spp.) belong to the same family as widow spiders (Theridiidae). They are distributed throughout Australia, South Africa, the Americas, and Europe .
Comb-footed spiders have a similar shape to widow spiders (and are sometimes mistaken for them), but their color is uniformly dark brown to black.
False black widow (Steatoda grossa)
The clinical syndrome caused by comb-footed spider venom is called steatodism. A study of 23 bite cases demonstrated that its symptoms are similar to those of latrodectism, but milder and without local sweating. They included [18, 17]:
The effects normally disappeared within 6 hours .
Five bite cases by spiders from the genus Achaearanea (also belonging to the Theridiidae family) caused similar symptoms to those of the widow and comb-footed spiders. This suggests that the venoms of these three groups contain similar toxins .
3) Recluse Spiders
Most species of recluse spiders (Loxosceles spp.) are found in South America. Additionally, these spiders are in Europe, southern Africa, parts of Asia, the south of Australia, and the US (especially in the central South) [19, 20, 21].
The main clinically relevant species are L. laeta (distributed throughout most of the South American continent), L. intermedia (found in Brazil and Argentina), and L. gaucho (restricted to Brazil) .
Recluse spiders are nocturnal and prefer dry, dark places under rocks, wood, or tree barks. They often invade houses, where their flat body helps them hide in clothes, bed sheets, and cupboards .
Brown recluse spider (Loxosceles reclusa)
Their venom contains phospholipase-D proteins and hyaluronidase as key components, triggering the following reactions that lead to skin necrosis :
- Release of inflammatory molecules (cytokines and prostaglandins) 
- Breakdown of red blood cells [27, 28]
- Platelet aggregation 
- Growth of the wound 
- Redness, swelling, and pain at the bite site
- Appearance of a piece of dead tissue (necrotic eschar) that lasts 2 to 3 weeks
- Formation of an ulcer that takes weeks or months to heal after eschar detachment
Progression of skin necrosis caused by a recluse spider bite. Pictures were taken at days: 1 (A), 9 (B), 16 (C), and 25 (D) .
In about 50% of the cases, systemic symptoms also occur within the first 48 hours. The most common ones are :
- Bleeding disorders
- Kidney failure
- Multiple organ damage
4) Australian Funnel-Web Spiders
Australian funnel-web spiders (Atrax and Hadronyche spp.) are the most venomous spiders in the world. They are distributed throughout the east of Australia, with the most venomous species living between southern New Wales and southern Queensland [35, 36].
Australian funnel-web spiders are medium to large (10 to 50 mm body length) spiders with prominent jaws and parallel fangs that live in logs, rocks, and leaf litter. They are nocturnal and very aggressive .
Sydney funnel-web spider (Atrax robustus)
Their venom contains δ-atracotoxins. These toxins target sodium channels on the surface of nerve cells and cause the excessive release and ultimate exhaustion of messenger molecules (such as hormones and neurotransmitters) .
Severe envenoming (sometimes referred to as atraxism) is rare, with only 5 to 10 cases per year (representing 10 to 75% of confirmed bites). It is considered an important medical condition because its effects are rapid and life-threatening. An effective antivenom is available .
When envenoming does not occur, there are still prevalent effects from the bite, including: :
- Severe pain at the bite site that may last 30 to 60 minutes
- Puncture marks
In some cases, the envenoming is only mild and causes the following systemic symptoms :
- Pins and needles
- Muscle twitches
The most frequent symptoms of severe envenoming found in a study of 8 different cases were :
- Excessive sweating (78% of cases)
- High blood tension (75%)
- Accelerated heart rate (59%)
- Fluid accumulation in the lungs (54%)
- Agitation (47%)
- Salivation (44%)
- Vomiting (41%)
- Tingling tongue (17%)
- Goosebumps (12%)
- Headache (10%)
- Low blood pressure (10%)
- Low heart rate (10%)
Severe envenoming is especially critical to children, who may die within 1 to 2 hours if left untreated .
In untreated adults, severe envenoming may result in a progressive, irreversible decrease of blood pressure, profound coma, continuous muscle spasm, and eventual death .
5) Mouse Spiders
Mouse spiders (Missulena spp.) are distributed throughout mainland Australia and often mistaken for funnel-web spiders due to their similar appearance .
Eastern mouse spider (Missulena bradleyi)
Their venom is similar to that of funnel web spiders and is treated with the same antivenom. Despite the similarities in venom, these spiders are significantly less dangerous to humans .
In a systematic review of 40 different confirmed bite cases, the effects were generally minor and included :
- Pins and needles
- Excessive sweating
Severe envenoming similar to that caused by funnel-web spiders occurred in only one case. This affected a 19-month-old child who developed high blood pressure, muscle spasm, arched back, and unconsciousness. The child responded well to funnel-web spider antivenom .
6) South American Armed Spiders
Armed spiders are large (30 to 50 mm body length), solitary, nocturnal spiders that construct no web and catch their prey by active hunting during long wanders. They are sometimes transported in banana boxes (and thus commonly referred to as ‘banana spiders’), and occasionally enter houses .
Their characteristic defensive behavior consists of raising 4 front legs, showing the fangs, bristling their leg spines, and rotating to continuously face the threat .
Brazilian wandering spider (Phoneutria nigriventer) in a defensive position
Armed spiders, especially Phoneutria nigriventer, cause thousands of bites in Brazil every year, mainly during their mating season (March and April) .
Their venom contains a mixture of proteins called phoneutriatoxins that block ion channels on nerve cells, thus preventing both the transmission of nerve signals and the release of messenger molecules .
In an observational study on 422 people bitten by armed spiders, 90% had mild envenoming. The most common symptoms were :
- Local, possibly radiating pain
- Excessive sweating at the bite site
- Redness at the bite site
Systemic, less common effects included :
- Accelerated heart rate
- High blood pressure
- Visual disturbances
- Prolonged erections
- Fluid buildup in the lungs
- Death (very rare)
Causes of Spider Bites
The target organisms of spiders are almost universally invertebrates. Human bites generally occur by accidental contact or when the spider is disturbed .
Most widow spider bites occur in and around dark, dry areas of the house when people put on clothing attire where a spider was hidden. The accidental compression of the spider between the fingers and external objects is another frequent cause of bite. In rural areas, widow spider bites are an occupational hazard of farm workers [11, 3].
Most bites by Australian funnel-web spiders occur when the highly venomous males (6-fold more than females) leave their burrows in search of females, which often brings them into houses .
South American armed spiders mostly bite when they feel threatened after being found in shoes, piles of sticks or rubbish, and construction material .
Risk Factors for Spider Bites
Children are more prone to severe envenoming by widow spiders and must be treated with antivenom up to 90 hours after the bite occurs .
In pregnant women, the symptoms can include :
- Stomach pain
- High blood pressure
- High protein levels in urine
Treatment with antivenom is required in these cases.
A pregnant woman bitten by a widow spider had signs of premature labor (regular, strong contractions), but the symptoms were resolved with antivenom.
In an observational study on 267 people envenomed by recluse spiders, 17 developed kidney failure. All of the people who died (4) were children under 14 years old. In another study, 6 adolescents bitten by recluse spiders developed anemia [32, 53].
Envenoming by both Australian funnel-web and mouse spiders is particularly life-threatening in children. Bites from funnel-web spiders are more dangerous on the torso than on the limbs, since pressure bandages cannot be applied [4, 42, 54].
Envenoming by armed spiders is especially dangerous to children and elderly patients .
Prevention of Spider Bites
The best prevention measure is to avoid contact with clinically relevant spiders .
The probability of human-spider encounters is higher during daytime outdoor activities in the spring and summer months. Bites may be prevented by wearing gloves, long-sleeved shirts, and long pants tucked into socks, especially when gathering firewood and clearing brush .
Chemical control may provide additional protection against spider bites. Pyrethroid pesticides are toxic to recluse spiders and more effective than commercial repellents for loxoscelism prevention. They are used to spray spiders, clothes, and outdoor or indoor surfaces [56, 57].
Indoor bites may be prevented by properly insulating homes, removing spider webs, and applying safe indoor insecticides .
It is important to note that flicking spiders with a finger is safer than crushing them against the skin, which by reflex causes the jaws to open with the fangs in biting position .
Genes Linked to Spider Bites
Neurexins are proteins on the surface of nerve cells that serve as receptors for the widow spider toxin and other substances. One variant (rs8019381) of the neurexin gene (NRXN3) is associated with decreased abundance of the protein. As a result, this genotype may reduce the effects of the toxin on nerve cells [58, 59].
The other group of receptors for widow spider toxins are latrophilins. A couple of variants (rs1397548 and rs2305339) of one of the latrophilin genes (LPHN3) generate shorter proteins.
The toxins of funnel-web spiders bind to the site-3 receptor of sodium channels. Mutations in the amino acids identified as critical for toxin binding (Glu1613, Glu1616, and Lys1617) may prevent or reduce envenoming [62, 63].
One of the armed spider’s toxins blocks calcium channels on nerve cells by binding to their ω-conotoxin binding site. Mutations within this region may lead to altered sensitivity to the spider toxin [64, 65].
Treatments for Spider Bites
If you suspect that you have been bitten by one of the spiders described in this article, speak with your doctor immediately so that he or she can diagnose and treat any conditions caused by the bite.
Local care including wound cleansing, ice pack application, painkillers, drugs that act against muscle spasm and stiffness (e.g., benzodiazepines), and tetanus shots, are recommended in most cases .
In people with suspected envenoming by Australian funnel-web spiders, a pressure bandage with immobilization must be applied until the patient is transferred to a hospital with antivenom .
Antivenoms are employed to treat envenoming syndromes, especially when they involve dangerous species (e.g., Australian funnel-web spiders) and/or high-risk patients (children, pregnant women, and elderly patients) .
1) Widow Spider Antivenom
The use of antivenom against the widow spider toxin varies in some countries depending on its availability, reported successful case studies, and perceived risk of adverse reactions. While it is extensively employed in Australia, the fear of allergic reactions restricts its use in the US [12, 66].
In 4 studies on over 200 people, widow spider antivenom was regarded as generally safe since it causes mild to moderate allergic reactions in only 5% of patients (including 1 to 2% of anaphylaxis) and serum sickness in 10% [67, 68, 69, 70].
In contrast, another trial on 31 people showed an increased efficiency of antivenom over placebo, especially when taken through the veins .
Additionally, a pilot trial on 24 people recorded a faster relieving effect of antivenom over placebo, which resulted in similar degrees of overall pain reduction by the end of both treatments .
The treatment of envenoming by comb-footed spiders is normally symptomatic with painkillers, but the injection of widow spider antivenom has successfully resolved the symptoms in some severe cases [71, 17].
2) Recluse Spider Antivenom
Antivenom against recluse spider toxins is available in Brazil (where it is most extensively used), Argentina, Peru, and Mexico .
In Brazil, antivenom injection is recommended within the first 72 hours in cases with extensive skin necrosis or systemic reactions. The treatment is normally combined with corticosteroids .
3) Australian Funnel-Web Spider Antivenom
Funnel-web spider antivenom is a rabbit-derived antibody against the venom of the Sydney funnel-web spider (Atrax robustus). It neutralizes the toxins of 10 different species from this group.
Patients bitten by big black spiders in eastern Australia must always be treated as potentially envenomed patients. This is because less harmful species like mouse spiders or trapdoor spiders can be mistaken for funnel-web spiders. After 2 to 4 hours in observation, patients can be discharged if they do not show symptoms of envenoming .
The funnel-web spider antivenom also neutralizes the venom of mouse spiders and efficiently resolved the symptoms of severe envenoming in a 19-month-old child bitten by an eastern mouse spider [41, 16].
4) South American Armed Spider Antivenom
Antivenom against armed spider toxins is reserved for moderate-to-severe envenoming. In a Brazilian observational study on over 400 people with confirmed bites, only 10 (2.3 %) were given the antivenom .
The antivenom, which is also used against recluse spiders bites and scorpion stings, resolves the envenoming symptoms within 24 hours .