Spider bites

Author: Vanessa Ngan, Staff Writer, 2006.

Spiders belong to the class of mainly terrestrial arthropods known as Arachnida. Medically significant classes of arachnids include spiders, ticks/mites and scorpions. Unfortunately through myths, legends and nowadays media, spiders have gained a reputation of being dangerous and harmful, and in some people instil a psychological fear known as arachnophobia. In reality, very few are dangerous to man and media reports exaggerating the dangers of spider bites are far out of proportion to the actual threat they pose.

What spiders bite and may be harmful to man?

The most important groups of spiders based on the medical consequences if bitten by one are shown in the table below.

Genus nameCommon NameCountry commonly found in where bites reported
Latrodectus spp. Widow spiders
  • Black widow (North America)
  • Katipo (New Zealand)
  • Red-back (Australia)
  • Shoe-button (South Africa)
Found throughout the world and known by many different common names according to country
Loxosceles spp. Violin spiders
Recluse spiders
Brown recluse spiders
Fiddleback spiders
South America, United States, Australia, commonly in the tropics
Tegenaria agrestis Hobo spider United States
Cheiracanthium Yellow sac spider United States
Phoneutria Banana spider Central and South America
Atrax and Hadronyche Australian Funnel-Web spider Australia

The venom produced by spider bites is generally either neurotoxic or cytotoxic. Web dwellers tend to have neurotoxic venom and non-web dwellers cytotoxic venom. Spiders of the Latrodectus genus produce neurotoxic venom, while the violin spider and yellow sac spider produce cytotoxic venom.

What are the signs and symptoms of spider bites?

The signs and symptoms of a spider bite depend on many factors, these include:

The signs and symptoms from a bite from a spider with neurotoxic venom differ to those produced by a spider with cytotoxic venom. The severity of the symptoms depends on the species of spider as the symptoms of bites from different species of Loxosceles can range from unremarkable (requiring no care), localised (usually self-healing), dermonecrotic (slow-healing ulcerated lesion requiring treatment), to systemic (vascular, renal damage and sometimes life-threatening).

Features of neurotoxic venom biteFeatures of cytotoxic venom bite
  • Affects neuromuscular junctions
  • Severe pain in chest and abdomen (cramp-like pains)
  • Breathing difficulties, heart palpitations
  • Nausea and vomiting
  • Sweating, fever, excessive salivation
  • Increased blood pressure
  • Rash may develop
  • Symptoms usually start about 1-3 hours after being bitten
  • More severely affected are children and elderly
  • Affects cellular tissue and usually restricted to area of the bite
  • Initial bite is painless but symptoms develop about 2-8 hours later, area becomes painful and swollen
  • Eventually a blister may form over a necrotic lesions which then sloughs to create an ulcerated wound (up to 10cm)
  • Ulcer will heal over months and leave behind a scar. In extreme cases, skin grafts may be necessary.
  • In severe cases, systemic conditions may occur, e.g. thrombocytopenia, DIC, renal failure

What are the dermatological features of spider bites?

Widow spider bites

The skin around the site of the bite is red and two fang marks may be visible where the skin was penetrated. In untreated cases a rash may develop after several days. Systemic symptoms are of more diagnostic value.

Violin or recluse spider bites

The dermatological features of these spider bites depend on the severity of the bite. In self-healing wounds, the bite site gets no worse than being swollen and red. With more serious bites a “bull's eye” wound may form. This is characterised by a central red swollen blister that is separated from a peripheral bluish region by a white zone of firm swelling. If the bite turns a purplish colour within the first few hours, this usually indicates severe localised tissue death (necrosis) may occur. Over days the blister forms a scab, which hardens and falls off to leave behind an ulcerated depression. Healing can take weeks to months.

Interestingly, it appears that bites that become systemic do not also develop necrotic wounds. It is thought that in necrotic wounds the venom is localised in the tissue whereas in systemic reactions the venom is distributed quickly throughout the body without any localised effects.

Other spider bites

SpiderDermatological features
Hobo Dermonecrotic lesions similar to violin/recluse spider bites
Yellow sac Painful, red, swollen and itchy bite that may produce a slightly necrotic wound that heals without scarring
Banana Few dermatological features, mainly neurotoxic symptoms
Funnel web Sudden onset of neurotoxic symptoms predominate, dermatological features insignificant in comparison
White tail May cause discomfort and redness but does not cause ulceration or any serious symptoms

Do spider bites cause bacterial infection?

Although many people attribute an episode of bacterial infection (especially cellulitis and necrotising fasciitis) to an unseen spider bite, they are falsely blamed. Documented spider bites have not led to skin these infections.

What is the treatment for spider bites?

One of the most important aspects in treating spider bites it to try and identify the offending spider. The venom of spider bites is quite variable hence identification of the spider can be of value in determining the management of the condition.

General measures that should occur after a spider bite include:

Depending on identification of the offending spider and the severity of the bite treatment may include:

Specific treatment for bites from certain spiders include:

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