Clinical Notes : Dermatology

120. Spider Bites

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.

Which spiders bite and may be harmful to man?

Latrodectus spp (widow spiders) are found throughout the world and known by many different common names according to country.

  • Black widow (North America)

  • Katipo (New Zealand)

  • Red-back (Australia)

  • Shoe-button (South Africa)


Loxosceles spp are found in South America, United States, Australia, and commonly in the tropics.

  • Violin spiders

  • Recluse spiders

  • Brown recluse spiders

  • Fiddleback spiders


Tegenaria agrestis (hobo spider) and Cheiracanthium (yellow sac spider) are found in the United States.

Phoneutria (banana spider) is found in Central and South America.

Atrax and Hadronyche (funnel-web spider) is found in 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 in the Loxosceles genus produce cytotoxic venom.



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

  • Neurotoxic or cytotoxic venom

  • Amount of venom injected

  • Health of the patient (e.g. any allergies)

  • Age of the patient (small children and elderly are more adversely affected)

  • Site of the bite

Features of neurotoxic 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

Features of cytotoxic venom bite :

  • 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


  • Widow spider bite

    • 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 bite

    • 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.

  • 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 infections.​​


Necrotic spider bite

Day 1

Day 2

Day 3

Management :


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:​​

  • Wash the area well with soap and water

  • Apply a cold flannel or ice pack wrapped in cloth to the site

  • Give paracetamol for pain

  • Seek immediate emergency care for further treatment

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

  • Muscle relaxants
  • Stronger pain relievers

  • Antihistamines to reduce swelling

  • Antibiotics for confirmed secondary infection

  • Supportive care

  • Antivenin


Specific treatment for bites from certain spiders include:

    • Antivenin is available for bites by venomous spiders of the Latrodectus and Loxosceles genera and is very effective if given soon after the bite.

    • Hydroxyzine (antihistamine) may be given to alter the necrotic lesion of bites from spiders of the Loxosceles genus.

    • It is possible that systemic steroids may be of benefit.

    • Intravenous calcium gluconate alternating with methocarbamol to relieve muscle cramps caused by spiders belonging to the Latrodectus genus (e.g. black widow) (controversial).


Vanessa Ngan 

DermNet NZ, 2006

Accessed June 2018


Ireland notes.png

there are over 370 species of spiders in Ireland - and all of them can bite.

very few of these species have enough venom that can cause harm to a human.


Missing sector orb spider

Also known as Zygiella x-notata, this spider is named because it spins an orb web with one full sector missing.

It is relatively small and is common around Ireland's houses and gardens.

This spider can be distinguished by its pale body and legs, with silvery-grey markings on its abdomen.

Usually seen indoors in the autumn and winter months, it prefers warmth.

How big are they ? : Up to 15mm

Are they harmful ? : No


Giant house spider

This relatively large spider is most common in the autumn months when the males leave their webs in search of females.

Often found in the bath, they can run extremely fast, but only for a limited length of time before they have to stop to recover from their exhaustion.

They build sheet like webs and may be found in garages, sheds, attics and cavity walls where they are less likely to be disturbed.

Giant house spiders do possess a potent venom and can bite, but they do not usually pose a threat to humans.

How big are they ? :120mm

Are they harmful ?:  Potentially, yes - but they're not at all aggressive


Daddy long legs spider

These spiders have small grey bodies and long, thin legs.

Although they can vary in size, they can potentially measure up to 45mm.

Urban myths exist that suggest the daddy long legs spider contains the most potent venom but that their fangs aren't strong enough to penetrate human skin.

Reports on research into this theory suggests that the spiders can bite - but the venom will only deliver a brief mild burning sensation - if anything at all.

How big are they ? : Up to 45mm

Are they harmful ? : No


Lace web spider

Usually found on outdoor walls and fencing, these spiders will retreat inside in the autumn months to find a mate.

Heavy rainfall can also force these spiders into the house, usually because they have been flooded out of their own home.

They generally grow to a size of around 20mm and are brown with yellow markings on the abdomen.

Be on your guard when you see one of these spiders, as they have been known to bite people in recent years.

Bites are reported to be painful but the symptoms usually just consist of localised swelling for around 12 hours.

How big are they ? : 20mm

Are they harmful ? : Yes - if they bite, you'll know about it


Zebra jumping spider

These eight-legged spiders are small, reaching a size of just 8mm.

Recognisable from their distinctive large eyes, white and black markings, and their confident jerky 'stop, start' motion.

They are usually seen from spring through to autumn and can be found on external walls, as well as indoors where they will enter through open doors and windows.

Unlike most other spiders, Zebra jumping spiders seem to exibit intelligent and flexible behaviour.

They rely on their excellent vision for hunting and navigation.

They are inquisitive and readily enter houses.

They are more likely to flee from humans than attack them, but they can bite - although the venom is not considered medically threatening.

How big are they ? : 8mm

Are they harmful ? : No


False widow spider

Ireland's most venomous spider.

The species, also known as Steatoda nobilis, or rabbit hutch spider, usually has an overall size of 20mm and is characterised by a dark brown colour and a bulbous abdomen.

Adult female false widow spiders are known to have bitten humans, although they are not usually aggressive and attacks on people are rare.

Adult males commonly wander through homes in search of mates, but they are also very unlikely to attack people or pets.

There are no reported ROI or UK deaths.

Symptoms of a bite can range from a numb sensation to severe swelling and discomfort.

A condition known as steatodism may follow a bit, the symptoms of which include generalised malaise for several days and local skin blistering.

In serious cases there can be various levels of burning or chest pains, which will depend on the amount of venom injected.

How big are they ? : 20mm

Are they harmful ? : Yes

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bite black widow 2.jpg
bite black widow.jpg

False black widow spider

initial bite

False black widow spider

evolving reaction to bite

False black widow spider

healing bite

Cardinal spider

The cardinal spider is the largest spider in the ROI and UK, growing to an overall length of 14cm in some cases.

Also known as Tegenaria parietina, it is known as the cardinal spider in Britain because of the legend that Cardinal Thomas Woolsey was terrified by this species at Hampton Court back in the 16th century.

Although they are mainly thought to be harmless to humans, these arachnids get a bad reputation because of their huge size, incredible speed and their nocturnal habits.

Bites from these spiders are rare, and painless.

How big are they ? : 14cm

Are they harmful ? : No


Money spider

From the biggest to the smallest, money spiders grow no more than 5mm long, with their leg span just 2mm.

They get their name from an old superstition that if one got stuck in your hair, it would bring you good luck and increased wealth.

The money spider weaves hammock shaped webs and bites its prey to paralyse it -  before wrapping it in silk and eating it.

The fangs on this spider are not anywhere near big enough to penetrate human skin.

How big are they ?  2mm

Are they harmful ? : No


Cupboard spider

Closely related to the false black widow, the Steatoda grossa is often mistaken because of its dark colour and similarly bulbous abdomen.

It usually grows to approximately 10mm in length and its appearance can vary slightly from purple to brown to black.

The female can lay egg sacs at least three times a year which typically contain between 40 - 100 eggs.

They have been known to bite humans, but are not usually agressive.

Although injuries are minor, symptoms can include blistering and generally feeling unwell - which can last a couple of days.

How big are they ? : Up to 12mm

Are they harmful ? :  A bit - they don't bite often, but when they do it hurts


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