What is Spider Bite

Spider Bite Spider bites can affect anyone. It is a clinical entity typically exaggerated in movies and the media. Although spider bites can be common depending on the geographical distribution of different spiders, they are generally benign and harmless.
Spider bites can affect many sites. Generally reactions to spider bites can be divided into 3 types:

  • local reactions: this means reactions that occur at bite sites
  • systemic reactions: this means reactions that happen widespread in the body
  • allergic reactions: this means exaggerated reactions that do not happen in normal individuals.

As described above, local reactions usually happen on the skin, while the latter two can affect any organ systems.

Statistics on Spider Bite

Spider bites are generally considered to be uncommon. However, in Australia, it is the single commonest reason for inquiries to the Victorian Poisons Information Centre, with more than 1300 calls recorded in 1997.

Risk Factors for Spider Bite

Predisposing factors for spider bites depend on the natural habitats of different species of spiders. For example, the most significant spider bites in Australia are caused by the Sydney funnel web spiders (Atrax robustus). They live in moist places, and can be distributed in up to 160km within the radius of Sydney.
The wide distributions of other spiders will not be discussed here.

Progression of Spider Bite

The local or systemic reaction can only be attributed to a spider bite after the criteria are met:

  • the spider has to be seen during the bite
  • the spider has to be recovered, collected and sent for identification by expert
  • other diagnoses such as vessel inflammation, infection, allergic reaction other than due to spider venom, anxiety and panic attack must be ruled out.

Because these criteria are rarely met, the ‘true’ diagnoses of spider bites can be few in between.
The different types of reactions can be described as below:

  • local reactions: after the bite, the typical lesion is characterised by fang markings (leaving 1-2 separate ports of entry). Within minutes, local inflammation occurs, leading to local bright red tender lump with subsequent hardening. Sometimes it can be accompanied by adjacent red plaques. This localised swelling usually lasts for 7-10 days. The symptoms of the localised lesion can be variable. Some may be severely painful, and some can go painless and unnoticed. Otherwise, a more severe reaction called necrotizing local reactions can occur due to spider bites from recluse or fiddleback spiders. When this occur, blistering can occur, and further complicated by formation of an ulcer with a crater. Tissue death occurs in the middle of the crater, and it can take several months to heal with scar formation. This whole series of event are sometimes termed ‘necrotising arachnidism’.
  • Systemic reactions: sometimes during the spider bites, venom components can be injected into the body in sufficient amounts. When this occurs, the majority of the venom travels through the circulating lymphatics. Therefore, a non-specific generalised systemic inflammation can occur, leading to symptoms such as fever, muscle pain, fatigue, enlarged nodes, etc. Other systemic effects are specific to the venoms produced by specific species – for example, the brown recluse spider (Loxosceles reclusa) can cause severe red blood cell breakdown. The notorious black widow spiders (Latrodectus sp) can cause a syndrome characterised by severe muscle spasms,
    nausea and vomiting. In both of these syndromes, death can occur but they are rare.
  • Allergic reactions: aside from specific syndromes caused by certain spider species (see systemic reactions), the most dangerous effect is allergic reaction either due to direct spider bite or contact with spider hair. This happens in tarantulas. The symptoms can range from mild hives to anaphylactic reactions. The latter can be life-threatening and is considered a medical emergency.


How is Spider Bite Diagnosed?

If the reactions are severe enough, basic investigations may be done to rule out other medical conditions. These include looking at the blood function, liver function, kidney function and blood electrolytes.
In some spider bites causing clotting disturbances, measurement of clotting profile can be done.

Prognosis of Spider Bite

Although mystified in movies and media, spider bites are generally benign. Although necrotising arachnidism can occur and lead to significant morbidity, it rarely cause death. The death rate caused by black widow spider bites is less than 1%. Anaphylaxis is always life-threatening but it happens very rarely.

How is Spider Bite Treated?

Treatment is symptomatic. Below are the different treatment options depending on the types of reactions:

  • local reactions: cleaning of the bite site with water and soap, applying dry ice to reduce inflammation, reassurance of the patient, and observe the patients for certain period for signs of systemic reactions. Pressure immobilization (eg splinting the affected limb) slows lymphatic spread of venom. Also, tetanus shot can be given. Necrotic lesions (necrotising arachnidism) – this means tissue death due to spider bites – can be treated with delicate washing, surgical removal of dead tissue, tetanus shot, pain killers, sterile dressing, rest and close follow up.
  • Systemic reactions: This involves treating the target organ damage, including maintaining adequate fluid status, and blood transfusion for extensive blood cell breakdown. Other measures include sufficient pain relief and calcium gluconate injection for relief of muscular pain.
  • Allergic reactions: this is treated as in any other type I hypersensitivity reaction, including anti-histamines, steroids for late phase reactions and adrenaline for life-threatening reactions like angioedema and anaphylaxis.

Muscle spasms due to black widow bites can be treated with antivenom. Antivenoms are used to counteract the action of spider venom. It is important to rule out previous antivenom administration (either to spider bites or snake bites) because of the possibility of reactions towards these. Side effects include fever, joint pain, muscle pain, lymph node enlargement and skin rash development.

Spider Bite References

  1. Anderson, PC. Spider bites in the United States. Dermatol Clin 1997; 15:307.
  2. Australian Family Physician: The funnel web and common spider bites [online]. 2004. [Cited 2005 October 20th]. Available from: URL: http://www.racgp.org.au/afp/downloads/pdf/april2004/20040413nimo.pdf
  3. Majeski, J. Necrotizing fasciitis developing from a brown recluse spider bite. Am Surg 2001; 67:188.
  4. Up to Date: Spider bites [online]. 2005. [Cited 2005 October 20th]. Available from: URL: http://www.utdol.com/application/topic.asp?file=allergy/2888&type=A&selectedTitle=1~6
  5. Victorian Poisons Information Centre Annual Report 1997. Melbourne: Royal Children’s Hospital, 1998.
  6. Wong, RC, Hughes, SE, Voorhees, JJ. Spider bites. Arch Dermatol 1987; 123:98.