Spider Bite

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What is 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.

Who gets 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.

Predisposing Factors

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

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.

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