Latrodectism is the most important spider envenoming syndrome worldwide and results from widow spider bites. Widow spiders occur on most continents and there are about 30 different species.
Redback Spider Envenoming
The redback spider (Latrodectus hasselti) is a widow spider and is easily recognised by its red markings on its dorsum (Figure 1). Redback spider envenoming or latrodectism is the commonest medically important envenoming syndrome in Australia.1, 2

Figure 1: Australian Redback spider (Latrodectus hasselti)

RedBackSep2002 027.JPG

Redback spider envenoming is characterised by pain that may be local, radiating or regional depending on the site of the bite. Pain is associated with non-specific systemic effects and less commonly unusual autonomic or neurological effects.
Table 1: Clinical effects of redback spider envenoming
Pain:
  • Bite site pain which develops over about an hours and may continue for hours to days;
  • Pain radiating from the bite site up the bitten limb;
  • Regional pain: abdominal, back or chest pain; bilateral foot or lower limb pain.
Diaphoresis:
  • Sweating at the bite site is a classic finding in redback spider bite
  • Regional diaphoresis in unusual distributions, such as bilateral below knee diaphoresis.
Bite site:
  • Local redness may occur uncommonly and there is often little evidence of the bite.
  • Fang marks are uncommon (<5%)
  • Piloerection can occur
Systemic symptoms:
  • Nausea and vomiting
  • Headache
  • Malaise and lethargy
Uncommon effects:
  • Irritability and agitation (often in children)
  • Hypertension, tachycardia, fever
  • Neurological: Muscle spasms, paraesthesia, patchy paralysis
  • Priapism
Initial treatment is with analgesia and the currently recommended approach is a combination of paracetamol, non-steroidal anti-inflammatories and oral opiates:
  1. Paracetamol 1g every 4 to 6 hours up to a daily maximum of 4g; 10mg/kg in children;
WITH
  1. Ibuprofen, initial dose of 800mg thence 200 to 400mg every 8 hours
AND
  1. Oxycodone 5 to 10mg every 4 hours or 0.1 to 0.2mg/kg in children.
If the pain does not respond to oral analgesia then titrate intravenous opiate analgesia may be required, such as morphine 2.5 to 5mg initially and then titrated every 5 to 10 minutes; 0.1 to 0.2 mg/kg in children.
The use of antivenom has been controversial until a recent large negative study. Two earlier randomised controlled trials comparing intramuscular versus intravenous antivenom showed no difference in clinical effect.3, 4 In one of these trials antivenom was measured in a subgroup of patients’ blood and only intravenous antivenom was detected after administration.5 This suggested that antivenom may not be effective via either route of administration. A recent randomised controlled trial of antivenom versus placebo in 224 patients found no difference in the improvement in pain or resolution in systemic effects. Immediate systemic hypersensitivity reactions occurred in about 4% of patients in all trials. Therefore current evidence would not support the use of antivenom.

References
1. Isbister GK, Fan HW. Spider bite. Lancet. 2011; 378(9808): 2039-47.
2. Isbister GK, Gray MR. Latrodectism: a prospective cohort study of bites by formally identified redback spiders. MedJ Aust. 2003; 179(2): 88-91.
3. Isbister GK, Brown SG, Miller M, Tankel A, MacDonald E, Stokes B, Ellis R, Nagree Y, Wilkes GJ, James R, Short A, Holdgate A. A randomised controlled trial of intramuscular vs. intravenous antivenom for latrodectism--the RAVE study. QJM. 2008; 101(7): 557-65.
4. Ellis RM, Sprivulis PC, Jelinek GA, Banham ND, Wood SV, Wilkes GJ, Siegmund A, Roberts BL. A double-blind, randomized trial of intravenous versus intramuscular antivenom for Red-back spider envenoming. EmergMedAustralas. 2005; 17(2): 152-6.
5. Isbister GK, O'Leary M, Miller M, Brown SG, Ramasamy S, James R, Schneider JS. A comparison of serum antivenom concentrations after intravenous and intramuscular administration of redback (widow) spider antivenom. British Journal of Clinical Pharmacology. 2008; 65(1): 139-43.