Redback Spider Envenoming

Redback spider bite is the most common envenoming in Australia with 5000–10,000 human bites occurring annually. Clinical features can be distressing and refractory to symptomatic treatment but not life‑threatening. Antivenom is curative.

• Latrodectus hasselti: Redback spider, Australia
• Latrodectus katipo: Katipo spider, New Zealand

Toxin

Venom of the Latrodectus genus contains Alpha‑latrotoxin. This toxin acts presynaptically to open cation channels (including calcium channels) and stimulate the release of multiple motor end‑plate neurotransmitters.

Clinical presentation and course

• Redback spider bites are not initially painful
• Intense local pain develops 5–10 minutes after the bite and is followed by sweating and piloerection within an hour. Puncture marks are not always evident and erythema, if present, is usually mild
• Systemic envenoming occurs in a significant minority of patients. Pain typically radiates proximally from the bite site to become regional then general (e.g. pelvic, back, abdominal, chest or shoulder pain). Autonomic features include severe sweating which may be regional (e.g. both legs) or generalised, mild hypertension and tachycardia
• Non‑specific features of envenoming include headache, nausea, vomiting and dysphoria
• Untreated, systemic envenoming may follow a fluctuating course lasting 1–4 days. Rarely, patients may feel unwell for up to a week. Very rarely, untreated patients report on‑going local symptoms that last weeks or months.

Management

• Reassure the patient, apply an ice pack and give simple oral analgesia such as paracetamol
• Do not apply a pressure immobilisation bandage (PIB)
• Refer to hospital if the patient has local symptoms refractory to simple analgesia, clinical features of systemic envenoming, or the diagnosis is in doubt

Resuscitation and supportive care

• Redback spider envenoming is not life‑threatening and resuscitation is rarely required

Antivenom

CSL Redback Spider Antivenom is the definitive treatment of envenoming by spiders of the Latrodectus genus.

• Give an initial two ampoules (2 × 500 units) IV or IM to all patients with systemic latrodectism or local symptoms unrelieved by simpleanalgesia
• Give further doses of two ampoules every 2 hours until symptoms are relieved.

Investigations

• Laboratory investigations do not assist in diagnosis or management.

Differential diagnosis

• Funnel‑web spider envenoming is potentially lethal. It is associated with immediate bite site pain, visible fang marks and the abrupt onset, within minutes, of a severe syndrome characterised by sweating, agitation, piloerection, cardiovascular and neurologicalchanges
• Envenoming by Steotoda species (cupboard spider or brown house spider) closely resembles redback spider envenoming.
• Non‑specific spider bites are associated with bite site pain and mild systemic symptoms, such as nausea, headache, malaise or vomiting. Significant cardiovascular, autonomic or neurological features do not occur
• Latrodectism has been mistaken for conditions such as acute surgical abdomen, acute myocardial infarction and thoracic aortic dissection.

Disposition and follow up

• Patients without clinical features of systemic envenoming or local pain do not require referral for medical evaluation
• Envenomed patients treated with antivenom are discharged when asymptomatic and advised to return if symptoms recur.

Handy tips

• The triad of local pain, sweating and piloerection increasing over the first hour are pathognomic of redback spider bite
• Symmetrical profuse sweating and pain in both legs following a bite to one lower limb is also characteristic of latrodectism
• Consider the diagnosis in any child with abrupt onset of inconsolable crying, acute abdomen or priapism.

Pitfalls

• Failure to consider the diagnosis when spider bite is not witnessed
• Failure to administer antivenom to children or pregnant women when indicated.

References

• Isbister GI, Gray MR. Lactrodectism: A prospective cohort study of bites by formally identified Redback spiders. Medical Journal of Australia 2003; 179:88–91
• Isbister GI, Sibbritt D. Developing a decision tree algorithm for the diagnosis of suspected spider bites. Emergency Medicine Australasia 2004; 16:161–166
• Hensley J. Has the bell tolled for redback antivenom? EBM Gone Wild
• Nickson C. Does Antivenom Work? CCC
• Long N. Redback Spider toxinology. LITFL
• Nickson C. Bitten by a Redback Spider. LITFL
• Nickson C. Is this a RedBack I see before me? LITFL