Australian funnel-web spiders (Atrax and Hadronyche spp.) are the most venomous spiders in the world, but the few species that have been associated with severe envenoming only occur in a small region of Eastern Australia. Funnel-web spiders are 3 to 5 cm black, primitive (mygalomorph) spiders. There are about five severe envenomings each year and only 13 fatalities have been reported, all prior to antivenom becoming available in 1981.1 Most bites occur from the male spiders when they come out in late Spring and Summer looking for females. See images at the Australian Museum. There are about 40 species distributed from Tasmania and Southern Australia to Queensland.
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Funnel-web spider venoms contain mostly small peptide neurotoxins that affect ion channels, including the medically important δ-atracotoxins which slow tetrodotoxin (TTX)-sensitive voltage-gated Na+ current inactivation. It is likely that these toxins are responsible for envenoming in humans.2

Severe envenomation has only been reported for six species of funnel-web spiders:
  • Sydney funnel-web spider (A. robustus)
  • Southern tree funnel-web (H. cerberea)
  • Northern tree funnel-web spider (H. formidabilis)
  • Port Macquarie funnel-web spider (Hadronyche sp. 14)
  • Blue Mountains funnel-web spider (H. versuta)
  • Toowoomba or Darling Downs funnel-web spider (H. infensa).1

Clinical Effects
Most funnel-web spider bites do not result in systemic envenomation and cause local pain, local bleeding and fang marks.
Severe envenomation is characterised by the rapid onset of neuromuscular and autonomic excitation. This is associated with pulmonary oedema which and possibly due to the autonomic overstimulation and was the main cause of death in the pre-antivenom era. Autonomic excitation results in a mixture of cholinergic and adrenergic effects, including tachycardia or bradycardia, mydriasis or miosis, hypertension, lacrimation, hypersalivation and diaphoresis. The combination of effects depends on the balance between sympathetic and parasympathetic hyperstimulation, so that a patient may have bradycardia or tachycardia, or mydriasis or miosis. Neuromuscular excitation is characterised by paraesthesia, fasciculations at the bite site, peri-orally and distally. Other effects include non-systemic symptoms (vomiting, headache, abdominal pain), agitation or anxiety and uncommonly cardiac effects likely secondary to catecholaminergic excess (see table).3 Coma, intractable hypotension and multiorgan failure have been reported prior to antivenom being available.

Treatment
First aid for funnel-web spider bites or any bite by a large black spider suspected to be a funnel-web is a pressure bandage with immobilisation. Patients should be observed for 2 to 4 hours and if there is no evidence of systemic envenoming they can be discharged.
Patients with systemic envenoming should be treated with 2 to 4 vials of funnel-web spider antivenom. Antivenom appears to be highly effective although this is based on the rapid reversal of neurotoxicity, the absence of deaths since its introduction and decreased length of hospital stay.1,4 Early antivenom reactions and serum sickness are rare with funnel-web spider antivenom.5 Atropine may be useful for the cholinergic effects, if antivenom is not available.

Table 1: Clinical effects of Australian funnel-web spider envenoming

Clinical Effects
Autonomic excitation including cholinergic and catecholaminergic effects
  • Bradycardia or tachycardia
  • Hypertension
  • Miosis or mydriasis
  • Generalised diaphoresis and piloerection
  • Hypersalivation, lacrimation
Neuromuscular excitation
  • Paraesthesia: bite site, distal and characteristically oral
  • Fasciculations: at the bite site or generalised; characteristically tongue fasciculations
  • Muscle spasms: local or generalised
Non-specific systemic symptoms
  • Abdominal pain, nausea, vomiting, headache
Pulmonary oedema
  • Non-cardiogenic/neurogenic pulmonary oedema
Cardiac Effects
  • Myocardial injury with pulmonary oedema: less common; probably due to catecholaminergic stimulation
Central nervous system effects
  • Agitation or anxiety
  • Coma: rare and usually occur late or in association with severe envenoming
Other effects
  • Hypotension, disseminated intravascular coagulation and multiorgan failure occurred late in life-threatening cases prior to antivenom.

References
1. Isbister, G.K., et al. Funnel-web spider bite: a systematic review of recorded clinical cases. Medical Journal of Australia 182, 407-411 (2005).
2. Nicholson, G.M. & Graudins, A. Spiders of medical importance in the Asia-Pacific: Atracotoxin, latrotoxin and related spider neurotoxins. Clin.Exp.Pharmacol.Physiol 29, 785-794 (2002).
3. Isbister, G.K. & Warner, G. Acute myocardial injury caused by Sydney funnel-web spider (Atrax robustus) envenoming. Anaesthesia and Intensive Care 31, 672-674 (2003).
4. Dieckmann, J., Prebble, J., McDonogh, A., Sara, A. & Fisher, M. Efficacy of funnel-web spider antivenom in human envenomation by Hadronyche species. Medical Journal of Australia 151, 706-707 (1989).
5. Miller, M.K., Whyte, I.M. & Dawson, A.H. Serum sickness from funnelweb spider antivenom. Medical Journal of Australia 171, 54 (1999).