Funnel Web Spiders
Funnel-web spiders (Hexathelidae, Atracinae: Atrax and Hadronyche species) are the most dangerous spiders in Australia. Severe envenoming has only been reported from southern Queensland to southern New South Wales, but it is rare (5-10 cases annually requiring antivenom). However, funnel-web spider envenoming is an important clinical condition because of the life-threatening effects, rapid onset and the availability of effective antivenom.
Funnel-web spider bites cause immediate local pain, and usually puncture marks and local bleeding. In many cases this is the only effect because severe envenoming develops in only a proportion of cases. In some cases mild envenoming occurs with local neurotoxicity (paraesthesia, numbness or fasciculations) and/or non-specific systemic effects. Severe envenoming is characterised by:
- autonomic excitation – generalised diaphoresis, hypersalivation, lacrimation, piloerection, hypertension, bradycardia or tachycardia, miosis or mydriasis
- neuromuscular excitation – paraesthesia (local, distal and oral), fasciculations (local or generalised, commonly tongue fasciculations), muscle spasms
- non-specific systemic effects – abdominal pain, nausea, vomiting, headache
- pulmonary oedema and less commonly myocardial injury
- central nervous effects – agitation/anxiety, and less commonly drowsiness or coma.
Severe envenoming has been reported for six species, including the Sydney funnel-web spider (Atrax robustus), the southern tree funnel-web spider (Hadronyche cerberea) and northern tree funnel-web spider (Hadronyche formidabilis). The Sydney funnel-web spider causes severe envenoming in 17% of cases, but the two tree funnel-web spiders cause severe envenoming in over half of cases.
First aid for funnel-web spider bite is a pressure immobilisation bandage and rapid transport to hospital. The mainstay of treatment is funnel-web spider antivenom, admission to a critical care area and monitoring for 12-24 hours until all evidence of envenoming has resolved. Funnel-web spider antivenom appears to be effective in bites by Atrax and Hadronyche species. Premedication is not required and early allergic reactions and serum sickness are rare6 The initial dose of antivenom is two vials which can be repeated every 15-30 minutes until envenoming has resolved.
Patients with funnel-web spider bites without symptoms of severe envenoming, or bites by unidentified big black spiders in eastern Australia, should initially be treated as suspected cases of envenoming. These patients should be observed for 2-4 hours and the pressure immobilisation bandage can be removed once funnel-web spider antivenom is available. If there is no evidence of severe envenoming after two hours, it is unlikely to occur6 but it is prudent to observe the patient for four hours.
Mouse spider bites can cause local neurotoxic effects (paraesthesia, numbness) and non-specific systemic effects in some cases. However, because they are large black spiders, the bites should be treated as suspected funnel-web spider bites.
A clinical toxicologist can be contacted for advice on managing severe envenoming through the Poisons Information Centre (phone 13 11 26).