Funnel-Web Spider

Funnel-web spiders comprise of 40 species capable of causing a lethal envenomation. Unfortunately they look very similar to other BIG BLAKC spiders such as the trap door and mouse spiders. Therefore you need an approach to the big black spider bite in areas where the funnel-web lives.

The pathognomonic presentation is a painful bite with abrupt onset sweating, piloerection, coma and fasciculations. Also any bite by a big black spider in New South Wakes or Southern Queensland is assumed to be a Funnel Web until proven otherwise.

Resus

Potential early life threats include:

• Cardiac arrest – give undiluted antivenom as a rapid IV push, all available ampoules should be given (at least 4 ampoules).
• Respiratory failure
• Hypotension or hypertension
• Pulmonary oedema
• Coma

Risk Assessment

Typical symptoms include:

• Often there is a history of a bite from a big black spider with fangs
• Local pain is severe and the fang marks are often present but localised swelling or erythema are not a usual feature
• Severe systemic envenoming develops within 2 hours, usually 30 minutes and consists of:
• General: Agitation, vomiting, headache and abdominal pain
• Autonomic: Sweating salivation, piloerection and lacrimation
• Cardiovascular: Hypertension, tachycardia, hypotension, bradycardia and pulmonary oedema
• Neurological: muscular fasciculation, oral paraesthesia, muscle spasm and coma
• Children may present with sudden inconsolable crying, salivation, vomiting or collapse

Supportive Care

• Apply a pressure bandage with immobilisation the the affected limb and only remove once in an area/hospital capable of managing a funnel web spider envenomation.
• Analgesia Simple analgesia including paracetamol and ibuprofen can be given but often opitates are required like oxycodone 0.1 mg/kg (max 10mg) every 4 hours have been effective before proceeding to IV opiates (0.1 mg/kg of morphine).
• Salivation can be treated with atropine until antivenom is given.
• With severe systemic envenomation aggressive supportive care maybe required while antivenom is sourced or the patient is still critical. This may include non-invasive ventilation, blood pressure control with either GTN or inotropes all the way through to intubation and ventilation

Investigations

1. Laboratory investigations should selected in the context of trying to rule out other differentials e.g. other reasons for a collapse on the beach if the history is unclear.
2. However, if the patient is haemodynamically unstable check FBC, EUC, magnesium, calcium, phosphate, CK, troponin and chest X-ray

Differential Diagnosis:

• Other big black spiders can cause bite site pain and mild systemic symptoms like nausea, headache, vomiting but significant cardiovascular, autonomic or neurological features do not occur.
• Redback spiders can present very similar with pain, sweating, piloerection but lethal envenoming with coma, fasciculations or pulmonary oedema do not occur

Antivenom

• CSL Funnel-web Spider Antivenom
• Initial dose of 2 ampoules to all patients with evidence of neurological, autonomic or cardiovascular features.
• Patients are then observed for response to treatment. Ongoing symptoms would prompt a further 2 ampoules
• Patients with severe envenoming should start with 4 ampoules

Disposition

• Patients without clinical features of systemic envenomation can be discharged at 4 hours
• Patients treated with antivenom need to be observed for 12 hours from resolution of symptoms and can then be discharged in daylight hours.

References and Additional Resources:

Additional Resources:

• Funnel-web Antivenom
• Chris Nickson – Spiders and stingers – SMACC podcast

References:

• Isbister GK, Gray MR, Balit CR et al. Funnel-web spider bite: a systematic review of recorded clinical cases. Medical Journal of Australia 2005; 182:407-411
• Isbister GK, Sibbritt D. Developing a decision tree algorithm for the diagnosis of suspected spider bites. Emergency Medicine Australasia 2006; 16:161-166
• Nicholson GM, Graudins A, Wilson HI et al. Arachnid toxinology in Australia: from clinical toxicology to potential applications. Toxicant 2006; 48:872-898
• Rosengren D, White J, Raven et al. First report of a funnel-web spider envenoming syndrome in Brisbane. Emergency Medicine Australasia 2008; 20:164-166
• Isbister GK. How to treat suspected spider bite. Australian Doctor 2004 Sept. 27-35