Spider Bites

There are over 34,000 species of spider world-wide (except Antarctica). Almost all are fanged and venomous. Fortunately less than 0.5% are able to penetrate human skin and, of those, only a handful are considered dangerous. Most spiders are shy and not naturally aggressive, most bites occurring when the spider is provoked or trapped. However, more spiders are being kept as exotic pets and occasionally they turn up unexpectedly in supermarket fruit shipments. In England there are very, very few dangerous spiders and these tend to be less venomous than their overseas cousins. The vast majority of spider bites, despite much hype, are not medically significant.¹

• Widow spiders (Latrodectus spp.) - including the American black widow (L. mactans), and the Australian redback (L. hasselti) - these have distinctly marked abdomens with red/orange dorsal stripe and ventral hourglass patterns.
• Funnel web spiders of Australia - the Sydney funnel-web (Atrax robustus), Mouse spiders and relatives - fairly large, black and aggressive.
• Necrotising arachnids - includes the American brown recluse (Loxosceles reclusa), South American brown spider (Loxosceles laeta) - these have a violin pattern on their backs.
• The large South American armed-banana or wandering spider (Phoneutria nigriventer & other spp.)
• Other large spiders such as the Huntsman (Heteropodidae spp.) or the many varied orb weavers may look fearsome but are generally timid and harmless to humans.

Any bite can be painful and usually causes no more than local swelling and pruritis. Nausea, vomiting, sweating and dizziness may sometimes occur. Severe allergic reactions are rare, but potentially life threatening. Instead of biting, tarantulas can also defend themselves by flicking fine barbed abdominal hairs using their legs. These can cause significant irritation of eyes, skin and respiratory tract.
Even with the most venomous spiders, often more than 80% of the bites may be 'dry' or with insufficient venom for systemic envenomation. Significant bites have traditionally been classified in to two syndromes depending on the mode of action of the venom, though the number of spiders that can actually cause these types of envenomations is decreasing as the evidence is re-examined and trials completed.

Neurotoxic arachnidism (widow, funnel-web, Phoneutria spiders)

Widow spiders, some Steatoda ('false widow') species.

The characteristic feature is that of pain. Initially the bite can go unnoticed or be perceived as a sharp pinprick. The pain can be local or spread proximally from a bite on the limb to the torso, causing chest or abdominal pain. There are often nonspecific systemic features (nausea, vomiting, headache, lethargy, and malaise), local and regional diaphoresis, and less commonly other autonomic and neurologic effects. If a spider bite is not considered, the diagnosis may be missed especially in younger patients where communication is limited. Other rarer features of full blown latrodectism include tachycardia, hypertension, irritability, psychosis, priapism, renal failure, respiratory compromise and cardiac failure. The facies latrodectismica is a painful grimace caused by facial spasm and trismus associated with swollen eyelids, congested conjunctivae, flushing and sweating.
Brown house or false widow spiders, Steatoda grossa in Australia and Steatoda nobilis in England, have caused mild neurotoxic arachnidism and, in the case of the former, has been treated successfully with Redback antivenom.
The widow neurotoxin, active against vertebrates, opens cation channels (including calcium channels) presynaptically, causing increased release and then depletion of multiple neurotransmitters affecting somatic and autonomic nerves.

Funnel-web & related spiders

The initial bite is usually immediately painful from the larger fangs and acidic venom. The Australian mouse spider (Missulena occatoria) may have a venom as dangerous as the funnel-web but to date only a very few serious envenomations have been reported.
Primates are particularly sensitive to funnel-web venom. It contains a distinctive peptide with hyaluronidase and other components and causes rapid massive release of neurotransmitters at autonomic and neuromuscular junctions with associated uncontrolled autonomic hyper-reactivity and muscle twitching, followed about 2 hours later by neurotransmitter depletion and weakness. Pulmonary oedema, hypotension, hypoxaemia and cardiac arrest can ensue.

Phoneutria spiders

These aggressive spiders can give a very painful bite. The venom is a complex mixture of amino acids, hyaluronidase, serotonin, and other unidentified kallikreinkinin activating factors that stimulate peripheral and central nervous system neurones. Shortly after localized sweating and piloerection appears at the bite site, pain radiates up the bitten extremity to the trunk. Following this, tachycardia, hypertension, profuse diaphoresis, hypothermia, salivation, nausea, vomiting, vertigo, visual disturbances, priapism (especially in young boys), and rarely death can occur within 2 to 12 hours.

Necrotic arachnidism

This should probably be called loxoscelism as only Loxosceles species have good evidence for causing ulcers.² Other spiders have gained notoriety for causing dermonecrosis (e.g., Hobo spiders (Tegenaria agrestis), white-tailed spider (Lampona cylindrata), wolf spider (Lycosa spp), and black window or house spider (Badumna spp.)) but the evidence has not held up to scrutiny. Also the jury is still out on another accused, the yellow sac spiders (Cheiracanthium spp.).
The site of the bite burns, swells and develops a characteristic macular erythematous halo lesion which either resolves over a few days or becomes purple and then a dark eschar that sloughs off over week, sometimes leaving a necrotic ulcer that can be recurrent and slow to heal. Rarer systemic loxoscelism can give rise to fever, morbilliform rash, jaundice, intravascular haemolysis associated with spherocytosis, haemoglobinuria/renal failure, seizures, and uncommonly DIC.
Sphingomyelinase D was thought to be responsible for the necrosis but studies have found that this is not present in some Loxosceles species that have caused necrotic bites. Other active enzymes are present but a single or common dermonecrotic component is not yet known. Necrosis, of course, may also result from secondary infection of a spider bite.

• General: other bites & stings, anaphylaxis.
• Necrotic bites: erythema multiforme toxic epidermal necrosis, herpes infections, HSV.
• Neurotoxic bites: causes of shock / hypertensive crises / acute abdomen, priapism, back pain, ischaemic heart disease, hypocalcaemia.
• Tarantula hairs: conjunctivitis, uveitis/iritis, corneal abrasion, dermatitis.

First aid treatment

Depends on the spider. With funnel-web and other rapidly acting venoms, firm crepe bandaging and splinting of the bitten limb may delay venom spread. For the widow and brown recluse use an indirect icepack instead. Reassure the patient. If possible take the spider (dead or alive) to hospital for identification.

Supportive treatment

For all bites includes analgesia (NSAIDS, opioids) as required, tetanus prophylaxis, and local wound care. Antihistamines, Beta-blockers, benzodiazepines and atropine may also be useful for more significant local and systemic symptoms. Antibiotics are not advocated unless there is evidence of secondary infection.

Specific treatment

Antivenom (SE: anaphylaxis and serum sickness ³) is available in some countries for widows, funnel-webs, Loxosceles and Phoneutria. Neurotoxic arachnidism seems more responsive to antivenom than does the necrotic type. Redback antivenom can be given several days after the bite to good effect. Many therapies for necrotic lesions have been advocated but there is little clear evidence of their effectiveness ^4,5.

Antivenom has dramatically cut the mortality and almost no deaths now occur where it is readily available. Accurate data on the morbidity and mortality is difficult, as spider bites are not a reportable condition in most countries, and simple tests to determine envenomation are not available. Most fatalities are in children and the elderly and are likely to be related to the faster acting potent venoms. Funnel-web fatalities (all before introduction of antivenom in 1980) died 15 minutes to 6 days after the envenomation.

Image 1. Redback Spider - Image © Dr A M Bonsall - Used with Permission