CSL Antivenom Handbook


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Necrotic Arachnidism

This condition encompasses a broad spectrum of responses to spider bite, from very mild local skin damage through to major skin damage and systemic illness. It is a phenomenon seen in many parts of the world, but particularly in the Americas, where it is caused by the recluse spiders (Loxosceles reclusa, L. laeta, L. gaucho and others; "Loxoscelism"). In Australia local skin damage following presumed spider bite is not rare, with probably hundreds of cases each year. Most of these are minor, with ulcers of less than 2cm in size, but there are a few cases with large areas of skin loss requiring prolonged hospitalisation and ultimately, skin grafting. Several spiders have been suggested as causes of these ulcers (eg. white tailed spider) but until recently, none was proven. Recently in South Australia there have been cases where the spider was caught at the time of biting and in these cases it was a relative of the recluse spiders (fiddleback spider; Loxosceles rufescens). This spider is not native to Australia but has been accidentally introduced and is establishing itself in metropolitan areas. However, it cannot be said that all cases of "necrotic arachnidism" in Australia are due to this spider. Indeed, it is likely that in many cases the cause of the ulcer is secondary infection.

A typical case of necrotic arachnidism will present with a small, often painful, area of skin, usually found in the morning. It is presumed the bite occurred overnight while the patient was asleep. They often have erythema and a central blister at the presumed bite site. Over the next few days this may either resolve or develop a central ulcer, which is both painful and slow to heal. Secondary infection may occur and warrant treatment. The wound should be kept clean and the patient regularly reviewed. They should be told that this ulcer may take weeks or even months to fully heal. Occasionally the local lesion is more alarming, with extensive blistering and darkening of the skin. The darkened skin, which may be deep blue to black, may well become fully necrotic, leaving a large area of painful full thickness skin loss. Such cases require hospitalisation, bed rest, careful cleaning, antibiotics (after culture), and DELAYED surgical debridement. There is limited evidence that hyperbaric oxygen may assist in the healing process, which in any case, may be prolonged.