Necrotising arachnidism describes the syndrome of blistering and ulceration or destruction (necrosis) of the skin following spider bites. In some cases the problem can be severe with ongoing pain and tissue destruction requiring extensive debridement and skin grafting. The white-tailed spider (Lampona sp.) and the black window or black house spider (Badumna sp.) have been blamed for necrotising arachnidism in Australia. Wolf spiders (Lycosa sp.) have also been suspected in at least one case of suspected necrotising arachnidism. However, it appears that the majority of people bitten by these spiders suffer only minor local reactions. As the majority of spider bites are not reported, the proportion of bites that result in necrosis is unknown.
The reasons why some people develop more extensive skin lesions are unknown. It may be that only some species of spiders are associated with skin necrosis (there are several species of white-tailed spiders, all of a similar appearance.) It is possible that the spider's sex or geographic location may be important, or that venom composition or contents of the spiders' gastrointestinal tract may vary over time. Patient factors such as underlying vascular disease or immune response to the bite may also be involved. There is as yet no definitive treatment for this syndrome. It is likely that the overwhelming majority of such alleged cases are not even related to the effects of the spider venom.
Necrotising arachnidism, if this is a real phenomenon at all, is at present poorly understood in Australia. This is in contrast to loxoscelism, a syndrome of tissue necrosis sometimes accompanied by systemic illness and occasional deaths in the Americas. This syndrome is caused by the brown recluse or fiddleback spider (Loxosceles reclusa) in the United States, and by other Loxosceles species in South America. It is believed to occur after approximately 10% of bites by these spiders.
The number of documented cases of necrosis where the spider bite was witnessed is small. A series of 15 cases of ulceration or necrosis following witnessed spider bite has recently been published by the Australian Venom Research Unit in the Medical Journal of Australia. The spider involved was positively identified in only 4 cases, and well described or recognised by the patient in the remaining cases. Fourteen of the cases presented were associated with white-tailed spiders, and one with a black house spider.
Research is hampered by the lack of reliability in reproducing necrotic effects in experimental models to date. Another major difficulty in characterizing the syndrome of necrotising arachnidism in Australia is the lack in most clinically suspected cases of a definitive history of spider bite and/or a positive identification of the spider involved. In addition, there is as yet no test or assay for spider bite or spider venom, so that retrospective diagnosis is impossible at this time. Many patients present with an area of blistering or necrosis, usually on the limb, but usually without a definite bite history or without identification of the offending creature if a bite has been felt.
The diagnosis of necrotising arachnidism, therefore, is one that must be considered circumstantial in many cases, and care must be taken to exclude other treatable causes of necrotic lesions . A list of possible differential diagnoses is given below.
Differential diagnosis of Necrotic Lesions suspected of being due to Necrotising Arachnidism
- vascular ulcers (arterial or venous insufficiency)
- diabetic ulcer
- focal vasculitis
- foreign body
- infection: bacterial (e.g. streptococcus, staphylococcus); mycobacterial (e.g. M. ulcerans, anthrax; fungal; viral?
- drug reaction
- physical/ mechanical trauma (may be deliberate)
- burns (especially chemical burns)
- pyoderma gangrenosum
- alpha 1 antitrypsin deficiency
- fat herniation with infarction
- injection of toxin (accidental or deliberate)