For centuries spiders have been feared by humans but their notoriety has been fuelled by anecdotal reports rather than scientific evidence. The findings of a Seminar, published Online First in The Lancet, highlight how misinformation about the alleged dangers of spiders has led to the misdiagnosis of spider bites as a common cause of necrotic lesions in primary care and emergency departments, resulting in harmful delays in appropriate treatment with potentially life-threatening consequences. Furthermore, a lack of well-conducted research to test antivenom therapy is putting patients at risk of ineffective treatment and allergic reactions.
Over the past few decades, there has been a misconception among both the public and the medical community that bites from many types of spiders are responsible for necrotic ulcers, yet most suspected cases have more common causes (eg, methicillin-resistant Staphylococcus aureus and Lyme disease) that can result in far worse outcomes, explain Geoffrey Isbister from the University of Newcastle, New South Wales, Australia and Hui Wen Fan from Instituto Butantan, São Paulo, Brazil.
Furthermore, because of a delay in clinical presentation serious arachnidisms (skin conditions resulting from spider bite venom) such as loxoscelism (a condition produced by the bite of Loxosceles spiders generally known as recluse, fiddle-back, or brown spiders) are not being recognised by clinicians and treatment is often delayed. The authors say: "Future research should focus on reporting of definite cases of spider bite to improve definition of clinical syndromes and help clinicians recognise envenomation."
Despite there being over 41 000 species of spider, nearly all spider bites are harmless and require no specific treatment. Although a large number of harmless spiders have been wrongly implicated as causing necrotic bites (eg, wolf spider, white-tailed spider, hobo spider), only Loxosceles spiders have good evidence for occasionally causing necrotic ulcers.
There is no established treatment for the bite of a Loxosceles spider. Although antivenom is widely used, supportive evidence is lacking and there have been no placebo-controlled trials in humans. Little is known about the optimum timing and dose of antivenom and results of animal studies are conflicting. Moreover, antivenom is unlikely to be effective against loxoscelism because many cases are diagnosed at a late stage when patients have already developed irreversible skin lesions and systemic complications.
Although antivenoms exist for many groups of spiders a lack of research into their effectiveness has limited their success. In Australia, widow spider antivenoms have been used extensively for decades for the treatment of severe latrodectism (a clinical syndrome caused by the bite of Latrodectus spiders such as the Black Widow spider) despite a lack of evidence for their effectiveness. Conversely, exaggerated fears about allergic reactions have restricted their use in the USA.
The authors conclude: "Well designed clinical trials to test antivenom therapy in both latrodectism and loxoscelism need to be undertaken urgently. Such studies are needed to prevent the unnecessary use of ineffective antivenom, which puts patients at risk of allergic reactions, and to better define the timing and dose of antivenom when it is effective."
Dr Geoffrey Isbister, University of Newcastle, New South Wales, Australia. T) +61 438 466471 E) geoff.isbister@gmail.com
Journal
The Lancet