Latrodectism: a prospective cohort study of bites by formally identified redback spiders

Redback spiders, Latrodectus hasselti, are widow spiders,1 arguably the most medically significant spiders in the world, and certainly in Australia.2-5 Two major issues relating to redback spider bite remain unresolved. The first is the proportion of bites causing severe effects, and the second is the indications for antivenom and its route of administration.6 Many doctors in Australia believe that most redback spider bites (80%) cause minor effects,7,8 and, although a safe and effective intramuscular antivenom exists, doctors are reluctant to use it unless there is severe or systemic envenomation.9,10 Severe pain is often not regarded as an indication for administering the antivenom.

Redback spider antivenom is usually given intramuscularly, which remains the recommendation of CSL Ltd.11 However, redback spider antivenom was introduced without controlled studies to demonstrate efficacy or safety. Its introduction was based on a retrospective collection of 167 cases reported by Australian doctors suggesting it was effective.12

Here we report an analysis of the subset of bites by L. hasselti from a larger prospective cohort of 750 spider bites in Australia4 to better describe the clinical features, proportion of severe cases, and outcomes of redback spider bite. We also address two further issues: the effect of intramuscular antivenom in a smaller cohort of patients presenting to hospital and the diagnostic utility of early features of bites by L. hasselti which distinguish them from other spider bites.

Currently, it is believed that many redback spider bites cause minor effects and require neither treatment nor hospital assessment.7,8 Although redback spider antivenom is believed to be effective, doctors are reluctant to use it unless there is severe systemic envenomation.

Two studies of hospitalised patients, involving 150 adults and 156 children with definite redback spider bites, demonstrated that 29% and 22%, respectively, had systemic envenomation; 21% of subjects in both studies received antivenom.10,17 In a small prospective study of nine patients with redback spider bites, more than half had severe effects, and eight received antivenom.18 These three studies, based on emergency department presentations, suffer from sample bias, and patients were not followed up. The largest study, of 2144 cases,19 consisted of reports to the antivenom manufacturer, and did not define the true spectrum of definite redback spider bite.

Our study shows that, when only definite cases are included, a much higher proportion of redback spider bites causes severe and ongoing effects than previously reported.10,17 Two-thirds of patients had pain persisting beyond 24 hours and, in 32%, pain was severe enough to prevent sleep in the first 24 hours. Thirty-five per cent of patients had systemic effects, similar to the proportion in previous studies, but our study showed that these lasted 1–2 days (earlier studies had not examined duration of systemic effects). Our study demonstrates that the severity of envenomation is related not only to the severity of pain and systemic features, but also to the duration of these effects.

Recently, the intramuscular use of redback spider antivenom has been questioned. One series of four patients demonstrated that intramuscular antivenom was ineffective, but intravenous antivenom administered after failure of intramuscular antivenom was effective.6 In our study, intramuscular antivenom was no better than no treatment when all patients were followed up over a week.

Because of the small number of patients in our cohort, the study lacked power to definitively demonstrate no difference between intramuscular antivenom and no treatment. However, we believe that an acceptable treatment effect would be that 80%–90% of patients are pain-free at 24 hours. We found that this is not occurring, with 17% (95% CI, 0–64%) pain-free at 24 hours. This is a poor treatment effect for any antivenom, as they usually have dramatic effects. There are considerable implications of this finding, and the use of intramuscular antivenom9 needs review. A randomised controlled trial of intravenous versus intramuscular antivenom would now be appropriate.

Previous bite reports have invariably involved female redback spiders, and it was widely believed that male spiders are too small to bite and envenom humans.8,9 This article includes the first report of bites by male redback spiders, one causing severe pain. Our study confirms that bites by male spiders are rare compared with bites by females. This is probably due to the small size of males (3–4 mm body length, indicative of smaller jaws, compared with females, measuring up to 14 mm), rather than the male spider being incapable of delivering an effective bite or lacking venom of similar potency to females.

Our study has some limitations, including whether the sample truly represents all patients with redback spider bites. However, there was clear case definition with expert identification of spiders, and the prospective nature and follow-up of all patients were superior to previous studies. Bites were included from poisons information centre calls and from emergency department presentations. As these populations may differ, it is important to include both. Spider bites for which the patient did not seek any medical attention were not included, and this may have biased the study to more severe bites.

Our study demonstrates that, in many patients, early referral to hospital for assessment of severe pain is appropriate. We believe antivenom should be administered in most cases for pain relief alone, although the smaller cohort analysis challenges the current practice of using intramuscular antivenom, which is based on anecdotal experience. Controlled trials are required to compare the efficacy and safety of intramuscular and intravenous antivenom, because it may be more appropriate to use the latter. It will be a challenge, particularly for primary care physicians and poisons information centre specialists, in identifying cases of redback spider bite and in determining which patients should be advised to seek further treatment. However, we have demonstrated that early features of bites can be used to assess whether the spider is a redback (Box 4).

1: Geographical distribution of redback spider bites

The map includes only the 56 cases collected in the study of all spider bites between 1 February 1999 and 30 April 2001.4 The 12 cases collected later are not included because the recruitment site differed, so the first 56 bites provide a more accurate geographical representation.