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

OBJECTIVE To determine the spectrum of severity and early diagnostic predictors of redback spider bites (Latrodectus hasselti), and to examine the effect of intramuscular redback antivenom. DESIGN AND SETTING Prospective cohort study of calls to New South Wales, Queensland and Western Australian poisons information centres and presentations to Royal Prince Alfred Hospital and Royal Darwin Hospital emergency departments. PATIENTS 68 people with definite redback spider bites in which the spider was immediately collected and expertly identified (1 February 1999 to 30 April 2002). INTERVENTIONS Intramuscular redback spider antivenom in a smaller cohort of hospitalised patients. MAIN OUTCOME MEASURES Pain severity and duration, local effects and systemic envenomation (effects, prevalence, and persistence > 24 hours). RESULTS The median duration of effects was 48 hours (interquartile range, 24-96 hours). Pain occurred after all bites and was severe in 42 (62%). Forty-five patients (66%) had pain lasting longer than 24 hours, and 22 (32%) were unable to sleep because of pain. Systemic effects occurred in 24 (35%). Increasing pain over one hour occurred in 37 cases (54%), and local/regional diaphoresis in 23 (34%); both these features were highly predictive of L. hasselti bites compared with bites of other spiders. One of six patients treated with intramuscular antivenom (17%) had no pain at 24 hours, compared with two of 17 untreated patients (12%) (difference, 5%; 95% CI, -36% to +64%; P = 0.95). There was no difference in duration of systemic effects with antivenom administration. CONCLUSIONS Most redback spider bites cause severe and persistent effects. Intramuscular antivenom appears to be less effective than previously thought and its use by this route needs review.