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Spider Envenomations, Redback

Author: Alexandr Rafailov, MD, Staff Physician, Department of Emergency Medicine, State University of New York Downstate/Kings County Hospital
Coauthor(s): Mark A Silverberg, MD, FACEP, MMB, Assistant Professor, Assistant Residency Director, Department of Emergency Medicine, State University of New York Downstate College of Medicine; Consulting Staff, Department of Emergency Medicine, Staten Island University Hospital, Kings County Hospital, University Hospital, State University of New York Downstate at Brooklyn
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Updated: Jan 8, 2007

Introduction

Background

The redback spider (Latrodectus hasseltii) belongs to the family Theridiidae, the comb-footed spiders. Its genus Latrodectus also includes black widows, whose name may be more familiar to North American readers. The redback spider can be found throughout Australia, although it is more commonly seen in the temperate regions than the colder, southern areas. The spider exists in higher numbers in Australia's urban and suburban areas and is virtually absent in the continent's forests. Outside of Australia, similar species of Latrodectus include Karakurt in the Central Asia, Malmignatte in Europe, the Koppie spider in South Africa, and the Night Stinger in New Zealand.

The redback spider bite is the most common envenomation requiring antivenom in Australia. The female redback spider is responsible for most occurrences of envenomations. She is usually 10 mm in length and has a small cephalothorax and a large, globular abdomen that bears a red, orange, or brown stripe. The male redback spider is considerably smaller than the female and is only occasionally able to cause mild envenomation.

Pathophysiology

The redback spider can cause a clinical condition referred to as latrodectism following a bite. The active ingredient in the redback's venom responsible for its toxic properties in vertebrates is a 130-kd protein, alpha-latrotoxin (aLTX). A potent neurotoxin, aLTX aggregates into tetramers that form pores in neuronal presynaptic cell membranes allowing calcium influx into the cytosol and resulting in exocytosis of neurotransmitters. The membrane pores formed by aLTX may also be large enough for a direct efflux of small intracellular compounds that are vital for cytoplasm function.

The monomeric aLTX can also act by activating latrophilin (LPH), an aLTX receptor found on the cell surface of neuronal cells, without incorporating into the cell membrane. LPH is a G protein-coupled receptor that activates phospholipase C, which, in turn, increases the cytosolic concentration of IP3 leading to release of calcium from intracellular stores. This rise in cytosolic calcium increases the rate of spontaneous exocytosis of neurotransmitters and the amplitude of evoked release. Alpha-latrotoxin is a potent venom with an LD-50 in mice, which is 20-40 µg/kg of body weight.

Frequency

United States

Only the black widow spider, a close relative of the redback spider, lives in the United States. These arachnids cause approximately 2500 envenomations each year.

International

The redback spider is found in Australia, New Zealand, and southern Asia. In Australia, the spider has been blamed for 250 envenomations requiring antivenom annually. Perhaps many more cases are mild or unrecognized and do not receive antivenom.

Race

All races are susceptible to redback spider envenomation if the patient lives in an endemic area inhabited by the redback spider.

Sex

In one study of redback envenomations in Australia, 60% of victims were female. However, most other sources do not quote a male/female sex discrepancy.

Age

Redback envenomation may occur at any age; the median age is 35 years. Envenomation may be more dangerous in babies and small children because of the difficulty in making a specific diagnosis in that group of patients in addition to the small body size bearing the same dose of injected poison as an adult would tolerate.

Clinical

History

• In Australia, most bites occur during the warmer months between December and April. Bites to the limbs comprise approximately three quarters of cases, and bites to the distal limbs are twice as common as to the proximal limbs.
• The cardinal symptoms of redback spider envenomation include immediate pain at the site of the bite with or without erythema, which usually progresses over hours to involve the entire limb and draining lymph nodes in the axilla or groin. The pain may persist longer than 24 hours with a median duration of 36 hours. Other common complaints include nausea, vomiting, abdominal pain, headache, and migratory arthralgias.
• In infants, nonspecific symptoms may be present, including inconsolable crying and refusal to eat.
• Most cases are mild or unrecognized and do not receive antivenom. The only way to accurately diagnose a redback spider bite is for the patient to recognize the typical markings on the spider's abdomen or bring the spider in with the patient for proper identification.

Physical

• Common physical findings in a patient with redback spider envenomation include tenderness and erythema at the bite site. Occasionally, one can see localized or generalized sweating, which may be unrelated to the bite site. Local piloerection may also be present.
• Systemic findings in redback spider envenomation, seen in approximately 35% of all bites, include fever, hypertension, and tachycardia.
• Neurological symptoms may include restlessness and insomnia, muscle weakness and twitching, and paralysis. The median duration of all effects is 48 hours.
• Rare complications include myocarditis, rhabdomyolysis, and death.

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