Redback Spider Envenomation 

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 in 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 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.^{[1, 2]}

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). aLTX is a potent neurotoxin that works in 2 ways to produce efflux of neurotoxins from presynaptic nerve cells.

In one mechanism of action, aLTX aggregates into tetramers that form pores in neuronal presynaptic cell membranes allowing calcium influx into the cytosol and resulting in exocytosis of neurotransmitters such as norepinephrine, dopamine, acetylcholine, glutamate, and GABA. 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. Latrophilin 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.^{[3, 4, 5]}

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.

Mortality/Morbidity

Accurate data regarding morbidity/mortality is difficult to gather, as there is no mandatory reporting of spider bites in Australia. There appears to be greater morbidity in pediatric and elderly patients. There have been no recorded deaths caused by redback spider bites in the past several decades.^[6]

Race

All races are susceptible to redback spider envenomation if the patient lives in an endemic area inhabited by the redback spider. One article shows more severe envenomation in Aboriginal patients, but states that this is due to a significantly longer time from envenomation to presentation in ED as compared to non-Aboriginal patients.^[7]

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.^[8]

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.^[8]