Chapter 134: Spider and Arthropod Bites

High-Yield Facts

• Black widow spider bites result in painful muscle spasms, secondary to neurotoxicity, that are responsive to antivenin.

• Brown recluse spider bites result in hematotoxicity and manifest locally as skin necrosis.

• Scorpion stings cause severe localized pain with occasional systemic effects in children.

• Hymenoptera stings from bees and wasps can result in severe anaphylactic reactions and are responsible for more adverse outcomes and fatalities in children than any other arthropods.

• Fire ant stings can cause painful localized skin reactions.

Introduction

In North America, bites and stings by arthropods occur frequently. Approximately 50,000 bites or stings occur every year with about half of these caused by spiders. There are more than 41,000 species of spiders,¹ most of which cannot inflict serious bites to humans.² The majority of exposures are unnoticed and do not need treatment. There are a few medically relevant spiders that produce toxic venoms, which can lead to local reactions, systemic illnesses, and neurotoxicity.

Black Widow Spiders

Anatomy

The Latrodectus genus of spiders includes five primary species found in North America: Latrodectus bishop, L. geometricus, L. hesperus, L. variolus, and L. mactans.² They live in dimly lit, secluded areas such as woodpiles, barns, and stonewalls. They are present in every US state except Alaska.³ Black widows are described as shiny jet black with a characteristic red hourglass mark on the ventral aspect of the abdomen (Fig. 134-1). The red hourglass is specific to L. mactans, and other species have distinctive ventral markings, such as triangles and spots. There is a seasonal variation in the number of black widow bites, starting to rise in spring, peaking in September, and reaching a nadir in January–February.³

Pathophysiology

Black widows are overall shy, nocturnal, and only bite when their web is disturbed. The female black widow is generally considered poisonous to humans. The male black widow spider, with its smaller jaw and minimal venom, is not significantly poisonous to humans. These spiders use striated muscles to control the amount of venom they inject, and about 15% of bites do not deliver venom.⁴ The venom's toxicity is due to the presence of α-latrotoxin. This toxin facilitates exocytosis of synaptic vesicles and the release of the neurotransmitters norepinephrine, γ-aminobutyric acid, and acetylcholine.⁵ The toxin also causes degeneration of motor end plates, resulting in denervation. The venom destabilizes nerve cell membranes by opening ion channels, causing a massive influx of calcium into the cell, which may lead to hypocalcemia.

Clinical Presentation

Latrodectism is the clinical syndrome that follows a black widow ...