LOXOSCELES
Members of the Loxosceles genus, also called recluse spiders or fiddleback spiders, are non-aggressive spiders characterized by a dark brown marking on their cephalothorax in the shape of a fiddle or violin . The brown recluse spider, L. reclusa, is the most well-known member of this genus and is most abundant in the American Midwest and Southeast. Other medically significant members of this genus include L. deserta (Desert recluse), L. rufescens (Mediterranean recluse), L. kaiba (Grand Canyon recluse) and L. arizonica (Arizona recluse). Recluse spiders are so named because they will often seek out shelter in undisturbed places such as closets, attics, and storage areas for bedding and clothing. Bites usually occur when the spider feels threatened or provoked, such as when someone tries to put on clothing that contains the spider.
Clinical Findings.
Bites of the brown recluse spider (L. reclusa) vary from mild, local reactions to severe ulcerative necrosis, a reaction known as necrotic arachnidism. After a bite, transient erythema may develop with the formation of a central vesicle or papule. The hallmark “red, white, and blue” sign of a brown recluse bite is characterized by a central violaceous area surrounded by a rim of blanched skin that is further surrounded by a large asymmetric erythematous area . In a small percentage of cases, the initial wound may progress to necrosis , which usually begins 2 to 3 days after the bite, with eschar formation occurring between the fifth and seventh days. Eventually deep ulcers develop . The bite reaction may mimic pyoderma gangrenosum or erythema migrans of Lyme disease. Cutaneous anthrax and chemical burns have been mistaken for a brown recluse spider bite. The potential for misdiagnosis has led to the development of a sensitive enzyme-linked immunosorbent assay for Loxosceles species venom that may eventually be available for clinical application.
Brown recluse venom contains a number of proteins, including sphingomyelinase D, esterase, hyaluronidase, and alkaline phosphatase, which all contribute to tissue destruction. Sphingomyelinase D, the major component of the venom, cleaves sphingomyelin to form cermade-1-phosphate and choline and also hydrolyzes lysophosphatidylcholine to produce lysophosphatidic acid. Lysophosphatidic acid then triggers a pro-inflammatory response and causes platelet aggregation and increased vascular permeability. Sphingomyelinase D is also capable of inducing complement-mediated hemolysis. Systemic symptoms may develop within 1 to 2 days after envenomation and include nausea, vomiting, headache, fever, and chills. Rare but serious sequelae of brown recluse bites include renal failure, hemolytic anemia, hypotension, and disseminated intravascular coagulation.
Management.
General treatment measures for recluse bites include cleansing the bite site and the application of cold compresses. Patients may also require
analgesics to control pain. Antibiotics may be useful in reducing secondary bacterial infection of the wound site. Warm compresses and strenuous exercise should be avoided. Although Loxosceles antivenins have been developed and are frequently used in South America, there is little evidence to support their effectiveness, particularly against local cutaneous effects. A number of treatment modalities have been suggested, including hyperbaric oxygen, dapsone, intralesional and systemic corticosteroids, colchicine, and diphenhydramine. However, one study that used a rabbit model to compare dapsone, colchicine, intralesional triamcinolone, and diphenhydramine showed no effect on eschar size from any of these medications. Necrotic wounds heal slowly, sometimes over many months, and may require surgical excision and reconstruction to close the resulting defect. Surgical interventions should be delayed until the wound has stabilized.
TEGENARIA
Tegenaria agrestis, the hobo spider or “aggressive house spider,” is the predominant cause of necrotic arachnidism in the Pacific Northwest of the United States and can be found in an area ranging from Alaska to Utah. Although Loxosceles species are not typically found in the same geographic distribution, bites from hobo spiders are often mistaken for brown recluse bites. These spiders are brown in color with a gray herringbone pattern on the abdomen. The hobo spider typically builds funnel-shaped webs in crawl spaces, basements, wood piles, and bushes. Most hobo spider bites occur from July to September when the more venomous male spiders are seeking mates.
Clinical Findings.
The local cutaneous effects after envenomation by the hobo spider, which can range from mild to serious, are similar to those caused by the brown recluse. The initial bite is often painless. Induration and paresthesia of the bite site may develop within 30 minutes. A large erythematous area may form around the site. Vesicle formation often occurs during the first 36 hours. Eschar formation may follow in severe cases, with necrosis and sloughing of the underlying tissue.
Management.
Wounds usually heal within several weeks. The most common systemic symptom after a hobo spider bite is a severe headache, which can persist for up to 1 week. Other symptoms may include fatigue, nausea, vomiting, diarrhea, paresthesias, and memory impairment. Although rare, death may ensue due to severe systemic effects, including aplastic anemia.