Black Widow Spider Envenomations
A review of the diagnosis and treatment of envenomations of black widow spiders.
This article will review the diagnosis and treatment of envenomations of black widow spiders. Whether an individual clinician will commonly treat these envenomations depends on where in the country they are practicing. In the United States, anti-venom for black widow spider has helped reduce morbidity and mortality.
Black Widow Spiders
In North America the Latrodectus species or black widow spider is a cause of significant envenomation. The spider can be identified by its black body and red hourglass-shaped marking on the abdomen. The black widow moniker results from observation of the female frequently devouring the male after a mating session. Black widow spiders typically range from 0.5 to 1.5 inches in length and have fangs and venom. The female has larger venom glands, longer fangs, and can be many times larger in size than the male and as such is more of a threat to humans. Males may have red or yellow spots or bands on the back rather than an hourglass-shaped marking.
The two species that most commonly cause envenomation within the United States are the Latrodectus mactans, or the southern black widow spider, found in the Southeast US and the Latrodectus hesperus or the western black widow spider, found in most of the Western U.S. A third species Lactrodectus variolus or northern black widow spider is found in northern Florida up to the Middle Atlantic states and occasionally in southern Canada. In variolus species the red hourglass pattern on the female is typically separated into two distinct triangles.
Black Widow Spider Venom
The venom of the black widow spider consists of a number of biologically active substances. Alpha-latrotoxin is the major component which binds irreversibly to receptors on presynaptic neurons. It causes an influx of calcium ions releasing massive amounts of neurotransmitters, which cause most of the symptoms in an envenomation.
Black Widow Spider Envenomation
Black widow spider bites can occur while hiking, gardening, in garages, and frequently in outdoor bathrooms. Fang marks may be seen and within minutes of being bitten there is pain, and the wound may become erythematous and edematous. There may be local diaphoresis around an area of central clearing. Severe muscle pain and cramping usually occur within an hour. Increased autonomic function leading to tachycardia, tachypnea and hypertension may occur.
When patients develop systemic symptoms the term latrodectism is used. There may be diffuse muscle rigidity and cramping, tenderness, burning around the bite, truncal and abdominal tenderness, diaphoresis, headache, nausea, and vomiting. Occasionally black widow bites have been misdiagnosed as an acute intra-abdominal emergency such as appendicitis due to the muscle pain and rigidity. The pain associated with latrodectism spreads from the bite site, so if someone is bitten on the foot the pain will initially move proximally up the leg and then to the rest of the body. Lab values are generally non-specific but may show an elevated white blood cell count, hematuria, and elevated liver enzymes. Occasionally rhabdomyolysis or myocarditis may occur.
In one case series of over 23,000 exposures to black widow venom, 65% of patients presented with minor clinical effects, 33.5% moderate effects, and 1.4% had major potentially life-threatening symptoms.
Initial management of black widow envenomation includes local wound management, ice packs, and tetanus prophylaxis. Calcium gluconate and methocarbamol, which were recommended in the past for the resulting muscle spasms have been shown to be ineffective and are no longer recommended. Pain management may be required with oral analgesics and occasionally opioids. Benzodiazepines may be required for muscle spasms.
There is an antivenom that can be used for cases of latrodectism that is very effective but allergic reaction, serum sickness, and anaphylaxis may occur as it is made out of horse proteins. The drug information sheet recommends a skin or conjunctival test be performed prior to administration to check for allergies. The dose is one vial intravenously which can be repeated if needed. A more purified antivenom has been developed and tested to try to reduce the possibility of allergic reactions, but is not available commercially, and currently in clinical testing.[26,27]
The prognosis for black widow bites is good. Most pain and systemic symptoms are self-limited and recovery is typically within 24 to 48 hours.
Fortunately, most of the envenomations from black widow spiders are not severe. However, severe systemic toxicity can occur. Deaths are rare, especially with the availability of anti-venom for black widow spider envenomations.
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 Dart RC et al. The Efficacy of Antivenin Latrodectus (Black Widow) Equine Immune F(ab')2 Versus Placebo in the Treatment of Latrodectism: A Randomized, Double-Blind, Placebo-Controlled, Clinical Trial. Ann Emerg Med. 2019 Sep;74(3):439-449. Retrieved from: https://www.fresno.ucsf.edu/wp-content/uploads/2019/04/EfficacyAntivenin.pdf
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