Abstract

Latrodectism is the illness caused by the bite of "Latrodectus" spiders (the black widow spider and related species). Pain, muscle rigidity, vomiting, and sweating are the symptoms of latrodectism. Contrary to popular conception, latrodectism is very rarely fatal to people though domestic cats have been known to die with convulsion and paralysis.

There are several spider species all named black widow: southern black widow spider ("L. mactans"), the European black widow ("L. tredecimguttatus"), Western black widow spider ("L. hesperus"), Northern black widow spider ("L. variolus"). Other "Latrodectus" that cause latrodectism are the Australian redback spider ("L. hasselti"), and the New Zealand katipo spider ("L. katipo"). Several other members of "Latrodectus" genus are not commonly associated with latrodectism including the cosmopolitan brown widow ("L. geometricus").

Signs and symptoms

A bite of "Latrodectus" may not inject any venom (known as a dry bite) and so no illness occurs. About 75% of "wet" bites will have localized pain and nothing more. If, however, there is a substantial dose, a bite can cause latrodectism. The main symptoms are generalized muscle pain, stomach cramps, nausea and vomiting.

Initially a pinprick or burning sensation can be felt when bitten by widow spiders. If there was enough venom injected, pain worsens over the next hour. The area will develop localized sweating and gooseflesh piloerection. The pain may spread and become generalized.

The typical duration is three to six days. Some people who do not receive antivenom may feel unwell, be weak, and have muscle pain for weeks.

Pathophysiology

Spider venoms are a complex collection of toxic agents. Unique to the widows is Latrotoxin. The venom acts on nerves causing the massive release of the neurotransmitters acetylcholine, norepinephrine, and GABA. The release of these neurotransmitters leads to pain, cramps, sweating and fast pulse. Latrotoxin acts on presynaptic nerve membranes (See Chemical synapse) and through the cell's signalling protein (calcium-independent receptor of alpha-latrotoxin CIRL). Thus initial pain is often followed by severe muscle cramps. Contraction of musculature may extend throughout the body, though cramping in the abdomen is frequently the most severe. Latrotoxin may act on muscles directly preventing relaxation, promoting tetany—constant, strong, and painful muscle contractions.

At high doses (in the lab) the venom also deforms human red blood cells, an effect common to the venom of bees, the blue-ringed octopus, and a range of snakes.

Diagnosis

There are no tests required to diagnose widow spider bites, or latrodectism symptoms. The diagnosis is clinical and based on historic evidence of widow spider bites. Pathognomonic symptoms such as localized sweating and piloerection provide evidence of envenomation. Unlike the brown recluse, the widow species are easily identified by most people.

Diagnosis is obvious in most people reporting contact with a "Latrodectus" spider. However, without a spider, either through inability to communicate or unawareness, the diagnosis may be missed as symptoms overlap with a variety of other serious clinical syndromes such as tetanus or acute abdomen. Blood values are typically unimportant but may be needed to show myocarditis or dehydration from vomiting.

Treatment

People who have been bitten by a black widow spider are recommended to seek professional medical assistance for symptoms. Symptoms self-resolve in hours to days in a majority of bites without medical intervention.

Medical treatments have varied over the years. Some treatments (e.g. calcium gluconate) have been discovered to be useless. Currently, treatment usually involves symptomatic therapy with pain medication, muscle relaxants, and antivenom. When the pain becomes unbearable, antivenom is administered. Antivenom historically completely resolves pain in a short time. Antivenom is made by injecting horses with latrodectus venom over a period of time. The horse develops antibodies against the venom. The horse is bled and the antibodies purified for later use. Doctors recommend the use of anti-inflammatory medications before antivenom administration, because antivenom can induce allergic reactions to the horse proteins. The efficacy of antivenom has come under scrutiny as patients receiving placebo have also recovered quickly.

Antivenom is used widely in Australia for redback bites; however, in the United States it is less commonly used. Antivenom made from prior spider bite victims has been used since the 1920s. Opiates such as morphine relieve pain and benzodiazepines ease muscle spasm in most patients.

Prognosis

The vast majority of victims fully recover without significant lasting problems (sequelae). Death from latrodectism is reported as high as 5% to as low as 0.2%. In the United States, where antivenom is rarely used, there have been no deaths reported for decades.

Despite frequent reference to youth and old age being a predisposing factor it has been demonstrated that young children appear to be at lowest risk for a serious bite, perhaps owing to the rapid use of antivenom. Bite victims who are very young, old, hypotensive, pregnant or who have existing heart problems are reported to be the most likely to suffer complications. However, due to the low incidence of complications these generalizations simply refer to special complications (see Special circumstances).

Epidemiology

Bites from "Latrodectus" occur usually because of accidental contact with the spiders. The species are not aggressive to humans naturally, but may bite when trapped. As such, bite incidents may be described as accidents. Reports of epidemics were associated with agricultural areas in Europe in the last two centuries. However the European spider is associated with fields and humans come in contact only during harvest. For example, in the 1950s researchers believed that three bites happened each year and with an epidemic up to 180 each year.

Conversely, redback and North American black widows live in proximity with people and several thousand black widow bites are reported to Poison Control in the United States each year. About 800 are reported by medical personnel. Amongst those 800 bites only a dozen had major complications and none were fatal.

In Perth, Australia, for example there were 156 bites in children from redback spiders over 20 years. Twice as many boys were bitten as girls, mostly toddlers. A third of the children developed latrodectism and there were no deaths.