The Southern Black Widow spider (Latrodectus mactans), a cause of latrodectism
|Classification and external resources|
|MeSH||bites / diagnosis spider bites / diagnosis|
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 hallmarks of latrodectism. Contrary to popular conception, latrodectism is very rarely fatal to people. 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 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 under the weather, be weak, and have muscle pain for weeks.
- Intense local pain develops 5–10 minutes after the bite and is followed by sweating and piloerection (goosebumps) within an hour. Neither puncture marks nor redness are necessarily seen.
- A few people go on to have widespread symptoms. Pain typically starts at the bite site then travels up (e.g. from foot to thigh to trunk), followed by generalized pain (in back, trunk, chest or shoulder). The venom directly affects nerves leading to the unusual feature of severe sweating, which may be regional (e.g. both legs) or generalized. Changes in adrenaline can lead to mild increase in blood pressure and pulse.
- Non-specific features of latrodectism include headache, nausea, vomiting and feeling ill and weak.
- Symptoms may wax and wane over the next one to four days. Rarely, patients may feel unwell for up to a week. Very rarely, untreated patients report ongoing bite site pain that last weeks or months.
- During pregnancy the pain and abdominal cramps may be confused with other conditions. A case of preterm labor initiated by a redback spider was relieved by antivenom.
- Early medical reports of latrodectism were described in men using an outhouse. The genitals were often the site of the bite. No direct injury to site is reported.
- Myocarditis (inflammation of the heart muscle) has been associated with one medically reported death in the last 50 years.
- Rhabdomyolysis (rapid skeletal muscle tissue breakdown).
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.
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.
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 — are used to reduce symptoms, as the calcium injections do very little for the pain and do not significantly reduce the recovery time.
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).
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 3 bites happened each year and with an epidemic up to 180/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.
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