Public Health Importance

Scorpion sting cases can be categorized as two general types: those involving only localized, transitory symptoms usually lasting from a few minutes to several hours, and those involving systemic reactions. Localized responses are characterized by immediate pain followed by moderate swelling at the sting site, often likened to the sting of a wasp or bee. In some cases, the sting may result in a raised, reddened, indurated lesion, even in the case of relatively harmless scorpions (e.g., Vaejovis carolinianus). In cases involving cytolytic toxins (e.g., scorpionids and ischnurids), swelling may persist up to 72 hr, followed by development of hemorrhages and blood-filled blisters near the sting site. Sloughing of skin may occur, but this varies greatly in severity. Other localized effects include gooseflesh, sweating, and muscle spasms near the sting site. In cases of buthid stings, pain usually radiates from the site of the sting up the affected limb. The pain tends to concentrate in the joints, especially the armpits and groin, and often crosses from one armpit to the other.

In cases of systemic reactions, the clinical signs and symptoms are highly variable, ranging from mild to life-threatening. Systemic reactions commonly are mild and are not necessarily indicators of a serious problem. Often there is no appreciable swelling or discoloration of the skin at the sting site. An intense aching and burning sensation may spread to adjacent tissues, which in turn often throb, sometimes becoming numb. The acute pain at the sting site turns into a chronic, dull pain accompanied by a feeling of numbness around the edge of the sting site, which may persist for one to several days. Numbness in the face, mouth, and throat is fairly common. Muscles may become spasmatic, resulting in muscular twitching, slurred speech, difficulty swallowing, tightness or cramps in the chest and back, rapid heartbeat, and nausea. Often these systemic responses persist less than an hour after the sting and are not considered serious.

In more severe systemic reactions, neurologic effects can lead to profuse sweating and salivation, restlessness, extreme nervousness, respiratory and cardiovascular problems, mental confusion, and convulsions. As the clinical symptoms indicate, the principal components of the venom of dangerous scorpions are neurotoxins. These toxins act on the autonomic, sympathetic, and neuromuscular systems, causing the wide range of systemic reactions reported in sting victims. They act by disrupting the voltage-sensitive sodium and potassium channels of nerves, which in turn causes neural depolarization, prolonged action potentials, repetitive firing, and uncontrolled release of vasodilators and neurotransmitters, which affect virtually every major organ system. The effect on neurotransmitters results in a depletive release of catecholamines (e.g., adrenaline, noradrenaline) that severely damages the heart and other organs. The most commonly reported cause of death in scorpion sting cases is cardiac failure. In other cases, respiratory failure may be the cause, especially in patients with upper respiratory infections or related problems. Death usually occurs several days after envenomation. If symptoms subside during the first 2—12 hr following a sting, the prognosis for recovery is generally good. Mortality rates are quite variable, depending on the species and amount of venom injected. The rates are much higher among children than adults. For further details on the clinical toxicology and symptoms of scorpion stings, see Dehesa-Davila et al. (1995) and Ismail (1995).

Scorpion venom is a very complex mixture of substances which differs significantly among the various taxa, within families, and among genera. Differences also occur in different geographic populations of the same species and even within the same population. The toxins are low-molecular-weight proteins which are among the most powerful toxins known. They are comparable in some species to the neurotoxins of certain deadly snakes. Two recognized types of neurotoxins are a-scorpion toxin, characteristic of the genera Androctonus (Fig. 20.7), Leiu -rus (Fig. 20.8), and Buthus, and Jl-scorpion toxin, characteristic of Centruroides. Tityus species appear to have both types. The effects of envenomation by any given scorpion species can differ significantly among individual cases, owing to a wide range of contributing factors.

in the United States. Deaths from its sting, however, are rare. In fact, the toxicity and effects of its venom are very similar to that of the striped scorpion C. vittatus. The latter is the most widely distributed of all American scorpions, occurring in the southern United States from the Mississippi River west to New Mexico, as far north as Kansas and Missouri, and well south into northeastern Mexico.

Among the more than two dozen Centruroides species and subspecies in Mexico, the following taxa are of particular concern because of the seriousness of sting cases: C. electa ns,C. exilic auda, C. infamatus, C. noxius, and C. suffusus. All are closely related to one another (Exil-icauda group). The most notorious are C. suffusus and C. limpidus, both of which are capable of causing human deaths. Despite its small size, usually less than 5 cm, C. noxius is considered very venomous.

Tityus species are similar to Centruroides species in size, general appearance, and behavior. As a group of over 100 species, they occur throughout South America and the Caribbean Basin. In most places where they occur, all Tityus species are considered dangerous. The most venomous is the Brazilian species T. serrulatus, which is common in urban areas and readily enters homes. Second only to T. serrulatus in its medical importance is another house-infesting scorpion in Brazil, T. bahiensis. Related Tityus species which also are highly venomous include T. cambridgei, a forest-dwelling species in the Amazon Basin and northern South America, T. trinitatis, which can be a serious problem in coconut groves and cane fields in Venezuela and Trinidad, and T. trivittatus, a house-infesting scorpion in Argentina.

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