A silent killer: The Gadim scorpion is the most dangerous scorpion of Iran

A 1 in 10 chance of surviving – these are the odds victims are facing after being stung by the highly venomous Gadim Scorpion. It alone is responsible for up to 67% of the scorpion-related deaths in Iran. And with Iran being among the top countries in the world most affected by scorpion-related envenomation, these numbers are alarming.

A unique scorpion species

Although there are more than 2000 known scorpion species in the world, only around 20 of them are medically important and considered dangerous to humans. All of these scorpion species belong to the Buthidae family, except the Gadim scorpion (Hemiscorpius lepturus) which belongs to the Hemiscorpiidae family.

Another characteristic of this scorpion is its exceptional painless sting. In almost every scorpion envenomation case, a painful sensation at the sting site is the first sign, which makes patients seek medical attention. But the Gadim scorpion sting is painless for the first 24-72 hours, providing the venom enough time to silently move around and damage the body. The explanation of this delay in the onset of clinical sign can be found in the distinct composition of the venom.

All the deadly scorpion species affect the human body in almost the same way because their venoms are very similar. Their venom is mainly composed of neurotoxins, which target nerve cells, and this is why they cause pain and inflammation at the sting site. However, the venom of the Gadim scorpion is mainly composed of cytotoxins and hemotoxins.

Cytotoxins are a group of toxins, which have a direct toxic or destructive effect on certain cells of the body. Cytotoxins of this scorpion mainly affect the skin and kidney cells. In skin, they produce necrosis, large open wounds, and sloughs, and in the kidney, they can cause acute renal failure. Besides, its hemotoxins affect the red blood cells and lyse them. Together, the cyto and hemotoxins may eventually cause renal failure and excretion of blood in the urine, which changes its color from pale yellow to red or brown. In absence of pain or any other sign of envenomation, this is the most important sign, which makes people seek medical attention.

The Gadim scorpion (Hemiscorpius lepturus) is the most dangerous scorpion of Iran. Notice its unique bead-shaped jointed tail and very small size of the stinger. Photo: Morteza Johari

How potent is the venom of the Gadim scorpion?

The lethal dose (LD50), an index of venom potency, is defined as the amount of venom required to kill half of the test animals in a cohort that have been challenged with the LD50. The lower the LD50, the more potent the venom (as less venom is needed to cause 50% death). The interesting part is that LD50 of Gadim Scorpion is way higher than other scorpions. This theoretically means its ability to kill is low, but the number of people killed by this scorpion is higher than any other scorpion species in Iran. Why is that?

One of the main reasons is the absence of pain at the early hours following envenomation. In this time period, the toxins start damaging cells in an irreversible way, which means that many cells have already been destroyed when the feeling of pain introduces itself. Thus, especially in children that have less body weights and old people that are generally weaker, even receiving anti venom may not result in any notable recovery, as the venom is already working full throttle. This is why it is recommended that all the patients stung by the Gadim Scorpion should receive antivenom as soon as getting to the hospital, even in the absence of clinical manifestations of envenoming.

Mysteries surrounding the Gadim scorpion

Venomous animals use their venom for two main purposes: hunting and self-defense. But in general, it seems that they use their venom quite considerately. For instance, it has been shown for some species of the Buthidae family of scorpions that they do not sting prey that can easily be handled by their pedipalps (pincers). Even when they have to use their sting, they can choose between a dry sting and a wet sting based on the situation. In case of a dry sting, the scorpions only insert their stinger inside the body of the victim but no venom is injected. This kind of sting is usually used for defense and deterrence. In contrast, a wet sting involves the insertion of the stinger and injection of venom into the body of the victim or prey. This type of sting is often used for hunting. But this is not the whole story!

Tricolor Scorpion (Opistophthalmus wahlbergii). Scorpions usually do not spend their precious venom for prey that can be handled by pedipalp.

It has been demonstrated that some scorpion species are able to control the amount and the composition of the venom that they inject in each envenomation situation. For relatively small prey items, or in low threat situations, they inject a transparent pre-venom of low toxicity that is mainly composed of salts and small peptides. This type of envenomation generally only produces local pain and is not lethal. However, for large or very active prey items, as well as high risk situations, the scorpions use their milky venom of high toxicity, which contains abundant toxic proteins and requires more energy to produce. Therefore, scorpions regulate their venom expenditure to conserve as much energy as possible not to waste this energy on venom frequent regeneration. During the period of 8 days, which is necessary for re-filling the venom glands, scorpions may be disadvantaged by a depleted supply of venom, as they may be unable to capture prey or defend themselves against predators.

In contrast to scorpion species from the Buthidae family, it seems that the Gadim scorpion rarely uses its venom for foraging purpose. The main function of the venom for this scorpion species seems to be self-defense against predators. In animals that use their venom for this purpose (such as bees) the main function of venom is to exert an extreme pain sensation locally to distract or deter the predator, while the scorpion can escape. However, the evolutionary reason for having a venom that does not induce pain in the victim and has a very slow mechanism of action is an unanswered question.

Only a few phylogenetic studies have been conducted on the Gadim scorpion. More research in this area can help to broaden our knowledge about the prey-predator relationship and the role of natural selection in divergence of Hemiscorpidae and Buthidae families. The reason why this scorpion species has followed a completely different evolutionary pathway than all its medically important scorpion cousins remains, an unresolved mystery. 

By Shirin Ahmadi, Christoffer Vinther Sørensen, and Andreas H. Laustsen

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