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Tarantulas Dangerousness

by Christophe Lallier

Although people seem more and more interested in spiders, as we see arachnology societies emerging with new subscribers, spiders still suffer from their bad reputation.

One of the main reasons is merely that we have not yet collected enough informations on them, and especially on their venomous functions. Of course we are able to identify the most dangerous species which may involve (only in a few cases!) a fatal result to humans (Latrodectus, Atrax, Phoneutri), but there is a lack of knowledge on the Theraphosidae family, commonly named Tarantula family.

There are approximately 800 to 1000 species of tarantulas, and none of its members are known to be deadly to humans. But on the other hand, all of them seem to have an efficient venom against vertebrates. Most of the species located in the south and central America area have a less active venom. This is related to the fact that they have a more dissuasive means of defence, constituted by urticating hairs which are located on a precise area of the abdomen. When the spider is attacked or merely disturbed, it "bombards ", meaning that it rubs its rear legs on the abdomen, releasing urticating hairs in the air. They have a real dissuasive effect when contacting your eyes, and may also involve a respiratory allergic response. They may enter into the skin, inflicting wounds on the area affected. With time, the wounds can become more serious.

Necrosis and swelling may occur. All those symptoms and pain intensity depend on the efficiency of urticating hairs, depending itself on the species (Avicularia urticans, for example).

 That is why it is generally advised those species must be handled carefully and hands must be cleaned with soap after handling to avoid any allergens.

Asian and African species seem to have a more efficient venom, which may be related
to the fact they have no urticating hairs on the body.

Although no human death have been reported as resulting from a Theraphosidae bite, some genus may be classified as really dangerous for human. Efficiency of venom may not be related to aggressiveness. When looking at reports dealing with accidents inflicted to amateurs, an empirical classification may be compiled.

The most dangerous genus are :

- Poecilotheria (India, Sri Lanka).
- Stromatopelma (Sierra Leone).
- Haplopelma (Thailand, Myanmar).
- Pterinochilus (Kenya, Tanzania).

Poecilotheria seem to be the most dangerous genus, as a bite may involve a coma (a two day coma has been reported recently!).

The more commonly symptoms of a serious bite are :

- An immediate serious local pain after the bite.
- Stiffness and great difficulty to move the bitten member.
- An intense burning feeling covering all the thorax.
- Cardiac distress (often reported in case of a Stromatopelma bite).
- Cramps in calfs, and adductors at night for approximatively eight days.
- Pain generally still exists one month after the bite and even more in a
few cases.

Theraphosidae 's venoms seem to be less convulsing and tetanizing than other spiders, but it has more necrotic effects. An injection of Phormictopus cancerides venom (1/5 of its gland) in a mouse involved a brief moment of violent (excitement) movements, followed by stupor. Then, the mouse lost consciousness, suffered from respiratory distress (slower and irregular breathing) with hypothermia, and finally died, without real paralysis. Symptoms are the same with birds.

However, an experience made with Pterinochilus murinus showed that its venom has similar effects compared to those resulting from Latrodectus mactans venom.

The mouse 's death occurs after a period of great excitation, followed by tetanic convulsions, dyspnea, salivation and tear secretions, and signs of paralysis are noticed.

A kidneys, liver, and brain examination shows a cellar degeneration.

All this shows only a few species are dangerous for humans, and all still has to be discovered on this topic.

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