SNAKEBITE, SNAKE VENOMS AND THEIR ANTIDOTES:nuerotoxins,hemotoxins, myotoxins, cytoxins and other toxins

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Around the world, between 94,000 and 125,000 people die annually from snakebites. Most snakebite victims are poor rural farmers or children in the developing world. The countries with the highest incidences of snakebite deaths include India, Sri Lanka, Nepal, Myanmar, Nigeria, Mali, Togo, Benin, Senegal, and Papua New Guinea. No data exist for Indonesia, but death rates there are also assumed to be high.

Rather than going to hospital, victims will often visit a local shaman or medicine man in the vain hope that he can save them. Even for those people who survive a snakebite, the prognosis may not be good. Some snake venoms cause massive tissue destruction, leading to limb deformity or loss—each year, up to 400,000 snakebite victims may be disabled in this way. Snakebites are terrible to endure, but they are not without a cure.

Modern antivenoms are produced from the antibodies formed when horses and sheep are injected with increasing doses of snake venom, and are very effective for saving life and reducing the damage done by snake venoms, provided the victim gets to hospital quickly. Unfortunately, however, some Western drug companies are stopping the production of antivenoms, as they are less profitable than drugs for diseases like obesity, cancer, and heart disease. The world—and Africa in particular—may consequently be entering an antivenom crisis.

SNAKE VENOMS

Snake venoms are complex cocktails of different proteinaceous toxins that are designed to target prey. With the exception of spitting cobras, which spray jets of venom into the eyes of a perceived enemy and then effect an escape, snake venoms are not purposefully defensive. There are several different venom types, which are summarized below:

Neurotoxins

These paralyze the nervous system, preventing the passage of messages along nerves and leading to death through respiratory paralysis. There are two types: presynaptic neurotoxins, which destroy the transmitter sites on the “upstream” side of the synaptic gap; and post-synaptic neurotoxins, which block the receptor sites on the “downstream” side of the gap. These venoms are primarily found in the elapids—cobras (Naja), mambas (Dendroaspis), and taipan (Oxyuranus)—but also in some rattlesnakes, including the Mohave Rattlesnake (Crotalus scutulatus).

Hemotoxins

These toxins affect the blood and circulatory system. Anticoagulants cause prolonged bleeding by preventing blood coagulation, while procoagulants do the same by using up all the clotting factor in blood. Platelet inhibitors prevent normal blood clotting, and when combined with hemorrhagins (which puncture holes in the blood vessels), the result can be massive blood loss. Hemolytic toxins break down the red blood cells, causing blockage of the kidney tubules, leading to renal failure. Many vipers produce hemotoxins, but they are also found in some elapids such as taipans.

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Extracting the venom from a cobra for antivenom manufacture, a process usually called “milking.”

Myotoxins

These toxins affect the muscles, acting like neurotoxins by causing paralysis, or like hemotoxins by breaking down muscle tissue. They are primarily found in seasnakes.

Cytotoxins

These toxins digest protein, leading to massive tissue destruction. They are found in large vipers that need to digest bulky mammalian prey, and in the venoms of spitting cobras.

Other toxins

Sarafotoxins are cardiotoxins that cause a narrowing of the cardiac arteries; they are found in the venoms of the side-stabbing snakes (Atractaspis). The St. Lucia Lancehead (Bothrops caribbaeus) produces a cardiotoxin that causes arterial thrombosis, while the venom of the Gwardar (Pseudonaja mengdeni) contains a nephrotoxin that directly attacks the kidneys. Snake venoms are highly complex compounds.

 

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