Snake venom usually contains two types of poison: hemolytic toxins which attack the walls of blood vessels and neurotoxins which attack the nerves.
Hemolytic toxin attacks blood vessel walls, allows serum to escape into the surrounding tissues, and causes clotting within the vessels. The result is severe swelling, pain, and discoloration at the site of the bite. In the few cases where hemolytic toxins cause death, the actual cause is likely to be shock. The effects of hemolytic toxin are immediate and primarily localized. Symptoms will be obvious.
Neurotoxins produce much less obvious immediate symptoms, at times fooling the victim into believing envenomation has not occurred. But systemic symptoms can appear later. Neurotoxins produce much less local reaction than do hemolytic toxins. On the other hand, they can affect nerves quite removed from the site of the bite. In extreme cases they can cause respiratory arrest, although this is uncommon with the bites from most North American snakes. However, respiratory distress without actual arrest may to occur in neurotoxin victims. Less severe symptoms from neurotoxins include tingling or prickly feelings and eyelid paralysis.
All snake venom probably has some of each kind of toxin. But, most pit vipers have a higher fraction of hemolytic toxin, and elapids have more neurotoxin. The Mojave rattlesnake, a pit viper, is an exception; see below. The potency of venom will vary, with species, with time of year and with geographic area.
The typical snake mouth is no cleaner than a human's. So, they tend to induce microbial contamination into bites. Although it is common for a snake to bite without injecting venom, microbial contaminants will always be present and should always be treated. Such contamination seems to be much less of a problem in bites by nonvenomous snakes, perhaps because their bites do not penetrate so deeply.
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