A dry bite is a bite by a venomous animal in which no venom is released. Dry snake bites are called "Venomous snake bite without envenoming". Dry bites can occur from all snakes, but their frequency varies from species to species. For example, Australian eastern brown snakes (Pseudonaja textilis) can inflict dry bites 80% of the time while taipans inflict dry bites only 5% of the time. About 25% of snakebite cases can be dry bites. They are characterized by fang and tooth marks and the absence of injected venom.
Dry bites are often confusing for the attending physician and the victim. The phenomenon is exploited by quack doctors as evidence for the effectiveness of supposed miracle cures.
In practice it is not necessarily simple to tell a dry bite from a dangerously or at least harmfully venomous bite. This is not much of a problem where there is no specific antivenom, because then main options are prevention of infection and reactive treatment of symptoms as they occur; but where one is dealing with say, a black mamba bite, where the venom dosage often is large, the effects are rapid, and an effective antivenom is available, one seldom can afford to delay treatment long enough to confirm whether the bite is dangerous. Also, bites such as those from snakes with venom that cause blood or tissue damage, such as many Viperidae, may take a long time to kill, but do not take long for the tissue damage to become irreparable, possibly leading to the loss of a limb, or worse. Such envenomation should be treated as soon as practical, not only after symptoms become severe.
One of the most difficult problems associated with dry bites, is that it is not simply a matter of some bites being dry and others venomous; some species have quite sophisticated mechanisms for controlling the dosage of venom injected, which can vary widely and unpredictably. Jumping to conclusions on the basis of just a few experiences of a given species or a group of related species can be fatal. There are many variables; anecdotally, very young venomous snakes for example, tend to have disproportionately dangerous bites—there is reason to suspect that they routinely inflict a full bite.
Dry bites from spiders such as tarantulas and large Sparassidae are common and, where correctly identified, can simply be ignored or, if appropriate, treated using mild antiseptics. On the other hand, some reports clearly suggest that some of their bites cause marked neurotoxic effects. For example, in South Africa the common "Rain Spider" Palystes castaneus and similar species, is usually described as negligibly venomous, and certainly it is at the least difficult to find documented cases of serious effects.
A case in Cape Town is typical of evidence of the dangers of generalizing. In her garden with her bare arm in bushes, a woman felt a stinging sensation and withdrew hurriedly. Seeing nothing, she thrust her arm in again and felt a worse sting; inspection revealed a Palystes nest-cocoon, with the female on guard. There was local redness at the site of the bites, but no other ill effects. Later that evening, however, there was dizziness and nausea, and her arm became swollen and painful, with local discoloration at the sites of the bites. The pain and swelling took several days to subside.
Given that the typical Palystes bite is less traumatic than a bee sting, it is altogether plausible that many are dry bites, but as in this case, when there were two bites, possibly less dry than usual, the putative effects make it altogether reasonable to suspect that the venom as such is non-negligible. It is not possible to carry out controlled studies on humans, and the effects on non-human subjects cannot be trusted because there are major unpredictable differences between species in such matters.
In the event of a dry bite, anti-venom should not be taken, as it has unneeded side effects.
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