PRINCIPLES OF TOXICOLOGY - Biology East Borneo

17.7 ANIMAL VENOMS AND TOXINS 425action of the cardiotoxin and phospholipase A. Generally there is little or no localized edema soonafter the bite, as in crotalid envenomations, which sometimes leads to an incorrect initial perceptionthat the life of the victim is not endangered.Although antivenin therapy remains the most powerful approach towards treating snake envenomations,in many situations the antivenin is not immediately available, so a rational therapeutic approachbased on knowledge of the actions of the toxic constituents is required.Most amphibians possess skin toxins serving as some chemical defense against predators, but onlya few species present a danger to humans. Some of the brightly colored tropical South American frogspossess extremely potent toxins, and touching these may be enough to become intoxicated! Apparently,some of these frogs are collected for the exotic pet market and kept in vivariums as pets; fortunatelyfor the owner, these frogs soon lose their toxicity in captivity, which suggests that they make theirtoxins from precursor molecules in their natural diet. Batrachotoxin (Tables 17.1 and 17.2), whichcomes from one of these frogs, is a lipophilic sodium channel activator, making it popular in thepreparation of poison darts by Indian hunters. Human symptoms of intoxication, although they havenot been reported, should be similar to those caused by the grayanotoxins or the veratrum alkaloidsfound in the false hellebore (Table 17.1 and 17.3). Another frog alkaloidal toxin, histrionicotoxin,causes neuromuscular paralysis by binding to the open channel of the skeletal muscle nicotinic receptor.Toads of the genus Bufo possess a very potent venom in their skin and parotid glands behind theireyes. The major toxic constituents are cardiac glycosides called bufotoxins, but there also are biogenicamines, including epinephrine and bufotenin, a methylated form of the neurotransmitter serotonin.Because bufotenin is hallucinogenic, some enthusiasts have taken up “toad licking.” This is adangerous way to get high, as the white milky venom is rich in bufotoxins!Fish Venoms and ToxinsOnly a relatively small proportion of fish species are venomous, and in all cases the venoms are useddefensively to deter predators. Probably the most commonly encountered venomous fishes are thecatfishes and sting rays. Experienced fisherman are aware of the irritating stings caused by marinecatfish venom, but novices often learn the hard way. Little is known about the active constituents,although a recent paper reports smooth muscle stimulating and hemolytic activity of a large proteintoxin. Sting rays contain a dorsal spine near the base of their tail; when the ray is stepped on in shallowwater, the tail is thrust upward so that the spine can penetrate the skin of the intruder. When wadersshuffle their feet along the surface of the bottom, the sting rays almost always are frightened away, sothis is the best way of avoiding this fish. In contrast, the tropical Pacific stonefish (Synancega sp.) isnot easily frightened, and simply raises its spine when it senses the presence of an intruder. Like catfishvenom, stonefish and sting ray spine venoms probably contain several protein toxins that cause smoothmuscles to contract and cause inflammation. The stonefish toxin has recently been isolated and shownto be a large protein that enhances neurotransmitter release from nerve terminals. While these stingsare quite unpleasant, they are rarely life-threatening, and can usually be treated with antiinflammatorydrugs such as antihistamines and corticosteroids.Tetrodotoxin is certainly one of the most potent fish toxins. Pufferfish are considered a dangerousdelicacy in Japan, and consequently cooks must be carefully trained in the removal of poisonous visceraand skin when preparing “fugu” flesh for consumption. In the United States pufferfish are rarelyconsumed, but several cases of poisoning have been reported over the years. A person intoxicated whileconsuming pufferfish will generally experience tingling and numb sensations in the mouth area withinan hour after ingestion. Muscular weakness also develops, and the victim can be completely paralyzed.Endoscopic removal of the consumed fish is recommended if it can be done without delay. Treatmentis otherwise supportive; bradycardia and hypotension can be countered with atropine, intravenousfluids, and oxygen. Anticholinesterases may restore neuromuscular function if it is not entirely blocked.While tetrodotoxin is usually present in puffers, regardless of the place or season, some other toxinslike ciguatoxin are less predictable in their occurrence, as they are slowly passed up the food-chainfrom algae or bacteria to herbivores, then predatory fish and marine mammals.