A remarkable number of marine organisms, venomous animals produce venom in a specialized gland. Venom is a mixture of mainly protein and peptide toxins. In contrast, poisonous animals may have special glands that produce toxins but more often accumulate toxic compounds from the environment in their bodies.
These substances are known as poisons and have to be ingested to be effective because the animal has no specialized organ to deliver them. They are further classified as neurotoxic, hemotoxic, cytotoxic or myotoxic.
Human poisoning may occur from several invertebrates viz. Cnidaria (jelly fish), Porifera (sponges), Echinodermata, Mollusca and Annelida.
The phylum Cnidaria contains approximately 10,000 species. All four classes of Cnidaria contain venomous species. Almost 100 species are medically important. The class hydrozoa has worldwide distribution and includes Physalia, (Portuguese and Pacific man of war or blue bottle), the hydroids (Millipora, stinging corals), limnomedusae and Gonionemus.
Physalia causes thousands of envenomation in Florida, South America, parts of Asia and Africa, Australia and Portugal. Physalia has large gas filled floats which suspend multiple tentacles. These tentacles bear nematocysts. Each nematocyst contains a small amount of venom.
A physical or chemical stimulus triggers the release of hollow, sharply pointed thread like tube from the contained nematocyst. This process is very rapid (1/100 second) and results into the penetration of the tube into the skin and delivering venom simultaneously.
Hydroid stings are minor but can occur even after superficial contact. It causes discomfort in humans Lytocarpus and Aglaophenia are locally called as fireweeds. They have featherlike branches lined with nematocyst bearing polyps. A number of other medically important hydrozoans include Gonionemus and Olindias. Gonionemus causes envenomation in Japan. Olindias stinging is known to occur in Brazil, Uruguay and Argentina. Stings cause erythematous and edematous reactions and mild pain.
The class cubozoa contains box jelly fish. They possess a cube shaped body with tentades attached to each of the four corners. Chirodropid box jelly fish (Chironex fleckeri) is the world’s most venomous animal. It is found along the northern coast of Australia, another group of box jelly fish belong to the order Carybdidae. The group is identified by Carukia barnesi from Australia which causes IRUKANDJI syndrome.
Example # 2. Scyphozoa (True Jelly Fish):
Several species of true jelly fish are known to be poisonous. They include sea nettle, hair jelly fish, blubber jelly fish, mauve stingers and moon jelly fish.
Example # 3. Sea Anemones:
Sea anemones are often found in tidal pools. They have numerous nematocysts on their tentacles. Stings are characterized by local pains. The pain may last for few hours and local tenderness may remain for days.
Example # 4. Echinoderms:
The phylum Echinodermata contains organisms like sea urchins, star fish and sea cucumbers. Echinoderms contain a variety of toxins including steroid glycosides and terpenes but only a few of these animals are known to cause envenomation. In the small group of sea-urchins, the venom apparatus varies from short sharp spines with venom glands on their tips. Systemic features have been reported including nausea, vomiting, paresthesia, muscular paralysis, hypotension and respiratory distress.
Sea-stars only cause a minor traumatic injury. The animals are covered with sharp rigid spines that can passively deliver a variety of substances when they penetrate skin. The region may become dusky or discolored particularly with multiple spine injuries.
Sea-cucumbers do not have a venom apparatus but direct contact may induce a contact dermatitis. Injury to the cornea and conjunctiva may cause intense inflammation.
Example # 5. Sponges:
Sponges are the simplest of the multicellular organisms. They do not have a specialized venom gland but many species produce crinotoxins (slimes or surface liquids) and a few species can cause skin irritation and dermatitis.
Application of sponge extract to the rabbit cornea results in opacity and blindness demonstrating potential hazard to eyes. Neofibularia mordens causes the most severe stings in Australasian region. Stings from several sponges have been reported from United States, Hawaii and the Caribbean islands.
Example # 6. Annelida:
Phylum Annelida includes the bristle worms. Their body is segmented and covered with bristle like setae that become erect on contact. These setae or spines detach from the worm and are able to penetrate human skin. Initially there is a burning sensation followed by intense skin inflammation. Localized safe tissue edema and itchiness may occur. In some cases the effect may last for many weeks.
Example # 7. Molluska:
The phylum Molluska contains un-segmented soft bodied invertebrates that include the snails and slugs, chitons, bivalves, octopi, squids, and related species. Two classes of mollusks have a venom apparatus the gastropods (cone snails and nudibranchs) and the cephalopods (Octopi).
Cone snail shells mainly occur in the Pacific Ocean. Cone snails are often classified as- (a) piscivorous (b) molluskivorous (c) vermivorous. The venom they produce consists of a mixture of neurotoxic components called as conotoxins. They have a broad range of pharmacologic effects, including blockade of Na+, Ca+, and K+ channels and antagonism at nicotinic and serotonin receptor sites.
The only cephalopod that has been reported to cause significant envenomation belongs to the genus Hapalochlaena, the blue ring Australian octopus. The octopus can inject its neurotoxin which is stored in modified venom glands. The symptoms are nausea, dizziness, malaise, jerky muscular movements and finally respiratory failure.
Several species of fish are known to be poisonous. They include catfish, stonefish, lion fish, bull trout, weaver fish, stingrays, puffer fish, shell fish and sea hares.
Approximately 1000 species of catfish inhabit fresh and software although many do not contain venomous spines. Spines at dorsal and pectoral fins can inflict wounds in fishermen and less commonly in water sports participants. The venom glands are attached in the leading edges of the spines and are contained within integumentary sheaths.
Penetration of the spine into the skin and sub-dermal tissues simultaneously causes the rupture of the spine integument and venom is then injected into the wound many spines have a series of razor sharp teeth that act as barbs making extraction of these structures difficult.
Serious envenomations have been reported from Arabian Gulf Catfish (Aurius thalasinus) and the oriental or striped cat fish (Plotosus lineatus). The strike causes muscular spasm, respiratory distress, neurotoxicity, hemolysis and death in laboratory animals but these effects are rarely reported in humans.
Stone fish (Synanceia spp) occur throughout tropical warm temperate oceans. The venom apparatus of stonefish is highly developed and consists of paired venom glands that are associated with 13 dorsal spines of the fish. Each spine produces 6 mg of venom. It can cause cardiotoxicity, myotoxicity and neurotoxicity.
Lionfish are probably the most commonly reported fish to cause stings in Untied States. They are the most colorful venomous fish with large fan like fins. Therefore, they are kept in aquariums. Most of the injures have been reported from aquarium keepers.
Scorpion fish occur worldwide but most commonly in tropical and temperate oceans and seas. At least 80 members of the fish have been reported to be poisonous. They exhibit camouflage and may be stationary or slow moving. Their venom apparatus varies between species with most having 10 to 15 dorsal spines, 2 pelvic and 3 anal spines associated with venom glands.
Soldier-fish (Gymmapistes marmoratus) occur in southern Australia. The venom apparatus consists of 13 dorsal, 3 anal, 2 pectoral, and 4 opercular spines. The venom has the same effects as described for stone fish.
The bull trout (Notesthes robusta) lives in tidal estuaries of eastern Australia and belongs to the Scorpaenidae family. They occur in rocks and weeds. It has 15 spines that erect when fish is disturbed. Their venom is called nocitoxin that contain a 170 kd protein.
Weaver fish are marine fish found in the Mediterranean and European coast of Chile. They are bottom dwellers and sting when stepped on. Weaver fish have five obviously visible dorsal spines and a few less visible ones that are responsible for stings. These spines can pierce a leather boot. The venom is proteinaceous and has hemolytic properties. Sting may cause pain and local inflammation for upto a week.
Stingrays have a characteristic dorsoventrially flattened appearance with pectoral flaps that they use for propulsion. Gills are confined to the ventral surface. The venom apparatus of stingrays is a whip like striking organ (spine) attached to the dorsal surface of the fish. The sting can produce a significant laceration and less commonly a punctured wound.
The traumatic wound itself is dangerous and increases the risk of infection. Venom is simultaneously injected. It may cause cardiovascular collapse as well as neurologic effects when injected intravenously or intraperitoneally to rats. Divers sustain injuries to the chest or abdomen.
There are other several venomous fish. Scats occur in the Indo Pacific Ocean. Stings cause immediate pain. They are kept in aquariums. Zebra fish has also been implicated in spine injuries. Australian shark has venomous spines at front of each of the dorsal fin. Puffer fish poisoning is mainly confined to South East Asia and is more common in Japan where fugu is a delicacy and mainly consumed as a food.
Shellfish poisoning is a medical and economic problem that affects many fisheries especially in Japan, Southeast Asia, Central and North America, North Africa and Europe. Shellfish are the vectors for a number of infections including viral and bacterial infections, allergies and toxin poisoning. Four major toxic syndromes that result from shell fish poisoning are – paralytic shellfish poisoning, diarrheic shellfish poisoning and encepalopathic shellfish poisoning.
Amongst Amphibians, toads are the only poisonous animals. They are found all over the world except Madagaskar, New Guinea, New Zealand and Polynesia. Their skin secretes the venom. It may be stored in the parotid glands behind the eyes. Toxicity can occur if toad’s skin or secretions are handled or ingested.
Dried and powdered toad skin have been used as cardiac medicines, hallucinogens, expectorants and diuretics and also for the treatment of toothaches, sinusitis, and bleeding gums. Eyes, nose and throat pain and irritation may follow on contact exposure. Cardiac arrest, atrial fibrillation bradycardia, ventricular fibrillation and hypotension have been described. Seizures have been reported, as have salivation and vomiting.
Another amphibian known to be poisonous is poison dart frog. They are found in tropical rain forests of Central America and South America. The most poisonous amongst them is Phyllobates terribilis. Poison dart frogs secrete batracotoxin that depolarizes electrical membranes through increasing the permeability to sodium ions.
Salamanders secrete a poison known as samandrin. It is a potent neurotoxin. Newts secrete a potent toxin TTX in their skin. Poisoning is caused by the ingestion of flesh, viscera or skin. Ascending paralysis develops which leads to respiratory paralysis.
Amongst lizards, helodermatods are known to be poisonous. They are large, slow moving and primarily nocturnal. Adult lizards can reach to the length of 55 cm. The skin has bead like scales, which can be black or brown in colour.
The lizard has a set of venom glands which can be found in the anterior lower jaw. Heloderma suspectum and H. cinctum inhabit the southwestern parts of the United States. H. horridum is native to Mexico and Guatemala and is generally larger than the sub species found in U.S. The lizard has a powerful bite, and the teeth may cause several small puncture wounds in the skin of the victim.
Envenomation by adult helodermatids produce pain and local swelling at the site of the bite. Lymphangitis, hypotension, and weakness may occur. The venom contains serotonin, amine oxidase, phospholipase, hyaluronidase, protease and salivary kallikrein. Unique compounds include gilatoxin and helothermine. No antivenom is available for helodermatid envenomation.
Example # 11. Snakes:
Of the approximately 3000 species of snakes in the world, about 600 or 20% are venomous. Nearly all of them possess specialized teeth called as fangs. Venomous snakes are found on all continents except Anarctica. Venomous snakes belong to four families: Colubridae, Elapidae, Atractaspididae and Viperidae.
Family Colubridae is the largest family of snakes, with 1864 species in a global distribution. Most of them are nonvenomous but a minority is venomous. They include vine or bird snakes (Thelotornis sp.), red necked keelback (Rhabdophis sp.). Montpelier snake (Malpolon monspessulanus), Argentine black headed snake (Elapomorphus bilineatus) and Boomslang (Dispholidus typus).
Next family of elapid snakes encompass a wide variety of species with 297 species on a global distribution, including African and Asian cobras, mambas, kraits and coral snakes, Australian snakes and sea snakes. In most species fangs have an enclosed groove, acting like a hypodermic needle.
The snake has the ability to control venom release. Indian cobra (Naja naja), king cobra (Ophiophagus hannah), kraits (Bungarus sp.), water cobra (Naja melanoleuea), tree cobras (Pseudohaje spp), burrowing cobra (Paranaja multifasciata) American coral snakes (Micrucoides euryxanthus) Asian coral snake (Maticora spp.) are a few major species of medically important elapid snakes.
Family Atracaspididae have a limited distribution in Africa and the Middle East. They are fossorial, usually hunt the prey beneath the surface. Their fangs come out of the side of the mouth to allow a sideways strike. Most are probably harmless to humans but a few in the genus Atractaspis have toxic venom with unusual components notably the sarafotoxins.
Vipers (Viperidae) are the second family of venomous snakes. All have mobile fangs in front of the mouth on hinged maxillae, allowing the fang to fold away against the root of the mouth when not in use. Their fangs are longer than those of elapid snakes.
These long fangs are often coupled with large venom glands, allowing large quantities of venom to be injected but “dry bites” can also occur. There are 3 typical vipers and pit vipers. Pit organ is located in front of their head. It allows infrared detection of the warm blooded prey in total darkness. Rattlesnakes are best known amongst pit vipers.