The toxins that are produced by animals may produce a variety of reactions and work by many mechanisms.  NOTE that toxins that are directly introduced through bites are often termed venoms.

Microbial Toxins -- The most well know toxin produced by bacteria is the botulinum toxin of Clostridium botulinum.  Persons are most often affected by eating contaminated food.  It causes a chemical denervation by blocking the release of acetylcholine.  This causes muscle paralysis and death is most often due to paralysis of the respiratory muscles.

Aquatic Species

Fish -- The most toxic substance found in fish is tetrodotoxin (its neurotoxicity has been discussed previously, along with sejudotoxin).  It is a sodium channel blocking agent.  Ingestion (from improperly prepared sushi and other sources) will cause a tingling in the mouth.  This is followed within 10-45 min by muscle incoordination, salivation, skin numbness, vomiting, diarrhœa, convulsions, and respiratory paralysis.  Treatment is supportive and symptomatic.

Blue-Ringed Octopus -- Found in the South Pacific and Indian Oceans, also produces tetrodotoxin.

Crown of Thorns Starfish -- Indian and Pacific Oceans -- may cause fever, vomiting, and paralysis.

Sea Urchins -- West Indies -- paralysis and death

Sea Cucumbers -- North American Coast -- blindness and death

Coneshell Snail -- The toxin (omega-conotoxin) is a calcium channel blocker producing muscle relaxation and paralysis.  It is injected into the victim by a modified tooth that is fired from the proboscis of the snail like a dart.  It may cause human death.

Box Jellyfish -- This is the most toxic of the jellyfish, although the entire family is poisonous to some extent.  It injects a necrotising neurotoxin by thousands of nematocycts that are present on the tentacles.  Stings from a box jellyfish are extremely painful and may result in death within a few minutes if untreated.  If the victim survives, the necrosis may cause scarring that will be present for as long as 10 years following exposure.  First aid treatment is the application of a weak acid, such as acetic acid in the form of vinegar, which neutralises the toxin.  Treatment with an anti-venin should follow as quickly as possible.  It is most active in the Australian spring and summer (October to May).

Ants and other insects (wasps, bees) -- Ants will inject formic acid as well as numerous other toxins and venoms, including apamin, melettin, phospholipases, hyaluronidases, histamine, and serotonin.  The initial bite may be quite painful, due to the histamine and serotonin that is introduced into the victim.  However, the primary cause of concern from these animals is for those victims that have developed sensitivity to the venoms that is severe enough to cause anaphylactic shock.  This is best treated with epinephrine A.S.A.P., followed by parenteral anti-histaminics and glucocorticoids along with supportive therapy.

Centipedes, millipedes, and caterpillars rarely, if ever, bite humans.  However, some may possess spikes or external toxins that produce reactions.  Similar to the Hymenoptera discussed above, death will generally occur only if the person goes into anaphylactic shock.

Arachnids

Ticks -- Generally, ticks are most toxic by their role as a vector for bacterial diseases such as Rocky Mountain Spotted Fever or Lyme Disease.  A second cause for concern is secondary bacterial infection arising from the bite.  However, one tick -- the paralysis tick of Australia -- does produce a neurotoxin that can cause paralysis in humans, evening causing death in infants.
Spiders
Latrodectus spp.  (Black Widow and Australian Redback Spiders) -- Only the females possess fangs that are large and strong enough to penetrate skin.  Consequently, only the female is toxic to humans.  In the U.S.A. there are approximately 500 reported bites per year, with less that 1% of these resulting in fatalities.  The venom contains a neurotoxin along with many other constituents.  Signs and symptoms of envenomation include muscle spasms near the bite within 30 minutes.  These may progress to involve back and abdominal muscles to cause severe muscle pain.  Other symptoms include nausea, salivation, and sweating.  Treatment is generally supportive and the antivenin is reserved for severe toxicity.
Loxosceles reclusa (Brown Recluse or Fiddle-back Spider) -- Only the female is toxic to humans.  The venom contains a mixture of toxins including hæmotoxins and neurotoxins.  Signs and symptoms of a bite include delayed pain (2-8 hr after envenomation).  The patient may experience hæmolysis, cyanosis, hæmoglobinuria, fever, chills, weakness, nausea, vomiting, joint pain and delirium, although these symptoms will not be exhibited by all victims.  The more common reaction includes vesiculation (blistering) at the site, redness, swelling, and ulceration, which may progress to include destruction of tissue as deep as the bone.  This toxicity is chronic and progressive and can result in loss of limb function, if severe.  Acute treatment includes dexamethasone (4 mg IM and 4 mg intradermal near the bite).  Dapsone has been effective in limiting the ulceration.  Debridement to inhibit ulceration has proven most effective, but must not be performed too soon, since additional damage may occur.
Atrax robustus (Sydney Funnel Web) -- This Australian spider is probably the most toxic spider, and also one of the most aggressive.  The toxin, atraxatoxin, is an N-type calcium channel antagonist and a neurostimulant.  Envenomation may cause tongue spasms, retching, diaphoresis, salivation, dyspnœa, muscle fasciculations, hypertension, coma, and death within 15 min.  The development of an antivenin in the early 1980s has led to 0 fatalities since 1984.
 
Scorpions (Centruroides spp. in the U.S.A.) -- C. sculpturatis is a deadly scorpion approximately 2 3/4 inches long.  The larger (4 inch) vinegaroon is not deadly but may produce a painful sting, mediated by the toxins formic acid, chlorine, and acetic acid.  South American, African, Middle Eastern, and Indian species may grow up to 7 inches in length.  In general, if the scorpion has thick, heavy pincers, the sting is painful but not deadly.  If, however, the pincers and slender and weak, the scorpion is more likely to be lethal.  The toxin contained in the stinger of scorpions contains a mixture of substances.  Initial pain, which may be absent in children, will cause extreme sensitivity to touch (hyperalgesia), restlessness, abnormal head and neck movements (chorea), nystagmus, and increased startle reflex.  This symptoms, which may present in adults is much more common in children.  Both children and adults may exhibit tachycardia, hypertension, muscle fasciculations (especially of facial and larger muscles), ataxia, slurred speech, salivation, respiratory distress proceeding to paralysis, and convulsions.  Treatment is with anti-venin.

Reptiles

Snakes
Generally, snake toxins may be classified as
myotoxins -- generally necrotising, causing tissue destruction of not only muscle but other tissues
neurotoxins -- which may cause CNS depression or occasionally stimulation
hæmotoxins -- which may cause hæmolysis OR thrombosis
Many snake venoms will contain a mixture of these toxins, so that a combination of symptoms will present following envenomation.  The "danger" rating of a snake is determined by the strength of its toxin (and the length of fangs), the temperament of the particular snake, and its exposure to humans.

Families of Snakes
Crotalids -- Including Agkistrodon (copperhead and cotton mouth or water moccasin), Crotalus (rattlesnakes) and Sistrurus (pygmy rattlesnake) spp.
Envenomation will present as swelling, œdema, and pain within 10 min.  This is generally followed by ecchymoses/hæmorrhaging, weakness, fainting, nausea, lymphadenopathy, and numbness of tongue, mouth, and scalp.  Death is generally due to respiratory paralysis or convulsions.  Treatment is anti-venin and supportive as needed.
Elapids -- There is only one elapid species found in the Southeastern part of the U.S.A., the Eastern Coral Snake (Micrurus -- which resembles the King snake, giving rise to the warning red and yellow can kill a fellow, since the King snake exhibits red and yellow markings separated by black stripes).
Envenomation presents as pain.  Onset of swelling is much slower than with the crotalids.  The patient then presents with drowsiness, weakness, salivation, paralysis of facial muscles, lips, tongue, and larynx; hypotension, respiratory difficulty, blurred vision, and headache.  Death is usually due to convulsions.  Treatment is anti-venin and supportive.
Viperids -- Not found in the U.S.A.  Bites produce local pain, swelling (especially above the knees and elbows), oedema, skin discolouration, ecchymoses, and bleeding from the wound and the gums.
European Snakes -- Primarily vipers such as adders and asps.  The blunt-nosed viper can kill a camel.

Indian Snakes -- Indian has 230 species of snakes with 50 of them venomous including Russel's viper, the saw-sealed viper, common krait, and the common cobra.  Human bites from the King Cobra are rare, but the venom may kill an elephant.

Southeast Asian Snakes -- Similar to Indian snakes, with the addition of the Malayan Pit Viper.

Sub-Saharan African Snakes

Colubrids -- The boomslang and birdsnakes are arboreal. Bites initially cause clotting and then internal haemorrhage, often after the patient appears to have recovered.

Elapids -- The "spitters" include black and green mambas, the Cape cobra, and Ringhals snake.  These produce neurotoxins.

Vipers -- puff adder and bush viper.


Central and South American Snakes -- Predominantly vipers, including the palm pit viper, and various coral snakes (elapids).

Australian Snakes -- Primarily elapids (with the exception of the Sea snake).  Including the sea snake, which is found off the Australian shore, the top 10 venomous snakes are found in Australia.

Sea Snake -- Normally docile.  When it strikes, it may control the amount of venom injected into the victim.  Bites may cause rapid paralysis and death.
Death Adder -- Myo- and neuro-toxins, death is usually due to respiratory failure.
Brown Snake (Eastern and Western) -- This snake is extremely fast and extremely aggressive.  Its venom is a neurotoxin.
Fierce Snake (Inland Taipan) -- Its venom is the most toxic of any snake (on a concentration basis).  However, it only bites once and is found only in areas that are very sparsely settled.
Taipan -- Regarded as the most dangerous snake of Australia, it is the most aggressive, striking and biting repeatedly.  It also resides near heavily populated urban areas.  Its venom is primarily a hæmotoxin.
Tiger Snake -- More human bites are reported from this snake than any other in Australia.  It possesses a hæmo- and neuro-toxin and also resides in urban areas.
 
Sea Snakes -- Envenomation causes skeletal muscle pain (especially with movement), paraesthesias, ptosis, and myoglobinuria.

LD₅₀s of various snakes (mg/Kg ip/iv in mice)

Region	Snake	LD50
North America
	Eastern Diamond Back Rattler	1.68
	Western Diamondback	3.71
	Cottonmouth	4.00
	Copperhead	10.5
	Eastern Coral	0.97
Asia
	Asian cobra	0.4
	Indian krait	0.09
	Russel's viper	0.08
	Malayan pit viper	6.2
Africa
	Puff adder	3.68
	Eastern green mamba	0.45
Australia
	Tiger snake	0.04
	Inland taipan	0.009
Sea	Beaked Sea Snake	0.01

Lizards -- Gila Monster
This lizard of the American Southwest has a venom composed of serotonin, phospholipase, and hyaluronidase.  It causes little tissue breakdown, but may cause hypotension, reflex tachycardia and respiratory distress.  Death, occurring in approximately 1% of bites, is generally due to decreased ventricular contractility.

Monotremes -- Duck-billed platypus and Echidna -- The males of these egg-laying mammals of  Australia possess a spur on the inner aspect of the hind leg.  This is connected to a venom gland in the groin of the animal.  Stinging by the spur causes the venom gland to be emptied.  The victim experiences pain and swelling, which may re-appear following apparent recovery.  No human deaths have been reported, although several canine deaths have occurred.

Shrews and Solenodons -- These animals of the Caribbean resemble rats but have cat or dog-like teeth.  Their saliva provides the toxic response observed following envenomation.

Hedgehogs -- The quills of these animals may exert a toxic effect.  The toxin may be saliva of the animal, which it licks onto the quills.  Some species are also known to collect toad venom and coat their quills with it.