Antivenom (or antivenin, or antivenene) is a biological product used in the treatment of venomous bites. It is created by injecting a small amount of the targeted venom into an animal such as horses, sheep, goats, or rabbits; the subject animal will suffer an immune response to the venom, producing antibodies against the venom's active molecule which can then be harvested from the animal's blood and used to treat envenomation in others.

Therapeutic use

Antivenoms typically need to be administered as soon as possible after the venom has been injected to be effective (up to 4-5 hours). Since the advent of antivenoms, some bites which were previously inevitably fatal have become only rarely fatal provided that the antivenom is administered soon enough.

The antivenin is usually purified by several processes but still may contain other serum proteins that can act as antigens. Some individuals may react to the antivenin with an immediate hypersensitivity reaction (anaphylaxis) or a delayed hypersensitivity (serum sickness) reaction and should therefore be used with caution.

Antivenins can be classified into monovalent (when they are effective against a given species' venom) or polyvalent (when they are effective against a range of species, or several different species at the same time). Antivenins for therapeutic use are usally preserved as freeze-dried ampules and must be injected intramuscularly to achieve effect, but some of them are available only in liquid form and must be preserved in a cold chain .

The principle of antivenim was based on that of vaccines, developed by Louis Pasteur. The first antivenom for snakes (called a anti-ophidic serum) was discovered by Albert Calmette, a French scientist of the Pasteur Institute working at its Indochine branch in 1895, against the Indian naja snake . Vital Brazil, a Brazilian scientist developed in 1901 the first monovalent and polyvalent antivenoms for Central and South American Crotalus, Bothrops and Elaps genera, as well as for certain species of venomous spiders, scorpions and batrachia. They were all developed in a Brazilian institution, the Instituto Butantan, located in São Paulo, Brazil.

Availability of antivenoms

Antivenom have been developed for the venoms associated with the following animals:

Spiders

• Sydney funnel-web spider
• Venomous funnel-web tarantula
• Antilicosic serum: wolf spider (Lycosidae genera).
• Antictenidic serum: Brazilian wandering spider (Phoneutria genera).
• Antilatrodectic serum: Red-back spider (Latrodectus genera).

Snakes

• Mamba
• Death adder
• Taipan
• Anticrotalic serum: Rattlesnake (Crotalus durissus)
• Antibothropic serum: Central- and South American Jararaca (Bothrops)
• Antielapidic serum: True coral snake (Elapidae)
• Polyvalent antibothropic-anticrotalic
• Polyvalent antibothropic-antielapidic

No antivenom is yet available against Tetrodotoxin, produced by the blowfish.

Natural and acquired immunity

Although individuals can vary in their physiopathological response and sensitivity to animal venoms, there is no natural immunity to them in humans (some animals, particularly the ophiophagic ones, i.e., that feed on venomous snakes, are genetically immune to some species, by the presence of antihemorrhagic and antineurotoxic factors in their blood).

In theory, immunity can be acquired by humans by administering small and graded doses of venom (according to Greek mythology, this was done by King Mithridates in order to protect himself against attempts of poisoning, therefore this procedure is often called mithridatization). There is, however, no practical purpose and favorable cost/benefit ratio for this, unless vaccination is carried out with zoo handlers of venomous animals, researchers and circus artists. This has been tried with success in Australia and Brazil and total immunity has been achieved even to multiple bites of extremely venomous cobras and pit vipers.

External link

• Snakebite. eMedicine.