What are snake antivenoms and how do they work?

What is an antivenom?

If you get bitten by a venomous snake, antivenom can be the difference between life, death or lifelong injury. Just a couple of drops of snake venom can have devastating consequences.

Antivenom is a very old kind of medicine described as an “antique” of the drug world. The first snake antivenom was made in the 1890s and the method to make them hasn’t changed much since.

Antivenoms are administered into the blood to bolster the immune system with antibodies to the snake venom. The antibodies work by binding to the venom or its components and then neutralising them, stopping the damage they can do.

What kinds of antivenom are there?

Some antivenoms are monovalent, meaning they treat the bite of a single species of snake. This means that the person has to know what snake they were bitten by in order to get the appropriate treatment.

Other antivenoms are polyvalent and can be used against multiple species. All of the antivenoms in our investigation were polyvalent.

There are some attempts to make a “universal antivenom”, which could combat the venom of any snake from anywhere. However, some experts feel these are a way off – and not everyone believes they can work.

How are antivenoms made?

A snake is “milked” for its venom – and yes, milking snakes is someone’s job.

A small amount of snake venom, not enough to be deadly, is then injected into a large animal – usually a horse, but sheep and llamas are also used. The animal will be given a series of injections, with the amount of venom usually increasing each time until the horse becomes hyper-immunised, meaning its immune system is pumping out as many antibodies as possible.

Why are horses used to make antivenoms?

Horses have strong immune systems that respond to the snake venom by making antibodies. The antibody-packed blood is then drawn from the horse and the plasma, which holds the antibodies, is separated from other parts of the blood, like blood cells. (The rest of the blood is returned to the horse so that it doesn’t become anaemic.)

The plasma is then purified and developed into an antivenom. Polyvalent antivenoms contain a cocktail of different antibodies, some of which neutralise the venom or venoms it is intended to treat.

Scientists are trying to develop antivenoms that could be made in a lab and without the need to use horses. But for now the bulk of antivenom supply is made using animals.

Why does it matter where the snake comes from?

Snake venom varies from species to species, and within the same species – so venom from a snake in one country can be very different to the venom of a snake of the same species in a different country.

Diet, local environment, stress and genetics all influence the venom a snake produces. Researchers have even found differences between the venom of baby snakes and adult snakes of the same species.

So, because antivenom is made using the venom of specific snakes, an antivenom that’s made to treat a snake from one country can be useless in the treatment of a snake from another area or of a different species – something research has demonstrated time and again (including the Bureau of Investigative Journalism (TBIJ)’s own research).

What else affects how well antivenoms work?

The snake isn’t the only variable – there’s the horse (or sheep, or llama) too. The health of the horse, its diet and how it is kept can all affect how it responds to the venom and thus how useful its antibodies are for making an antivenom. If a horse has a cold, the antibodies it develops in response to that cold will be included in the plasma drawn from it.

In 2016, inspections authorised by the Animal Welfare Board of India found thousands of horses were being kept in terrible conditions across several facilities, including ones used by the some of manufacturers in TBIJ’s investigation.

The last living variable is, of course, the person that receives the antivenom. The human body doesn’t necessarily like having foreign matter injected into it. Antivenoms have been known to trigger such a strong allergic response that patients are unable to be treated any further.