Antivenom is made by injecting small, increasing doses of snake venom into a large animal (usually a horse), collecting the antibody-rich blood plasma after several weeks, and then purifying those antibodies into a form safe enough to give to humans. The core process has remained largely unchanged for over a century, which is one reason antivenom remains expensive. In the United States, a single vial can cost anywhere from $433 to over $3,800 depending on the product type, and a full course of treatment can exceed $50,000.
Step 1: Collecting Venom From Snakes
The process starts with “milking” venomous snakes kept in specialized facilities. At Brazil’s Butantan Institute, one of the world’s oldest and largest antivenom producers, snakes are milked every 60 days. Technicians place the snake in a container saturated with carbon dioxide gas for about five minutes, which sedates it without causing harm. Once the snake stops moving, a handler massages its venom glands by hand, a process that takes only five to eight seconds per snake.
For pit vipers, the venom drips into a glass beaker covered with thick plastic, kept cold in an ice bath. The plastic cover is swapped between each snake to prevent cross-contamination. After extraction, an antiseptic is sprayed on the snake’s fangs to prevent infection from tiny tissue tears, and the snake is weighed and measured before returning to its enclosure. Coral snakes require a different approach because their venom glands are so small. Handlers administer a drug beforehand that stimulates gland secretion, then use tiny pipette tips fitted directly onto the fangs to collect microliter quantities of venom into chilled tubes.
Safety gear is everywhere. Handlers use hooks, restraining tubes, and tongs, all of which are soaked in disinfectant after each use. Staff enter extraction rooms wearing protective aprons and shoe covers to keep pathogens out of the animal facility.
Step 2: Immunizing the Host Animal
Once enough venom is collected and processed, it’s injected in tiny amounts into a large animal. Horses are the most common choice, though donkeys, sheep, and camels are also used in different parts of the world. The animals typically weigh 200 to 370 kg and are selected for general health and age (usually 3 to 10 years old).
Immunization follows a carefully escalating schedule. In one well-studied protocol, horses received venom doses starting at just 1 milligram and gradually increasing to 7 milligrams, injected under the skin at multiple sites on the neck every two weeks. The horse’s immune system recognizes the venom proteins as foreign and begins producing antibodies against them. Blood antibody levels rise rapidly and typically plateau around the eighth week, at which point the horse’s blood is rich enough in anti-venom antibodies to be useful.
This is where the distinction between monovalent and polyvalent antivenoms matters. A monovalent antivenom is made by immunizing with venom from a single snake species, producing antibodies effective only against that one venom. A polyvalent antivenom uses a cocktail of venoms from several species, so the resulting antibodies can neutralize multiple types of bites. Polyvalent products are especially valuable in regions where multiple dangerous species overlap and bite victims can’t always identify what bit them.
Step 3: Collecting and Processing Plasma
Once antibody levels peak, blood is drawn from the immunized animal. The plasma (the liquid portion containing the antibodies) is separated from the red blood cells, which are typically returned to the animal so it can recover and be immunized again in future cycles.
Raw plasma can’t be injected into a human patient. It contains too many non-antibody proteins that would trigger severe immune reactions. So the plasma goes through a multi-step purification process designed to isolate the useful antibodies while stripping away everything else.
Step 4: Purifying the Antibodies
Purification typically involves three main stages: removing unwanted proteins, breaking the antibodies into smaller fragments, and a final polishing step.
- Removing contaminants: A fatty acid called caprylic acid is added to the plasma at a concentration of about 2%. This causes nearly all non-antibody proteins to clump together and fall out of solution, leaving the antibodies floating in a clear liquid above.
- Enzyme digestion: The isolated antibodies are then treated with pepsin, a digestive enzyme. This clips each Y-shaped antibody molecule into fragments, removing a portion (called the Fc region) that is most likely to cause allergic reactions in patients. The remaining fragment retains the ability to bind and neutralize venom. Getting this step right requires precise control: the enzyme-to-antibody ratio, acidity level (pH 3.2), and incubation time (about 90 minutes) all need to be balanced to avoid destroying the useful parts of the antibody while fully removing the problematic ones.
- Final polishing: The antibody fragments pass through a chromatography column, a kind of molecular filter. Residual impurities (including leftover pepsin) stick to the column while the purified antibody fragments flow through. Additional filtration steps remove any remaining traces of contaminants.
Not all manufacturers use the same approach. Some products retain the whole antibody molecule rather than digesting it into fragments. In the U.S., the two main snakebite antivenoms represent both designs: one uses smaller Fab fragments, while the other uses slightly larger F(ab’)2 fragments. Each has different dosing profiles and costs.
Testing Before Release
Every batch of antivenom must prove it works before reaching hospitals. The standard test measures two things: how lethal a given dose of venom is (the LD50, or dose that kills 50% of test subjects) and how much antivenom is needed to neutralize it (the ED50, or dose that protects 50% of subjects). These values are combined to express potency in units per milliliter, where one unit represents the volume of antivenom needed to neutralize a set amount of venom (commonly 10 micrograms). Historically, these tests required mice, though newer methods using embryonated chicken eggs are being validated as alternatives.
Storage and Shelf Life
Finished antivenom comes in two forms: freeze-dried powder that must be reconstituted before use, or ready-to-use liquid. Freeze-drying was the traditional approach because it makes the product more resistant to heat, which matters enormously in tropical regions where most snakebites occur and refrigeration isn’t always reliable.
Liquid formulations have improved significantly. Modern liquid antivenoms stored at refrigerator temperature (4°C) remain fully potent for at least 3.5 years, and some studies have found liquid antivenoms still active after 60 years in storage. Even brief exposure to tropical heat (37°C for a month) doesn’t measurably reduce their antibody binding activity, which is critical for regions where the cold chain can break down during distribution.
Why Antivenom Is So Expensive
The manufacturing process is slow, labor-intensive, and dependent on living animals at every stage. Snakes must be housed and milked regularly. Horses must be carefully immunized over weeks. Purification involves multiple costly biochemical steps. And every batch needs potency testing before release.
But the manufacturing cost is only a fraction of what patients actually pay. In the U.S., the 2023 Medicare reimbursement rate was $2,078 per vial for one type of antivenom and $433 per vial for another. At average wholesale prices, those numbers jump to $3,838 and $1,584 respectively. Since a typical snakebite requires multiple vials, total antivenom costs alone can reach tens of thousands of dollars, often representing 72 to 75% of the entire treatment bill. Patents, regulatory requirements, and limited competition all contribute to keeping prices high.
Why Side Effects Still Occur
Even highly purified antivenom contains foreign animal proteins, and the human immune system sometimes reacts to them. Early reactions, occurring within hours, can result from residual protein aggregates or antibody fragments that trigger the immune system’s alarm pathways. These reactions range from mild skin flushing to, rarely, full anaphylaxis.
A second type of reaction, called serum sickness, appears 5 to 14 days after treatment. The patient’s immune system gradually recognizes the horse (or sheep) proteins as foreign and mounts its own antibody attack against them, causing joint pain, fever, and rash. The risk of serum sickness increases with the total amount of foreign protein administered, which is one reason manufacturers work so hard to remove non-essential proteins during purification. Products made from smaller antibody fragments from sheep appear to cause less sensitization than whole-antibody products from horses, though no antivenom is completely free of this risk.