Sheep blood is used to make antivenom, and it has been for decades. The question likely stems from confusion about the word “lamb,” but the animals used are adult sheep, not young lambs. Producers immunize sheep with small, escalating doses of venom, then collect the animals’ blood plasma, which by then is rich in venom-targeting antibodies. Those antibodies are purified and processed into a form safe enough to inject into a snakebite victim. Horses have historically been the more common production animal, but sheep-derived antivenoms have gained ground because they can be cheaper to produce, sometimes more effective, and potentially safer for the patient.
Why Sheep Instead of Horses
Most of the world’s antivenom supply still comes from horses. They are large, produce big volumes of blood, and have been used for this purpose since the late 1800s. But sheep offer several practical advantages that have made them increasingly attractive to manufacturers. For one, sheep are cheaper to house and feed. A research review in BioDrugs noted that using sheep instead of horses results in a less expensive product because of lower animal-maintenance costs.1PubMed. Antivenoms That same review observed that sheep-derived preparations should, in theory, cause fewer allergic reactions in patients, because ovine protein tends to be less immunogenic to humans than equine protein.
The performance comparison is not one-sided, though. A head-to-head study in mice found that antibodies raised in sheep against the venom of several medically important snake species provided better protection than their horse-derived counterparts. Ovine antivenoms against the South American rattlesnake, the saw-scaled viper, and the European adder, among others, outperformed equine versions in preventing death and blocking harmful venom activities like hemolysis and abnormal blood clotting. However, equine antivenoms against the monocled cobra and the western diamondback rattlesnake were more protective than the sheep product.2PubMed. A comparison of ovine and equine antivenoms The takeaway is that neither animal is universally superior. The best host depends on the specific venom being targeted.
How Sheep Are Immunized
Making antivenom starts months before any blood is drawn. The sheep must first develop a strong immune response to the venom, which means receiving carefully controlled injections of it over a long period. This is not a single shot. It is a multi-dose immunization program designed to push the animal’s immune system into producing large quantities of highly specific antibodies without making the animal dangerously ill.
A typical protocol, used in a Brazilian government facility for scorpion antivenom production, illustrates what this looks like in practice. Sheep received six doses per immunization cycle. The first injection mixed venom with an adjuvant, a substance that amplifies the immune response. Two more adjuvant-boosted doses followed at three-week intervals. Then three booster shots without adjuvant were given at shorter intervals of about three days each. After the cycle, the resulting blood serum was tested for its ability to neutralize the venom. If the antibody levels were not strong enough, the entire cycle was repeated.3Journal of Veterinary Science. Toxicity of crude and detoxified Tityus serrulatus venom in anti-venom-producing sheep
For snake antivenoms, a similar approach applies. One production process for European adder antivenom immunized sheep monthly with relatively small amounts of venom and then pooled the resulting antisera.4Toxicon. An affinity purified ovine antivenom for the treatment of Vipera berus envenoming Another program immunized young adult male sheep over a two-year period following World Health Organization guidelines, confirming that the animals maintained high levels of specific antibodies throughout.5PubMed Central. The development and evaluation of the efficacy of ovine-derived experimental antivenom immunoserum against Macrovipera lebetina obtusa (MLO) venom The sheep used are typically young adults, around one and a half to two years old and weighing about 40 kilograms, not lambs in the common sense of the word.
From Blood to Injectable Medicine
Once a sheep’s blood contains adequate antibody levels, plasma is collected. But you cannot inject raw sheep plasma into a human. The plasma undergoes several processing steps to isolate and refine the antibody components, and the specific type of antibody fragment produced makes a real difference in how the antivenom behaves in a patient’s body.
Whole antibodies, known as immunoglobulin G or IgG, are large Y-shaped molecules. Many modern sheep-derived antivenoms do not use the whole molecule. Instead, they use a smaller piece called a Fab fragment, which is the part of the antibody that actually binds to the venom toxin. To produce Fab fragments, manufacturers first precipitate the immunoglobulin from the plasma using a salt solution, then use an enzyme called papain to cleave the antibody into its component parts. Additional purification steps, such as column chromatography, remove unwanted proteins and yield a concentrated Fab product.6PubMed Central. Production of an Efficient Enzymatically Fab Fragment Antivenom against Cobra Snake (Naja naja oxiana) Venom The European adder antivenom produced from sheep uses this same core process: salt precipitation followed by papain digestion.4Toxicon. An affinity purified ovine antivenom for the treatment of Vipera berus envenoming
Why bother with Fab fragments instead of using whole antibodies? The answer comes down to how these molecules move through the body and how quickly they are cleared. Fab fragments have a larger volume of distribution, meaning they spread more readily from the bloodstream into surrounding tissues where venom toxins may have migrated. This is an advantage when venom has already begun diffusing away from the bite site. Whole IgG molecules stay in the bloodstream longer but penetrate tissues less effectively. The tradeoff is that Fab fragments are cleared from the body faster, primarily through the kidneys, which sometimes means a patient needs repeat doses if venom continues to be released from the bite site.7PubMed. Pharmacokinetic-pharmacodynamic relationships of immunoglobulin therapy for envenomation
Real-World Use in Humans
Sheep-derived Fab antivenoms are not experimental. They have been used in clinical settings and studied in humans. One of the best-known examples is the antivenom developed for European adder bites, which was tested in a multicenter clinical study. The results showed that specific sheep Fab fragments were both safe and effective in countering the effects of adder envenomation in people.8PubMed. First clinical experiences with specific sheep Fab fragments in snake bite. Report of a multicentre study of Vipera berus envenoming CroFab, the primary antivenom for pit viper bites in the United States, is also a sheep-derived Fab product. It is made by immunizing sheep with the venoms of four North American pit viper species and then processing the resulting antibodies into Fab fragments.
The safety profile of sheep Fab antivenoms is a meaningful improvement over older horse-derived whole-IgG products. Early horse serum antivenoms were notorious for causing serum sickness, an immune reaction triggered by the patient’s body recognizing the foreign equine proteins. Sheep-derived Fab fragments carry less of the non-binding portion of the antibody, which is the part most likely to provoke an allergic response. Reactions still occur, but the overall rate of serious adverse events tends to be lower.
Cross-Reactivity and Polyvalent Antivenoms
A snakebite victim does not always know exactly which species bit them, and hospitals in regions with multiple venomous species need antivenoms that work against more than one snake. This is where cross-reactivity becomes important. An antivenom made against one species’ venom sometimes neutralizes a related species’ venom as well, because many snake venoms share similar toxic proteins.
A study of five medically important Iranian viper species found that cross-reactivity varied widely depending on the source venom. Antivenoms raised against the blunt-nosed viper and the spider-tailed viper showed the broadest cross-reactivity, binding well to venoms from several other species. In contrast, antivenom against the Caucasus viper was effective only against its own venom and essentially useless against the others. The polyvalent antivenom, made by immunizing animals with a mixture of all five venoms, bound broadly across every species tested.9PubMed Central. Immunological Cross-Reactivity and Neutralizing Efficacy of Antivenom Against Five Medically Important Iranian Viper Venoms
This inconsistency is one reason why polyvalent products, made from multiple venoms, are the standard in most tropical hospitals. It also explains why a single “universal antivenom” does not exist. The protein makeup of venoms differs enough across species, and even across geographic populations of the same species, that no single antibody preparation covers them all equally well.
Not Just for Snakebites
Sheep are used to produce antivenoms against more than just snake venom. Scorpion antivenom, as mentioned in the immunization section, is another major application, particularly in Latin America, North Africa, and the Middle East where dangerous scorpion species are common. Researchers have also used sheep as models for evaluating next-generation antivenom formats, such as single-chain antibody fragments engineered in the lab rather than harvested from an immunized animal. In one study, sheep served as large-animal models for testing the pharmacokinetics of a lab-produced single-chain antibody fragment designed to neutralize scorpion toxins.10PubMed. Pharmacokinetic evaluation of a single chain antibody fragment against scorpion toxins in sheep The use of sheep in these studies reflects how central the species has become to antivenom science, both as a production animal and as a research platform.
Why “Lamb’s Blood” Is a Misleading Term
The internet search that likely brought you here probably used the phrase “lamb’s blood,” and it is worth clarifying why that term is a bit off. In antivenom manufacturing, producers use adult sheep, not lambs. A lamb is a sheep under one year old, and its immune system has not had enough time or exposure to develop the robust, high-titer antibody response needed for effective antivenom production. The immunization protocols themselves take months and sometimes years, meaning the animals are well into adulthood by the time their blood is drawn for production. One study explicitly described using male sheep aged one and a half to two years and weighing about 40 kilograms.5PubMed Central. The development and evaluation of the efficacy of ovine-derived experimental antivenom immunoserum against Macrovipera lebetina obtusa (MLO) venom That said, in casual conversation, “lamb” and “sheep” are often used interchangeably, and the confusion is understandable.
There may also be a cultural or religious dimension to the question. The phrase “lamb’s blood” carries strong symbolic weight in several traditions, and some people encountering the fact that sheep blood is used in medicine may connect it to those traditions. The reality is purely practical: sheep produce useful antibodies, are economical to maintain, and yield a product that works well in humans.
The Future Beyond Animal Blood
Animal-derived antivenoms work, but they come with real limitations. Production is slow. Each batch depends on the health and immune response of individual animals. Supply chains are fragile, especially in the tropical low-income countries where snakebite kills the most people. And there is always the risk of adverse reactions to foreign animal proteins, even with refined Fab products. Researchers are pursuing two main alternatives.
The first is monoclonal antibodies, which are lab-produced antibodies designed to target specific venom toxins without ever involving an immunized animal. A review of this approach described monoclonal antibodies and their fragments as a possible alternative for producing antivenoms regardless of the specific venom involved.11PubMed Central. Antibodies as Snakebite Antivenoms: Past and Future The advantage is consistency and scalability. Every batch would be identical, unlike animal-derived products that vary from animal to animal and season to season. The disadvantage is cost. Monoclonal antibodies are expensive to develop and produce, and snake venoms contain dozens of different toxic components, meaning an effective monoclonal product may need to combine multiple different antibodies.
The second approach sidesteps antibodies entirely. Small-molecule drugs that directly inhibit the enzymes in venom are being tested as early interventions that could be given before a patient ever reaches a hospital. Two repurposed drugs, varespladib and marimastat, have shown striking results in animal models. Varespladib blocks phospholipase A2, a key enzyme in many snake venoms, while marimastat inhibits metalloproteinases, another major venom enzyme family. Used in combination, a single dose prevented death in mice envenomed by the most dangerous vipers of Africa, South Asia, and Central America.12Nature Communications. A therapeutic combination of two small molecule toxin inhibitors provides broad preclinical efficacy against viper snakebite A more recent study confirmed that these drugs also counter the venom of Russell’s viper across different geographic populations in India, even when treatment is delayed.13PubMed Central. Preclinical evaluation of small molecule inhibitors as early intervention therapeutics against Russell’s viper envenoming in India
These small molecules could be manufactured as pills or simple injectable solutions, stored without refrigeration, and administered by non-specialists in rural settings where snakebite deaths are most common. They are not a full replacement for antivenom, because they target only specific enzyme classes and would not neutralize all venom components. But as a bridge treatment given in the critical hours before reaching a hospital, they could save thousands of lives. Both drugs are already approved for other uses in humans, which shortens the path to clinical trials for snakebite.
How the Animals Are Treated
The welfare of production animals is a legitimate concern, and it shapes how antivenom manufacturing is regulated. The World Health Organization publishes specific guidelines for the production, control, and regulation of antivenom immunoglobulins, and facilities are expected to follow them. Sheep used in antivenom programs are typically screened for infections and diseases before entering the immunization program. The WHO guidelines cover housing, feeding, veterinary care, and the volume and frequency of blood collection.
For horses, where more detailed data on bleeding protocols has been published, the traditional approach involves collecting six to eight liters of blood per day for three consecutive days, with eight-week rest periods between cycles. After plasma is separated, the red blood cells are resuspended in saline and transfused back into the animal to reduce the risk of anemia. Sheep blood collection follows a similar principle of taking only what can be safely replaced, though the volumes are proportionally smaller given the animals’ body size. One long-term study of 25 sheep maintained high antibody levels over a two-year immunization period, suggesting the animals tolerated the program well enough to remain productive throughout.
Still, advocacy groups have raised concerns about the cumulative stress of repeated venom injections and blood draws. The development of recombinant and small-molecule alternatives is partly motivated by the desire to reduce dependence on animal production systems entirely. Until those technologies are ready for widespread clinical use, sheep and horses remain the backbone of antivenom manufacturing worldwide.