A cow develops mad cow disease, formally called bovine spongiform encephalopathy (BSE), almost exclusively by eating feed that contains tissue from an already-infected animal. The infectious agent is not a bacterium or virus but a misfolded protein called a prion, and the main vehicle that carries it into a cow’s body is a protein-rich feed supplement known as meat-and-bone meal. That seemingly simple chain of events, one cow’s remains recycled into another cow’s food, powered the massive BSE epidemic that swept through British herds in the late 1980s and 1990s and ultimately jumped to humans.
Meat-and-Bone Meal and the Recycling Loop
Meat-and-bone meal is a rendered product made from slaughterhouse leftovers: bones, offal, nervous-system tissue, and other parts not sold for human consumption. For decades, the livestock industry used it as an inexpensive, high-protein feed additive for cattle, poultry, and other farm animals. The problem began when carcasses from BSE-infected cattle entered the rendering pipeline. Prions survived the rendering process and ended up in the finished meal, which was then fed to healthy cattle, infecting them in turn.1PubMed Central. Usage of meat and bone meal in animal, poultry and fish feeds: A survey and risk analysis for the occurrence of Bovine Spongiform Encephalopathy in Bangladesh
This created what researchers describe as a feedback loop: an infected cow is slaughtered, its remains are rendered into feed, and that feed infects the next generation of cattle. During epidemic conditions, the loop amplified rapidly because the more infected cattle entered slaughterhouses, the more contaminated feed was produced.2PubMed. A quantitative assessment of the risk of exposure to bovine spongiform encephalopathy via meat-and-bone meal in Japan Epidemiological data from the UK outbreak consistently pointed to tainted meat-and-bone meal as the common cause, and feeding cattle this material remains the only confirmed risk factor for developing classical BSE.3PubMed Central. Bovine spongiform encephalopathy: A review of current knowledge and challenges
What a Prion Actually Does
Prions are unlike any other infectious agent. They carry no DNA or RNA. Instead, a prion is simply a normal brain protein that has folded into the wrong shape. Cattle naturally produce a protein called PrP in their nervous systems. When a misfolded version of that protein enters the body, it acts like a template, forcing the cow’s normal PrP molecules to refold into the same abnormal shape. Those newly misfolded proteins then convert still more, and the process cascades.
The misfolded proteins clump together into dense, sticky aggregates that accumulate in brain tissue. Over months and years, these aggregates destroy neurons and leave the brain riddled with tiny sponge-like holes, which is where the name “spongiform encephalopathy” comes from. The damage is irreversible, progressive, and invariably fatal. There is no treatment and no vaccine.
How Prions Travel from the Gut to the Brain
When a cow eats contaminated feed, the prions first encounter the lining of the gastrointestinal tract. From there, they need to reach the brain to cause disease, and researchers have studied the mechanisms of that journey. Current evidence suggests that prions cross the gut wall, interact with immune cells, and then spread along nerves toward the central nervous system.4PubMed Central. Transmission of prions within the gut and towards the central nervous system The transit is slow. Incubation periods in cattle are long, typically ranging from about four to six years. A cow can be carrying the infection and silently accumulating misfolded prions in its brain for years before showing any outward signs.
Symptoms, when they finally appear, tend to include nervousness, aggression or unusual fearfulness, difficulty standing, loss of coordination, and weight loss despite normal appetite. The popular name “mad cow disease” comes from the behavioral changes that can make infected animals appear agitated and disoriented. Once clinical signs emerge, deterioration is rapid and death typically follows within weeks to months.
Atypical BSE and Spontaneous Cases
The epidemic form caused by contaminated feed is known as classical BSE, or C-type. But cattle can also develop two rarer forms called L-type and H-type BSE, collectively known as atypical BSE. These are caused by prions with different molecular shapes than the classical strain.5PLOS Pathogens. The ultrastructure of infectious L-type bovine spongiform encephalopathy prions constrains molecular models
The critical difference is that atypical BSE does not appear to require contaminated feed. Evidence suggests that L-type and H-type BSE arise spontaneously, much like sporadic Creutzfeldt-Jakob disease in humans, either through a random misfolding event or through a genetic predisposition in the affected animal.6PubMed Central. Atypical BSE: Current Knowledge and Knowledge Gaps These cases are extremely rare and tend to occur in older cattle. Whether atypical BSE can spread naturally between animals remains an open question. Experimental transmission by injecting prion material directly into the brain of healthy cattle has been accomplished in laboratory settings, with both L-type and H-type producing nervous-system disease that shares some similarities with classical BSE but also shows distinct pathological features.7PubMed Central. Experimental H-type and L-type bovine spongiform encephalopathy in cattle: observation of two clinical syndromes and diagnostic challenges
One persistent hypothesis is that atypical BSE in a single cow may have been the original spark that, once recycled through the rendering system, gave rise to the classical BSE epidemic. That idea is plausible but unproven. What is clear is that atypical BSE means the disease cannot be entirely eliminated by feed bans alone; a very low baseline rate of spontaneous cases will likely continue to appear in aging cattle populations worldwide.
Do Some Cows Have Genetic Protection?
In sheep and humans, specific genetic variations in the prion gene strongly influence susceptibility to prion diseases. Certain sheep breeds are nearly resistant. The situation in cattle, however, is more ambiguous. Research has found no strong evidence that mutations in the coding region of the bovine prion gene confer resistance to or increase risk of classical BSE. Two variations in a non-coding region of the gene have been linked to some difference in susceptibility, but the effect is modest compared to what is seen in sheep.8PubMed Central. Genetics of Prion Disease in Cattle
In practical terms, this means you cannot breed a herd of cattle that is reliably immune to BSE the way you can with certain sheep flocks and scrapie. Genetics plays a role at the margins, but feed safety remains the primary defense.
Why Prions Are So Hard to Destroy
Part of what made the BSE epidemic so difficult to control is the extraordinary durability of prions. They are not alive, so you cannot “kill” them in the traditional sense. Standard methods of sterilization that work on bacteria and viruses, such as boiling, autoclaving, chemical disinfection, and UV radiation, are far less effective against prions. Misfolded prion proteins can persist in soil and on surfaces for years and resist temperatures that would destroy any conventional pathogen.9PubMed Central. Prion diseases: Lessons from historical outbreaks and potential emerging ones
This resilience had direct consequences in the real world. Rendering plants that processed cattle carcasses operated at temperatures that would have sterilized virtually any bacterial or viral contaminant, yet prions sailed through. Contaminated equipment in slaughterhouses and feed mills was difficult to decontaminate. And because prions can bind to soil particles, pastures grazed by infected animals remain a theoretical concern, though the oral dose required to infect cattle through grazing on contaminated ground is thought to be very high.
Detecting BSE in Living Animals
One of the most frustrating aspects of BSE for both farmers and regulators is that there is no practical way to screen a living cow for the disease. Prions accumulate primarily in the brain and spinal cord, and the definitive diagnostic test involves examining nervous-system tissue after the animal has been killed.3PubMed Central. Bovine spongiform encephalopathy: A review of current knowledge and challenges In surveillance programs, suspect animals are euthanized and their brains are tested for the presence of misfolded prion protein.
The traditional laboratory approach, the bioassay, involves injecting suspect material into a test animal’s brain and then waiting, sometimes for years, for signs of disease to develop.10TrAC Trends in Analytical Chemistry. It’s a mad, mad, mad, mad cow: a review of analytical methodology for detecting BSE/TSE Faster immunological tests that detect prion protein in brain samples now allow results in hours rather than years, and these rapid tests form the backbone of modern national surveillance programs. But all of them still require brain tissue, which means testing can only happen after slaughter. During the long, silent incubation period, an infected cow looks and behaves normally, and there is no blood test, no imaging scan, and no clinical marker that reliably flags the disease before symptoms appear.
This diagnostic gap has real consequences for food safety. Surveillance programs work by testing a sample of slaughtered cattle, especially older animals and those showing neurological symptoms. The goal is to estimate how much disease exists in a national herd, not to test every single animal. Countries that discovered BSE cases in their herds, including the UK, Japan, Canada, and the United States, have relied on this statistical surveillance approach combined with strict feed bans to manage the risk.
Feed Bans and How the Epidemic Was Controlled
Once the link between contaminated meat-and-bone meal and BSE was established, governments moved to break the recycling loop. The UK banned the use of ruminant-derived protein in cattle feed in 1988 and progressively tightened the rules over the following decade. Other countries followed with their own feed bans. The logic was straightforward: if cattle never eat feed containing prions, the epidemic cycle stops.
It worked, but slowly. Because of the years-long incubation period, cattle infected before the ban continued to develop symptoms and enter the food chain for years afterward. The UK epidemic peaked in 1992 with nearly 37,000 confirmed cases that year. Numbers then declined steadily as the feed ban took hold and older, pre-ban animals aged out of the herd. By the 2010s, classical BSE had become vanishingly rare in countries that enforced feed restrictions.
Modern regulations in most countries go beyond simple feed bans. Specified risk materials, the parts of a cow most likely to harbor prions, like the brain, spinal cord, eyes, and certain intestinal tissues, are removed from all cattle at slaughter and destroyed rather than entering the rendering or food supply. These combined measures have been remarkably effective at suppressing classical BSE, though the occasional atypical case in an older cow reminds regulators that the disease has not been entirely eradicated.
Jumping to Humans and Other Species
BSE is not just a livestock problem. The disease can cross species barriers. During the UK epidemic, BSE prions spread to domestic cats, zoo cats, zoo ruminants, and, most alarmingly, to humans.11PubMed. Bovine spongiform encephalopathy (BSE)–infectious, contagious, zoonotic or production disease? The human form, called variant Creutzfeldt-Jakob disease (vCJD), was first identified in 1996 and is presumed to result from eating beef or beef products contaminated with BSE prions.12PubMed Central. Prion diseases as transmissible zoonotic diseases vCJD is distinct from the classical forms of Creutzfeldt-Jakob disease that arise spontaneously in older people. It tends to strike younger individuals and produces a different pattern of brain damage.
The total number of confirmed vCJD cases worldwide has remained relatively small, roughly 230 as of recent counts, with the vast majority occurring in the UK. But the long and variable incubation period, which may stretch to decades in some individuals, means that the final toll is still uncertain. Researchers have tracked the human fallout from the epidemic closely, and the concern has influenced blood-donation policies in many countries: people who lived in the UK during peak BSE years are still barred from donating blood in some nations, decades after the epidemic subsided.13PubMed. Tracking the human fallout from ‘mad cow disease’
Chronic Wasting Disease and Related Concerns
While BSE in cattle has been brought under control through feed regulation, a related prion disease in deer and elk called chronic wasting disease (CWD) is spreading across North America and has appeared in parts of Scandinavia. CWD raises some of the same cross-species concerns that BSE did, particularly because prions persist in the environment and the disease is transmitted horizontally between animals, not just through feed.9PubMed Central. Prion diseases: Lessons from historical outbreaks and potential emerging ones
There is currently no confirmed case of CWD jumping to cattle or humans, but laboratory studies have shown that species barriers are not absolute, and the BSE experience taught the scientific community to take prion diseases seriously long before a jump is confirmed. The inability to detect prions in living animals, the extraordinary durability of the infectious agent in the environment, and the absence of any treatment all make prion diseases a category of risk that regulators continue to watch carefully. CWD in deer populations is the prion problem that keeps veterinary and public-health researchers up at night now, even as BSE itself has faded from headlines.