Pigs die from a surprisingly wide range of causes, from fast-moving viral diseases to something as simple as a hot day or a mother sow rolling over. For farmers and smallholders, the biggest killers fall into a few major categories: infectious diseases, environmental stress, bacterial infections, feed contamination, and physical hazards in the first days of life. Here’s what poses the greatest threat at each stage.
Crushing and Starvation in Newborn Piglets
The single most common cause of death in piglets before weaning has nothing to do with disease. On commercial farms in England, farmers reported that crushing by the sow accounted for 55% of all pre-weaning deaths among live-born piglets. Sows are heavy animals, and newborn piglets are small, slow, and drawn to warmth. When a sow lies down or shifts position, piglets that don’t move quickly enough are trapped underneath. Starvation and hypothermia account for most of the remaining deaths in this early window, often because smaller piglets are pushed off the teat by stronger littermates.
Viral Diseases With Near-Total Kill Rates
Several viruses can tear through a herd with devastating speed. The most feared is African swine fever (ASF), which causes severe internal hemorrhaging and has a case fatality rate approaching 100%. There is no vaccine and no treatment. ASF spreads through direct contact with infected pigs, contaminated feed, and even water sources. The virus entered the Caucasus region in 2007 through contaminated pork products and later spread across parts of Europe, with contaminated water from the Danube River linked to an outbreak on a farm housing roughly 140,000 pigs in Romania. The infectious dose through liquid consumption is remarkably low, because the virus replicates in the tonsils after contact with contaminated water or feed.
Porcine epidemic diarrhea virus (PEDv) is another major killer, though its impact depends heavily on the age of the pig. In suckling piglets under 10 days old, mortality can reach 95 to 100%. The virus causes rapid, severe diarrhea that dehydrates tiny piglets faster than they can recover. Piglets older than 10 days fare much better, with mortality dropping below 10%. Adult pigs rarely die from PEDv, though they may develop watery diarrhea for several days. Highly virulent strains swept through 31 U.S. states between 2013 and 2015, causing significant losses in breeding operations.
Porcine reproductive and respiratory syndrome (PRRS) has circulated in commercial herds since the late 1980s and remains one of the costliest swine diseases worldwide. In young pigs, it causes respiratory disease and death. In pregnant sows, it triggers late-term abortions and premature farrowing. The virus targets immune cells in the lungs, leading to severe pneumonia and internal bleeding. Healthy adult pigs with some prior exposure often survive, but immunologically naive herds can experience explosive outbreaks.
Bacterial Infections in Nursery Pigs
Once piglets are weaned and moved into group housing, bacterial infections become a leading threat. Streptococcus suis is one of the most common bacterial killers in nursery-age pigs. It causes septicemia, meningitis, joint infections, and inflammation around the heart. Clinical signs appear rapidly, and sudden death without obvious warning signs is a frequent finding. In a typical herd, only a small percentage of pigs show clinical disease at any given time, but outbreaks affecting more than 20% of at-risk animals do happen.
One six-month study tracking nearly 2,800 nursery piglets recorded an average post-weaning mortality of 14.4%, with most deaths occurring during the second through fourth week after entry into the nursery. After the fourth week, mortality dropped sharply. The timing matters because weaning is an intensely stressful transition. Piglets lose maternal antibodies, change diets, and encounter new pathogens in a shared environment, all within days.
Respiratory infections are the top killer in older growing and finishing pigs. USDA survey data found that respiratory problems accounted for nearly 48% of reported deaths in grower-finisher pigs. In nursery pigs, scours (severe diarrhea) and respiratory problems were nearly tied as the leading causes, each responsible for roughly a quarter of deaths.
Heat Stress
Pigs are uniquely vulnerable to heat because they cannot sweat effectively. When the ambient temperature rises above a pig’s upper critical temperature, evaporation through panting becomes their only route of heat loss, and it’s not very efficient. The comfort zone varies by size: smaller pigs tolerate warmer temperatures, while large finishing pigs and breeding sows overheat more easily.
What makes heat stress particularly dangerous is what happens internally. High temperatures damage the lining of the intestines, making the gut “leaky.” Bacteria and their toxic byproducts then cross into the bloodstream, triggering a systemic inflammatory response. In severe cases, this cascade of inflammation and organ stress is what actually kills the pig, not the temperature alone. Providing shade, ventilation, and water misters are the primary defenses for outdoor and poorly ventilated operations.
Contaminated Feed and Mycotoxins
Mold-produced toxins in grain and feed are a persistent, sometimes invisible, killer. The three most important mycotoxins for pigs are aflatoxins, deoxynivalenol (commonly called vomitoxin), and fumonisins. All three suppress the immune system, damage the gut lining, and kill cells directly. Young pigs are the most sensitive. Regulatory limits reflect this: in the U.S., finished feed for nursery pigs must contain no more than 0.02 mg/kg of aflatoxins, while feed for pigs over 100 pounds is allowed up to 0.2 mg/kg.
European limits are even stricter, capping aflatoxin B1 (the most toxic and most common form) at 0.01 mg/kg for young pigs. Fumonisins are allowed at higher concentrations, up to 5 mg/kg in Europe and 10 mg/kg in the U.S. for nursery feeds, because they’re less acutely toxic but still cause chronic damage over time. The practical risk is highest when grain is stored in warm, humid conditions that promote mold growth, or when feed ingredients are sourced from regions with poor quality control.
Interestingly, pigs are more resistant than cattle or sheep to certain plant-based toxins. They’re generally not susceptible to poisoning from sorghum-type plants, and they handle nitrates better than ruminants because of differences in how their digestive systems process these compounds. That said, pigs with access to pasture can still be poisoned by specific plants like cocklebur seedlings or water hemlock if they root up and eat them.
Predators
For pigs raised outdoors or feral populations, large predators pose a real but limited threat. Mountain lions, wolves, coyotes, and bears will take piglets and occasionally adult pigs. Feral pig populations are large enough that some predators, particularly mountain lions, rely on them as a food source. For domestic pigs in outdoor systems, the main predator risks come from coyotes and stray dogs targeting young piglets, especially at night. Secure fencing and enclosed farrowing areas reduce these losses significantly.