Pig diarrhea (often called “scours”) has a long list of possible causes, from dietary changes and stress to bacterial infections and parasites. The cause depends heavily on your pig’s age, what it’s been eating, and whether other symptoms like fever or blood in the stool are present. Understanding these patterns can help you figure out what’s going on and how urgently your pig needs help.
Dietary Changes and Protein Overload
One of the most common and least dangerous causes of pig diarrhea is a sudden change in diet. When you switch feeds abruptly or introduce new foods, the bacterial community living in your pig’s gut hasn’t had time to adjust. Undigested protein passes into the large intestine, where bacteria ferment it into toxic byproducts like ammonia and hydrogen sulfide. These compounds damage the intestinal lining and trigger inflammation, pulling fluid into the gut and producing loose stool.
High-protein feeds are a particular culprit, especially in younger pigs whose digestive systems aren’t fully mature. The excess protein essentially feeds the wrong types of bacteria, allowing harmful species to multiply while beneficial ones decline. If you recently changed your pig’s feed, that’s likely your answer. Transition gradually over 7 to 10 days, mixing increasing amounts of the new feed with the old, to give the gut microbiome time to adapt.
Weaning Stress in Piglets
If your pig is a recently weaned piglet, the timing alone may explain the diarrhea. Post-weaning diarrhea is one of the most common health problems in young pigs, typically appearing 3 to 10 days after weaning in piglets aged 3 to 5 weeks. In one monitored study, 34% of piglets developed spontaneous diarrhea after weaning, with new cases peaking around day 5.
The cause is a perfect storm: the piglet loses the protective antibodies from its mother’s milk, its immature gut is suddenly processing solid food, and the stress of separation disrupts normal immune function. This combination creates an opening for E. coli and other opportunistic bacteria to colonize the intestine. Even piglets that seemed healthy during nursing (though 21% in the same study had mild diarrhea while still suckling) can develop significant scours once weaned.
Bacterial Infections
Several bacterial species cause diarrhea in pigs, and they’re widespread. In herd-level testing, Salmonella was found in 51% of herds with diarrhea history, Lawsonia intracellularis in 46.5%, and Brachyspira hyodysenteriae in 37.2%. Each produces somewhat different symptoms.
Salmonella infections cause yellow, watery diarrhea that may contain blood and mucus, particularly as the illness progresses. Affected pigs typically run a fever, stop eating, and become dehydrated. Swine dysentery, caused by Brachyspira, starts with soft yellow-to-grey stool containing large amounts of mucus, then progresses to watery feces with blood and fibrous material. It’s most common in growing and finishing pigs rather than very young ones. Lawsonia causes a condition called proliferative enteropathy. The acute form can cause sudden death with bloody diarrhea and anemia, while the chronic form produces persistent loose stool and poor weight gain.
Viral Causes
Porcine Epidemic Diarrhea Virus (PEDV) is one of the most devastating viral causes, especially in young pigs. It spreads through the fecal-oral route, attacks the lining of the small intestine, and causes severe watery diarrhea with vomiting and rapid dehydration. In suckling piglets, the mortality rate ranges from 50% to 100%. Older pigs can also catch it, but they typically show milder symptoms and are far more likely to survive.
Outbreaks of watery diarrhea with high death rates among nursing piglets are often the first sign PEDV is circulating. Rotavirus and transmissible gastroenteritis virus (TGE) cause similar symptoms and also hit youngest pigs hardest. If multiple pigs in your herd develop profuse watery diarrhea at the same time, a viral cause is likely.
Coccidiosis in Young Piglets
If your piglet is between 6 and 10 days old and develops diarrhea, coccidiosis is a strong possibility. This parasitic infection of the intestinal lining is caused by a single-celled organism and follows a very predictable timeline, with diarrhea beginning in that narrow age window and persisting for roughly five days. The parasite damages the cells that absorb nutrients, so undigested food ferments in the gut, producing acidic, foul-smelling stool. Affected piglets often grow poorly even after the diarrhea resolves because the intestinal damage takes time to heal.
Roundworms and Other Parasites
Ascaris suum, the large roundworm, is the most common internal parasite in pigs. Heavily infected pigs may show difficulty breathing (sometimes called “thumps”), weight loss, slow growth, and a generally unthrifty appearance. You might even see whole worms passed in the manure. While roundworms don’t always cause obvious diarrhea on their own, a heavy parasite load disrupts normal digestion and can contribute to loose stool, especially in combination with other stressors. Regular deworming and fecal testing through your veterinarian are the best ways to keep parasite burdens in check.
Cold Stress and Environment
Temperature plays a bigger role than many pig owners realize, particularly for piglets. Research shows that piglets with diarrhea lose their ability to maintain body temperature in cold environments more quickly than healthy ones. This creates a dangerous feedback loop: cold stress weakens the gut’s defenses, making diarrhea more likely, and diarrhea then makes the piglet less able to stay warm. Diarrhea-affected neonatal piglets need even more warmth and thermal support than healthy ones, not less. If your piglet housing is drafty or poorly heated, environmental stress could be a contributing factor or making an existing problem worse.
What the Stool Looks Like Matters
The color and consistency of your pig’s diarrhea offer real diagnostic clues. Watery yellow stool points toward Salmonella or viral infections. Grey, mucus-heavy stool suggests swine dysentery. Acidic, pasty stool in a piglet under two weeks old is classic coccidiosis. And bloody diarrhea is always a red flag.
Blood in the stool can indicate several serious conditions: Clostridium perfringens type C (which can kill piglets within hours of the first symptoms), the acute form of proliferative enteropathy, or advanced swine dysentery. Piglets with Clostridium infections become weak, reluctant to move, and hypothermic, with body temperature dropping to 95°F (35°C) or below. Their abdominal skin may darken before death. Any pig with bloody diarrhea, a dropping body temperature, or refusal to eat and drink needs veterinary attention immediately.
Keeping Your Pig Hydrated
Regardless of the cause, dehydration is the most immediate threat from diarrhea. A pig that’s still drinking can often be supported with an oral rehydration solution while you work to identify and treat the underlying problem. The World Health Organization’s standard oral rehydration formula, which works for pigs as well as other animals, calls for dissolving 3.5 grams of table salt, 2.5 grams of potassium chloride, 2.9 grams of sodium bicarbonate (baking soda), and 20 grams of glucose or sugar in one liter of clean water.
If you don’t have all those ingredients on hand, a simpler version using 30 grams of ground corn boiled in a liter of water for 20 minutes with 3.5 grams of salt added after cooling provides sodium and energy. Supplement with potassium-rich foods like banana if available. The goal is to replace lost fluids and electrolytes quickly, before mild dehydration becomes a life-threatening crisis. A pig that won’t drink at all, or that has sunken eyes and skin that doesn’t spring back when pinched, needs fluids administered by a veterinarian.