Pigs are susceptible to a wide range of diseases, from fast-spreading viruses that can devastate entire herds to bacterial infections that pick off individual animals. The most common problems fall into a few broad categories: respiratory infections, digestive diseases, skin conditions, parasites, and systemic viral syndromes. Understanding what each disease looks like and how it spreads is the first step toward keeping a herd healthy.
Porcine Reproductive and Respiratory Syndrome (PRRS)
PRRS is one of the most economically damaging diseases in commercial pig production worldwide. First recognized in the United States in 1987, it targets two groups especially hard: pregnant sows and young piglets. In sows, infection during late gestation causes premature farrowing and delivery of stillborn, mummified, or weak piglets. In neonatal pigs, the virus causes severe pneumonia with interstitial thickening in the lungs that makes breathing progressively more difficult.
Pneumonia from PRRS tends to be worse in young pigs than in adults, and the real danger often comes from secondary bacterial infections that pile on once the immune system is compromised. Adult pigs may show only mild respiratory signs or reduced appetite, which makes the disease easy to miss until reproductive losses start showing up. There is no cure once infection takes hold; management relies on vaccination, strict biosecurity, and herd monitoring.
Swine Influenza
Three subtypes of influenza A currently circulate in pig populations globally: H1N1, H3N2, and H1N2. The illness itself is relatively short, typically lasting six to seven days with fever, respiratory distress, and weakness that resolve within a few days in otherwise healthy animals. The bigger concern is that flu weakens pigs enough to open the door for secondary bacterial pneumonia, which can be far more serious.
Swine flu spreads rapidly through a barn by direct contact and airborne droplets. Outbreaks often hit most of a herd within a week or two, causing a noticeable dip in feed intake and growth. Vaccination is standard practice on commercial farms, and timing matters. Breeding animals are typically vaccinated several weeks before breeding, while piglets receive their first dose at around three weeks of age.
African Swine Fever
African swine fever (ASF) is in a different league. It is a highly infectious hemorrhagic disease with extremely high mortality rates, and there is currently no approved vaccine or treatment. The 2018–2019 outbreak in China led to the culling of 1.2 million hogs and economic losses exceeding $200 billion. From China, the disease spread through Southeast Asia and into parts of Europe. After being absent from the Americas for four decades, ASF reappeared in 2021 with confirmed cases in the Dominican Republic and Haiti.
The global trade consequences are severe. In 2023, countries including Armenia, Australia, Japan, the Philippines, Taiwan, and Ukraine halted pork imports from Sweden after ASF was detected in wild boars there. For pig farmers, the only defense is rigorous biosecurity: controlling who and what enters the farm, sourcing feed carefully, and reporting sick animals immediately.
Porcine Circovirus Type 2 and Wasting Syndrome
Porcine circovirus type 2 (PCV2) is the virus behind post-weaning multisystemic wasting syndrome, or PMWS. Affected piglets, usually in the nursery stage, progressively lose weight, develop swollen lymph nodes, and fail to thrive despite adequate feed. The disease has been reported worldwide for over a decade and remains a significant cause of nursery mortality.
What makes PCV2 tricky is that the virus alone doesn’t reliably produce severe disease in experiments. Additional stressors are needed for clinical illness to develop, including co-infections, overcrowding, or poor colostrum intake. Researchers have observed a strong sow effect, meaning the quality of the mother’s colostrum and antibody transfer plays a major role in whether her piglets get sick. Vaccination of both sows and piglets is now routine on most commercial farms and has dramatically reduced losses.
Digestive Diseases and Diarrhea
Diarrhea is one of the most frequent reasons pigs need treatment, and the cause depends largely on the animal’s age. In pre-weaning piglets, colibacillosis caused by E. coli is a leading problem. The bacteria colonize the small intestine and produce toxins that cause watery diarrhea, rapid dehydration, and death if untreated. Some strains of E. coli (specifically F18-producing strains) can also cause edema disease, a condition that affects the nervous system and causes sudden swelling and death in recently weaned pigs.
In growing and finishing pigs, ileitis is one of the most common gut diseases. Caused by the bacterium Lawsonia intracellularis, it produces thickening of the intestinal lining and chronic diarrhea that leads to poor feed conversion and slower growth. The disease is present on farms worldwide and can range from mild, subclinical infections that quietly reduce performance to acute hemorrhagic forms that cause bloody stool and sudden death. Both colibacillosis and ileitis are manageable with a combination of hygiene, strategic antibiotic use, and vaccination where available.
Greasy Pig Disease
Exudative epidermitis, better known as greasy pig disease, is a skin infection caused by toxin-producing strains of Staphylococcus hyicus. The toxins attack a structural protein in the skin, causing it to flake, ooze a greasy secretion, and form dark crusts that can eventually cover the entire body. It primarily strikes piglets, and while it occurs sporadically on most farms, occasional major outbreaks can affect large numbers of animals at once.
The disease typically enters through skin wounds, including scratches from fighting, rough flooring, or teeth clipping. Mildly affected piglets may have localized patches behind the ears or on the flanks, but severe cases become systemically ill, stop nursing, and can die from dehydration and secondary infections. Good hygiene at birth, prompt treatment of skin injuries, and reducing overcrowding are the main preventive strategies.
Internal Parasites
The large roundworm, Ascaris suum, is the most economically important internal parasite of pigs. It has a high prevalence on farms worldwide, and its lifecycle includes a damaging migration phase: larvae hatch in the gut, travel through the liver, pass through the lungs (causing coughing), and return to the intestine to mature. This migration leaves white scarring on the liver, which leads to condemnation at slaughter, a direct financial loss.
Even when worm burdens aren’t high enough to cause obvious illness, infected pigs absorb nutrients less efficiently. Lower weight gains in infected animals are thought to result from this impaired nutrient absorption, combined with reduced feed conversion. Regular deworming programs and all-in, all-out management that allows thorough cleaning between groups are the standard approach to keeping parasite loads under control.
The Role of Barn Environment
Many pig diseases are made worse, or even triggered, by poor housing conditions. Ammonia buildup in poorly ventilated barns is a major contributor to respiratory problems. Exposure to ammonia at 50 parts per million for just three hours can cause coughing, irritation of the eyes and airways, and reduced feed intake. While the traditional safe threshold has been 25 ppm, more recent guidance suggests keeping levels at or below 10 ppm to protect both pig and human health.
High ammonia concentrations damage the lining of the respiratory tract, making pigs more vulnerable to infections like PRRS and swine flu. Overcrowding compounds the problem by increasing both ammonia production and the speed at which pathogens spread. Adequate ventilation, proper manure management, and appropriate stocking densities are foundational to disease prevention, sometimes more effective than any vaccine.
Zoonotic Risk: Streptococcus Suis
One pig disease that deserves attention for a different reason is Streptococcus suis, a bacterium that can jump from pigs to humans. People acquire the infection through handling pigs during slaughter, processing raw pork, or eating undercooked pork products. Those at highest risk include slaughterhouse workers, butchers, and pig breeders. Skin injuries provide a direct entry point, and about one quarter of documented human cases involved a cut or wound.
In humans, S. suis most commonly causes meningitis, accounting for 68% of reported cases. Other manifestations include bloodstream infection, joint inflammation, and heart valve infection. The overall case fatality rate is about 12.8%, and survivors frequently suffer lasting consequences: hearing loss occurs in roughly 39% of cases, and balance problems in about 23%. Wearing gloves during slaughter, covering open wounds, and cooking pork thoroughly are straightforward protective measures, particularly in regions of Southeast Asia where food-borne transmission is more common.
Core Vaccination Practices
Vaccination forms the backbone of disease prevention on most pig farms. The three pathogens most commonly vaccinated against are influenza, PCV2, and Mycoplasma (a bacterium that causes chronic pneumonia). These are often available as combination products, simplifying the process. Gilts and sows are vaccinated several weeks before breeding to ensure piglets receive protective antibodies through colostrum. Boars are typically vaccinated every six months. Piglets and feeder pigs receive their first doses at three weeks of age or older, with some protocols calling for a second dose for influenza and Mycoplasma.
Beyond these core vaccines, farms dealing with specific problems may add vaccines for E. coli, erysipelas, or other pathogens depending on their herd history. Vaccination works best as part of a broader strategy that includes biosecurity, good nutrition, clean housing, and parasite control. No single tool prevents all disease, but combining these measures dramatically reduces losses and keeps pigs growing efficiently.