Hog cholera is a highly contagious viral disease of pigs, now more commonly called classical swine fever (CSF). It has nothing to do with human cholera. The disease attacks the pig’s immune system and can range from a rapid, fatal infection to a slow, lingering illness depending on the virus strain involved. It affects both domestic pigs and wild boar, and outbreaks can devastate herds, making it one of the most economically significant pig diseases worldwide.
What Causes It
Hog cholera is caused by the classical swine fever virus, a small, enveloped RNA virus in the same family as the viruses behind bovine viral diarrhea in cattle. It only infects pigs. The virus enters through the mouth or nose, begins replicating in the tonsils, then spreads through the bloodstream into the lymphatic system. It has a strong preference for immune cells, including T cells, B cells, and a type of white blood cell called monocytes. By destroying these cells, the virus cripples the pig’s immune defenses, which is what makes the disease so dangerous and opens the door to secondary infections.
How It Spreads
Direct contact between sick and healthy pigs is the most common route. Infected animals shed the virus in saliva, nasal secretions, urine, and feces. But direct contact is far from the only concern. The virus can travel on boots, clothing, vehicles, pens, and feed that have been contaminated. It also spreads through the transport of infected animals, both legal and illegal.
One particularly important route is contaminated pork. The virus survives in refrigerated meat for months and in frozen meat for up to four years. Feeding pigs kitchen scraps or swill containing infected pork products has historically been a major source of outbreaks. Pregnant sows can also pass the virus to their piglets in the uterus, and those piglets may shed the virus for months after birth without showing obvious signs of illness, silently spreading it through a herd.
How Long the Virus Survives Outside a Pig
The virus is moderately fragile in open environments. Sunlight and drying kill it relatively quickly, and it typically remains infectious in pig housing for only a few days under normal conditions. Cold weather is another story. In winter, it can survive in pens for up to four weeks. At refrigerator temperatures (around 4°C), the virus remains viable for about 84 days. At room temperature (17°C), it lasts around 70 days. It can persist in feces for six weeks or longer and in urine for up to 18 days. In moist, protein-rich environments like meat and body fluids, survival times stretch dramatically.
Signs and Symptoms in Pigs
Symptoms typically appear four to seven days after infection, though it can occasionally take up to 10 days. The disease takes several different forms depending on the virus strain’s aggressiveness and the pig’s age and overall health.
Acute Form
This is the most commonly described version. In the first two weeks, pigs develop high fever, stop eating, become weak, and may develop red, watery eyes. Gastrointestinal problems are common. These early signs are nonspecific, meaning they look like many other pig illnesses, which makes early detection difficult. Around two to four weeks after infection, more distinctive signs emerge: stumbling and loss of coordination, paralysis, and seizures. At the same time, purple or reddish skin discolorations from hemorrhaging may appear on the ears, legs, and belly. These late-stage signs are the “textbook” markers of hog cholera.
Acute-Lethal Form
With highly virulent strains, the disease resembles a hemorrhagic fever. Pigs develop severe bleeding under the skin, fluid in the lungs, and massive drops in blood platelet counts. Blood vessels begin leaking. Death typically occurs two to four weeks after infection.
Chronic and Persistent Forms
Less aggressive virus strains can cause a drawn-out illness where pigs linger for weeks or months with vague symptoms, poor growth, and recurring secondary infections. The persistent form is especially insidious: piglets infected in the womb may appear healthy at birth but carry and shed the virus continuously, acting as hidden reservoirs within a herd.
How It Differs From African Swine Fever
The two diseases look strikingly similar in sick pigs, and African swine fever was originally mistaken for the same disease before being identified as a separate entity in Kenya in the early 1900s. Despite the overlapping symptoms (fever, hemorrhaging, high mortality), the viruses behind them are completely unrelated. Classical swine fever is caused by a small RNA virus, while African swine fever is caused by a large, complex DNA virus. The practical difference that matters most is vaccination: effective vaccines exist for hog cholera, but no commercially available vaccine is widely approved for African swine fever. Pigs that recover from classical swine fever develop strong, lasting immunity. Pigs that survive African swine fever often gain protection only against the exact strain they were infected with, not against other strains.
Prevention and Vaccination
In countries where hog cholera is still present, the primary control strategy is vaccination with a modified live vaccine. The most widely used version, known as the C-strain, has been effective for decades. It produces strong immunity, but it has one significant drawback: standard blood tests cannot distinguish a vaccinated pig from one that was actually infected and recovered. This matters enormously for international trade, since importing countries want proof that pigs are disease-free, not just vaccinated.
To solve this problem, newer “marker” or DIVA vaccines (short for “differentiating infected from vaccinated animals”) have been developed. These include subunit vaccines based on a single protein from the virus’s outer coat, and chimeric vaccines built by combining genetic material from the hog cholera virus with a related cattle virus. These allow laboratories to tell vaccinated animals apart from infected ones through blood testing, which is critical for trade and eradication programs.
Countries that have successfully eliminated the disease, including the United States, Canada, Australia, and most of Western Europe, generally prohibit routine vaccination. Instead, they rely on strict import controls, surveillance, rapid detection, and culling of infected herds to keep the virus out. Bans on feeding swill to pigs have also been a key preventive measure in many regions. In parts of Asia, South America, and Eastern Europe where the disease remains endemic, vaccination programs continue to be the frontline defense.
Why It Matters Economically
Hog cholera is classified as a notifiable disease by the World Organisation for Animal Health, meaning any confirmed case must be reported internationally. A single outbreak in a previously free country can trigger immediate trade bans on pork exports, costing the industry hundreds of millions of dollars. Beyond trade, the direct losses from pig deaths, culling of exposed herds, and movement restrictions on farms in quarantine zones add up rapidly. This economic threat is why biosecurity on pig farms, including controlling who and what enters the property, proper disposal of food waste, and quarantine procedures for new animals, remains so important even in countries that haven’t seen a case in years.