Pseudorabies is a viral disease caused by an alphaherpesvirus that primarily infects pigs but can spread to nearly all other mammals, often with fatal results. Despite its name, the disease has no connection to rabies. It’s also known as Aujeszky’s disease, named after the Hungarian veterinarian who first described it in 1902. In pigs, the virus causes respiratory illness, neurological problems, and reproductive failure. In other animals like dogs, cats, and cattle, infection is almost always deadly within days.
The Virus Behind the Disease
Pseudorabies virus (PRV), formally classified as Suid alphaherpesvirus 1, belongs to the same broad family as herpes simplex virus in humans. It’s one of the most intensively studied alphaherpesviruses in science, partly because of its economic impact on the pork industry and partly because researchers use it as a model to understand how herpesviruses invade the nervous system. Like other herpesviruses, PRV can establish lifelong latent infections, meaning a pig that recovers can carry the virus silently and reactivate it later, shedding it to other animals without showing symptoms.
How It Affects Pigs
The severity of pseudorabies in pigs depends heavily on age. Newborn piglets are hit hardest. Infection in piglets under two weeks old causes severe inflammation of the brain (viral encephalitis), and mortality rates in this age group can approach 100%. Piglets may show trembling, seizures, paddling movements, and loss of coordination before dying within 24 to 48 hours of symptom onset.
Weaned pigs and growing pigs fare better but still develop significant illness, typically showing fever, respiratory distress, sneezing, coughing, and sometimes neurological signs. Mortality drops as pigs get older, though the disease still causes weight loss and setbacks in growth. In adult breeding stock, the most common consequences are respiratory disease and reproductive failure: sows may abort litters, deliver stillborn piglets, or fail to conceive. Adult pigs rarely die from the infection, but they become carriers capable of spreading the virus for life.
Why It’s Called “The Mad Itch”
Pseudorabies earned its dramatic nickname from the behavior it triggers in non-pig species. Dogs, cats, cattle, sheep, and other mammals that contract the virus develop an agonizing, uncontrollable itching sensation, typically on the head or hindquarters. The itching is so intense that affected animals will gnaw, bite, or scratch the area until they cause massive tissue destruction, sometimes exposing muscle and bone. This self-mutilation is one of the most recognizable signs of the disease in livestock other than pigs.
The itch isn’t caused by anything happening at the skin’s surface. It’s neuropathic, meaning the virus itself damages sensory nerve cells. After entering the body, PRV travels along nerve fibers into sensory ganglia (clusters of nerve cells near the spinal cord and brainstem), where it triggers a fierce inflammatory response. Immune cells flood the infected nerve tissue, and the damaged neurons release signaling molecules that amplify the itch sensation far beyond anything a normal skin irritation could produce. The inflammation spreads into the brainstem and surrounding membranes, and the combination of nerve damage and brain inflammation is what makes the disease fatal, typically within two to three days of the first symptoms appearing.
Which Animals Are at Risk
Pigs are the only natural reservoir host, meaning they’re the only species that can survive the infection and continue spreading the virus. For virtually every other susceptible mammal, pseudorabies is a death sentence. Dogs, cats, cattle, sheep, goats, rabbits, foxes, mink, raccoons, and horses have all been documented with fatal infections. The World Organisation for Animal Health notes that the disease spares humans and tailless apes (gorillas, chimpanzees, orangutans, and bonobos), though recent evidence has complicated the picture regarding humans.
Cattle are particularly vulnerable when housed near pigs. Dogs on farms may become infected after eating raw pork scraps or contacting pig carcasses. There is no treatment or vaccine for non-pig species, and by the time symptoms appear, the outcome is already determined.
How the Virus Spreads
PRV transmits through several routes. The most common is direct nose-to-nose contact between pigs. The virus replicates in the tonsils and throat, then sheds through oral and nasal secretions. Infected pigs can spread it through saliva, nasal discharge, and reproductive fluids. Aerosol transmission over short distances is well documented, and in at least one case involving horses, aerosolized pig waste (slurry) was implicated as the source of infection.
Indirect transmission plays a significant role as well. The virus can survive on contaminated surfaces, equipment, and clothing, making biosecurity lapses a major risk factor. Wild boars and feral pigs serve as reservoir populations, capable of transmitting PRV to domestic herds through fence-line contact or shared grazing areas. Raccoons can also carry and transmit the virus to each other through direct contact. Even insects like winter ticks have been investigated as possible mechanical vectors, and consumption of contaminated wild boar meat has been identified as an infection route for some wildlife species.
Eradication in Commercial Herds
The United States declared its commercial swine herds free of pseudorabies in 2004, following decades of testing and vaccination programs across all 50 states. This success relied heavily on a clever vaccine strategy known as DIVA, short for “differentiating infected from vaccinated animals.” Vaccines used in eradication programs are engineered with specific viral genes deleted. When a pig is vaccinated, it develops antibodies against most of the virus but not the deleted gene. A blood test can then distinguish vaccinated pigs (missing that specific antibody) from truly infected ones (which have antibodies to the complete virus). This allowed regulators to identify and remove infected animals from herds without being confused by vaccinated ones.
Diagnosis relies on two main approaches. Antibody-based blood tests detect immune responses in exposed animals, while PCR testing identifies the virus’s genetic material directly, with sensitivity down to about 10 copies of viral DNA per reaction. The antibody tests are simpler and widely used for herd screening, while PCR is the go-to for confirming active infections.
Despite the eradication success in commercial operations, feral swine remain a persistent problem. Wild pig populations across the United States still harbor the virus, and sporadic infections continue to pop up in outdoor production herds or farms where domestic pigs can contact feral ones. This ongoing wildlife reservoir is the primary reason pseudorabies hasn’t been fully eliminated from the country.
The Emerging Question of Human Infection
For decades, pseudorabies was considered essentially harmless to humans. That understanding has shifted. As of 2023, 49 laboratory-confirmed human cases have been documented worldwide, with 32 of those concentrated in China between 2011 and 2023. Since 2017, novel PRV variants circulating in Chinese pig populations have shown the ability to cross species barriers into humans, causing encephalitis (brain inflammation) and severe eye infections with high rates of death and lasting disability.
Human infections have occurred through direct contact with infected animal tissues, saliva, nasal secretions, or other bodily fluids, with the virus entering through skin wounds or mucous membranes. Exposure to contaminated wastewater from pig farms has also been linked to ocular infections. There is no evidence of human-to-human transmission. The risk to the general public remains very low, but for people who work closely with pigs, particularly in regions where newer PRV variants are circulating, the concern is no longer theoretical.