A feral swine family of male, female and two piglets in a field.

Feral Swine and Pseudorabies

Samantha ShieldsLivestock diseases, Wildlife

When I was young, my parents would tell me to stay away from wild animals. They said that I could catch a disease, so I heeded their warnings but never really understood the reasons why. As a college student, I am grateful for their wisdom after learning about diseases that wild animals can spread to domesticated animals and humans. A recent incident in Vermont involving a feral pig brought this fact closer to home: On March 25, 2019, the United States Department of Agriculture (USDA) shot and killed a feral pig that had been on the loose for months in Lyndonville, VT (Feral swine shot by USDA, 2019). The lone feral pig tested positive for pseudorabies virus (PRV). PRV has been eradicated in domestic swine populations since the 2000s.

Occasional outbreaks have occurred since then in states that have feral swine populations, because they became reservoirs of PRV and other diseases. Feral swine are a risk for the reintroduction of pseudorabies into domestic swine populations.

What is Pseudorabies Virus?

Pseudorabies virus—also known as Aujeszky’s disease—is a neurological disease unrelated to the rabies virus that affects swine and other livestock. PRV is highly transmissible through direct contact, aerosols, contaminated feed and water, and ingestion of infected tissue. Although this virus does not affect humans, it can be passed from swine to cattle, sheep, deer, bears, cats, dogs, and a number of other mammals. PRV can cause a respiratory disease and in some cases be fatal.

Pseudorabies virus became a big problem during the 1960s. The devastating impact on domestic swine, as well as, the economy had the nation in an uproar trying to solve the problem. Early attempts to control the disease were initiated including quarantines, controlled movement, and vaccinations. However, these attempts only slowed down the continued spread. The National Eradication Plan was created in 1987 with the intention of stopping the disease. The United States Animal Health Association (USAHA) and the United States Department of Agriculture Animal and Plant Health Inspection Service USDA APHIS worked together to develop the eradication program road map (Anderson, et al. 2008). The stages of the eradication road map included:

  1. Preparation – states developed a basic procedure to control and eradicate PRV.
  2. Control – states continued to cooperate within the program guidelines with the goal of identifying infected herds and beginning herd cleanup.
  3. Mandatory Herd Cleanup – cleanup of the infected herds became mandatory for all farmers.
  4. Surveillance – after successfully controlling PRV, efforts were focused on surveillance of the disease.
  5. Free – states were considered free of PRV after one year of no new cases.

The eradication of PRV was successful in domestic pigs and most states were declared free of PRV. There were still a few outbreaks though, in states with substantial feral swine populations.

Domesticated to Wild Pigs

Feral swine may be descendants of escaped or released farm pigs, or hybrids between the feral domestic pigs (Sus scrofa domestica), and the Eurasian wild boar (Sus scrofa linnaeus). There are between 5 and 6 million feral pigs across more than 35 states (Holmstrom, 2013). In Vermont, the Agency of Agriculture, Food and Markets was notified on April 29, 2019 about the positive test of PRV in the lone feral pig. Vermont does not have an established feral swine populations at this time, but the incident concerned many people. Feral pigs co-occur in locations across the country with more than half of all U.S. livestock. This co-occurrence creates danger because it can lead to greater levels of introduced pathogens into domestic livestock populations.

Feral Swine Control Efforts

In response to the feral swine threat, state agencies have been cooperating with the USDA Veterinary Services and USDA Wildlife Services, to introduce solutions for the management of feral swine populations. However, each state has their own combination of control methods based on national standards Some of the control methods include:

  • Monitoring and surveillance of feral swine populations in order to assess the risk of potential disease spread.
  • Sampling feral and domestic swine herds that faced potential exposure from feral swine.
  • Encouraging active trapping and hunting programs of feral swine. Hunters who capture or kill a feral pig are asked to alert officials so that diagnostic testing for PRV can be done.

Pseudorabies is one example of diseases that can be transmitted from feral swine to livestock. According to APHIS, feral swine have been known to carry or transmit over 30 diseases and 37 parasites to livestock, people, pets, and wildlife. Pseudorabies and swine brucellosis are two diseases that have been eradicated in domestic swine populations. However, feral swine continue to pose a risk for reintroduction of both diseases into domestic livestock.

Preventing Pseudorabies Infection

In order to prevent this risk, steps must be taken to protect domestic animals from infection:

  • Report feral swine activity to state APHIS Wildlife Services officials.
  • Report nervous, respiratory, unexplained death issues with sows and piglets to a veterinarian for diagnostic testing.
  • Farmers should take adequate  biosecurity measures for their facilities, to prevent the accidental release of domestic livestock and interactions with feral swine.
  • Biosecurity measures can include: vaccination; proper surveillance and monitoring of premises; strict movement; feed and water management; quarantines.

Prevention is key when dealing with PRV and other feral swine diseases. PRV can be devastating to breeding animals and livestock. However, in the case of an outbreak, following proper control methods and reaching out for help are important steps. Preventative measures can protect the herd and save owners money, time, and their livelihood.


Anderson, A. L, Anderson, L. P, Annelli, F.J, Battrell, M, Beran, W.G, Bigelow, T.T, et al. (2008, October). Pseudorabies (Aujeszky’s Disease) and Its Eradication: A Review of the U.S. Experience. United States Department of Agriculture Animal and Plant Health Inspection Service. Retrieved from https://naldc.nal.usda.gov/catalog/7207242

Holmstrom, L. (2013, April 15-17). Feral Swine and Foreign and Emerging Animal Diseases. Retrieved from https://www.slideshare.net/trufflemedia/

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About the Author

Samantha Shields

Samantha Shields was an undergraduate student at the University of Vermont, studying for a Bachelor of Science in Biological Science, and a minor in Animal Science. She worked with ADBCAP Director Julie M. Smith, DVM, PhD, as an online outreach assistant intern. Samantha assisted with content development for the Healthy Farms Healthy Agriculture website by evaluating, summarizing, and presenting information about protecting animal health. At the University of Vermont, she was an active member of many programs including RALLYTHON, University of Vermont Program Board, Campus Recreation, and the Women’s Club Hockey. Samantha is interested in pursuing the field of epidemiology after graduation. She has a particular interest in the research of different factors that result in diseases, as well as public health emergency planning and response.

About the Editor

Joanna Cummings

Joanna Cummings received a Bachelor of Science in Horticulture from The Pennsylvania State University (PSU), with a specialization in vegetable crop and greenhouse production. At PSU, she was a research technician on no-tillage vegetable crop experiments, and a greenhouse assistant in the All-American Selections Research Gardens. Her career in the agriculture industry includes field research, work with dairy and vegetable farms, and as a greenhouse manager, estate gardener, landscaping and market garden entrepreneur. She transitioned to the science communication field after receiving a master of science degree in environmental communications from Antioch University New England. At Antioch, Joanna was a field botany laboratory teaching assistant, manager of the herbarium, and editor of the department's student newsletter, Notes and Niches. She currently works with Research Professor Julie M. Smith as a communications professional in the University of Vermont Animal and Veterinary Sciences Department. She is the webmaster for the Healthy Farms Healthy Agriculture website.

About the Editor

Dr. Julie Smith

Dr. Julie Smith is a research professor at the University of Vermont. Julie received her B.S. in biological sciences, D.V.M., and Ph.D. in animal nutrition at Cornell University. Since 2002, she has applied her veterinary background to programs in the areas of herd health, calf and heifer management, and agricultural emergency management. She has taught courses in animal welfare, calf biology and management, and ABCs of biosecurity to undergraduates, mostly in the Department of Animal and Veterinary Sciences. As a veterinarian and spouse of a dairy farmer, Julie is well aware of the animal health and well-being concerns of dairy animals. Julie has conducted trainings for Extension educators, livestock producers, and community members on the risks posed by a range of animal diseases, whether they already exist in the United States, exist outside of the United States, or pose a risk to both animal and human health. In all cases, she emphasizes the importance of awareness and prevention. In addition, she has led a number of projects on biosecurity and emergency disease preparedness.