Pseudorabies Virus in Feral Hogs

Pseudorabies virus (PRV), also known as Aujeszky’s disease, is an infectious viral disease that is caused by the Suid herpesvirus 1 (SuHV-1).  The name “pseudorabies” means “rabies like” or “false rabies;” however, PRV is a herpes virus and is not related to the rabies virus.  Although humans cannot be infected, PRV is found in several species of mammals (for example, cattle, sheep, goats, deer, bears, cats, dogs, mink, skunks, raccoons, rabbits and rats) as well as swine.  It represents a significant disease of domestic swine and is considered to be the most economically significant viral disease of concern for swine in areas where hog cholera (also called classical swine fever) has been eradicated.  In the United States, PRV has been almost completely eliminated from domestic swine herds.  However, infection commonly occurs in feral hogs (also called wild hogs; Sus scrofa) in this country. 

Swine are reported to be the only species that can serve as a reservoir for PRV.  This disease is highly contagious and transmission is through various forms of contact.  In feral hogs, this transmission is through sexual contact.  In confined or penned domestic herds, transmission is through direct contact, aerosols, contaminated feed, water, infected tissue ingestion, or contaminated footwear, clothing or trailers.  The current concern with PRV is that feral hogs represent a potential source of transmission of this pathogen to non-infected domestic swine.  Further, the recent range expansion and population increase of feral hogs in the United States has increased the potential for this contact with domestic swine.

Symptoms of PRV include loss of appetite, excessive salivation, spasms, and convulsions.  Infections in swine can vary from asymptomatic appearances to fatalities, depending upon the age of the animal.  The mortality rate in very young piglets (less than one month old) is almost 100 percent; however, for piglets between one and six months old, the mortality rate drops to less than 10 percent.  Adult hogs rarely die, but exhibit fever and upper respiratory tract infection.  Infected pregnant sows will either abort or deliver mummified fetuses.  In cattle, the acute infection of the nervous system causes persistent scratching, biting or rubbing, which led to the common name of “mad itch” for this disease.  PRV in non-suid hosts is almost 100 percent fatal.

Diagnosis is usually made on clinical signs and a variety of serological tests.  Primarily based on such serologic data, feral hog populations in numerous states have been shown to be positive for this disease.  Researchers have also demonstrated that PRV can be persistent for long periods of time in these feral populations.

The successful control of PRV in feral hogs is difficult if not impossible to achieve at present.  There is no cure.  In infected domestic swine, lethal elimination of the entire herd is the only successful method of treatment.  In addition, the vaccination of domestic swine herds has not always proved to be effective, and quarterly vaccinations are needed to control this disease.  No vaccines have been developed to date that could be used in feral populations.  Due to the potential high prevalence and latent nature of these infections, the probability of moving this virus with feral hogs during translocation is great.  The only options related to PRV control in feral hog populations are either eradication or the control of illegal relocations by man.  Both of these options would be extremely challenging to realistically accomplish.