DISEASE RISKS ASSOCIATED WITH 

INCREASING FERAL SWINE 

NUMBERS AND DISTRIBUTION IN THE 

UNITED STATES 

Thomas Hutton, USDA-APHIS-Wildlife Services 

Dr. Thomas DeLiberto, USDA-APHIS-Wildlife Services 

 Dr. Sheldon Owen, USDA-APHIS-Wildlife Services 

And 

Bruce Morrison, Nebraska Game and Parks Commission 

For the 

Midwest Association of Fish and Wildlife Agencies 

Wildlife and Fish Health Committee 

July 11, 2006
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DISEASE RISKS ASSOCIATED WITH INCREASING FERAL SWINE 

NUMBERS AND DISTRIBUTION IN THE UNITED STATES 

 

INTRODUCTION 

Feral hogs or “wild boars” come from several sources and include released or escaped 

domestic swine and the truly wild European boar. When free-roaming in North America, all 

are included in the term “feral swine,” as are hybrids of the two types. Although 

morphologically distinct, both the feral swine and European wild swine are recognized as Sus 

scrofa. The physical damage caused by feral swine has been well documented and includes 

damage to vehicles, vineyards, tree plantings, archaeological sites, agricultural crops, turf, 

soils, rare plant communities, and wildlife habitat (Seward et al. 2004). In addition they 

compete with livestock and native wildlife for food resources; prey on domestic animals and 

wildlife; and carry diseases that affect pets, livestock, wildlife and people (Seward et al. 

2004). Texas, the state with the largest feral hog population, reports the annual damage to 

agriculture at $51.8 million (Adams et al. 2005). The total damage caused by feral swine in 

the United States is estimated to be approximately $800 million annually (Pimentel et al. 

2000). This estimate is approximate, and probably conservative, because environmental 

damage costs attributable to feral swine are not easily quantified nor are the costs of potential 

disease outbreaks. 

The Midwest Association of Fish and Wildlife Agencies, Wildlife and Fish Health 

Committee, is greatly concerned about the expanding populations and range of feral swine in 

member states and elsewhere. This review of the potential disease impacts was undertaken 

to inform administrators and biologists of the threats this exotic mammal poses to companion 

animal, livestock, native wildlife, and human health. Feral swine could also play a role in the 

spread and amplification of several foreign animal diseases, which could severely limit 

opportunities for hunting, fishing, and other outdoor recreation; reduce agency income; 

require substantial public funds for eradication and thereby severely reduce resources 

available for traditional wildlife management. 

FERAL SWINE POPULATIONS AND DISTRIBUTION 

Feral hogs in North America are believed to have originated from domestic hogs introduced 

by early settlers from Europe. Christopher Columbus probably made the first introductions 

in 1493 in the West Indies and Desoto in 1593 in Florida (Towne and Wentworth 1950). 

Domestic hogs reportedly were also released in California by Spanish explorers in 1769 

(Hutchinson 1946) and in the Hawaiian Islands by Captain Cook in 1778 (Kramer 1971). 

Since their introduction, feral swine populations in the United States have increased both in 

size and distribution. This expansion has been possible because of their omnivorous feeding 

habits, reproductive capacity, behavioral adaptability, the absence of large predators over 

most of their range, and through releases by humans (Bill Kohne, Missouri Conservation 

Department, pers. com., Mayer and Brisbin 1991). 

Pimental et al. (1999) thought that there were 4 million feral pigs in the United States, while 

Muller et al. estimated in 2000 that 3 million were present in Texas alone. In 1988, twenty-
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three states reported having feral swine populations (SCWDS 1988, Mayer and Brisbin 

1991). Invasive swine have continued to expand their range and were present in more than 

30 states in 2002 (Bergman et al. 2002) and in 39 states in 2004 (SCWDS 2004). Since the 

2004 update, feral swine have also been reported in Iowa (Bill Bunger, Iowa DNR, pers. 

com.), Michigan (Timothy Wilson, APHIS, pers. com.), Pennsylvania (Chris Croson, 

APHIS, pers. com.), Maryland (Dan Emanueli, APHIS, pers. com.) and New Jersey (Beth 

Jones, APHIS, pers. com.). In most states, feral swine populations are distributed unevenly, 

with densities varying from abundant to sparse, depending on the quality of the habitat and 

the history of the local population. Feral swine and European boar are also held in enclosed 

hunting preserves or private collections in several states (Nettles 1997) and have been 

released in several locations within the continental United States, particularly in the 

Appalachian Mountains (Lewis et al. 1965). The expansion of feral swine range can be 

attributed to several factors; 1) intentional releases by misguided citizens attempting to create 

sport hunting opportunities, 2) range expansion as population numbers increase, 3) escapes 

from domestic swine facilities and wild boar shooting operations, and 4) habitat alteration 

due to human activities and global warming. 

In the Midwest, feral swine have been reported to occur in Iowa, Michigan, Nebraska, 

Missouri, Kansas, Wisconsin, Illinois, Indiana, Kentucky, and Ohio, with populations in most 

of these states occurring within the last 10 years. The continued advance of feral swine 

range, coupled with habitat alterations related to human activity and global warming will 

insure that this range expansion continues unless action is taken to halt it soon. Once a 

population of feral swine becomes established in an area, eradication is difficult, time 

consuming, and expensive. Prevention of escapes and releases and timely eradication of new 

populations is the best management practice known at this time. 

 

The Southeast Cooperative Wildlife Disease Study mapped feral swine distribution in 1982, 

1988, and 2004 (SCWDS 1982, 1988, 2004a, 2004b). Figure 1 depicts the increased 

distribution from 1982 to 2004. While the increase is evident and substantial, the 2004 status 

is more discernable in Figure 2 that shows feral hog distribution by county (SCWDS 2004b). 
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Figure 1. US Feral Swine Distribution per SCWDS – 1982 & 2004. 
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Figure 2. ’04 Feral Swine Distribution by County. 
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FERAL SWINE DISEASES 

Feral Swine are highly mobile disease reservoirs and can carry at least 30 important viral and 

bacterial diseases (Davidson and Nettles 1997, Samuel et al. 2001, Williams and Barker 

2001) in addition to a minimum of 37 parasites that affect people, pets, livestock, or wildlife 

(Forrester, 1991). Witmer et al. (2003) compiled the list of important feral swine diseases in 

Table 1 from Williams and Barker (2001). 

Table 1. A partial list of viral and bacterial diseases to which feral swine are susceptible. 

 * Indicates zoonotic disease 

Viral Diseases Bacterial Diseases

Bovine herpesvirus* Anthrax* 

Classic swine fever (hog cholera) Brucellosis* 

Coronaviral infections* Erysipelothrix infections* 

Encephalomyocarditis* Helicobacter spp.* 

Foot-and-mouth disease* Leptospirosis* 

Influenza* Bovine tuberculosis* 

Louping-ill virus* Pasteurellosis* 

Malignant catarrhal fever Plague* 

Menangle virus* Salmonellosis* 

Papillomavirus infections* Yersiniosis* 

Parainfluenza virus* 

Pestivirus infections 

Pseudorabies (Aujeszky’s disease) 

Rabbit hermorrhagic disease 

Rinderpest 

San Miguel sea lion virus 

Swinepox virus 

Vesicular stomatitis* 

Zoonoses 

Feral swine carry several diseases that can infect humans including brucellosis, balantidiasis, 

leptospirosis, salmonellosis, toxoplasmosis, trichinosis, trichostrongylosis, sarcoptic mange 

(Seward et al 2004), tuberculosis, tularemia (Hubalek 2002, Stevens 1996), anthrax, rabies 

(Luangtongkum et al. 1986, Helmers and Van Essen 2002) and plague (Burns and Loven 

1998). 

 

Some authors (Amass 1998) report that human infection with swine diseases is rare based on 

the lack of reported human cases. There are also undoubtedly illnesses contracted from swine 

that people perceive as the common flu that are untreated, unreported, or misdiagnosed. A 

recent swine brucellosis incident in Iowa was discovered only when a pork producer was 

diagnosed with Undulant fever (brucellosis) when he went to a hospital for treatment. He had 

contracted the disease from his swine, which had been suffering abortions for six months and 

had been infected by a feral boar (Kevin Petersburg, APHIS, pers. com.). Although human 
7

infection directly from feral swine may be rare, secondary infections through a third host can 

occur. Feral swine may transmit many diseases to other wild mammals, birds, and reptiles, 

which in turn may transmit them to either domestic livestock or humans. 

While serious diseases that pass from swine to humans may be rare in the United States due 

to modern livestock production, disease control, medical technology and limited feral swine 

distribution, diseases like brucellosis, anthrax, rabies, plague, tuberculosis and tularemia can 

be fatal for the infected individual. 

Livestock & Wildlife Diseases 

 

The US livestock industry is currently most concerned with pseudorabies, leptospirosis, 

swine brucellosis, bovine tuberculosis, and vesicular stomatitis that could be spread by feral 

pigs (Seward et al, 2004). Pseudorabies represents a serious threat to domestic swine 

operations as it can cause massive production losses and swine brucellosis can be contracted 

by both humans and domestic livestock. Several other diseases can cause loss of production 

in domestic livestock operations, death to commercial pigs, and large-scale economic loss to 

the producers. Wildlife can be affected by brucellosis, pseudorabies, leptospirosis, plague, 

anthrax (Davidson et al. 1997), tuberculosis, vesicular stomatitis, and tularemia (Dudley and 

Woodford 2002, Hubalek 2002). Wild swine could also play a role in the spread and 

subsequent eradication of foreign animal diseases like classical swine fever, African swine 

fever, Foot-and-Mouth Disease, Rinderpest, Rift Valley fever and Nipah virus (Dudley and 

Woodford 2002), if they emerged in the United States. The emergence of any of these 

diseases in the United States would cause the closure of export markets, be expensive or 

impossible to eradicate, and cause substantial damage to America’s economy and social 

system in the process. Additionally, the passage of various disease agents from feral swine 

to native wildlife species could devastate the local population of infected species. 

Swine as Virus Reassortment Vessels 

As described above, feral swine currently play a role in the spread of many diseases, some 

with significant implications. Feral swine could also play a role in the emergence of new 

Type A Influenza varieties since a variety of avian-like, human-like and swine influenza 

viruses/virus gene segments have been isolated from swine. Several researchers have 

identified the presence of such material in domestic swine in Canada, Italy, Great Britain, 

and the United States (Wright et al. 1992, Webby et al. 2000, Zhou et al. 1999 and Hatchette 

2004). Brown (2004) reviewed current evidence of the transmission of influenza viruses 

between pigs and other species. He concludes: 

Pigs serve as a(sic) major reservoirs of H1N1 and H3N2 influenza viruses which are endemic in 

pig populations worldwide and are responsible for one of the most prevalent respiratory diseases 

in pigs. The maintenance of these viruses in pigs and the frequent exchange of viruses between 

pigs and other species is facilitated directly by swine husbandry practices which provide for a 

continual supply of susceptible pigs which have regular contact with other species particularly 

humans. The pig has been a contender for the role of intermediate host for reassortment of 

influenza A viruses of avian and human origin since they are the only mammalian species which 

are domesticated, reared in abundance and are susceptible to, and allow productive replication of, 
8

avian and human influenza viruses. This could lead to the generation of new strains of influenza 

some of which may be able to transmit to other species including humans. This concept is 

supported by the detection of human-avian reassortant viruses in European pigs with some 

evidence for subsequent transmission to the human population. Following interspecies 

transmission to pigs some influenza viruses may be extremely unstable genetically, giving rise to 

many virus variants, which could be conducive to the species barrier being breached a second 

time. Eventually a stable lineage derived from the dominant variant may become established in 

pigs. Genetic drift occurs in the genes of these viruses; particularly those encoding the external 

glycoproteins, but does not usually result in the same antigenic variability that occurs in the 

prevailing strains in the human population. Finally, it would appear that adaptation of a ‘newly’ 

transmitted influenza virus to pigs can take many years. Both human H3N2 and avian H1N1 were 

detected in pigs many years before they acquired the ability to spread rapidly and become 

associated with disease epidemics in pigs. 

 

While research on virus reassortment has not been conducted on feral swine, they are the 

same species with the same physiology and similar proclivities for disease as domestic swine, 

though without as much intimate contact with humans. At the same time, since they are freeroaming, feral swine have behaviors and habitat preferences that bring them into contact with 

native quail, turkey, wild waterfowl, domestic swine, poultry, and humans and make them 

possible vectors for influenza virus transfer between and among species. The current strain 

of highly pathogenic H5N1 influenza virus circulating in Asia, Europe, Africa, and the 

Middle East is known to infect domestic swine, which act as reassortment vessels for the 

virus. It is feared by many that the swine will be the location of reassortment of the H5N1 

virus into one that is easily transmitted from human to human. 

POTENTIAL ECONOMIC IMPACT OF DISEASES CARRIED BY FERAL SWINE 

Agricultural Economy 

The United States Department of Agriculture’s Economic Research Service forecasts cash 

receipts from the sale of U.S. hogs and cattle at $14.2 and $49.6 billion respectively (2005). 

Obviously, the potential economic impact of disease outbreaks caused by feral swine 

depends on the disease and its variant involved, its rate of spread, the animal species affected 

and population size, the geographic area involved, the ease of eradication, and disposal and 

cleanup costs. For mild infections of one or two people or livestock the economic cost might 

be minimal with slightly lower job productivity with a day or two of sick leave for people, or 

decreased weight gain or reproductive output for livestock. In the Iowa brucellosis situation 

described above, depopulation and indemnification costs amounted to approximately $60,000 

not counting state or federal employee time, lost productivity of the swine operation, or the 

infected operator’s medical costs. For the more serious foreign animal diseases, the ultimate 

cost would undoubtedly be considerably more and would include human health costs, lost 

productivity in the workplace, in addition to agricultural losses. 

Large populations of widely distributed feral swine also jeopardize ongoing efforts to control 

a number of livestock diseases and the considerable investments that support those efforts. 

For example, the United States’ domestic swine recently achieved pseudorabies-free status 

after a 17-year effort and the expenditure of approximately $200-250 million dollars (John 

Korslund, APHIS, pers. comm.). A similar multi-year, multi-million dollar effort is 
9

underway for brucellosis in swine and cattle and the presence of infected feral swine may 

complicate and delay the final success of that program. Witmer et al. (2003) summarized 

surveillance studies of feral swine populations in the United States and reported infection 

rates of 0-46 and 0-53% for pseudorabies and swine brucellosis respectively. With these 

rates of infection feral swine serve as a reservoir for disease reintroduction and pose a 

constant threat to the progress of disease eradication programs in domestic livestock. The 

most serious, potential disease situation would be the reemergence of Foot-and-Mouth 

Disease (FMD) in portions of the United States inhabited by feral hogs. In such an 

eventuality the costs would be greatly magnified and affects on society and the economy 

would be disastrous and many faceted. The economic losses attributed to the FMD outbreak 

in the United Kingdom in 2001 is estimated at over $12 billion. An earlier outbreak in 

Taiwan in 1997 cost $25 billion, almost wiping out Taiwan’s entire hog industry (Pearson et 

al. 2005). Paarlberg et al. (2002) estimated that a FMD outbreak in the U.S., like that in the 

U.K., would decrease U.S. farm income by $14 billion assuming a 10% decrease in 

consumption. If a 20% decrease in consumption occurred, farm income is estimated to 

decrease by $20.8 billion dollars. Although the likelihood of the reemergence of FMD in the 

U. S. is impossible to quantify, FMD is widely distributed in Asia, Africa, and South 

America (Pearson et al. 2005) and could be introduced into the United States accidentally or 

intentionally by terrorists. Current livestock marketing and production systems would 

quickly spread it over wide areas where feral swine would likely contract it. Although the 

role of feral swine in the spread of FMD or other foreign animal diseases is poorly 

understood, their presence poses a definite risk when they interact with domestic livestock 

and wildlife. Additional impacts would be felt in the social sector of our country as livestock 

producers, health care workers, and animal health professionals would be overworked and 

stressed in round-the-clock attempts to stamp out the disease. 

Swine as Amplifiers of FMD 

Pigs usually become infected by eating FMDV contaminated products, by direct contact with 

another infected animal, or by being placed in a heavily infected environment (Kitching and 

Alexandersen 2002). Once FMD is established in the herd, transmission via oral, mucosal 

and through damaged epithelium can be very rapid (Kitching and Alexandersen 2002). Feral 

swine would not only spread FMD mechanically and through these routes, they would also 

excrete large quantities of aerosol virus--up to 108.6 TCID50/24 hours (Donaldson 1999), 30-

100 more times than do cattle and sheep (Andrews 2001), 100 times more than sheep for the 

2001 UK FMD outbreak (Donaldson et al. 2001), and up to 3,500 times more than ruminant 

species (Donaldson 1999) depending on the FMD strain. This output greatly increases the 

risk of airborne transmission of the disease (Sellers and Parker 1969), and would exacerbate 

its eradication and all of the resulting impacts. Fortunately, pigs are relatively resistant to 

infection with airborne FMDV compared to sheep and cattle (Alexandersen and Donaldson 

2002). 

 
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ECONOMIC COSTS TO TOURISM/OUTDOOR RECREATION 

As with agricultural losses, the disease impact on tourism will depend on the disease variant 

involved, its infectivity to humans, its rate of spread, the geographic area involved, the size 

of the quarantine area(s), if any, and the time required for eradication. The most serious 

impacts would again be from an outbreak of Foot-and-Mouth Disease. 

While such an outbreak would affect U.S. tourism in general, the greatest impact 

undoubtedly would fall on outdoor recreation, including fishing, hunting, and wildlife 

watching because of the strict quarantine(s) necessary to control the disease. The cessation 

of hunting, fishing, and/or outdoor recreation activities would economically impact private 

business that relies on these activities for all or a portion of their income as well as 

decreasing revenue to state natural resource agencies. This outcome would have serious 

economic impact since non-residential, wildlife-related recreation is enjoyed by 59.6 million 

people who spend over $105 billion per year (U. S. Fish & Wildlife Service 2001). Nonwildlife related outdoor recreation would also be affected. The Outdoor Industry Foundation 

reports an additional 64 million Americans, 32 million identified as enthusiasts, participated 

at least once in outdoor activities like bicycling, hiking, car camping, canoeing, backpacking, 

climbing, or skiing in 2004 (Outdoor Industry Association 2005). 

UNQUANTIFIABLE BUT REAL SOCIAL COSTS 

The Productivity Commission of the Australian Government (2002) has identified the social 

impacts of severe disease outbreaks and they include the following from lower livestock 

prices and FMD control measures. 

-Financial losses cause attendant social and mental stresses for farmers, their families, 

neighbors, suppliers, and communities. 

-Elimination of livestock as part of disease control would prove traumatic for farmers and 

their families who’s whole lives are invested in the animals. 

-Quarantine and buffer control areas would result in the loss of control for livestock owners 

in the operation of their farms and their daily lives. This would add to feelings of anxiety, 

grief, hopelessness, anger, and frustration. 

-Emergency service personnel may face extraordinary pressures because of the long hours 

and shared trauma from implementing eradication activities and enforcing quarantines. 

-Disruption a family routines and school and cancellation of recreational activities would 

increase travel times and reduction in normal social interactions. 

-Stress would increase health and mental disorders, suicide and alcohol and drug abuse. 
11

SUMMARY 

Feral Swine in the United States are expanding their range, both on their own and with 

human assistance. They are a destructive invasive species that affects many human, 

agricultural, and natural resources but adequate funding for addressing this serious threat has 

not been available. While the physical damage they cause is substantial in itself, the most 

compelling justification for aggressive control of feral swine populations comes from their 

associated disease risks and the role they may play in emergence of new viruses that can 

potentially affect human and animal health and the world’s economy. In consideration of 

these substantial risks, prudence requires that the United States mount a coordinated, 

comprehensive feral swine control program. Such a program will require at minimum 

legislation/regulation changes; a sustained, multi-dimensional public education effort; overt 

and covert law enforcement, and an aggressive, adequately funded, control effort to succeed. 

The Midwest Association of Fish and Wildlife Agencies, Wildlife and Fish Health 

Committee recommends an aggressive effort, conducted by both state and federal agencies, 

to attempt to halt the spread of feral swine populations and, where possible, reduce or 

eliminate existing populations. This effort can only be undertaken by the infusion of 

additional funds from state and federal levels and the establishment of a national, coordinated 

effort with all entities cooperating. 

 

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