Implications for Humans and Livestock
DONALD S. DAVIS, Department of Veterinary Pathobiology
Texas A&M University, College Station, Tx.
Feral swine like all animals wild or domestic are susceptible to a wide range of infectious and parasitic diseases. Some of these diseases are specifically limited to pigs (Sus scrofa) and while some of the other diseases are shared with other species of wildlife and domestic livestock and a few diseases of feral swine are also shared with humans. The diseases of animals that are transmissible to or shared with humans are known as zoonotic diseases. This manuscript will review some of the diseases of importance that feral swine share with domestic livestock and also diseases of swine transmissible to humans.
Although not an indigenous wild species in North America, populations of feral swine (domesticated pigs that have returned to the wild state) are of sufficient number, ecological impact, and distribution to consider them wildlife in many areas, particularly in the Southeastern United States. Recent estimates of the number of feral swine in the United States vary from 1 to 3 million. The largest number of feral swine are believed to be in Texas, Florida, and California. Feral swine are commonly hated and loved by different segments of society in the same locations. As the numbers and geographic distribution of feral swine continues to increase, it is certain that the number of contacts between feral swine and domestic livestock will also increase as will the probability of human exposure to feral swine directly or indirectly. Not a great deal of information has been documented on the diseases of feral swine let alone any information of diseases spread from feral swine to livestock and humans. Brucellosis and pseudorables are two diseases of feral swin that have been fairly well documented because of ongoing Federal Eradication Programs.
Brucellosis is a bacterial infectious disease of animals and humans caused by members of the Genus Brucella. The effects of the disease in the primary host caused by the various species of Brucella is generally limited to abortions and reproductive organ infections. In other host species such as humans, the disease clinically may mimic severe flu and may vary to crippling arthritis or meningitis. There is no cure for brucellosis in animals and humans are treated with very high doses of antibiotics for extended periods to hopefully clear the infection. Animals and humans are exposed to the Brucella bacterium by handling or contact with infected placentas, amneonic fluids, vaginal discharges, milk semen, reproductive tissues, and exudates from infected animals usually just prior to and after an abortion.
Brucellosis in feral swine in the continental United States was first reported by Wood et. al. (1976) in South Carolina. Eighteen percent of 255 of the feral hogs tested were reactors as determined by the card, complement fixation, and the rivanol precipitation tests. Brucella suis biovar 1 was isolated from the lymph nodes of one boar. Twenty-four percent (10/42) of feral swine in Hawaii had been previously found to have low agglutination titers to Brucella antigens by Nichols in 1962. Becker et al. in 1978 tested 95 serums from feral swine trapped in Florida by the card, standard tube, complement fixation, and rivanol precipitation test and found 50 (53%) to be positive to at least one test. Tissues were collected from 9 swine anb B. suis biovar 1 was isolated from the lymph nodes of all 9. The standard tube test was the only serologic technique utilized that detected Brucella antibodies in all culture positive swine. Bigler et al. (1977) while investigating epidemiologic trends of brucellosis in Florida, reported 22% (6/27) human cases of brucellosis during 1974-1975 as attributable to hunter contact with swine. Lawhorn (1984) reported thta 8 of 76 feral swine from southeast Texas were positive for brucellosis. National Veterinary Services Laboratory, Ames, Iowa has isolated B. suis from several cattle herds in Florida (personal communications, Darla Ewalt, NVSI.), and B. suis was also isolated from a cow in Texas (personnel communications, Jim Alexander, Texas Animal Health Commission). The wide geographic distribution of feral swine and the fact that they are known to be capable of transmission of brucellosis to humans and domestic livestock should be of some serious concern to governmental agencies responsible from the control and/or eradication of this disease.
Pseudorabies is an infectious, often very acute alphaherpes viral disease of the central nervous systems of feral swine that also is found in domestic livestock, cats, and dogs. Pseudorabies is not a zoonotic disease so humans are not infected. Pseudorabies is also known as Aujesky’s disease, mad-itch, and pseudohydrophobia. The disease was first reported in naturally infected oxen, cats and dogs. Up to the early 1960’s in the United States, pseudorabies virus (PRV) was found in young domestic swine and caused limited amounts of mortality. After that date more virulent strains began to occur and losses among adult swine were observed. Now PRV commonly causes abortions and mortality in adult sows. The disease in swine may be spread by asymptomatic carriers, and in carnivores are readily infected by contact or ingestion of infected tissues or carcasses.
The signs of pseudorabies vary widely from species to species, but anorexia, excessive salivation, spasms and convulsions are usually observed in all species. Mad itch is commonly seen in species other than swine, and pseudorabies is almost always fatal. Transmission of the PRV may occur through direct contact, aerosals, contaminated feed, water, ingestion of infected tissues, or contaminated footwear, clothing or trailers. Diagnosis is usually made on clinical signs and a variety of serological tests.
Prevention and control programs for PRV in domestic swine vary from test, isolation, removal, and slaughter methods that may or may not be combined with a vaccination program to increase herd immunity and prevent the shedding of hte PRV. The only method to control PRV in feral swine to date is population control.
PRV virus has been isolated from feral swine in Texas and Florida, and serologic evidence of PRV has been demonstrated in other areas. The spread of PRV from feral swine to domestic cattle has been observed on multiple occasions in Florida and Texas. Contamination by feral swine of supplemental feed spread on the ground for cattle is suspected as the source of infection. This spread from feral swine to domestic swine and cattle is seen as a serious obstacle to the Federal Pseudorabies Eradication program. The exact infection rates of PRV in feral swine populations, the precise amount of transmission from feral swine to livestock, and the geographic distributions of PRV infected swine populations is not well known.
Other Diseases and Parasites
Examples of other infectious diseases of swine that are now foreign to the United States but ones that could present a threat to the entire domestic swine industry if they should reach the continental United States are African swine fever, hog cholera, and foot and mouth disease. The last reintroduction of foot and mouth disease into the United States was in the 1920’s in California. That resulted in the eradication of over 22,000 black-tailed deer in the Stanislaus National Forest. If that incident was to reoccur today in the southcast with the 3 million feral swine and elevated deer populations, I doubt if it could be easily contained.
Swine can carry a variety of parasitic diseases that are also capable of infecting people, but these have not proven to date to be a problem in feral swine. Feral and wild swine in most instances probably do not occur a particular area for sufficient time and in sufficient numbers to facilitate large numbers of internal parasites. Therefore the transmission or spread to humans is also fairly unlikely.
In summary, feral swine are known to be infected with some serious diseases of humans and domestic livestock. Extreme caution should be taken by individuals such as wildlife biologists, guides, hunters, trappers, and game processors in handling the reproductive tracts of feral swine. Disposable gloves or rubber gloves are recommended when the handling of feral swine reproductive tissues is involved. Gloves are suggested as a precaution if one is frequently eviscerating feral swine. Spatial or temporal separation of domestic livestock and swine are also highly recommended. Do not feed cattle or wildlife by placing the food on the ground in areas known to be habituated by feral swine. To do so invites fecal contamination of the feed by droppings and exudates from possibly infected feral swine.
Becker, H.N., R. C. Belden, T. Bruault, et al. 1978. Brucellosis in Feral swine in Florida, J. Am. Vet. Med. Assoc., 173: 1181.
Bigler, W. J., G. L. Hoff, W. H. Hemmert, J. A. Tomas, and H. T. Janowski 1977. Trends of brucellosis in Florida an epidemiologic review, Am. J. Epidemiol., 105(3):245.
Lawhorn, B., 1984. Pseudorabies and brucellosis in East Texas piney wood rooter hogs, Park Producers, July-August, p. 4, 1984.
Nichols, L., Jr. Ecology of the wild pig. 1962. Hawaii Division of Fish and Games, PR Project W5-R-13, pp. 20.
Wood, G.W., J. B. Hendricks, and D. E. Goodman. 1976. Brucellosis in feral swine, J. Wildl. Dis., 12:579.