CHOICES
A publication of AAEA
Pandemics are increasing in frequency and magnitude as the human population grows. Three main drivers of pathogen emergence are wildlife engagement (trade, hunting, and recreational uses), agricultural intensification and expansion into previously wild areas, and destruction of tropical rainforests (Bernstein, 2022). One common link between these factors is the presence of wildlife. Many zoonotic diseases, which are caused by pathogens that can be transmitted between animals and humans, originate in wildlife. It is estimated that at least 79% of reportable domestic animal diseases have a wildlife component, and at least 40% are zoonotic (Miller et al., 2017). Monitoring the health of wildlife populations can help researchers identify potential sources of zoonosis, develop strategies to prevent or control their spread, and provide a means of measuring the economic efficiency of managing diseases in wildlife to prevent economic impacts when these diseases are transmitted to livestock.
The competitiveness of U.S. livestock in domestic and international markets is constantly threatened by diseases. With approximately 78 million domestic swine raised in 2020, approximately 26% of which is exported (USDA-FAS, 2021), this livestock industry is the second largest in the United States (Figure 1). Because the industry is at heightened risk of disease spread due to their invasive relatives, wild pigs, disease surveillance of wild pigs has increased sharply in the past 2 decades.
Wild pigs (Keiter, Mayer, and Beasley, 2016), are an invasive species comprised of escaped domestic swine, Eurasian wild boar, and hybrids of the two (Smyser et al., 2020). Millions of wild pigs are distributed across many states, causing extensive damage to crops, depredating livestock and native wildlife, property, and ecosystems (Harper, 2016; Anderson et al., 2019; McKee et al., 2020; Shwiff et al., 2020; Vercauteren et al., 2019). U.S. federal, state, and territorial agencies are involved in operational activities to reduce wild pig populations where they currently exist and prevent expansion and establishment into naïve areas.
Wild pigs are particularly problematic from a disease transmission perspective as wild and domestic swine are equally susceptible to pathogens, including several foreign animal diseases such as African swine fever, classical swine fever (CSF), and foot-and-mouth disease (FMD) (Miller et al., 2017). Additionally, wild pigs are a highly gregarious and social species, consistently interacting with livestock, wildlife, and humans (Mclean et al., 2021), making them a particularly risky reservoir for disease. The Animal and Plant Health Inspection Service (USDA-APHIS) Wildlife Services (WS) National Feral Swine Damage Management Program (NSFP) conducts surveillance for a variety of diseases in wild pigs including CSF, pseudorabies (PRV), and swine brucellosis (SB).
CSF, also known as hog cholera, is a contagious viral disease in pigs that was eradicated from the United States in 1978. The severity of the illness varies with the strain of the virus, the age of the pig, and the immune status of the herd. This disease is of economic concern for the United States because its introduction could have global economic impacts, particularly through trade restrictions (McKee et al., 2023). PRV is a contagious herpesvirus, and pigs are the only natural host. The virus primarily infects the central nervous system and the respiratory tract and death can reach 100% in piglets. Pigs can display a fever, anorexia, and weight loss, causing significant production losses. PRV was eradicated from the U.S. commercial swine industry in 2004 but is endemic in wild pigs (USDA-APHIS, 2023). SB is a zoonotic, infectious bacterial disease of swine that causes chronic inflammatory lesions in the reproductive organs, leading to abortions, infertility, and low milk production. The commercial U.S. swine industry is free of SB; however, wild pigs remain a reservoir for the disease (Robbins, Almond, and Byers, 2014).
Wild pigs are lethally removed by USDA-APHIS Wildlife Services (WS) to protect agriculture, property, natural resources, and human health and safety. As a part of
these operational activities to manage damage, blood samples are collected from a subset of the wild pigs removed and samples are tested for antibodies against CSF, PRV, and SB. Additionally, WS personnel collect metadata correspondent to each sample, including collection site and animal age class and sex. The WS official system of record for feral swine disease surveillance data, DART (Data And Results Transmission), maintains wild pig sample data for 40 states and three territories (Puerto Rico, Guam, and the U.S. Virgin Islands). Wild pig sampling began in 2006.
National disease surveillance in the United States for wild pigs provides an early detection system for ASF, CSF, and FMD (Miller et al., 2023). Each county within the invaded range of wild pigs is assigned a surveillance priority; 50%, 20%–30%, and 10%–20% of samples must come from high, medium, and low priority counties, respectively. The surveillance prioritization is based on risk of introduction comprised of (1) the presence of landfills, seaports, and airports and (2) mail and passenger cargo interdiction data as well as host risk, which is determined by (1) the presence of domestic livestock and (2) wild pig populations. The annual sample target is 6,000, and samples must be distributed both spatially and temporally. Upon collection, samplesare sent to the National Wildlife Research Center(NWRC), where quality control and data integrity are checked prior to the samples being sent to laboratories for diagnostics. Diagnostic testing for CSF is conducted at the Foreign Animal Disease Diagnostic Laboratory (FADDL) at Plum Island in New York; PRV and SB samples are sent to the Kentucky Federal Brucellosis Laboratory for testing. For each disease, the diagnostics used are antibody based, indicating historical infection, such that the results are reported as number of positive samples over the total number of samples tested. Lab results are then reported through DART 3–6 weeks after submission.
Between October 1, 2021, and September 30, 2022 (fiscal year 2022), Wildlife Services sampled more than 6,000 wild pigs and tested them for CSF, PRV and SB antibodies (Table 1). The national seroprevalence for U.S. wild pigs sampled was 18.7% and 6.8% for PRV and SB, respectively. This finding aligns with the seroprevalence findings over the prior 4 years (fiscal years 2018–2021), in which national seroprevalence for PRV fell between 18.6%–24.4% and 5.7%–14.5% for SB.
Specifically, PRV had the highest seroprevalence among the three disease types with Hawaii, Florida, and Guam at 53.3%, 38.6%, and 36.2%, respectively (Figure 2). Feral swine in Kansas had a 0% positivity for PRV, with Mississippi, North Carolina, and Missouri also having low seropositivity at 2.9%, 3.7%, and 4.0%, respectively (Figure 2). Guam, Hawaii, and Florida were also found to have the highest seroprevalence of SB in feral swine, with 19.7%, 17.2%, and 13.5% of samples having antibodies, respectively; however, Missouri, California, and Kansas exhibited the lowest SB seroprevalence of 0.5%, 0.8%, and 0.9%, respectively (Figure 3). The United States is currently free of CSF; no samples were found to have antibodies against CSF (Table 1).
The United States is the world’s third-largest producer and consumer of pork and pork products, with hog and pig inventory spread predominantly across the Midwest and North Carolina (Figure 3). The U.S. pork industry exports approximately $7.7 billion annually (USDA-FAS, 2020), with China the primary recipient. Surveillance of these diseases in wild pigs has the potential to significantly reduce an economic disturbance caused by an outbreak, thereby preventing losses in the billions.
While opportunistic sampling in a subset of wild pigs removed for damage management provides only a partial assessment of the true landscape risk of spillover from wild pigs to domestic swine, results from this study
indicate that economically important diseases like SB and PRV are present in wild pigs in states like North Carolina, which has significant hog and pig inventory. Data collected by WS is shared with federal and state animal health officials, who use these data to communicate to backyard swine producers and hunters about the impacts to domestic swine and human safety, respectively. WS disease data and sound estimates of the comprehensive costs of collecting these data can be used to guide and inform management decisions for agriculture and natural resources. The presence of these diseases makes continuous disease surveillance vital focus in the livestock industry.
Bovine spongiform encephalopathy (BSE, more commonly referred to as mad cow disease) illustrates the potential economic consequences of a livestock disease outbreak. In 2003, U.S. beef exports averaged $325 million per month. Following the discovery of BSE in a single U.S. cow in late 2003, average monthly exports plummeted to a monthly average of $59 millionduring 2004, an 82% reduction (Fox et al., 2005). In April 2007, beef exports finally recovered to levels seen prior to the discovery of BSE in the United States (Chen et al., 2020).
As the human population continues to expand at the human-wildlife-domestic livestock interface at increasing rates, the risk of disease transmission is heightened. Understanding the landscape epidemiology of diseases endemic in wild pigs but eradicated in domestic swine (PRV and SB) informs the risk of pathogen spillover. Further, surveillance for foreign animal diseases in feral swine (CSF) provides national and international confidence in detection and reporting ensuring the access to trade. Disease surveillance in wildlife plays a crucial role in safeguarding human health, protecting wildlife, and livestock, as well as promoting a One Health approach to management that considers the interconnections between humans, other animals, and the environment.
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