Persistence of pseudorabies virus in feral swine populations

Serologic surveys for evidence of exposure to pseudorabies virus (PRV) in feral swine were conducted from November 2001 to April 2002 at 10 sites in the southeastern United States, where evidence of previous PRV exposure had been documented during 1979-89. Sera were tested in the field on the day of collection by latex agglutination. Maximum sample size per site was to be 30 animals, but sampling was discontinued before reaching this number when positive results were obtained. Positive results were obtained at all of the study sites, demonstrating long-term persistence of PRV in feral swine populations. Overall, 38 of 100 (38%) animals were positive for antibodies. Consistent results from latex agglutination tests conducted in the field and laboratory demonstrated that this test was useful as a rapid and reliable diagnostic tool when used in the field.     

Increased prevalence of Brucella suis and pseudorabies virus antibodies in adults of an isolated feral swine population in coastal South Carolina

Two hundred twenty seven adult (> 8 mo) feral swine (Sus scrofa) trapped from April through July 1999 at three locations on a coastal South Carolina (USA) peninsula with restricted ingress and egress were tested for Brucella suis and pseudorabies virus (PRV) antibodies. Approximately 44% of the animals tested positive for B. suis antibodies and 61% tested positive for antibodies to PRV. Previous surveys (1976 and 1992) of feral swine at the same location with similar methods indicated lower seroprevalences (28% and 18% for B. suis and 0% and 19% for PRV). We also found 39% of feral swine seropositive (n = 179) for Trichinella spiralis and 49% seropositive (n = 181) for Toxoplasma gondii. Results of repeated sampling demonstrated that seroprevalence to pathogens can increase with time in an isolated, unhunted population of feral swine suggesting an increased risk to local domestic livestock and potentially to human health.     

Venereal transmission of pseudorabies viruses indigenous to feral swine

Between 1995 and 1998, we designed a series of studies in which we attempted to determine the main routes of transmission involved in the natural infection of pseudorabies virus (PRV) indigenous to free-ranging feral swine (Sus scrofa). Naturally infected feral sows transmitted the infection to uninfected feral boars, with which they had been commingled for a 6-wk period. Pseudorabies virus was isolated from boar preputial swabs, but not from nasal swabs. Three of the same PRV-infected feral sows did not transmit the infection to domestic boars during a 16 wk commingling period, despite the fact that they became pregnant. Feral boars, naturally infected with PRV transmitted the virus to domestic gilts while penned together during 6 wk. Pseudorabies virus was isolated from vaginal swabs, but not from nasal swabs of gilts, after 2 and 3 wk of commingling. When the same infected boars were commingled with either feral or domestic boars for 13 wk, PRV transmission did not occur. None of the exposed boars developed neutralizing antibodies or yielded virus from their preputial or nasal swabs. Our results indicate that PRV indigenous to feral swine is preferentially transmitted to feral or domestic swine of the opposite sex by the venereal route. This mode of transmission differs from that seen in the natural transmission of PRV prevalent in domestic swine, where contaminated secretions, excretions and aerosols are responsible for the spread of the virus. Based on these results, we feel that as long as feral swine do not come into direct contact with domestic swine, PRV-infected feral swine probably pose only a limited risk to the success of the National Pseudorabies Eradication Program. The fact that PRV is usually transmitted from feral to domestic swine at the time of mating would indicate that the isolation of domestic herds by the use of a "double fence," should be adequate protection against reinfection with PRV.     

Potential sites of virus latency associated with indigenous pseudorabies viruses in feral swine

Free-ranging feral swine (Sus scrofa) are known to be present in at least 32 states of the USA and are continuously expanding their range. Infection with pseudorabies virus (PRV) occurs in feral swine and the primary route of transmission in free-living conditions seems to be venereal. Between 1995 and 1999, naturally infected feral swine and experimentally infected hybrid progeny of feral and domestic swine, were kept in isolation and evaluated for occurrence of latent PRV indigenous to feral swine in sacral and trigeminal ganglia and tonsil. Sacral ganglia were shown, by polymerase chain reaction (PCR) amplification of the thymidine kinase (TK) gene of PRV, to be the most frequent sites of latency of PRV. Nine (56%) of 16 sacral ganglia, seven (44%) of 16 trigeminal ganglia, and five (39%) of 13 tonsils from naturally infected feral swine were positive for PCR amplification of TK sequences of PRV. These tissues were negative for PRV when viral isolation was attempted in Vero cells. DNA sequencing of cloned TK fragments from the sacral ganglia of two feral swine, showed only one nucleotide difference between the two fragments and extensive sequence homology to fragment sequences from various domestic swine PRV strains from China, Northern Ireland, and the USA. The hybrid feral domestic swine, experimentally inoculated with an indigenous feral swine PRV isolate by either the genital or respiratory route, acquired the infection but showed no clinical signs of pseudorabies. Virus inoculated into either the genital or respiratory tract could, at times, be isolated from both these sites. The most common latency sites were the sacral ganglia, regardless of the route and dose of infection in these experimentally infected hybrids. Nine of 10 sacral ganglia, six of 10 trigeminal ganglia, and three of 10 tonsils were positive for PCR amplification of TK sequences. No virus was isolated from these tissues in Vero cells. The demonstration of the sacral ganglia as the most common sites of latency of pseudorabies viruses indigenous to feral swine, supports the hypothesis that these viruses are primarily transmitted venereally, and not by the respiratory route as is common in domestic swine, in which the trigeminal ganglia are the predominant sites of virus latency.     

Aerosol transmission of a viable virus affecting swine: Explanation of an epizootic of pseudorabies

A Gaussian diffusion model was applied to an epizootic of pseudorabies in ten swine herds located in Decatur County, Indiana, USA to test the hypothesis that the virus can be spread via aerosol. The epizootic occurred during January to March, 1988, spreading through ten farms across an area of about 150 km². The model included a receptor component that provided an estimate of viruses received by the pig within an enclosed barn. Results show that the diffusion model can explain the spread of the virus during the epizootic for all nine farms to which the virus spread.     

Visceral helminth communities of an insular population of feral swine

Nine species of helminths, all nematodes, were recovered from the viscera of 48 feral swine (Sus scrofa) from Cumberland Island, Georgia. Both the overdispersed frequency distributions and the abundances of the four common species of helminths (Stephanurus dentatus, Metastrongylus apri, M. pudendotectus and Gongylonema pulchrum) did not vary significantly across the main and interactive effects of host sex and/or seasons. Whether or not the present low population densities of feral swine on Cumberland Island has influenced the pattern of fluctuations in abundances of helminth species across seasons as often observed in helminth communities from other hosts was not resolved. The apparent recent decline in prevalences and abundances, and the loss of certain species from the helminth communities of feral swine on the island may be explained partially by the decreasing transmission potentials of direct life cycle species caused by a recent marked reduction of numbers of individuals in the host population. Conversely, the apparent increased prevalence and abundance of three species of helminths (S. dentatus, M. apri and M. pudendotectus) may be related to their common utilization of earthworms as paratenic or intermediate hosts. Gongylonema pulchrum was the only helminth in which abundances seemed to remain unchanged. This was the only species that was not strictly host specific to feral swine. We found no evidence that helminth infections were responsible for morbidity or mortality in this feral swine population.     

Possible role of wild mammals in transmission of pseudorabies to swine

Of 73 wild and domestic mammals tested from an area endemic for pseudorabies in swine, 16 showed natural pseudorabies virus infection, 8 from farms with no pseudorabies history. In transmission experiments with swine and raccoons (Procyon lotor), pseudorabies was not transmitted between raccoons but was transmitted reciprocally between raccoons and swine by contact and when either consumed infected carrion of the other. The fluorescent antibody tissue section test proved valuable in diagnosis of pseudorabies, especially when employed with the virus isolation test.     

A serosurvey for Brucella suis, classical swine fever virus, porcine circovirus type 2, and pseudorabies virus in feral swine (Sus scrofa) of eastern North Carolina

As feral swine (Sus scrofa) populations expand their range and the opportunity for feral swine hunting increases, there is increased potential for disease transmission that may impact humans, domestic swine, and wildlife. From September 2007 to March 2010, in 13 North Carolina, USA, counties and at Howell Woods Environmental Learning Center, we conducted a serosurvey of feral swine for Brucella suis, pseudorabies virus (PRV), and classical swine fever virus (CSFV); the samples obtained at Howell Woods also were tested for porcine circovirus type 2 (PCV-2). Feral swine serum was collected from trapped and hunter-harvested swine. For the first time since 2004 when screening began, we detected B. suis antibodies in 9% (9/98) of feral swine at Howell Woods and <1% (1/415) in the North Carolina counties. Also, at Howell Woods, we detected PCV-2 antibodies in 59% (53/90) of feral swine. We did not detect antibodies to PRV (n=512) or CSFV (n=307) at Howell Woods or the 13 North Carolina counties, respectively. The detection of feral swine with antibodies to B. suis for the first time in North Carolina warrants increased surveillance of the feral swine population to evaluate speed of disease spread and to establish the potential risk to commercial swine and humans.     

Prevalence and transmission of hepatitis E virus in domestic swine populations in different European countries

ABSTRACT: BACKGROUND: Hepatitis E virus (HEV) genotype 3 and 4 can cause liver disease in human and has its main reservoir in pigs. HEV investigations in pigs worldwide have been performed but there is still a lack of information on the infection dynamics in pig populations. FINDINGS: The HEV transmission dynamics in commercial pig farms in six different European countries was studied. The data collected show prevalence in weaners ranging from 8% to 30%. The average HEV prevalence in growers was between 20% and 44%. The fatteners prevalence ranged between 8% and 73%. Sows prevalence was similar in all countries. Boar faeces were tested for HEV only in Spain and Czech Republic, and the prevalence was 4.3% and 3.5% respectively. The collected data sets were analyzed using a recently developed model to estimate the transmission dynamics of HEV in the different countries confirming that HEV is endemic in pig farms. CONCLUSIONS: This study has been performed using similar detection methods (real time RT-PCR) for all samples and the same model (SIR model) to analyse the data. Furthermore, it describes HEV prevalence and within-herd transmission dynamics in European Countries (EU): Czech Republic, Italy, Portugal, Spain, The Netherlands and United Kingdom, confirming that HEV is circulating in pig farms from weaners to fatteners and that the reproductive number mathematical defined as R0 is in the same range for all countries studied.     

Detection of Clostridium difficile and Salmonella in feral swine population in North Carolina

We sampled 161 feral pigs in eastern North Carolina, USA, to determine the prevalence and antimicrobial resistance profile of Clostridium difficile and Salmonella. Seven (4.4%) and eight (5.0%) pigs tested positive for C. difficile and Salmonella, respectively, highlighting the importance of determining the epidemiology of these pathogens in feral pigs.     

Health status of a recently discovered population of feral swine in Kansas.

Twenty feral hogs (Sus scrofa) from a newly discovered population on Fort Riley Army Base (Kansas, USA) were shot and examined from November 1993 through February 1994 to assess the health of the population. The hogs were generally healthy, although serologic evidence indicated that some individuals had been exposed to parvovirus, enterovirus, and swine influenza. We found no indications of brucellosis, pseudorabies, or porcine respiratory and reproductive syndrome. Lung worms (Metastrongylus spp.), round worms (Ascaris suum), and whipworms (Trichuris suis) were found in nine, four and two of the hogs, respectively. Seven hogs had infestations of lice (Haematopinus suis). Fence-line contacts were documented between four wild boars and domestic sows, and in three cases wild boars entered pens containing domestic sows. We recommend that hogs be examined periodically from this and other wild populations to monitor health status since new animals may enter populations through deliberate translocation, escape from shooting preserves or domestic swine producers, or dispersal from other feral populations.     

Live attenuated pseudorabies virus expressing envelope glycoprotein E1 of hog cholera virus protects swine against both pseudorabies and hog cholera

To investigate whether live attenuated pseudorabies virus (PRV) can be used as a vaccine vector, PRV recombinants that expressed envelope glycoprotein E1 of hog cholera virus (HCV) were generated. Pigs inoculated with these recombinants developed high levels of neutralizing antibodies against PRV and HCV and were protected against both pseudorabies and hog cholera (classical swine fever).     

Antibodies to vesicular stomatitis virus in populations of feral swine in the United States

From 1979 to 1985, 941 feral swine (Sus scrofa) from 53 locations in 15 states were serologically tested for antibodies to vesicular stomatitis virus (VSV). Antibodies to New Jersey serotype VSV were present in 75 swine from five locations in Arkansas, Florida, Georgia, and Louisiana. Within these populations, antibody prevalences ranged from 10 to 100%. No antibodies to Indiana serotype were detected.     

Rapid morphological change in an insular population of feral sheep

Artificially selected qualities can reduce fitness in a wild setting, thus feral domesticates should experience strong selective forces. Domestic sheep Ovis aries have frequently become feral on islands, which differ substantially from mainland environments. We examined changes in body mass and wool traits in feral sheep inhabiting Santa Cruz Island (SCI), California for ≥90 years. To elucidate the influence of nutrition, we compared the mass of feral island sheep with that of island sheep raised in farm conditions. We found that feral sheep on SCI were smaller than purported founder breeds, and that most documented populations of insular feral sheep worldwide have converged to similar body sizes (within 6 kg). SCI rams attained greater mass in farm conditions but ewes did not, suggesting phenotypic plasticity in ram body mass. Ewes exhibited self-shedding of wool at a greater frequency than rams, and sex differences and shedding patterns were consistent with thermoregulation and the risk of fly strike disease as benefits of wool loss. Pigmentation rates did not increase, further supporting the influence of heat stress on wool traits. These changes occurred in <25 generations and may have had a genetic basis, representing a potential example of rapid evolution in insular feral sheep.     

L-particle production during primary replication of pseudorabies virus in the nasal mucosa of swine

Different tissue culture cell lines infected with a number of alphaherpesviruses produce, in addition to virions, light particles (L particles). L particles are composed of the envelope and tegument components of the virion but totally lack the proteins of the capsid and the virus genome; therefore, they are noninfectious. In this electron microscopy report, we show that L particles are produced during primary replication of the alphaherpesvirus pseudorabies virus (PRV) in the nasal mucosa of experimentally infected swine, its natural host. Although PRV infected different types of cells of the respiratory and olfactory mucosae, PRV L particles were found to be produced exclusively by epithelial cells and fibroblasts. We observed that formation of noninfectious particles occurred by budding of condensed tegument at the inner nuclear membrane and at membranes of cytoplasmic vesicles, resulting in intracisternal and intravesicular L particles, respectively. Both forms of capsidless particles were clearly distinguishable by the presence of prominent surface projections on the envelope and the higher electron density of the tegument, morphological features which were only observed in intravesicular L particles. Moreover, intravesicular but not intracisternal L particles were found to be released by exocytosis and were also identified extracellularly. Comparative analysis between PRV virion and L-particle morphogenesis indicates that both types of virus particles share a common intracellular pathway of assembly and egress but that they show different production patterns during the replication cycle of PRV.     

Influenza exposure in United States feral swine populations

Swine play an important role in the disease ecology of influenza. Having cellular receptors in common with birds and humans, swine provide opportunities for mixed infections and potential for genetic reassortment between avian, human, and porcine influenza. Feral swine populations are rapidly expanding in both numbers and range and are increasingly coming into contact with waterfowl, humans, and agricultural operations. In this study, over 875 feral swine were sampled from six states across the United States for serologic evidence of exposure to influenza. In Oklahoma, Florida, and Missouri, USA, no seropositive feral swine were detected. Seropositive swine were detected in California, Mississippi, and Texas, USA. Antibody prevalences in these states were 1% in Mississippi, 5% in California, and 14.4% in Texas. All seropositive swine were exposed to H3N2 subtype, the predominant subtype currently circulating in domestic swine. The only exceptions were in San Saba County, Texas, where of the 15 seropositive samples, four were positive for H1N1 and seven for both H1N1 and H3N2. In Texas, there was large geographical and temporal variation in antibody prevalence and no obvious connection to domestic swine operations. No evidence of exposure to avian influenza in feral swine was uncovered. From these results it is apparent that influenza in feral swine poses a risk primarily to swine production operations. However, because feral swine share habitat with waterfowl, prey on and scavenge dead and dying birds, are highly mobile, and are increasingly coming into contact with humans, the potential for these animals to become infected with avian or human influenza in addition to swine influenza is a distinct possibility.     

Entry of pseudorabies virus: an immunogold-labeling study

Herpesviruses infect cells by fusion of the viral envelope with cellular membranes, primarily the plasma membrane. During this process structural components of the mature virion are lost from the invading nucleocapsid, which then travels along microtubules to the nuclear pore. We examined the penetration process by immunoelectron microscopy and analyzed which of the major tegument proteins remained associated with the incoming capsid. We show that the UL36, UL37, and US3 proteins were present at intracytoplasmic capsids after penetration, whereas the UL11, UL47, UL48, and UL49 tegument proteins were lost. Thus, the three capsid-associated tegument proteins are prime candidates for viral proteins that interact with cellular motor proteins for transport of nucleocapsids to the nucleus.     

Glycoprotein gI of pseudorabies virus promotes cell fusion and virus spread via direct cell-to-cell transmission.

Mutants of pseudorabies virus defective in either glycoprotein gI or gIII are only slightly less virulent for mice and chickens than is wild-type virus, while mutants defective in both gI and gIII are avirulent. To clarify the reason for the lack of virulence of the gI- gIII- mutants, we have analyzed in some detail the interactions of these mutants with their hosts. The results obtained showed that the gI glycoprotein is an accessory protein that promotes cell fusion. This conclusion is based on the findings that in some cell types, syncytium formation is significantly reduced in mutants deficient in gI. Furthermore, despite efficient replication, gI- mutants form significantly smaller plaques on some cell types. Finally, while wild-type and gI- virus are neutralized similarly by antisera, the size of the plaques formed by gI- mutants, but not by wild-type virus, is reduced by the presence of neutralizing antibodies in the overlay. Passive immunization of mice with neutralizing antipseudorabies virus sera is also considerably more effective in protecting them against challenge with gI- mutants than in protecting them against challenge with wild-type virus. These results show that gI- mutants are deficient in their ability to form syncytia and to spread directly by cell-to-cell transmission and that these mutants spread mainly by adsorption of released virus to uninfected cells. Wild-type virus and gIII- mutants, however, spread mainly via direct cell-to-cell transmission both in vivo and in vitro. We postulate that the lack of virulence of the gIII- gI- virus is attributable to its inability to spread by either mode, the defect in gIII affecting virus spread by adsorption of released virus and the defect in gI affecting cell-to-cell spread. Although a gI- gIII- mutant replicates as well as a gIII- mutant, it will be amplified much less well. Our results with in vitro systems show that this is indeed the case.     

Prevalence of hepatitis e virus in swine fed on kitchen residue

The aim of this study was to investigate the prevalence of swine hepatitis E virus (HEV) in pigs fed different feedstuffs (kitchen residue or mixed feeds) and genetic identification of HEV isolated in Hebei province, China. Serum and fecal samples were collected from adult swine. Anti-HEV antibody was evaluated by double sandwich antigen enzyme immunoassay. HEV RNA was extracted from fecal samples and amplified by nested RT-PCR. The reaction products were sequenced, and the sequence analyzed. Virus-like particles were distinguishable by negative staining in the electron microscope. Histopathological observation and immunohistochemical localization were used in the animal models. Overall, the anti-HEV positive percentage of serum samples from pigs fed on kitchen residue was 87.10% (27/31), and 53.06% (130/245) from pigs fed on complete feed. The HEV RNA positivity rate of fecal samples from pigs fed on kitchen residue was 61.54% (8/13), but zero for pigs fed on complete feed. Sequence analysis of these eight samples and comparison with the published sequence showed that there were eight groups that belonged to genotype 4 d and the nucleotide identity was 95.6-99.3%. swHE11 is most closely related to strain CCC220, and the other seven HEV isolates were most closely related to strains swGX40, SwCH189 and V0008ORF3, which are isolates from human and pigs. Histopathological observation showed that there was liver damage in the experimental group, and immunohistochemistry indicated that the HEV antigens were strongly positive at 7 days after infection. The results demonstrated that the prevalence of HEV in pigs fed on kitchen residue was higher than in those fed on complete feed (P<0.05).