Patrick G. Halbur DVM, MS, PhD
Iowa State University, College of Veterinary Medicine
Department of Veterinary Diagnostic and Production Animal Medicine
Ames, Iowa 50011

Introduction
The selection of disease to discuss in this presentation is largely based on trends in submissions to the Iowa State University Veterinary Diagnostic Laboratory (ISU-VDL). Approximately 70 percent of the 45-60,000 annual cases we handle at the ISU-VDL are from swine. I will discuss four emerging diseases and four problematic recurring diseases. The emerging diseases include hepatitis E virus, porcine circovirus-associated postweaning multisystemic wasting syndrome, H3N2 swine influenza virus, and Actinobacillus suis. The recurring diseases that I will discuss include porcine reproductive and respiratory syndrome, Mycoplasma hyopneumoniae, Streptococcus suis, Bordetella bronchiseptica, and Pasteurella multocida.

Swine Hepatitis E Virus Update
Human Hepatitis E Virus. Hepatitis E virus is one of the causative agents of enterically transmitted non-A, non-B hepatitis and is currently unclassified (Pringle, 1998) although it was once classified in the family Caliciviridae (Purcell, 1996). Clinical disease due to human HEV is rare in North America and most of the cases have occurred in people who have traveled abroad (Purcell, 1996). Clinical cases of human hepatitis E are observed predominately in developing countries such as Asia, Africa, India and Mexico. However, sporadic cases of domestic hepatitis E in the United States have recently been reported (Kwo et al., 1997; Schlauder et al, 1998). Hepatitis E generally affects young adults and usually is not fatal, although mortality rates of up to 20% have been reported for pregnant women (Arankalle et al., 1995; Hussaini et al., 1997). Transmission is thought to be predominately fecal-oral and often occurs through contaminated water. Anti-HEV antibodies have been found in a significant proportion of healthy individuals in developed countries including the U.S. (Purcell, 1996). It is not clear if the presence of anti-HEV antibodies in healthy individuals in developed countries represents infection with a nonpathogenic human HEV strain or if it is due to crossreactivity with a related agent. Hepatitis E virus has been successfully transmitted to nonhuman primates (Tsarev et al., 1992; 1993, 1994; 1995). Domestic pigs (Balayan et al., 1990; Clayson et al., 1995), lambs (Usmanov et al., 1994), and laboratory rats (Maneerat et al., 1996) have reportedly been infected with a human strain of HEV.

Discovery of Swine Hepatitis E Virus. In a retrospective study of 15 commercial swine herds in the midwestern U.S., we found anti-HEV antibodies in pigs in all 15 herds tested and the majority of pigs over 3 months of age were seropositive (Meng et al., 1997). A prospective study was done in one of the infected herds where the majority of the sows were seropositive (Meng et al., 1997). Maternal antibody in the piglets suckling seropositive sows waned by 8-9 weeks of age and most pigs seroconverted to swine HEV by 14-21 weeks of age. Clinical disease was not observed. No significant gross lesions were observed in four pigs necropsied during the early stages of infection. Microscopic examination revealed mild lymphoplasmacytic hepatitis and enteritis in the four pigs. Prior to seroconversion, swine HEV ORFs 2 and 3 were amplified by RT-PCR from the sera of the naturally-infected pigs. The putative capsid gene (ORF2) of swine HEV shares about 79-80% sequence homology at the nucleotide level and 90-92% identity at the amino acid level with that of human HEV strains (Meng et al., 1997).

Experimental Inoculation of Pigs with Swine Hepatitis E Virus. We attempted to experimentally infect SPF pigs with swine HEV (Meng et al., 1998). Serum samples collected from naturally-infected pigs in the prospective study were used as the source of swine HEV. Pigs were inoculated intravenously with serum samples containing swine HEV. The inoculated pigs seroconverted to HEV 4-8 weeks postinoculation. Swine HEV was detected in nasal and rectal swabs samples as early as 2 weeks postinoculation and 4-8 weeks thereafter. Viremia appeared 4-6 weeks postinoculation and lasted 1-3 weeks. The disappearance of viremia was temporally related to the appearance of anti-HEV antibodies. The inoculated pigs appeared clinically normal and serum liver enzymes were not significantly elevated. In the sham-inoculated contact control pig, swine HEV was detected in rectal swabs 2 weeks after the virus first appeared in the feces of an inoculated pig.

Comparative Pathogenesis of Infection of Pigs with hepatitis E Viruses recovered from a pig and a human. To study the relative pathogenesis of the two hepatitis E viruses in swine (swine HEV and the US-2 strain of human HEV), SPF pigs were intravenously inoculated with one of the two strains of HEV. Both groups of HEV-inoculated pigs became viremic and shed virus in the feces and bile for 5 weeks post inoculation. All inoculated pigs developed anti-HEV IgG antibodies. Lesions characterized as mild-to-moderate multifocal lymphoplasmacytic hepatitis and focal hepatocellular necrosis or apoptosis were observed in both groups. Comparison of the severity and duration of microscopic liver lesions revealed that the US-2 strain of human HEV induced more severe and persistent hepatitis lesions in pigs than did the swine HEV. This work further confirmed that pigs may be an important reservoir for HEV, pig tissues may also represent a risk for transmission of HEV from pigs to human xenograft recipients, and that human and swine HEV strains differ in virulence (Halbur et al., 2001).

Evidence for Cross-Species Transmission by Swine HEV. The genetic sequence of swine HEV is very closely related to strains of human HEV (US-1 and US-2) recently isolated in the U.S. (Meng et al, 1997; 1998). We experimentally inoculated rhesus monkeys and a chimpanzee with swine HEV and demonstrated that both species became infected. The monkeys developed slight elevations of liver enzymes and microscopic lesions consistent with acute viral hepatitis. The chimpanzee was clinically normal but fecal excretion of swine HEV and seroconversion to HEV was demonstrated. In a reciprocal experiment, we inoculated SPF pigs with the recently isolated US-2 human strain of HEV and demonstrated that pigs could be infected with the human isolate (Meng et al, 1998). This provided the first evidence of cross-species transmission of swine HEV. The extremely high prevalence of swine HEV in pigs and its ability to cross species barriers may potentially put swine practitioners, pork producers, and pig handlers at risk for zoonotic infection by swine HEV. Subclinical infection of humans with swine HEV could explain the relatively high seroprevalence of HEV in apparently healthy individuals in the U.S. However, anti-HEV antibodies are also found in healthy individuals who have no contact with swine, suggesting that there may be other animal species that could serve as reservoirs for HEV.

Evidence for Infection of Wild Rats With Hepatitis E Virus. Because there is a relatively high seroprevalence of HEV in humans in some large U.S. cities, where contact with pigs is uncommon, other sources or reservoirs of HEV are suspected. Kabrane-Lazizi et al. (1999) recently tested the sera of wild rats sampled from widely separated regions of the U.S. They found that 77% of the rats from Maryland, 90% from Hawaii, and 44% from Louisiana were seropositive for anti-HEV. Rural and urban rats were both seropositive and three different species of rats were found to be seropositive. The virus was not recovered from rats, so it is not known how genetically related the rat HEV is to human or swine HEV. It remains to be determined what role rats play in the epidemiology of HEV. It is reasonable to suspect that rats are as important or possibly more important than swine in transmission HEV to humans.

Implications for the Swine Industry. A reliable differential diagnostic test to distinguish between human and swine HEV is essential to progress with epidemiological studies. Swine HEV cross reacts with antibody to human HEV and thus the ELISA serologic tests that are available today cannot distinguish between human and swine HEV serum antibodies. Veterinarians should be aware of the potential public health concern for zoonosis of swine HEV. There is also concern about the inadvertent transmission of swine HEV in pig organs that are transplanted to humans (xenotransplantation). From a more positive perspective, swine HEV infection of pigs may provide a useful animal model to study HEV infection. Swine HEV may also prove to be useful in developing vaccines against HEV infections of humans.

Porcine Respiratory Disease Complex
Porcine respiratory disease complex (PRDC) is the clinical manifestation of multiple disease agents working in concert with each other and associated deficiencies in environmental conditions and management strategies. The etiology of porcine respiratory disease complex (PRDC) varies between and within production systems and over time within the same system. In most farms with PRDC problems, one or two viruses, Mycoplasma hyopneumoniae, and several opportunistic bacteria work in combination to induce losses associated with respiratory disease. Most diagnostic laboratories with a swine emphasis have the tools to assist veterinarians and producers in defining the specific cause(s) of PRDC. The veterinarian and diagnostician then work together to prioritize the pathogens identified and determine which of the pathogens in the complex are the primary pathogens and which are opportunists. Once the primary pathogens are identified an appropriate vaccination and/or medication intervention strategy can be developed. It is equally important to identify and remedy environmental and production problems that enhance respiratory disease. This manuscript is an attempt to summarize current information on the infectious components of PRDC.

Respiratory Pathogen Trends. The table below summarizes 1993-2000 field case data from the ISU-VDL. A clear trend towards increasing case numbers of pneumonia due to porcine reproductive and respiratory syndrome virus (PRRSV), swine influenza virus (SIV), porcine circovirus type 2 (PCV2), Mycoplasma hyopneumoniae (M.hyo), Pasteurella multocida (P.mult.), Streptococcus suis (S.suis), Bordetella bronchiseptoca (Bord.), and Actinobacillus suis (A.suis) is demonstrated. The increased incidence of PRRSV (13X), PCV2 (456X), and A. suis (14X)-induced pneumonia is most remarkable. The 6-fold increase in SIV can in large part be explained by the introduction of subtype H3N2 into the U.S. in 1998. The number of cases of pneumonia due to the other primary pathogens (Pseudorabies virus (PRV), Actinobacillus pleuropneumoniae (APP), Salmonella choleraesuis (Salm)) is steady or decreasing.

Pneumonia case diagnoses at ISU-VDL from 1993-2000.

Porcine Reproductive and Respiratory Syndrome (PRRSV)
PRRSV is a member of the order Nidovirales, family Arteriviridae, genus Arterivirus. Several recent excellent reviews on PRRS are available (Done et al., 1996; Rossow et al., 1998; Zimmerman et al., 1997; Zimmerman et al., 1998). It is important to remember that considerable genetic, antigenic and virulence differences exist among PRRSV isolates.

Clinical signs. Many farms are subclinically infected with PRRSV while others experience severe reproductive and/or respiratory disease. Neonatal and nursery pigs may experience high fevers, anorexia, dyspnea, tachypnea, chemosis, conjunctivitis, and failure-to-thrive. Coughing is not a feature of uncomplicated PRRSV infection. Grow-finish pigs infected with PRRSV exhibit respiratory disease that varies from no detectable signs to fatal pneumonia depending on the strain of PRRSV and the types of coinfections.

Gross and microscopic lesions. Field and experimental data support a marked difference in pneumovirulence of PRRSV isolates within and between geographical areas across the world (Halbur et al., 1995). Neonatal and nursery pigs may have mild-to-severe, multifocal-to-diffuse, mottled-tan, rubbery lungs that fail to collapse. Experimentally induced lesions of pneumonia are evident by 3 days post inoculation (DPI), are most severe at 7-10 DPI, and if uncomplicated, the lesions induced by most isolates resolve by 14-28 DPI. Enlarged tan lymph nodes develop after 10 DPI and are the most consistent PRRSV-induced gross lesion. Microscopic examination of lungs reveals interstitial pneumonia in young pigs characterized by alveolar septal infiltration with mononuclear cells, type 2 pneumocyte hypertrophy and hyperplasia, and alveolar exudate consisting of mixed mononuclear cells and necrotic debris. Lymphohistiocytic encephalitis, myocarditis, and rhinitis may be observed with some strains.

Pathogenesis. Following oronasal exposure, PRRSV replicates in macrophages and dendritic cells in tonsils, upper respiratory tract, and lungs resulting in viremia by 6-12 hours post infection. Further replication occurs in the lungs, lymph nodes, spleen, thymus, bone marrow, heart and other tissues. Viremia may persist for several weeks despite the presence of circulating antibodies.

In growing pigs, PRRSV is shed in saliva, respiratory tract secretions, oropharyngeal secretions, and urine. Isolation of PRRSV from oropharyngeal samples for up to 157 days after experimental intranasal inoculation of conventional pigs provides evidence of persistent infection of growing pigs (Wills et al., 1997). Sows infected at 85-90 days of gestation may give birth to persistently infected pigs in which viral RNA could be detected for 210 days or more (Benfield et al., 1997). Preweaning mortality is high and respiratory disease is severe in these "Long Term Viremic Pigs" (LTVPs). The LTVPs may be an important source of virus dissemination and persistence.

PRRSV infection results in destruction and decreased function of pulmonary alveolar macrophages (PAMs) and pulmonary intravascular macrophages (PIMs), damage to the mucociliary apparatus, changes in T-cell subpopulations, and possibly in decreased function of antigen presenting cells such as dendritic cells and macrophages. PRRSV-induced apoptosis in mononuclear cells in the lung and lymphoid tissues has been demonstrated and might explain the reduction in number of alveolar macrophages and circulating monocytes (Sirinarumitr et al., 1998). PRRSV-induced damage to PIMs and PAMs results in increased susceptibility to bacterial pneumonia and septicemia (Thanawongnuwech et al., 1997; 1998; 1999). Differences in PRRSV-induced antibody response and severity of disease may be in part influenced by pig genetic factors (Halbur et al., 1998).

Diagnosis. PRRSV antigens can be detected by fluorescent antibody examination of frozen tissue sections or immunohistochemical examination of formalin-fixed tissue sections. In situ hybridization for detection of viral nucleic acid in formalin-fixed tissues is also available. PCR-based methods for detection of PRRSV nucleic acids in clinical specimens are becoming widely used. Virus isolation from serum, lung, lymphoid tissues, and bronchoalveolar lavage fluids is successful for several weeks post inoculation. Restriction enzyme analysis, monoclonal antibody panels, or sequencing can be performed to differentiate PRRSV isolates for epidemiological purposes. Serum antibody response to PRRSV can be detected by indirect fluorescent antibody (IFA), immunoperoxidase monolayer assay (IPMA), serum virus neutralization (SN), and enzyme-linked immunosorbant assay (ELISA), or most recently with a blocking ELISA. Serum antibody usually can be detected at 6-10 days post inoculation, peaks by 4-10 weeks, and persist for 5-12 months. Colostrum from immune sows appears to be protective and passive antibody wanes by 3-6 weeks of age. Immunological cross protection between strains of PRRSV ranges from excellent to poor.

Control: Control of PRRS involves first understanding the source of introduction of the virus into the herd and then the pattern of transmission within the herd. The most effective techniques currently utilized to stabilize sow herds involve a combination of gilt acclimatization, controlled exposure to resident strains of PRRSV, and possibly vaccination with commercial or autogenous vaccines. The ultimate goal is to effectively immunize the sow herd so that reproductive losses do not occur and PRRSV is not shed to suckling pigs and subsequently spreads downstream through the nurseries and finishers. Reports of successful elimination of PRRSV from stabilized herds suggest that this is a viable option in some herds and particularly for seedstock producers or producers in isolated areas (Dee et al., 2000; Henry and Tokach, 2001). Commercial and autogenous killed and modified live PRRSV vaccines are available for use on growing pigs for control of PRRSV-induced pneumonia. The antigenic, pathogenic, and genetic variation among PRRSV isolates poses a major obstacle in development of a widely efficacious vaccine for PRRS (Meng, 2000). Antibiotic treatment and/or strategic medication for concurrent bacterial infections are often necessary and in many cases are more beneficial than PRRSV vaccination. In particular, focusing on control of mycoplasmal pneumonia has been shown to be extremely important and quite successful in minimizing disease in PRDC cases associated with PRRSV and M. hyopneumoniae and opportunistic bacteria (Desrosiers 1998; Halbur 1998; Scheidt 1998; Thacker et al., 1999).

Swine Influenza Virus (SIV)
Swine influenza virus (SIV) is a member of the Family Orthomyxoviridae. Nucleoprotein and matrix proteins determine the type of influenza virus. SIV is a type A influenza virus. Subtype classification is by antigenic and genetic properties of surface proteins hemagglutinin (H) and neuraminidase (N). There are 15 hemagglutinins (H1-H15) and 9 neuraminidases (N1-N9). Subtype H1N1 is widespread in North America, Europe, and Asia. Prior to 1998, swine influenza in the U.S. was almost exclusively caused by subtype H1N1. Beginning in mid-1998, subtype H3N2 quickly became widespread in the U.S. Based on cases submitted to Iowa State University in 1998-2000, over 50% of the influenza cases were diagnosed as subtype H3N2 (Schneider et al., 2001). An even more recent report from Indiana in 2000 has confirmed the presence of H1N2 in the United States (Karasin et al., 2000).

There have been several recent examples of antigenic drift of SIV. Antigenic drift occurs within subtype and involves a series of point mutations. Mutations that affect neutralizing epitopes may produce antigenic variants. Since 1988 in Canada, unique pneumonia outbreaks described as proliferative and necrotizing pneumonia (PNP) has been associated with an antigenic variant of H1N1 SIV (Morin et al., 1990). Microscopic lesions of PNP are characterized by necrotizing bronchiolitis, marked type 2 pneumocyte proliferation, abundant alveolar exudate and hyaline membranes along alveolar septa. Analysis of a Quebec strain associated with PNP revealed a variant of H1N1 with considerable genomic divergence (drift) from current North American isolates (Rekik et al., 1994). In 1992 in the U.S., A/Swine/Nebraska/1/92 H1N1 was isolated and found to be antigenically and genetically distinct from classical swine H1N1 (Olsen et al., 1993).

Antigenic shifts are more dramatic changes in which an entirely new virus arises from reassortment of genes from two different viruses resulting in a completely new H or N component. In 1992 in England, H1N7 was identified in pigs and is thought to be the result of reassortment between human and equine isolates (Brown et al., 1994). An H1N2 virus was isolated from pigs experiencing respiratory disease in Scotland in 1994 and was determined to be a result of reassortment between human and swine viruses (Brown et al., 1995). Genetic reassortment between avian and swine influenza viruses has been reported in Italy (Castrucci et al., 1993). Genetic analysis indicates that the virus associated with recent swine flu outbreaks in the Midwestern U.S. (IA H3N2) is a reassortment of human H3N2 and swine H1N1. It appears to be distinct from European and Asian H3N2 viruses from pigs.

Cross species transmission of influenza virus to pigs. Evidence suggests that there has been continuous circulation of a human H3N2 virus in pigs in Europe for quite some time (Ottis et al., 1992). In January 1992, there was a sudden increase in respiratory disease in the swine population of England. The isolate A/swine/England/195852/92 was typical of the isolates from this outbreak and was distinguishable from classical and European swine viruses in hemagglutination inhibition tests using monoclonal antibodies to the H1 hemagglutinin (Brown et al., 1993). Convalescent sera from this outbreak tested negative in hemagglutination inhibition tests with the classical UK- prototype H1N1 and H3N2 swine influenza viruses. The isolate was later confirmed to be an avian H1N1 isolate based on genetic and antigenic analysis (Brown et al., 1997). Experimental inoculation of pigs resulted in disease and lesions similar to classical SIV (Brown et al., 1993). Avian H1N1 viruses have also been isolated from pigs in China (Guan et al., 1996).

Clinical signs. Severity of disease varies with the age and immune status of the pig, the particular influenza isolate involved, and with concurrent infections. Acute (epizootic) swine influenza is characterized by an acute onset of respiratory disease with high morbidity and low mortality. Dyspnea, labored abdominal respiration, paroxysmal barking cough, prostration, and fever are characteristic. Infection of naïve pregnant sows or gilts may result in abortion storms that persist for 2-3 weeks. Sickness in adult animals and subsequent reproductive disease appears to be more common and severe in recent U.S. cases of H3N2 SIV outbreaks. Rapid recovery of individual animals usually occurs in 2-6 days if uncomplicated. Passive antibody for SIV is generally protective and wanes by 8-12 weeks of age. Loss of protective passively acquired antibody explains why SIV-induced disease is most common in 12-24 week old pigs. If a naive sow herd is infected, influenza can be expected and is commonly observed in suckling and nursery pigs.

Gross and microscopic lesions. Swine influenza virus induces multifocal-to-diffuse pneumonia with dark red-tan, mottled areas affecting 20-100% of the lung tissue. Lesions are often cranioventral in distribution. The lungs are often congested and airways contain blood-tinged foam. Enlarged and hyperemic mediastinal and tracheobronchial lymph nodes are common. Microscopic examination reveals bronchointerstitial pneumonia characterized by necrotizing bronchitis and bronchiolitis, alveolar septal infiltration with mixed inflammatory cells, type 2 pneumocyte hypertrophy and hyperplasia, and filling of airways and alveolar spaces with proteinacious fluid and mixed inflammatory cells.

Pathogenesis. Swine influenza virus is most commonly spread pig-to-pig via nasopharyngeal secretions. The virus attaches to the cilia and viral replication begins in the epithelium of the upper respiratory tract. Infection spreads to bronchi and bronchioles resulting in loss of cilia, extrusion of mucus, exudation of neutrophils and macrophages, and necrosis and metaplasia of airway epithelium. Extension of virus infection to alveolar epithelium, endothelium, and alveolar macrophages results in flooding of alveoli with serofibrinous exudate. Pigs are predisposed to bacterial pneumonia due to damage to the mucociliary apparatus and decreased macrophage function.

Diagnosis. Diagnosis of swine influenza is based on observation of characteristic clinical signs, microscopic lesions, and antigen or virus detection. Fluorescent antibody examination of frozen sections or immunoperoxidase examination of formalin-fixed sections for SIV antigens work well in the early stages of disease. Antigen capture kits for rapid diagnosis from nasal or bronchial swabs are also available. Virus isolation from nasal swabs or lung tissue is successful in the early stages of disease. Paired serology (HI, ELISA, IFA) is required due to the persistence of passive antibody for 8-12 weeks. Hemagglutination inhibition test (HI) is the most commonly used serologic test. The HI test is subtype specific. The occurrence of antigenic variants within subtype, appearance of reassortment variants, and cross species transmission of influenza viruses to pigs needs to be considered when interpreting serology and antigen detection tests. It is important that practitioners check with the diagnostic lab and understand which SIV subtypes are detected by each test.

Control: Inactivated whole virus and subunit vaccines are available for SIV. These products have been used effectively in sows to protect pigs until 8-11 weeks of age. SIV vaccines are increasingly being used on growing pigs to treat and prevent PRDC outbreaks. Unless SIV is endemic in the population, practitioners more often choose to medicate and/or vaccinate for the concurrent bacterial infections and allow SIV to spread through the population and resolve in 1-2 weeks. Recent isolation of H3N2 SIV from U.S. herds vaccinated against H1N1 suggests limited heterologous protection. European SIV vaccines commonly contain both H1N1 and one or two H3N2 isolates. Several single antigen H1N1 commercial products are available in the U.S. at this time. Schering-Plough Animal Health received a conditional license in 1999 to market a single antigen H3N2 vaccine. Full licensure of the product is anticipated. Multiple antigen (H1N1 and H3N2) commercial products will likely be widely available in the U.S. by 2000. Use of autogenous killed H3N2 and H1N1 vaccines has recently increased considerably in the U.S.

Porcine Circovirus (PCV)
Porcine circovirus (PCV) is a very small, single-stranded, circular DNA virus in the Circoviridae family. PCV was originally detected as a noncytopathic contaminant of continuous PK/15 cells (Tischer et al., 1974) and that isolate is now known as PCV-1. Hines and Lukert (1994) associated PCV infection with congenital tremors in 1994. The congenital tremor strain of PCV was thought to be PCV-1; however, recently the presence of PCV-2 nucleic acid was identified in CNS tissues from pigs exhibiting congenital tremors (Stevenson et al., 1999). Interest in PCV has greatly increased since it has been associated with "Post Weaning Multisystemic Wasting Syndrome" (PMWS) in Canada (Ellis et al., 1998; Harding et al., 1997). Since that time, several groups in North America and Europe have associated porcine circovirus with PMWS (Kiupel et al., 1998; Morozov et al., 1998; Segales et al., 1997). Strains of PCV from cases of PMWS differ considerably from PK/15 strains based on sequence analysis (Hamel et al., 1998; Meehan et al., 1998; Morozov et al., 1998) and are tentatively classified as PCV-2. Within the PCV-2 genotype, several minor branches have been identified and appear to be associated with geographic regions of origin (Fenaux et al., 2000). All the US PCV-2 isolates examined to date appear to be closely related, but the Canadian isolates vary to some extent in their genomic sequences.

Clinical signs. The most common clinical signs of PMWS include progressive weight loss (wasting) and chronic pneumonia (tachypnea, dyspnea). Morbidity within a group is usually low (5-50%), however, mortality among affected pigs is often very high. Icterus, paleness, and diarrhea are less commonly reported. PCV has also been associated with gastric ulcers and with porcine dermatitis and nephropathy syndrome (Rosell et al., 2000). Pigs in the late nursery to mid-finishing phase (6-18 weeks) are most commonly affected. It can be very difficult clinically to distinguish PMWS from PRRSV and secondary infections. Based on cases submitted to the ISU-VDL, PRRSV and PCV are frequently detected together in cases of PMWS. It is still not clear whether PCV-2 is associated with reproductive disease.

Gross and microscopic lesions. Characteristic gross lesions of PMWS include markedly enlarged tan lymph nodes, rubbery-tan lungs that fail to collapse, and less commonly enlarged "waxy" kidneys or mottled-tan livers. Diagnosis of PMWS is based on identification of unique microscopic lesions (Rosell et al., 1999). Characteristic microscopic lesions include depletion of lymphoid follicles and granulomatous inflammation of the lymphoid tissues, liver, pancreas, and a variety of other tissues. Lung lesions are characterized by lymphohistiocytic interstitial pneumonia of variable severity. There are varying degrees of airway epithelial sloughing and fibroplasia in the lamina propria. Bronchiolitis and bronchitis obliterans fibrosa is common in severe cases. Multinucleate syncytial cells may be present in alveolar spaces. Macrophages in B-cell dependent areas of lymphoid tissues often contain clusters of variably sized basophilic intracytoplasmic "grape-like clusters" of inclusion bodies. The lymphoid depletion, granulomatous inflammation, and inclusion bodies are most easily identified in the tonsil and Peyer’s patch.

Pathogenesis. We are just beginning to understand the pathogenesis of porcine circoviruses. The PCV-1 strain from persistently infected continuous PK/15 cells was found to be nonpathogenic in colostrum-deprived pigs (Allan et al., 1995). Ellis et al. (1998) demonstrated a high degree of correlation between the presence of PCV-2 and lesions of PMWS in pigs from more than 70 herds in Canada. Several groups have attempted to experimentally reproduce characteristic PMWS lesions with filtered tissue homogenates or cell culture material. Ellis et al., (1999) reproduced most of the lesions typical of PMWS (lymphadenitis, pneumonia, hepatitis and nephritis) in gnotobiotic pigs inoculated with filtered cell culture material and filtered lymphoid tissues from pigs with naturally acquired PMWS. Both PCV-2 and porcine parvovirus (PPV) and antibodies to these viruses were detected in the experimentally inoculated pigs. Balasch et al. (1999) also reproduced some of the lesions consistent with PMWS in conventional pigs inoculated with tissue homogenates from PMWS pigs. Allan et al. (1999) and Kennedy et al. (2000) subsequently inoculated colostrum-deprived pigs with PCV2 alone, PPV alone, and the combination of PCV2 and PPV derived from Canadian pigs with PMWS and reproduced severe clinical disease and death and lesions typical of PMWS in the pigs dually-inoculated with PCV2 and PPV. Only mild lesions of PMWS were reproduced in pigs inoculated with PCV2 alone. Krakowka et al (2000) further confirmed the synergistic relationship PCV-2 and PPV in gnotobiotic pigs by reproducing clinical disease and lesions typical of PMWS in coinfected pigs. Mittal et al. (1999) recently demonstrated colocalization of porcine circovirus with porcine adenovirus in pigs with naturally occurring PMWS. Several groups are investigating the interaction of PRRSV and PCV-2 in experimental models.

Evidence continues to point towards PCV-2 as the primary cause or at least an integral component of PMWS. It appears that PCV-2 is essential but perhaps not sufficient by itself to induce PMWS (Krakowka et al., 2000). The importance of concurrent infection(s) with other pathogens such as parvovirus, adenovirus, PRRSV (Harms et al., 2001) and other pathogens is under continued investigation. Recent evidence suggests that activation of macrophages in lymphoid tissues may lead to potentiation of PCV-2 replication and development of clinical disease and lesions typical of PMWS (Krakowka et al., 2000; Allan et al., 2000).

Diagnosis. Diagnosis is based on observation of unique microscopic lesions. Immunohistochemistry, in situ hybridization, and PCR for detection of circovirus is used for confirmation of the presence of PCV antigen or nucleic acids in porcine tissue (Morozov et al., 1998; Sorden et al., 1999). Virus isolation is also offered by some diagnostic laboratories. An IFA serology test for PCV is available (Pogranichnyy et al., 2000) and ELISA tests are in development. Separate IFA tests are done for PCV-1 and PCV-2. The amount of cross reactivity between the IFA tests is unclear at this time. The limited serologic surveys done to date indicate that both PCV-1 and PCV-2 are widely distributed in the swine populations with and without clinical evidence of PMWS.

Control: It is difficult to recommend control procedures for PMWS at this point because so little is known about the epidemiology. Production strategies such as SEW are being evaluated. Preliminary work on in vitro evaluation of disinfectants suggests that many conventional disinfectants may not be effective (Royer et al. 2000). Treatment or vaccination for concurrent infections is essentially all that can be done in most cases. Anti-inflammatory drug treatment of affected pigs has also been reported to be beneficial. Removal of pigs that don’t respond to treatment is recommended. Development and use of autogenous and commercial PCV vaccines is expected in the near future.

Mycoplasma hyopneumoniae (MH)
Mycoplasma hyopneumoniae (MH) is the primary pathogen associated with enzootic pneumonia. Enzootic pneumonia results when MH is combined with opportunistic bacteria such as Pasteurella multocida. It has been demonstrated that 30-80% of slaughter swine have lesions of enzootic pneumonia making it the most economically important bacterial respiratory pathogen (Ross, 1992). Transmission occurs primarily through direct contact with respiratory secretions and by aerosolization. Herds established free of MH in pig dense areas usually are not able to stay free suggesting that area spread occurs. Chronic, sporadic, nonproductive cough, with high morbidity is characteristic of enzootic pneumonia.

Inhalation of MH results in infection of trachea, bronchi and bronchioles. Development of clinical disease is dose dependent. MH attaches to the cilia and surface of epithelium by adhesin proteins inducing clumping and loss of cilia, epithelial cell death, and reduced function of the mucociliary apparatus. This results in decreased clearance of normal lung secretions, inhaled particles, and pathogens. MH along with PRRSV and Pasteurella multocida is the most common combination of pathogens in PRDC. Recent research demonstrated that MH enhances the severity and duration of PRRSV-induced pneumonia (Thacker et al., 1999).

Diagnosis of mycoplasmal pneumonia is based on observation of characteristic chronic non-productive cough with poor performance and a spread in weights of the pigs. Gross lesions are characterized by the presence of well-demarcated, purple-to-grey-to-tan, depressed areas of cranioventral lung that exude viscous fluid in airways. Microscopic examination reveals bronchopneumonia with suppurative and histiocytic alveolitis and the presence of characteristic peribronchiolar and perivascular lymphoid hyperplasia. Confirmation is most often by histopathological observation of characteristic lesions and antigen detection by fluorescent antibody examination or immunohistochemical examination of lung sections. Isolation of MH can be done but is labor intensive, expensive, requires specialized media, and usually takes several weeks to complete. Cultures are also commonly overgrown by M. hyorhinis. PCR is increasingly used in combination with serology to more accurately determine the timing of colonization of pig populations with MH and thus better place strategic medication and vaccination programs. Serology has become more widely used since the development of the Tween 20 ELISA. CF and latex agglutination tests also are used. Seroconversion may take 4-8 weeks or more. There is concern about serologic cross-reaction with M. flocculare.

Control: SEW is successful in controlling MH if weaning age is less than 14-20 days and SEW is combined with all-in-all-out pig flow and various antibiotic medication programs (Clark, 1998; Dee et al., 1999; Desrosiers, 1998). Use of commercial MH vaccines has been very beneficial in control of PRDC associated with combined MH and PRRSV infection in the late finishing period (Desrosiers, 1998; Halbur, 1998; Thacker et al., 1999). Tiamulin, lincomycin, tilmicosin, tylosin and tetracyclines are often used for treatment and prevention of enzootic pneumonia.

Bordetella bronchiseptica (Bb)
Bordetella bronchiseptica (Bb) is a well-established cause of non-progressive atrophic rhinitis in young pigs. It is also a primary cause of pneumonia in young pigs and an opportunistic or secondary cause of pneumonia in grow-finish pigs. Bb colonizes ciliated epithelium of the respiratory tract resulting in decreased mucociliary apparatus function and pneumonia. Bb plays an important role in predisposing pigs to colonization by toxigenic strains of Pasteurella multocida. Virulence of Bb strains depends on their ability to colonize the turbinates and lungs and produce cytotoxins (dermonecrotic toxins). Clinical signs include sneezing, coughing and labored respiration. Typical gross lesions are a necrohemorrhagic pleuropneumonia in young pigs or a tan, firm bronchopneumonia in older pigs. The acute form in neonates grossly resembles APP with cranioventral distribution. The more chronic form seen in nursery and grower pigs is very firm and tan with fibrosis. Diagnosis is by isolation of the organism from nasal swabs, trachea, or lung.

Control: Pigs typically are exposed to Bb from shedding sows or gilts during the suckling period. Colostral antibody is important for protection. Bb bacterin toxoids are efficacious in prevention of Bb induced rhinitis and pneumonia. They are commonly used on sows and gilts prefarrow and sometimes on 1-6 week old pigs in problem herds. Lateral transmission of Bb occurs rapidly postweaning in comingled pigs of varying immune status. Antibiograms should be used to select the most appropriate antibiotic treatment. Tetracyclines and potentiated sulfonamides are often effective.

Pasteurella multocida (PM).
Six serotypes (A-F) of PM have been reported. Toxigenic PM type D and Bordetella bronchiseptica (Bb) are the primary causes of atrophic rhinitis in pigs. Toxigenic strains of PM type D induce progressive atrophic rhinitis manifest as severe turbinate atrophy and varying degrees of facial distortion or a "bull-nose" appearance in severe cases. Most lung isolates of PM are capsular serotye A and a few are D. PM is usually considered an opportunistic invader of lung that is rapidly cleared from the lungs of normal pigs. However, at the ISU-VDL, we are increasingly observing cases of severe bronchopneumonia often with pleuritis associated with singular infection with PM type A or type D. In cases such as these, it is suspected that certain PM isolates may be more virulent and should perhaps be considered as primary pathogens. Virulence factors of PM are poorly recognized. A polysaccharide capsule helps to resist phagocytosis. The role of PM dermatonecrotoxin in pneumonia is still in question. Infection of pigs with Mycoplasma hyopneumoniae, pseudorabies virus, swine influenza virus, or PRRS virus is thought to predispose pigs to pneumonic pasteurellosis.

Gross lesions typical of PM-induced pneumonia are red-grey cranioventral pneumonia with pus in airways. Focal dry, translucent pleuritis may also be observed. Lobular purulent bronchopneumonia is observed microscopically. Snouts can be sectioned at the level of the first upper premolar to evaluate the severity of atrophic rhinitis. Measurements of the amount of atrophy are taken and scored from 0-5 in severity and is often done as part of slaughter checks. Diagnosis is by isolation of the organism from nasal turbinates, tonsil, or affected lung tissue. Determination of toxin production by isolates from rhinitis cases is important and can be done by several techniques.

Control: Prevention of progressive atrophic rhinitis is best achieved through approaches to avoid introduction of seedstock carrying toxigenic strains of PM. Nasal and tonsillar swabbing for culture, ELISA and PCR techniques are used for this purpose. Control of atrophic rhinitis is usually achieved by vaccination of the dams prefarrow with Bb and PM type D bacterin-toxoids. Piglet vaccination and antibiotic injections are also done in more problematic herds. Strict adherence to all-in-all-out pig flow and proper ventilation will considerably minimize clinical disease associated with atrophic rhinitis and pneumonic pasteurellosis. SEW in combination with sow vaccination has been demonstrated to be very effective in control, and in many cases elimination, of atrophic rhinitis.

Streptococcus suis (SS)
There are 35+ capsular serotypes of Streptococcus suis. Type 2 is more commonly isolated than 1/2, 3, and type 8. Streptococcus suis invades tonsils and reaches the lymph nodes via lymphatics. Infected monocytes may distribute the organism throughout the body and to the brain. Virulence factors are not well characterized. The capsule inhibits phagocytosis and antigens associated with virulence include muramidase-release protein (MRP) and extra-cellular factor (EF). Other important capsular and cell wall associated products include fimbriae, adhesin, IgG binding protein, and albumin binding protein. Other secretory products that are thought to be involved in virulence include a hemolysin and a purine immunosuppressive factor (Brown, 1999). Streptococcus suis persists in tonsils and nasal cavities of carrier pigs.

Clinical signs are characterized by sudden deaths, fever, depression, dyspnea, arthritis, and central nervous system signs (meningitis). Gross and microscopic lesions of purulent bronchopneumonia with pleuritis or polyserositis and meningitis are characteristic of SS infection. Definitive diagnosis is achieved by isolation and serotyping of the organism.

Control: Efforts to eliminate or control SS by SEW have been unsuccessful (Clark, 1998). In fact, early weaning and segregation of weaned pigs from sows has been identified as a risk factor for development of disease associated with SS and HPS (Pijoan et al., 1997). Antibiograms should be used to select the most appropriate therapy. Bacterins are serotype specific and have been marginally effective. Both commercial and autogenous vaccines are widely used on pigs and dams. There is growing interest in avirulent live SS vaccines. In pigs coinfected with PRRSV and SS, antibiotic strategies focused on control of SS were considerably more effective than vaccination for PRRSV or SS (Halbur et al., 1999).

Actinobacillus suis (AS)
Actinobacillus suis is still relatively uncommon, however, it may well be an emerging pathogen particularly in high-health status herds. Clinical signs are characterized by sudden death, fever, tachypnea, cyanosis, swollen extremeties, widespread hemorrhages, and skin lesions resembling erysipelas. AS-induced gross lung lesions of fibrinohemorrhagic pleuropneumonia can easily be confused with those of APP. Polyarthritis and polyserositis are also commonly observed with AS. Microscopic examination reveals septic embolism, hemorrhage, and pleuropneumonia with characteristic streaming neutrophils. Definitive diagnosis is by isolation and identification of the organism from tissues.

Control: Losses associated with AS are usually sporadic and do not justify the use of vaccines. Some endemically infected herds claim to benefit from autogenous AS vaccines. Antibiotic treatment based on antibiogram results is usually effective if pigs are treated in the early stages of disease.

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