Outer membrane proteins and DNA profiles in strains of Haemophilus parasuis recovered from systemic and respiratory sites

Polyserositis caused by Haemophilus parasuis is an important disease that affects mostly weaned pigs. Recent studies have shown that virulence can differ among strains recovered from distinct body sites and also that it may be related to the presence of certain outer membrane proteins (OMPs). The objective of this study was to compare the OMP and DNA profiles of H. parasuis strains isolated from systemic and respiratory sites from diseased and healthy pigs. Strains evaluated in this study were processed using sodium dodecyl sulfate-polyacrylamide gel electrophoresis and repetitive-PCR techniques. Two experiments were conducted in order to better define the relationship among genotype, phenotype, and site of isolation. Experiment 1 included 53 H. parasuis isolates recovered from healthy and diseased pigs from unrelated herds. Experiment 2 included 31 isolates of H. parasuis obtained from diseased pigs involved in an outbreak in a large, multifarm system. Results showed that strains recovered from systemic sites had more homogeneous OMP and DNA profiles than those isolated from respiratory sites. Evaluation of isolates involved in the multifarm outbreak showed that only two H. parasuis strains were causing disease. These strains had homogeneous OMP and DNA profiles. However, it was noted that these two parameters were unrelated, since strains classified in the same genotype group expressed different OMP profiles. The homogeneity of OMP and DNA profiles of strains isolated from systemic sites strongly suggests the existence of clonal relationships between virulent strains and also suggests that expression of certain OMP profiles may be related to virulence.

Correlation between the presence of enzootic pneumonia lesions and detection of Mycoplasma hyopneumoniae in bronchial swabs by PCR

In many diagnostic laboratories the diagnosis of mycoplasmal pneumonia in pigs is based on clinical signs and the presence of gross and histopathological lesions. The objective of this study was to evaluate the nested-PCR technique as an adjunct to the histopathological diagnosis of Mycoplasma hyopneumoniae infection. Respiratory disease of 184 swine cases submitted to the Minnesota Veterinary Diagnostic Laboratory between 1 January and 30 June 1998 were used. Bronchial swabs were collected and the nested-PCR performed. Lung samples were graded PCR positive or negative. Histopathological lesions were scored 0-4, depending on the mycoplasma-like characteristics of the lesions, with category 4 demonstrating strong evidence of mycoplasma infection.Nested-PCR correlated well with histopathological lesions characteristic of M. hyopneumoniae in categories 3 and 4 and approximately half of the histopathological categories 1 and 2 were nested-PCR positive. The results demonstrate that the nested-PCR is a valuable adjunct in the diagnosis of M. hyopneumoniae infection when non-diagnostic microscopic lesions of mycoplasmosis are found.

Experimental exposure of young pigs using a pathogenic strain of Streptococcus suis serotype 2 and evaluation of this method for disease prevention

Control of Streptococcus suis infections and associated disease have proven to be a difficult challenge under most farm conditions. The objective of this study was to experimentally expose young pigs with a pathogenic strain of S. suis serotype 2 as a means of controlling the disease in a commercial swine farm. Prior to the start of the study, the pathogenic S. suis strain responsible for mortality in the farm was identified and used to experimentally inoculate baby piglets. Over a 3-week period, groups of pigs were selected (100 pigs/wk) and divided into 2 groups: control (50 pigs/week) and experimentally exposed (50 pigs/week). Pigs in the experimentally exposed group were inoculated at 5 d old by tonsillar swabbing with the pathogenic S. suis farm isolate. The effect of exposure with this pathogenic strain was evaluated during the nursery and finishing stages and was based on: morbidity (pigs with central nervous signs (CNS) and/or lameness), mortality and number of treatments required by pigs that had either CNS or lameness. The relative risk (RR) of acquiring disease due to S. suis infection was also calculated. Results showed that morbidity in the experimentally exposed groups was lower than in the control group and these results were statistically different (P = 0.006). Experimentally exposed pigs also showed a statistically significant reduction in lameness problems (P = 0.012), but not in CNS (P = 0.20) or mortality (P = 0.59). Pigs in the control group had an increased RR of 4.76, 8.77 and 2.7 for morbidity, to have lameness or to have CNS signs, respectively. In conclusion, experimental exposure of young pigs with the farm's pathogenic S. suis strain at a young age, had a positive effect in reducing clinical signs characteristics of S. suis infection. This method constitutes a novel approach to the control of S. suis infections in swine farms.

Application of a nested polymerase chain reaction assay to detect Mycoplasma hyopneumoniae from nasal swabs

The porcine respiratory disease complex (PRDC) is an increasingly important cause of decreased swine productivity and is characterized by slow growth, decreased feed efficiency, anorexia, cough, and dyspnea. Mycoplasma hyopneumoniae is among the most prevalent and important infectious agents associated with PRDC. Understanding of mycoplasmal pneumonia has been hindered by inadequate diagnostic methods. Many of the currently available tests are relatively insensitive or nonspecific when used in a diagnostic laboratory setting or are too costly or difficult for routine diagnostic use. Several polymerase chain reaction (PCR) assays have been described, but they are not sensitive enough to detect the microorganisms in live pigs, from either nasal or tracheal swabs. A nested PCR using 2 species-specific sets of primers from the 16S ribosomal DNA gave positive results with as little as 80 microorganisms and did not cross-react with other mycoplasma species or with other microorganisms commonly found in the respiratory tract of pigs. This assay was better suited for detection of M. hyopneumoniae from nasal swabs than was conventional PCR. Nasal swab samples were taken at different time periods following experimental challenge of 10 susceptible pigs. Only 2 of the 55 swabs examined gave a positive result with conventional PCR, whereas 30 of the 55 swabs gave a positive result using the nested PCR. Twenty of 40 (50%) nasal swabs from pigs experiencing a respiratory disease outbreak where M. hyopneumoniae had been diagnosed also gave a positive result with the nested PCR. To confirm that the amplified product was specific, 4 nested PCR products were purified, sequences were determined and aligned, and they were confirmed to be from M. hyopneumoniae.

Development of an oligonucleotide-specific capture plate hybridization assay for detection of Haemophilus parasuis

An oligonucleotide-specific capture plate hybridization assay has been developed to rapidly, specifically, and sensitively detect Haemophilus parasuis from nasal swabs. Several in vitro studies have been performed to determine the sensitivity and specificity of the test, and in vivo studies have validated this technique in pigs. Results suggest that the assay detects <100 colony-forming units/ml in a pure culture and gives a positive result when H. parasuis is present in a ratio of 1:10(3)-10(4) in a mixed culture, and the probe does not hybridize with other related species found in the upper respiratory tract. This assay is more sensitive than culture for detection of the microorganism from nasal swabs and lesions.

Porcine reproductive and respiratory syndrome virus and Haemophilus parasuis antigen distribution in dually infected pigs

Immunohistochemical, viral and bacterial isolation techniques were used to study the distribution and localization of porcine reproductive and respiratory syndrome virus (PRRSV) and Haemophilus (H.) parasuis in experimentally infected pigs. Thirty pigs seronegative to PRRSV and H. parasuis were divided into four groups. Group A pigs (10 animals) were inoculated with both virus and bacteria; group B pigs (10 animals) were inoculated with bacteria, group C pigs (five animals) were inoculated with virus and group D pigs (five animals) were kept as negative controls. All pigs of groups A and C became infected with PRRSV, according to virological techniques used (immunohistochemistry, virus isolation and virus serology). Lung, heart and tonsils were the most frequently immunolabeled tissues, and monocyte/macrophage lineage cells were the target for PRRSV in all tissues. All pigs in groups A and B also became infected with H. parasuis based on immunohistochemical and bacterial isolation results. Serosal surfaces, lung and tonsils were the most frequently immunolabeled tissues, and bacteria were found in monocyte/macrophage lineage cells as well as within neutrophil cytoplasm. No differences in terms of bacterial distribution or localization in tissues of pigs of groups A and B were detected. These results suggest that there is no influence of the previous infection with PRRSV in the occurrence of H. parasuis infection.

Protective role of maternal antibodies against Haemophilus parasuis infection

OBJECTIVE

To evaluate the impact of maternal antibodies after challenge exposure of baby pigs with a homologous serovar of Haemophilus parasuis.

ANIMALS

7 gilts and their litters from a high health status farm.

PROCEDURE

Gilts were vaccinated twice with a commercial bacterin that contained H parasuis serovar 4 and 5 or, as a control, adjuvant only. A group of pigs was also vaccinated similarly before challenge exposure. After early and late challenge exposure at 3 and 4 weeks, respectively, all pigs from vaccinated gilts were evaluated for clinical signs of infection, lesions, and antibody titer.

RESULTS

All pigs coming from control gilts had severe signs of H parasuis infection. Macroscopic lesions included polyserositis and pneumonia, and bacteriologic examination confirmed H parasuis as the etiologic agent. Vaccinated pigs born to vaccinated gilts did not have clinical signs of disease. However, some vaccinated pigs born to control gilts had signs of nervous system dysfunction and lameness. There was no difference in lesion scores between early or late challenge exposure, but lesions scores for pigs from vaccinated and control gilts were different (P < 0.01).

CONCLUSIONS

Under these experimental conditions, immune-naive and vaccinated pigs from vaccinated gilts were protected against systemic lesions when challenge exposed with a virulent strain of H parasuis.

CLINICAL RELEVANCE

Vaccination of the gilt and pigs protects the latter from polyserositis, but results are not different from those for nonvaccinated pigs from vaccinated gilts. Maternal antibodies did not seem to interfere with vaccination of pigs at 1 and 3 weeks of age.

Effect of porcine reproductive and respiratory syndrome virus infection on the clearance of Haemophilus parasuis by porcine alveolar macrophages

Porcine reproductive and respiratory syndrome virus (PRRSV) infection in young piglets is frequently associated with secondary infection due to various pathogens, especially those of the respiratory tract. One of the most important mechanisms in respiratory diseases is related to the alteration of function of porcine alveolar macrophages (PAMs). The objective of this study was to determine how PRRS virus infection affects the capabilities of PAMs in the phagocytosis and destruction of Haemophilus parasuis. Phagocytosis percentages were determined in vitro and ex vivo, after collected PAMs were directly exposed to the virus of if PAMs were collected from piglets previously infected with PRRSV. In vitro experiments demonstrated that H. parasuis uptake by PAMs is only increased in the early stages of PRRSV infection (2 h post-infection). In contrast, in the ex vivo experiments it was shown that PAMs from PRRSV-infected piglets do not seem to change in their phagocytic rate until the later stages of infection. Together with a decrease in the phagocytic rate, a marked decrease in the functional ability of PAMs to kill bacteria was observed 7 d post-infection. It is hypothesized that when animals are exposed to PRRSV, there is a marked decrease in the functional ability of PAMs to kill bacteria through the release of superoxide anion, indicating a possible negative effect of the virus, at least at the macrophage level.

Ultrastructural study of porcine alveolar macrophages infected in vitro with porcine reproductive and respiratory syndrome (PRRS) virus, with and without Haemophilus parasuis

Two experiments were designed to study ultrastructural changes in porcine alveolar macrophages (PAM) inoculated with porcine reproductive and respiratory syndrome (PRRS) virus (experiment 1) and with PRRS virus and Haemophilus parasuis (experiment 2). In both experiments, the viral infectious dose represented a "multiplicity of infection" of 1. Viral infection alone induced minimal ultrastructural changes at this dose, consisting only of an increase in lysosome numbers. Mixed viral and bacterial infection induced the production of greatly increased numbers of phagosomes and phagolysosomes. The PAM were of low efficacy in phagocytizing H. parasuis. PRRS virus infection had only a minimal effect on the phagocytosis of H. parasuis by PAM. It is suggested that the virus induces PAM activation rather than PAM destruction.

Evaluation of techniques for the detection of toxigenic Pasteurella multocida strains from pigs

Recently acquired field isolates and archived isolates from our collection of Pasteurella multocida were analyzed for production of dermonecrotic toxin. Detection of the toxin was carried out using a fetal lung feline (FLF) cell line and a commercial enzyme-linked immunosorbent assay (ELISA) kit. The dermonecrotic toxin gene (ToxA) was also detected using a polymerase chain reaction (PCR) technique. Results from the 3 methods were compared. Field isolates (group 1) came from a commercial herd that had clinical signs of atrophic rhinitis. Fifty-six (17.9%) strains were isolated from 312 nasal swabs. Thirty-five of these strains belonged to serotype A and the rest (21/56), although probably serotype D, were not characterized further. All of these strains were toxin negative based on both the ELISA and FLF cell culture results. Five isolates gave faint bands in the PCR reaction, and the rest (51/56) were PCR negative. PCR and ELISA were also performed from the initial swab cultures (mixed cultures); 7 samples gave faint PCR bands, but ELISA results were all negative. Archived strains (group 2) had been isolated from clinical cases of atrophic rhinitis and from cases of pulmonary pasteurellosis. A total of 76 strains were analyzed; 46 were serotype A, and the rest (30) were serotype D. ELISA and FLF cell culture tests were negative for all serotype A strains; however, 3 strains showed faint bands in the PCR reaction. Fourteen serotype D strains showed positive results in both the ELISA and the FLF cell culture tests. PCR from these samples also gave positive results showing a strong band in the gel. However, 4 strains that were ELISA and FLF cell culture negative showed a faint band in the PCR reaction. The 3 methods gave similar results in the detection of the P. multocida dermonecrotic toxin. However, complete agreement among the tests was achieved only when strong PCR bands were considered positive. This is the first report that demonstrates the use of FLF cell line for the detection of toxigenic P. multocida.

Colonization of suckling pigs by Streptococcus suis with particular reference to pathogenic serotype 2 strains

Three swine commercial farms with high mortality rates in nursery pigs due to Streptococcus suis serotype 2 were studied. Brain samples from diseased animals were collected for a period of 6 to 10 mo and used to isolate the strain that was responsible for the mortality (virulent strain) in each farm. Tonsil swabs from piglets at 5, 10 and 15 d were taken to assess both total colonization and colonization by the virulent strain. The effect of sow vaccination against S. suis on colonization was evaluated in 1 of the farms. All suspect tonsil isolates were identified biochemically and then tested against serotype 2. The genomic patterns of serotype 2 isolates were compared to that of the virulent strain using Rep-PCR. Results showed that total colonization by S. suis occurred very early in the pigs' life, with most animals being colonized by weaning age. Prevalence of colonization by serotype 2 strains was much lower than total colonization. After comparing serotype 2 isolates with the virulent strains, only 1 tonsillar isolate had the same genomic pattern as the virulent strain and it belonged to a 4-week-old weaned pig. The genomic pattern of the virulent strain was not found in any tonsillar isolate from 15-day-old or younger pigs. Although limited by sample size, sow vaccination against S. suis increased total colonization at the same time significantly decreasing colonization by serotype 2 strains. Even though most pigs are colonized early in age by S. suis, colonization by the virulent strain is of low prevalence and delayed in time. This could constitute a risk factor for developing the disease later in time, because animals would be colonized when maternal immunity is no longer present, allowing the organism to become systemic.

Airborne transmission of Actinobacillus pleuropneumoniae and porcine reproductive and respiratory syndrome virus in nursery pigs

OBJECTIVE

To document airborne transmission of Actinobacillus pleuropneumoniae and porcine reproductive and respiratory syndrome virus (PRRSV) infection in nursery pigs.

ANIMALS

32 two-week-old pigs obtained from 3 farms, but with similar Landrace X Yorkshire genetics for trial 1 of each experiment; 16 pigs for trial 2 of the A pleuropneumoniae experiment; and 14 pigs for trial 2 of the PRRSV experiment.

PROCEDURE

In experiment 1, pigs were inoculated with A pleuropneumoniae serotype 1 (6/8) or were left as contacts (2/8). At the beginning of trial 1, pigs were seronegative to A pleuropneumoniae serotypes 1 and 5 on the basis of results of an ELISA, but had positive results on the A pleuropneumoniae hemolysin I (Apx1)-neutralization test. Pigs in trial 2 had negative results on both tests. Pigs of trial 1 of experiment 2 were inoculated with a PRRSV virulent field isolate (MN-1b); pigs of trial 2 were inoculated with the virus reference strain VR-2332. Aerosol-exposed pigs were placed on the other side of the air duct and kept there for 2 to 7 weeks depending on evidence of airborne transmission.

RESULTS

In trial 1 of experiment 1, evidence of airborne transmission was not found. In trial 2, most airborne-exposed pigs died as a result of A pleuropneumoniae infection 12 days after initiation of the experiment. In trial 1 of experiment 2, all inoculated pigs (8/8) seroconverted, but only 2 of 8 contact-exposed pigs seroconverted. Aerosol-exposed pigs did not seroconvert nor was virus isolated. In trial 2, all inoculated and contact-exposed pigs seroconverted. All aerosol-exposed pigs seroconverted after 21 days, and virus was isolated at 16 days.

CONCLUSIONS

A pleuropneumoniae was transmitted by air at a distance of 1 m when pigs were fully susceptible to the organism. Transmission of PRRSV appeared to be strain dependent; when reference strain VR-2332 was used, airborne transmission of PRRSV was documented.

Immunohistochemical detection of Haemophilus parasuis serovar 5 in formalin-fixed, paraffin-embedded tissues of experimentally infected swine

An avidin-biotin complex immunohistochemistry technique was developed to detect Haemophilus parasuis serovar 5 in experimentally infected 18-21-day-old conventional pigs, using a rabbit polyclonal antiserum. Seven of 10 intratracheally inoculated animals developed a low to medium degree of fibrinous polyserositis; meninges and pleura were the most severely affected areas. Haemophilus parasuis was recovered from 9 of 10 pigs; in 2 of them H. parasuis was isolated from tracheal swabs only. Positive immunohistochemistry results, mainly observed as free bacteria or bacteria within inflammatory cell cytoplasm in the fibrinopurulent exudate, were observed in 8 of 10 animals. Cross-reactivity with Actinobacillus pleuropneumoniae was detected but not with other gram-positive and gram-negative bacteria tested. This immunohistochemistry technique seemed to be at least as sensitive as microbiologic cultures and could be useful in studies of pathogenesis and retrospective diagnosis. However, cross-reactivity with A. pleuropneumoniae means that positive immunohistochemistry results in lung tissue from field cases would be dubious.

Immunohistochemical demonstration of the spread of pneumotropic strain 4892 of Aujeszky's disease virus in conventional pigs

Sixteen pigs aged 5 to 7 weeks were inoculated intranasally with the pneumotropic strain 4892 of Aujeszky's disease virus (ADV) in a dose of 2 x 10(5) TCID50. Pigs died or were killed on day 4, 5, 6, 7, 8, 10, 12, 20 or 30 post-inoculation (PI). Two further pigs were kept as negative (uninfected) controls. Histopathological examination demonstrated meningoencephalitis, necrotizing rhinitis and multifocal systemic necrosis. Viral antigen was detected immunohistochemically, mainly in the central nervous system up to day 12 PI, and to a lesser degree in the lung, nasal mucosa and tonsil. ADV DNA was detected at days 20 and 30 PI by a nested polymerase chain reaction technique. This study indicated that the spread of the highly virulent, pneumotropic strain 4892 did not differ from that of other neurotropic or pneumotropic ADV strains.

Indirect fluorescent IgM antibody response of pigs infected with porcine reproductive and respiratory syndrome syndrome virus

IgG and IgM antibody responses were examined by an indirect fluorescent antibody method in pigs following inoculation with different porcine reproductive and respiratory syndrome virus (PRRSV) isolates or a vaccine virus. Viremia was also examined in the pigs. The IgG antibody was first detected between 9 and 14 days post inoculation (PI) and maintained high titers for at least 7 weeks PI. No change in IgG antibody titers was observed when the pigs were reinoculated with PRRSV 35 days PI. IgM antibody was detected between 5 and 28 days PI in the pigs. Reinoculation at 35 days PI caused a short term rise of IgM antibody. Virus was isolated from sera collected between 2 and 21 days PI. The IgM antibody was detected regularly in sera collected during viremia and up to 1-2 weeks after the viremic periods. These results suggest that pigs with detectable IgM antibody are probably pigs with recent infection and that routine testing of IgM antibody in purchased breeding pigs from seropositive farms may be useful in identification of pigs with recent infection.

Porcine reproductive and respiratory syndrome virus (PRRSv) interaction with Haemophilus parasuis

The interaction of bacteria and virus has been well demonstrated in the pathogenesis of respiratory disease in swine. The interaction between porcine respiratory and reproductive syndrome virus (PRRSv) and Haemophilus parasuis has not been studied. We initiated studies to evaluate a possible effect of the PRRSv on the pathogenesis of polyserositis caused by H. parasuis. A group of 30 three week old piglets were distributed in 4 groups. Group I (10 pigs) was inoculated with PRRSv and H. parasuis. Group II (10 pigs) was inoculated with H. parasuis alone. Group III (5 pigs) was inoculated with virus alone and group IV (5 pigs) was inoculated with culture media. Lesions consisted of a severe fibrinous polyserositis affecting 7 of 10 animals in group II and a mild fibrinous pleuritis in 1 of 10 animals of group I. Three of ten animals dually infected with the two agents died during the course of the study. These animals had pulmonary congestion and focal lung hemorrhages. No other animals died from other groups. Group III and IV had no macroscopic lesions. Microscopically group III had interstitial pneumonia. Immunomodulating virus effect may explain the differences in terms of lesions severity between groups I and II. Septic shock was suspected as cause of sudden death.

Effect of porcine reproductive and respiratory syndrome virus on subsequent Pasteurella multocida challenge in pigs

This trial was conducted to evaluate the effect of Porcine reproductive and respiratory syndrome virus (PRRSv) on a subsequent challenge with Pasteurella multocida in pigs. Sixteen, 3-4 week-old piglets, from a PRRSv and Aujeszky disease virus (ADV) free herd were used. Animals were equally and randomly allocated in four groups which were treated according the following schedule: Group I: negative controls; Group II: inoculation with only PRRSV; Group III: inoculation with PRRSV and P. multocida; Group IV: inoculation with ADV and P. multocida (positive controls). PRRSV and ADV were inoculated intranasally, at the doses of 10(4.6) and 10(4.5) TCID50/ml, respectively. Five days later, pigs from groups III and IV were inoculated intranasally, with two ml of a 10(9) CFU/ml suspension of equal parts of P. multocida, strains A52 and A24. No lesions were observed in piglets of group I. Microscopically, interstitial pneumonia was identified in all piglets of groups II and III and 3/4 piglets from group IV. Bronchopneumonia was detected in 3/4 of the piglets from group III and in all animals of group IV which, additionally, showed meningo-encephalitis and purulent rhinitis. Macroscopically, only piglets of groups III and IV had lung consolidation. However, much lower pneumonic scores (2.3%) were observed in group III, where 3 of 4 piglets were affected. On the other hand, all piglets of group IV showed some degree of pulmonary consolidation, with a mean score of 13.7%. Based on these results, it appears that the role of PRRSV as a initiator of secondary diseases is still undefined, but is probably mild. There was no clear interaction between PRRSv and Pasteurella multocida under the conditions and strains tested here.

Evaluation of the effects of nursery depopulation on the persistence of porcine reproductive and respiratory syndrome virus and the productivity of 34 farms

Nursery depopulation has been described as an effective strategy for improving the performance of weaned pigs. In order to assess whether the strategy was effective under a wide range of conditions, a study was carried out on 34 farms in the USA. Four groups with different depopulation protocols were designed on the basis of the location of the depopulated facility (on site vs off site) and the period for which the nursery remained empty (seven days vs 14 days). The changes in average daily liveweight gain, percentage mortality, feed efficiency and treatment cost per pig produced were assessed 12 months before and after nursery depopulation. The ability to eliminate porcine reproductive and respiratory syndrome (PRRS) virus was examined by indirect fluorescent antibody testing of the nursery pigs. Significant improvements (P < 0.0001) were detected in both average daily gain and percentage mortality after depopulation when the differences within an individual group were analysed, but no significant differences (P > 0.14) were observed between the study groups. Serological testing indicated that antibodies to PRRS virus were still present in 14 of the 34 farms after depopulation.

Prevalence of various phenotypes of Streptococcus suis isolated from swine in the U.S.A. based on the presence of muraminidase-released protein and extracellular factor

The present study describes the prevalence of muraminidase-released protein (MRP) and extracellular factor (EF) proteins associated with virulence of Streptococcus suis serotype 2 from a collection of USA strains. Sixty-six strains belonging to serotypes 1, 2, 3, 4, 7, and 10, were analyzed with a set of double antibody sandwich ELISAs and Western blots. Nineteen of 34 serotype 2 strains from cases of swine meningitis had the MRP+EF+ phenotype. Five of 7 serotype 2 strains isolated from lungs had an MRP*EF- phenotype. An MRP-EF+ phenotype was found in 4/34 strains isolated from swine meningitis. The MRP*EF- and MRP-EF+ phenotypes have not been reported previously. All strains of serotypes other than 2, including isolates from cases of meningitis, had the MRP-EF- phenotype, suggesting that these strains must have other, as yet undetected, virulence factors.

Expression of iron-regulated outer membrane proteins by porcine strains of Pasteurella multocida

The outer membrane protein (OMP) profiles of two strains of capsular type A Pasteurella multocida isolated from the lungs of pigs with enzootic pneumonia were studied. Sarkosyl extracted OMPs from P. multocida grown under iron-restricted and iron-replete conditions were subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blot analysis. Results showed that the iron-regulated outer membrane proteins (IROMPs) with molecular masses of 74 kDa, 94 kDa, 99 kDa and 109 kDa were expressed by strain A52, while 74 kDa, 82 kDa, 94 kDa and 99 kDa IROMPs were expressed by strain B80. Swine immune sera, obtained from pigs which were first immunized with a polyvalent P. multocida type A and type D bacterin and subsequently challenged with type A strain of P. multocida, contained antibodies against the IROMPs. These antibodies cross-reacted with the IROMPs expressed by avian strain P1059 of P. multocida. Convalescent-phase serum obtained from turkeys which survived fowl cholera, also cross-reacted with the IROMPs from porcine strains of P. multocida. These results suggested that IROMPs from porcine and avian strains of P. multocida may share common epitopes that were recognized by swine immune serum as well as turkey convalescent-phase serum.

Interaction between Streptococcus suis serotype 2 and porcine reproductive and respiratory syndrome virus in specific pathogen-free piglets

Secondary specific pathogen-free (sSPF) piglets were inoculated intranasally with Streptococcus suis serotype 2 alone, porcine reproductive and respiratory syndrome virus (PRRSV) alone, or with PRRSV followed by S suis. Uninfected piglets were used as controls. Pigs inoculated with PRRSV (ATCC VR-2332) followed by challenge with a virulent strain (87555) of S suis serotype 2 developed clinical signs, suppurative meningitis and large numbers of S suis in their tissues, including the brain and meninges. Pigs inoculated with PRRSV alone, S suis (87555) alone, or with PRRSV and the DH5 strain of S suis serotype 2 (lacking a protein associated with virulence) and the uninfected piglets did not develop clinical signs or lesions or have large numbers of bacteria in their tissues. The results suggest that PRRSV predisposes sSPF pigs to infection and disease caused by virulent S suis serotype 2. Co-infection of piglets with PRRSV and a virulent strain of S suis may provide a useful model for the study of S suis septicaemia and meningitis.