Genetic variation in swine influenza virus A isolate associated with proliferative and necrotizing pneumonia in pigs

Localization and immunohistochemical detection of swine influenza A virus subtype H1N1 antigen in formalin-fixed, paraffin-embedded lung tissues from naturally infected pigs

Background

Swine influenza A viruses (SIV) infection is among the leading causes of respiratory diseases in a number of animal species and human, and has been reported to cause substantial losses to pig industry. Previous reports of serological, molecular, and surveillance studies in commercial piggeries in Nigeria indicated the presence of SIV subtypes H1N1 and H3N2 in infected pigs; hitherto, there exists lack of studies on the pulmonary pathology and pathogenicity of SIV in Nigeria. This study investigates the presence of SIV subtype H1N1 antigen in the formalin-fixed paraffin-embedded lung sections obtained from apparently healthy pigs slaughtered at abattoirs located in Lagos, Ogun, and Oyo States, Southwest Nigeria using a streptavidin-biotin (ABC) immunoperoxidase (IP) staining. Two hundred four lungs consisting of 144 grossly pneumonic lungs and 60 apparently normal lungs were randomly collected, fixed in 10% neutral-buffered formalin, embedded in paraffin wax, and processed for histopathological examination and immunohistochemistry.

Results

The main gross lesions were marked pulmonary edema and mild bilateral consolidation of cranial lobes. Histopathology revealed suppurative bronchitis, and bronchiolitis with or without concurrent widespread degeneration and necrosis of epithelial cells (52.08%) and thickening of alveolar septa due to cellular infiltration consisting predominantly of neutrophils and mononuclear cells (macrophages and plasma cells) (39.58%). The lumina of most airways contained exudate consisting of neutrophils, desquamated epithelia cells, and necrotic debris. SIV antigen was immunohistochemically detected in 7/204 (3.43%) samples using SIV-specific (H1N1) monoclonal antibody. Positive cells exhibited a typical dark-brown reaction in the infected cells. A strong positive immunohistochemical staining was detected mainly in the alveolar macrophages and bronchial submucosal glandular epithelial cells while less intense staining was observed in the bronchiolar epithelial cells.

Conclusions

The present study describes the distribution and localization of SIV subtype H1N1 antigens in the lung tissues of the infected pigs and provides public awareness on the presence of the virus in pig population in Nigeria and the risk factors associated with the infection. Therefore, people working in pig farms should maintain high level of biosafety and personal hygiene. This is the first report of immunohistochemical detection of SIV subtype H1N1 antigen in naturally infected pigs in Nigeria and may indicate rapid dissemination of the virus in susceptible pigs in the study area. A further molecular epidemiological study to investigate other SIV subtypes circulating in Nigerian pig population is warranted.

The nucleotide sequence of the HA1 of the haemagglutinin of an HI avian influenza virus isolate from turkeys in Germany provides additional evidence suggesting recent transmission from pigs

The nucleotide sequence encoding the HA1 portion of the haemagglutinin gene of the influenza virus A/turkey/Germany/2482/90j isolated from birds kept in an area of many pig farms, was determined and compared with those of recent avian and swine influenza isolates. It was found to be closest to the 'avian-like' swine H1N1 influenza viruses that have been reported in Europe since the early 1980s and may represent good evidence for transmission of these viruses back to birds after they have become established in pigs.

Polymicrobial respiratory disease in pigs

Respiratory disease in pigs is common in modern pork production worldwide and is often referred to as porcine respiratory disease complex (PRDC). PRDC is polymicrobial in nature, and results from infection with various combinations of primary and secondary respiratory pathogens. As a true multifactorial disease, environmental conditions, population size, management strategies and pig-specific factors such as age and genetics also play critical roles in the outcome of PRDC. While non-infectious factors are important in the initiation and outcome of cases of PRDC, the focus of this review is on infectious factors only. There are a variety of viral and bacterial pathogens commonly associated with PRDC including porcine reproductive and respiratory syndrome virus (PRRSV), swine influenza virus (SIV), porcine circovirus type 2 (PCV2), Mycoplasma hyopneumoniae (MHYO) and Pasteurella multocida (PMULT). The pathogenesis of viral respiratory disease is typically associated with destruction of the mucocilliary apparatus and with interference and decrease of the function of pulmonary alveolar and intravascular macrophages. Bacterial pathogens often contribute to PRDC by activation of inflammation via enhanced cytokine responses. With recent advancements in pathogen detection methods, the importance of polymicrobial disease has become more evident, and identification of interactions of pathogens and their mechanisms of disease potentiation has become a topic of great interest. For example, combined infection of pigs with typically low pathogenic organisms like PCV2 and MHYO results in severe respiratory disease. Although the body of knowledge has advanced substantially in the last 15 years, much more needs to be learned about the pathogenesis and best practices for control of swine respiratory disease outbreaks caused by concurrent infection of two or more pathogens. This review discusses the latest findings on polymicrobial respiratory disease in pigs.

Characterization of H1N1 swine influenza viruses circulating in Canadian pigs in 2009

The 2009 pandemic H1N1 (pH1N1), of apparent swine origin, may have evolved in pigs unnoticed because of insufficient surveillance. Consequently, the need for surveillance of influenza viruses circulating in pigs has received added attention. In this study we characterized H1N1 viruses isolated from Canadian pigs in 2009. Isolates from May 2009 were comprised of hemagglutinin and neuraminidase (NA) genes of classical SIV origin in combination with the North American triple-reassortant internal gene (TRIG) cassette, here termed contemporary SIV (conSIV) H1N1. These conSIV H1N1 viruses were contiguous with the North American αH1 cluster, which was distinct from the pH1N1 isolates that were antigenically more related to the γH1 cluster. After the initial isolation of pH1N1 from an Alberta pig farm in early May 2009, pH1N1 was found several times in Canadian pigs. These pH1N1 isolates were genetically and antigenically homogeneous. In addition, H1N1 viruses bearing seasonal human H1 and N1 genes together with the TRIG cassette and an NA encoding an oseltamivir-resistance marker were isolated from pigs. The NS gene of one of these seasonal human-like SIV (shSIV) H1N1 isolates was homologous to pH1N1 NS, implicating reassortment between the two strains. Antigenic cross-reactivity was observed between pH1N1 and conSIV but not with shSIV H1N1. In summary, although there was cocirculation of pH1N1 with conSIV and shSIV H1N1 in Canadian pigs after May 2009, there was no evidence supporting the presence of pH1N1 in pigs prior to May 2009. The possibility for further reassortants being generated exists and should be closely monitored.

Drift in the nucleoprotein gene of swine influenza virus (H1N1) causing respiratory disease in pigs

The nucleoprotein (NP) gene of swine influenza H1N1 variant, A/Sw/Quebec/5393/91 (SwQc91) was sequenced. When compared with other H1N1 strains, 12 amino acid (aa) replacements were observed in the 101-484 aa region of the NP protein including two aas, 345 and 430, representing the unique lineage of swine viruses. Phylogenetic analysis showed a drift in the NP gene.

The emergence of novel swine influenza viruses in North America

Since 1997, novel viruses of three different subtypes and five different genotypes have emerged as agents of influenza among pigs in North America. The appearance of these viruses is remarkable because there were no substantial changes in the overall epidemiology of swine influenza in the United States and Canada for over 60 years prior to this time. Viruses of the classical H1N1 lineage were virtually the exclusive cause of swine influenza from the time of their initial isolation in 1930 through 1998. Antigenic drift variants of these H1N1 viruses were isolated in 1991-1998, but a much more dramatic antigenic shift occurred with the emergence of H3N2 viruses in 1997-1998. In particular, H3N2 viruses with genes derived from human, swine and avian viruses have become a major cause of swine influenza in North America. In addition, H1N2 viruses that resulted from reassortment between the triple reassortant H3N2 viruses and classical H1N1 swine viruses have been isolated subsequently from pigs in at least six states. Finally, avian H4N6 viruses crossed the species barrier to infect pigs in Canada in 1999. Fortunately, these H4N6 viruses have not been isolated beyond their initial farm of origin. If these viruses spread more widely, they will represent another antigenic shift for our swine population, and could pose a threat to the world's human population. Research on these novel viruses may offer important clues to the genetic basis for interspecies transmission of influenza viruses.

Genomic analysis of matrix gene and antigenic studies of its gene product (M1) of a swine influenza virus (H1N1) causing chronic respiratory disease in pigs

The nucleotide sequence of gene coding for the matrix protein (M1 and M2) of swine influenza (H1N1) virus, A/Sw/Quebec/5393/91 (SwQc91), associated with chronic respiratory disease in pigs, was determined. The deduced amino acid (aa) sequence was compared with the other North American swine strains including the A/Sw/Quebec/192/81 (SwQc81) strain associated with the chronic and acute respiratory disease in pigs. Separate analysis of the M1 and M2 gene products showed different evolutions. M1 had 2 aas changes among 252 aas and these were at positions 4 and 205. The mutation rate was 0.08%, aa changes per residue per year, and its homology with other strains was 99.2%. The M2 protein (97 aas) was relatively more variable than M1 with 5 substitutions. Differences observed were at positions 4, 16, 21, 54 and 95. The mutation rate was 0.51% and its homology with other strains was 94.8%. The M1 gene was cloned in the procaryotic plasmid pET21a and the recombinant plasmid was expressed in Escherichia coli under pre-determined optimal conditions. The recombinant M1 protein (RM1P) (approximately 28 kDa) comigrated as a single band on SDS-PAGE. RM1P was antigenic and reacted with polyclonal sera and 5 monoclonal antibodies (MAbs) spanning 4 epitopes including the membrane binding site and the transcription inhibition activity site. RM1P was immunogenic. The mouse anti-RM1P ELISA antibodies reacted with the purified viral M1 protein and the whole virus.

The epidemiology and evolution of influenza viruses in pigs

Pigs serve as major reservoirs of H1N1 and H3N2 influenza viruses which are endemic in pig populations world-wide and are responsible for one of the most prevalent respiratory diseases in pigs. The maintenance of these viruses in pigs and the frequent exchange of viruses between pigs and other species is facilitated directly by swine husbandry practices, which provide for a continual supply of susceptible pigs and regular contact with other species, particularly humans. The pig has been a contender for the role of intermediate host for reassortment of influenza A viruses of avian and human origin since it is the only domesticated mammalian species which is reared in abundance and is susceptible to, and allows productive replication, of avian and human influenza viruses. This can lead to the generation of new strains of influenza, some of which may be transmitted to other species including humans. This concept is supported by the detection of human-avian reassortant viruses in European pigs with some evidence for subsequent transmission to the human population. Following interspecies transmission to pigs, some influenza viruses may be extremely unstable genetically, giving rise to variants which could be conducive to the species barrier being breached a second time. Eventually, a stable lineage derived from the dominant variant may become established in pigs. Genetic drift occurs particularly in the genes encoding the external glycoproteins, but does not usually result in the same antigenic variability that occurs in the prevailing strains in the human population. Adaptation of a 'newly' transmitted influenza virus to pigs can take many years. Both human H3N2 and avian H1N1 were detected in pigs many years before they acquired the ability to spread rapidly and become associated with disease epidemics in pigs.

Differentiation between porcine reproductive and respiratory syndrome virus isolates by restriction fragment length polymorphism of their ORFs 6 and 7 genes

Three distinct antigenic profiles were identified by comparing the reactivities of 15 Canadian field isolates, the attenuated U.S. vaccine (Ingelvac MLV) strain and 2 European reference strains (Lelystad and Weybridge) of the porcine reproductive and respiratory syndrome virus (PRRSV) by indirect immunofluorescence with a set of 4 monoclonal antibodies to the nucleocapsid (N) protein and 2 other to the matrix (M) protein. In the present study, 9 Canadian isolates for which the sequences were determined appeared closely related to 2 U.S. reference strains (ATCC VR-2332 and ATCC VR-2385) with amino acid identities varying between 90 to 98% for the M and N proteins; substitutions in the nucleotide sequences were distributed randomly throughout the ORFs 6 and 7 genes, and most were 3rd base silent mutations. In comparison, more than 30% divergence was demonstrated with the Lelystad virus. Furthermore, differentiation between North American and European isolates, and between field isolates and the MLV strain could be achieved by cutting PCR-amplified products encompassing both ORFs 6 and 7 genes with 4 restriction endonucleases. When taken individually, BsaJI and AluI were the more appropriate restriction enzymes for distinguishing the vaccine strain from field isolates. The results obtained suggest that the restriction fragment length polymorphism of the genomic region covering the ORFs 6 and 7 genes may be a valuable tool to differentiate among PRRSV isolates.

Complete sequences of the neuraminidase genes of swine influenza viruses (H1N1) associated with the respiratory disease in pigs

The complete nucleotide sequences of neuraminidase (NA) of two swine influenza viruses (H1N1) are presented. A/Sw/Quebec/5393/91 (SwQc91) virus, associated with the chronic respiratory disease and A/Sw/ Quebec/192/81 (SwQc81) virus, associated with the acute respiratory disease, were used. The deduced amino acid sequences of NA of SwQc91 and SwQc81 viruses showed a high degree (>95%) of similarity. The NA gene of both viruses was a single open reading frame of 1459 nucleotides coding for 469 aa with a 5' noncoding region of 21 nucleotides and a 3' noncoding region of 28 nucleotides. The comparison of two sequences showed that there were 23 differences recorded for SwQc91 strain, of which 5, 6, and 12 differences were recorded in the hydrophobic, stalk and head regions, respectively. A potential antigenic determinant changed from Ala to Thr at position 453 and there was a new potential glycosylation site present at position 88 for SwQc91 strain whereas it was absent at position 50 when compared with SwQc81 strain. Estimates of genetic distance and phylogenic tree analysis showed that SwQc91 and SwQc81 viruses were closely related with each other and with the American strain, A/Sw/Wisconsin/4754/94. However, the swine viruses represented a distinct group that was considerably divergent from the group of human viruses.