Study Reveals Secrets of Swine Influenza Virus in Thailand

Researchers based at Chulalongkorn University have investigated the pathogenesis of Thai isolates of the swine influenza virus (SIV) sub-types H1N1 and H3N2 in weaned pigs. Editor of ThePigSite, Jackie Linden, summarises their report on this hottest of topics.

15 May 2009

5 minute read

Sreta and colleagues at Chulalongkorn University and the National Institute of Animal Health, both in Bangkok, have published their study of Thai isolates of the H1N1 and H3N2 sub-types of the swine influenza virus (SIV). Their study has been published in the latest issue of Journal of Virology.

The authors explain that swine influenza is an acute, highly contagious, respiratory disease caused by type A influenza virus infection. Currently, 16 haemagglutinin (HA) sub-types and nine neuraminidase (NA) sub-types are identified. Three main sub-types currently circulating in the pig population are classical swine influenza virus (SIV) and re-assortant viruses of H1N1, H3N2 and H1N2.

They point out that the pig plays a substantially important role in the ecology of influenza A virus since it can act as a 'mixing vessel'. The fear is that any new sub-type can be produced by antigenic reassortment in case of co-infections among human, avian or swine influenza viruses occur in a host – a very widely debated topic at the present time.

Typically, SIV infects the epithelial lining of the respiratory tract producing clinical signs consisting of cough, fever, lethargy and anorexia. The authors explain that SIV-associated gross lung lesions observed in pigs are characterised by multifocal well-demarcated purplish-red lesions in the cranioventral areas of lung lobes known as a checker-board lung. SIV-induced microscopic lesions consist of epithelial disruption and attenuation in the bronchioles with later found hyperplastic proliferation and bronchiolitis obliterans, they say. Mild to moderate peribronchiolar and perivascular lymphocytic infiltration occurs at nearly all levels of the airways. Viral antigen can be detected in epithelial cells of airways by immunohistochemistry (IHC) staining.

Study of SIV in Thailand

Sreta and colleagues write that in their country, H1N1 SIV was the first sub-type isolated from pigs with an influenza-like symptom in 1990. Both of the sub-types studied are commonly found among the pig population in Thailand. More recently, in 2005, a new sub-type (H1N2) was isolated from pigs in Saraburi province.

In Thailand, pathogenesis of SIV sub-type H1N1 and H3N2 infection in swine has never been studied, according to Sreta and colleagues. Since different sub-types of the influenza type A viruses isolated from pigs are found to cause different pathogenic levels in pigs, they set the objective of their study to investigate the pathogenesis of SIV (Thai isolates) sub-type H1N1 (A/swine/Thailand/HF6/05) and H3N2 (A/swine/Thailand/S1/05) in weanling specific pathogen-free (SPF) pigs. They also performed genetic characterisation of the HA gene of both the studied viruses.

Results

The study found that all pigs in the infected groups developed typical signs of flu-like symptoms at one to four days post-infection (dpi). The H1N1-infected pigs had greater lung lesion scores than those of the H3N2-infected pigs. Histopathological lesions related to swine influenza-induced lesions consisting of epithelial cells damage, airway plugging and peribronchial and perivascular mononuclear cell infiltration were present in both infected groups.

Table 1. Percentages of gross lung lesions and the presence of SIV-specific antigen based on immunohistochemistry (IHC) staining
	Mock *	H1N1 **		H3N2 **
dpi	Pig 1 (%)	Pig 1 (%)	Pig 2 (%)	Pig 1 (%)	Pig 2 (%)
2	0.0 (-)	36 (+)	33 (+)	20 (+)	2.0 (+)
4	0.0 (-)	5.0 (+)	3.0 (+)	2.0 (+)	1.0 (+)
12	0.0 (-)	6.0 (+)	0.5 (-)	0.0 (-)	0.0 (-)
n=1; *n=2 dpi = days post infection (-) negative antigen detection by IHC; (+) positive antigen detection by IHC)

Immunofluorescence and immunohistochemistry using nucleoprotein specific monoclonal antibodies revealed positive staining cells in lung sections of both infected groups at two and four days post-infection, say the report's authors. They added that virus shedding was detected two days after infection from both infected groups as demonstrated by RT-PCR and virus isolation.

SIV antigen staining by IHC, (A) negative control, (B) dark brown staining cells (arrow) of the SIV-positive control, (C) SIV-positive staining on alveolar epithelial cells and (D) bronchiolar epithelial cells

Conclusion

Sreta and colleagues says that their results demonstrate that both SIV sub-types were able to induce flu-like symptoms and lung lesions in weaner pigs. The severity of the diseases with regards to lung lesions both gross and microscopic lesions was greater in the H1N1-infected pigs than those infected with H3N2.

The added that, based on their phylogenetic analysis, haemagglutinin gene of sub-type H1N1 from Thailand clustered with the classical H1 SIV sequences and neuraminidase gene clustered with virus of avian origin, whereas, both genes of H3N2 sub-type clustered with H3N2 human-like SIV from the 1970s.

Reference

Sreta D., R. Kedkovid, S. Tuamsang, P. Kitikoon and R. Thanawongnuwech. 2009. Pathogenesis of swine influenza virus (Thai isolates) in weanling pigs: an experimental trial. Virology Journal, 6:34.