Friday, August 13, 2010

EID Journal: Asymptomatic H5N1 In Pigs



# 4803



Today an important study in the CDC’s EID Journal, epublished ahead of its scheduled October printing, that looks at the infiltration of H5N1 (bird flu) into Indonesia’s pig population.


The authors list reads like a `who’s who’ of influenza research – many being familiar to readers of this blog - including such names as C. A. Nidom and Yoshihiro Kawaoka.


First some excerpts (slightly reformatted for readability) from the abstract . . . then some discussion.


DOI: 10.3201/eid1610.100508
Nidom CA, Takano R, Yamada S, Sakai-Tagawa Y, Daulay S, Aswadi, D, et al. Influenza A (H5N1) viruses from pigs, Indonesia. Emerg Infect Dis. 2010 Oct;[Epub ahead of print]

Influenza A (H5N1) Viruses from Pigs, Indonesia


Pigs have long been considered potential intermediate hosts in which avian influenza viruses can adapt to humans. To determine whether this potential exists for pigs in Indonesia, we conducted surveillance during 2005–2009.


We found that 52 pigs in 4 provinces were infected during 2005–2007 but not 2008–2009.


Phylogenetic analysis showed that the viruses had been introduced into the pig population in Indonesia on at least 3 occasions. One isolate had acquired the ability to recognize a human-type receptor.


No infected pig had influenza-like symptoms, indicating that influenza A (H5N1) viruses can replicate undetected for prolonged periods, facilitating avian virus adaptation to mammalian hosts.


Our data suggest that pigs are at risk for infection during outbreaks of influenza virus A (H5N1) and can serve as intermediate hosts in which this avian virus can adapt to mammals.



Pigs have long been suspected as being ideal `mixing vessels’ for influenza because they are believed susceptible to a wide variety of human, swine, and avian flu viruses.


Swine possess both avian-like (SAα2,3Gal) and human-like (SAα2,6Gal) receptor cells in their respiratory tract, which scientists believe can facilitate a `bridging’ between avian and human strains.


The recent emergence of the 2009 H1N1 virus, after reassorting and bouncing around in pigs for years, has helped reinforce that theory.


But whether the H5N1 bird flu strain can readily adapt to, infect, and transmit among pigs has been an open question. 


In previous experiments, domestic pigs were demonstrated to have low susceptibility to the H5N1 virus, producing asymptomatic to mildly symptomatic infection of the respiratory tract and tonsils. 


Viral titers were lower than normally seen with swine influenzas, suggesting a limited ability to adapt and spread among pigs.


Today’s research found that during the period of 2005-2007 that 7.4% of pigs surveyed in Indonesia carried the H5N1 virus, and that phylogenic analysis showed at least 3 separate introductions into the pig population.


More recent surveillance, during 2008-2009 did not turn up any active infections, but 1% of pigs tested carried antibodies to the H5N1 virus.


The pigs (as in the study mentioned above) were asymptomatic, but the prevalence among pigs and genetic similarities among isolates were viewed as evidence of ongoing transmission within the pig population.


In at least one sample, the researchers found evidence of an adaptation of the virus to human-like receptor cells with via an Ala134Ser mutation (substitution of alanine with serine) at position 134.


While the entire study is of interest, and very much worth reading, a few notable quotes to whet your curiosity:


  • These findings show that although influenza A (H5N1) viruses may not have been extensively circulating in pigs in Indonesia recently, these animals are susceptible to influenza A (H5N1) viruses and can serve as asymptomatic reservoirs for these viruses.

  • We conclude that the 3 groups of viruses identified in this survey were likely established independently, suggesting at least 3 separate avian-to-pig episodes of transmission of influenza A (H5N1) viruses during 2005–2009 in Indonesia.

  • . . . when an outbreak of influenza A virus (H5N1) infection occurs on poultry farms, pigs on nearby farms should be evaluated for infection

  • We also found evidence of pig-to-pig transmission of influenza A virus (H5N1), particularly among animals sampled during the first surveillance period.

  • The lack of influenza-like signs in pigs infected with influenza A (H5N1) viruses has several public health implications.

    • In Indonesia, pigs are transported to different locations according to market needs. This movement is reflected in our finding that clusters of swine viruses collected after 2006 were not consistent with those common to the sampling region.

    • Indeed, viruses collected in North Sumatra, South Kalimantan, East Java, and Banten provinces showed identical or nearly identical genes, indicative of extensive transport of infected pigs throughout Indonesia.

    • Thus, pathogenic influenza A (H5N1) viruses could easily evade detection as they spread through Indonesia in asymptomatic pigs being transported from province to province.