# 4673
Most scientists believe that the two main ways that flu viruses mutate, change, or evolve are by either antigenic drift or by antigenic shift.
Drift occurs when when flu viruses make replication errors.
When a flu virus invades a host cell, it makes numerous of copies of itself, and after a few hours, those thousands of `offspring’ then burst out of the cell and move on to infect other cells.
Sometimes, however, errors creep into the duplication process.
If these errors result in a biologically fit virus that can compete successfully against the parent strain, we see a viable mutation.
Antigenic Drift is an incremental change and happens more or less continually.
It explains why the flu vaccine we used last year may not be protective against the same virus this year. And why immunity you gained from your last bout with flu may not protect you today.
Bigger, more abrupt changes come about through antigenic shift, which can occur when a novel virus makes a direct jump from another species to humans or (more likely) a reassortment of two flu viruses occurs in a host animal resulting in a new hybrid virus.
While rare, as any virologist will tell you. Shift Happens.
This is the process that, after having been compounded several times over a number of years in host pigs, resulted in the H1N1 `swine’ flu pandemic virus of 2009.
And while any susceptible host has the potential to serve as a `mixing vessel’ for influenza viruses, pigs appear particularly well adapted for this role.
Pigs are susceptible to many human, swine, and avian strains of flu and are in frequent contact with humans, which increased their chances of acquiring and passing on infection.
While reassortment is a frequent topic in this blog, a few notable stories from the past six months include:
- Last week the big news was a study out of Hong Kong on the reassorted 2009 H1N1 virus detected in a Chinese pig (see H1N1 Reassortment In Swine).
- In February we saw research that showed that the H3N2 seasonal virus could reassort with the H5N1 `bird flu’ virus, and produce virulent hybrids (see PNAS: H3N2 And H5N1 Reassortment) under laboratory conditions.
- In January we learned (via Helen Branswell) of a single human infection by a novel H3N2 swine flu virus in Iowa (see H3N2 Swine Flu)
All of which lays some foundation for a study which appears in the latest issue of the CDC’s EID Journal titled:
Novel Swine Influenza Virus Reassortants in Pigs, China
DOI: 10.3201/eid1607.091881
Yuhai Bi, Guanghua Fu, Jing Chen, Jinshan Peng, Yipeng Sun, Jingjing Wang, Juan Pu, Yi Zhang, Huijie Gao, Guangpeng Ma, Fulin Tian, Ian H. Brown, and Jinhua Liu
The abstract is disarmingly short.
Abstract
During swine influenza virus surveillance in pigs in China during 2006–2009, we isolated subtypes H1N1, H1N2, and H3N2 and found novel reassortment between contemporary swine and avian panzootic viruses. These reassortment events raise concern about generation of novel viruses in pigs, which could have pandemic potential.
A few excerpts flesh this out a little better. First, this is how the research was conducted.
During December 2006–February 2009 in the People's Republic of China, 3,546 samples from 3 main swine industry provinces—Fujian (765 samples), Guangdong (1,276 samples), and Shandong (1,505 samples)—were collected for influenza surveillance. Nasal and tracheal swab samples were collected from apparently healthy domestic pigs at abattoirs. Virus isolation and identification were performed as described (3).
Of 29 strains of influenza A virus obtained, 19 were subtype H1N1, 1 subtype H1N2, and 9 subtype H3N2. Subtype H1N2 was isolated from diseased pigs in Guangdong Province in 2006; the others were isolated from healthy pigs. Isolation rates for subtypes H1N1 and H3N2 were 0.54% and 0.25%, respectively, indicating that subtype H1N1 viruses were predominant in the sampled pig population.
And here are the author’s conclusions (slightly reformatted for readability).
Conclusions
Influenza A subtypes H1N1, H1N2, and H3N2 viruses co-circulate in China. Genetic analysis showed that the single subtype H1N2 virus and all subtype H3N2 viruses examined were either double- or triple-reassortant viruses, which have been rarely documented in China.
Finding a gene fragment ostensibly of highly pathogenic avian influenza (H5N1) virus in a subtype H3N2 virus implies that subtype H5N1 viruses may be able to contribute genes to virus pathogenic processes in pigs. Moreover, European avian-like swine (H1N1) virus undergoes reassortment with avian (H9N2) viruses.
Some researchers have hypothesized that pigs may serve as hosts for genetic reassortment between human and avian influenza viruses (4). Our results show that subtypes H3N2 and H1N2 and 1 European avian-like swine (H1N1) virus were all derived from relatively recent reassortment events.
The gene fragments of the subtype H3N2 viruses comprised those of human subtype H3N2 (A/Victoria/75-like and A/Moscow/99-like) and the strains H1N1 classical swine, Eurasian H5N1, and H9N2 avian.
Infection of pigs with avian H5N1 and H9N2 viruses in China has been reported, and swine H1 and H3 viruses appear widely established in the pig population in China and elsewhere in Southeast Asia (5–9). These findings raise more questions about the generation of novel viruses, which may have zoonotic potential, in pigs.
Pandemic (H1N1) 2009 virus probably resulted from reassortment of recent North American influenza subtypes H3N2 and/or H1N2 swine viruses with Eurasian avian-like swine viruses (2).
The current situation, therefore, presents continued risk for further reassortment of swine influenza virus in pig populations and continued spread of pandemic (H1N1) 2009 virus to pigs worldwide. Systematic influenza virus surveillance in pigs is needed in China.
Obviously, the detection of 29 strains of influenza A – including numerous double and triple reassortant viruses – is a worrisome finding, and discovery of fragments of avian flu viruses in some of these strains only compounds the concern.
Nature’s laboratory is open 24/7 - and unlike public health, scientific research, and surveillance programs - operates without budgetary constraints.
Which is why better surveillance (and funding) is needed if we hope to detect, and possibly head off, the next influenza pandemic before it jumps to humans.
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