# 5317
One of the biggest lessons learned from the past couple of years was that influenza viruses don’t always follow the script.
Not only did the last pandemic virus emerge from the `wrong’ species (pigs instead of birds), it came from the `wrong’ continent (North America instead of Asia) and from a completely unexpected lineage (H1N1).
While pigs have long been considered potential mixing vessels for influenza viruses, today that threat is taken even more seriously than it was before.
Over the past couple of years we’ve seen a number of reports of the now-human-adapted novel H1N1 virus circulating in pigs. Hardly surprising – given that the virus jumped from pigs - but concerning because pigs are exposed to a variety of influenza viruses (human, swine, and avian).
Although surveillance of influenza circulating in swine is severely limited around the world, last September we saw a study indicating a surprisingly high number of pigs tested in the UK carried various influenza viruses.
A brief excerpt from UK: Flu Prevalence In Pigs:
The most commonly detected virus was an avian-like H1N1, followed by H1N2, and with just one with H3N2. 18% of farms tested positive for two strains – H1N1 and H1N2 – which the authors state could either represent a cross-reaction or infection with both strains.
According to the Vetsonline article, the incidence of swine influenza detected in France in 2009 was far higher, with 97% of the 29 farms tested positive for at least one strain of the virus.
These strains are consistent with what is normally detected in pig herds, although the prevalence is higher than many had expected in the UK. This from the CDC Factsheet on swine influenza.
Key Facts about Swine Influenza (Swine Flu)
Over the years, different variations of swine flu viruses have emerged. At this time, there are four main influenza type A virus subtypes that have been isolated in pigs: H1N1, H1N2, H3N2, and H3N1. However, most of the recently isolated influenza viruses from pigs have been H1N1 viruses.
When two influenza viruses inhabit the same host at the same time, they can sometimes swap genetic material, and a new `hybrid’ virus can emerge.
This is called reassortment.
While `biologically fit’ hybrids don’t happen often, when they do, they have the potential to jump to humans. Which is exactly what happened with `swine flu’ in early 2009.
It is notable that on rare occasions over the past several years we’ve seen a few reassorted swine viruses show up in humans. So far, these appear to be mostly the result of direct pig-to-human transmission.
China: Single Novel Swine Flu Infection Reported
North America Influenza Surveillance - Week 4
It was just about a year ago that we learned that a sample taken from a pig in a Hong Kong slaughterhouse tested positive for a reassorted influenza virus (see Hong Kong: Swine Flu Reassortment).
Since then, Hong Kong has increased their surveillance of pigs – particularly those imported from the mainland – by taking routine samples twice a month at a local slaughterhouse.
Today a press release from Hong Kong’s Centre for Food Safety (CFS) on their last 3 month’s surveillance.
Results of regular influenza virus surveillance in pigs in past three months released
The Centre for Food Safety (CFS) today (February 16) announced results of the regular influenza virus surveillance programme on pigs conducted by the University of Hong Kong (HKU) in the past three months (from November 2010 to January 2011) at the Sheung Shui Slaughterhouse. Seven samples tested positive for human swine influenza (pandemic H1N1) virus, but no significant genetic re-assortment of viruses was found.
A spokesman for the CFS said that given the wide transmission of the pandemic H1N1 virus in humans, detection of the virus in pigs was not a surprise. It was expected that positive findings might appear from time to time in HKU's surveillance programme.
The spokesman said, "The CFS will continue to monitor reports of HKU's regular influenza virus surveillance programme on pigs and make announcements on a regular basis.
"Results will be announced immediately if there are significant public health impacts such as genetic re-assortment of viruses."
Under this surveillance programme, which aims to monitor the circulation of influenza virus in pigs, the CFS has been helping HKU researchers by collecting blood, tracheal and nasal swabs from pigs at the Sheung Shui Slaughterhouse twice a month.
Although the term `no significant genetic re-assortment’ may sound a bit vague, and suggests that perhaps some small genetic changes were detected, Hong Kong has a good record of transparency when it comes to disease surveillance.
Last summer (see EID Journal: Swine Flu Reassortants In Pigs) we saw a report on the prevalence of influenza viruses among swine in China in a study that appeared in 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 was 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.
Researchers in this study identified 29 strains of influenza A circulating in Chinese swine – including numerous double and triple reassortant viruses.
Granted, today’s report out of Hong Kong is reassuring.
Less so, however, is the knowledge that every year hundreds of millions of pigs are being raised in countries all around the globe.
And for the vast majority of them - surveillance for reassorted flu viruses is non-existent.
