Sunday, March 31, 2013

A Brief History Of H7 Avian Flu Infections

 

image

Site of recent H7N9 infections

 


# 7044

 

While it is far too soon to know if the breaking story out of Shanghai overnight (see China: Two Deaths From H7N9 Avian Flu) will have `legs’, the news of an H7 strain causing serious illness in humans is of some note.

 

In the past, when other H7 strains have been identified jumping to humans, they’ve generally only caused mild illness. 

 

Often little more than conjunctivitis. 

 

Ten years ago, the largest known H7 cluster was recorded in the Netherlands. In that outbreak, the culprit was H7N7.

 

Details on this cluster are reported in the December 2005 issue of the Eurosurveillance Journal.

 

Human-to-human transmission of avian influenza A/H7N7, The Netherlands, 2003

M Du Ry van Beest Holle, A Meijer, M Koopmans3 CM de Jager, EEHM van de Kamp, B Wilbrink, MAE. Conyn-van Spaendonck, A Bosman

An outbreak of highly pathogenic avian influenza A virus subtype H7N7 began in poultry farms in the Netherlands in 2003. Virus infection was detected by RT-PCR in 86 poultry workers and three household contacts of PCR-positive poultry workers, mainly associated with conjunctivitis.

 

Roughly 30 million birds residing on more than 1,000 farms were culled to control the outbreak. One person - a veterinarian who visited an infected farm – died a week later of respiratory failure.

 

The rest of the symptomatic cases were relatively mild.

 

The Fraser Valley H7N3 outbreak of 2004 resulted in at least two human infections, as reported in this EID Journal report:

 

Human Illness from Avian Influenza H7N3, British Columbia

Abstract

Avian influenza that infects poultry in close proximity to humans is a concern because of its pandemic potential. In 2004, an outbreak of highly pathogenic avian influenza H7N3 occurred in poultry in British Columbia, Canada. Surveillance identified two persons with confirmed avian influenza infection. Symptoms included conjunctivitis and mild influenzalike illness.

 

More recently, in Mexico we saw two mild human cases last summer (see see MMWR: Mild H7N3 Infections In Two Poultry Workers - Jalisco, Mexico).  The World Health Organization published this Summary and assessment as of 10 September 2012.

 

Sporadic human cases of influenza A(H7N3) virus infection linked with outbreaks in poultry have been reported previously in Canada, Italy and the UK, with H7N2 in US and the UK, and with H7N7 in the UK and the Netherlands. Most H7 infections in humans have been mild with the exception of one fatal case in the Netherlands, in a veterinarian who had close contact with infected birds.

 

While global surveillance and reporting on novel avian viruses in humans is spotty at best, some other known H7 cases include:

image

    Chart lifted and edited from CIDRAP’s excellent overview Avian Influenza (Bird Flu): Implications for Human Disease  showing known H7 avian flu infections in humans over the two decades.

     

    Of course – H7 flu strains - like all influenza viruses, are constantly mutating and evolving. What is mild, or relatively benign today, may not always remain so.

     

    In 2008 we saw a study in  PNAS that suggested the H7 virus might just be inching its way towards better adaptation to humans (see Contemporary North American influenza H7 viruses possess human receptor specificity: Implications for virus transmissibility).

     

    You can read more about this in a couple of blogs from 2008, H7's Coming Out Party and H7 Study Available Online At PNAS.

     

    Among the avian influenzas, H5N1 virus gets the bulk of the headlines, due to its high fatality rate. While a matter of some controversy (see Revisiting The H5N1 CFR Debate), among known human cases, the mortality rate has been a staggering 60%.

     

    Other strains that have demonstrated at least some ability to infect humans include H7, H9, H10, and H11.

     

    Currently, H5s and H7s are both reportable diseases (to the OIE) in poultry, due to their ability to mutate from a low pathogenic virus to highly pathogenic virus.

     

    But up until now, their ability to spark serious illness in humans has been limited. Which makes China’s announcement of 3 human infections - all resulting in serious and/or fatal illness - of particular interest.

     

    We will obviously be following the H7N9 story with interest. Whether this story has significant public health implications is something we may not know for some time.

     


    A final note - proving that timing is everything - a couple of weeks ago, in EID Journal: Predicting Hotspots for Influenza Virus Reassortment, we saw a study that looked at those areas around the world with the greatest potential of spawning new flu strains. 

     

    The Shanghai region was one of those areas identified.

     

    Potential geographic foci of reassortment include the northern plains of India, coastal and central provinces of China, the western Korean Peninsula and southwestern Japan in Asia, and the Nile Delta in Egypt.

    image

    No comments: