# 4488
It has now been a year since the first cases of novel H1N1 emerged, and while the pandemic of 2009 turned out to be less devastating than some feared, this new influenza virus continues to surprise doctors and scientists by its unusual characteristics.
Since novel H1N1 has (at least for now) apparently supplanted the previously circulating influenza A strains (H3N2 and seasonal H1H1), we need to recognize, respect, and learn to deal with its differences.
Over the past year we’ve seen a number of studies highlighting ways in which novel H1N1 differs from flu strains of the recent past.
Today, a brief review of some of those studies.
Influenza A viruses are notorious for their ability to mutate and adapt, and so the characteristics observed in this new virus may change over time.
The most obvious difference between novel H1N1 and seasonal influenzas of the past has been the age shift in infections, hospitalizations, and deaths.
Unlike seasonal influenza, novel H1N1 has taken its biggest toll on those under the age of 65.
While the total number of flu-related fatalities may prove to be less during this pandemic than in previous years, In terms of years of life lost, novel H1N1 was anything but benign (see Study: Years Of Life Lost Due To 2009 Pandemic).
The following chart comes from:
By Cecile Viboud, Mark Miller, Don Olson, Michael Osterholm et al (5 authors)
And here we have a breakdown of estimated infections, hospitalizations, and deaths by age groups from the CDC.
Both sources demonstrate the burden shift to a younger population with this virus. The median age of death from novel H1N1 related illness (37.4 years) has been half of that from seasonal flu.
In terms of years of life lost (YLL), the average pandemic flu death has a many fold greater impact than the average seasonal flu fatality.
Should novel H1N1 drift antigenically enough over time to evade the pre-existing immunity demonstrated by those over the age of 50, these trends could change.
Novel H1N1 also appears to differ from seasonal flu in how it is transmitted, at least according to researchers in Hong Kong who discovered that the novel H1N1 virus – unlike seasonal flu – easily infects and replicates in the conjunctival tissues of the eye (see I Only Have Eyes For Flu).
The study appears in The American Journal of Pathology is entitled:
Michael C.W. Chan*@, Renee W.Y. Chan*, Wendy C.L. Yu*, Carol C.C. Ho*, Kit M. Yuen*, Joanne H.M. Fong*, Lynsia L.S. Tang*, Wico W. Lai, Amy C.Y. Lo, W. H. Chui, Alan D.L. Sihoe, Dora L.W. Kwong¶, David S.H. Wong, George S.W. Tsao, Leo L.M. Poon*, Yi Guan*, John M. Nicholls, and Joseph S.M. Peiris**@
Conjunctivitis is sometimes associated with mild avian flu infections in humans, and in tests conducted on baby piglets in Thailand last May (see Casting Viruses Before Swine), conjunctivitis was one of the commonly observed symptoms with the novel H1N1 virus.
Another difference observed with novel H1N1 has been its unusually high rate of gastro-intestinal symptoms.
Diarrhea and vomiting have both been commonly reported with H1N1 pandemic influenza, symptoms that occur far less often with seasonal influenza.
Last August The Lancet published a study entitled:
Clinical characteristics of paediatric H1N1 admissions in Birmingham, UK
S Hackett a, L Hill a, J Patel a, N Ratnaraja b, A Ifeyinwa b, M Farooqi b, U Nusgen c, P Debenham c, D Gandhi c, N Makwana b, E Smit a d, S Welch a
Included in this study was a chart showing the incidence of symptoms observed in pediatric H1N1 admissions. I’ve modified it slightly to highlight the gastrointestinal symptoms.
Repeatedly we’ve seen nausea, vomiting, and diarrhea listed as atypical symptoms with novel H1N1, as in this report by Maggie Fox of Reuters.
US flu study confirms H1N1 more serious in youth
Thu Oct 8, 2009 10:41pm BST
* 45 percent of those hospitalized were under 18
* Diarrhea, vomiting in 42 percent of children with H1N1
* Quick drug treatment may save lives
By Maggie Fox, Health and Science Editor
WASHINGTON, Oct 8 (Reuters) - A study of people who became seriously ill and died with the new pandemic swine flu confirms it is hitting a younger population than the seasonal flu and causes often different symptoms.
It should be noted that there is some research that suggests that seasonal flu (A & B) may replicate in the G.I. tract of children as well (see Influenza’s Gastrointestinal Connection).
The incidence of G.I. involvement with novel H1N1 appears higher than with seasonal flu, however.
Beyond the gastrointestinal component, a number of other studies indicate that the novel H1N1 virus – in some small subset of victims – can produce profound lung damage.
In a study that appeared earlier this year in The Journal of Infectious Diseases researchers ast the Department of Virology, Erasmus Medical Centre and ViroClinics Biosciences BV, Rotterdam used a ferret model to test the pathogenicity of the novel H1N1 virus against seasonal and bird flu.
Severity of Pneumonia Due to New H1N1 Influenza Virus in Ferrets Is Intermediate between That Due to Seasonal H1N1 Virus and Highly Pathogenic Avian Influenza H5N1 Virus
Judith M. A. van den Brand,Koert J. Stittelaar, Geert van Amerongen,Guus F. Rimmelzwaan, James Simon, Emmie de Wit, Vincent Munster,Theo Bestebroer, Ron A. M. Fouchier, Thijs Kuiken, and Albert D. M. E. Osterhaus
Results.
Our results showed that the new H1N1 virus causes pneumonia in ferrets intermediate in severity between that caused by seasonal H1N1 virus and by HPAI H5N1 virus. The new H1N1 virus replicated well throughout the lower respiratory tract and more extensively than did both seasonal H1N1 virus (which replicated mainly in the bronchi) and HPAI H5N1 virus (which replicated mainly in the alveoli). High loads of new H1N1 virus in lung tissue were associated with diffuse alveolar damage and mortality.
Conclusions.
The new H1N1 virus may be intrinsically more pathogenic for humans than is seasonal H1N1 virus.
In December the NIH released information regarding autopsies performed on H1N1 victims, that showed severe pulmonary damage. A brief excerpt follows:
FOR IMMEDIATE RELEASE
Monday, Dec. 7, 2009Media Contact: Anne A. Oplinger
(301) 402-1663
niaidnews@niaid.nih.govNew York Autopsies Show 2009 H1N1 Influenza Virus Damages Entire Airway
In fatal cases of 2009 H1N1 influenza, the virus can damage cells throughout the respiratory airway, much like the viruses that caused the 1918 and 1957 influenza pandemics, report researchers from the National Institutes of Health (NIH) and the New York City Office of Chief Medical Examiner. The scientists reviewed autopsy reports, hospital records and other clinical data from 34 people who died of 2009 H1N1 influenza infection between May 15 and July 9, 2009. All but two of the deaths occurred in New York City. A microscopic examination of tissues throughout the airways revealed that the virus caused damage primarily to the upper airway—the trachea and bronchial tubes—but tissue damage in the lower airway, including deep in the lungs, was present as well. Evidence of secondary bacterial infection was seen in more than half of of the victims.
And as far back as September, pathologists were talking about the remarkably severe lung damage they were seeing from novel H1N1.
Once again, Maggie Fox of Reuters reported:
Swine flu deaths show this flu is different – experts
Tue Sep 15, 2009 10:40pm BST
By Maggie Fox, Health and Science Editor
WASHINGTON (Reuters) - Autopsies on people who have died from the new pandemic H1N1 flu show this virus is different from seasonal influenza, even if it has not yet caused more deaths, experts told a meeting on Tuesday.
Americans who died from swine flu had infections deep in their lungs, Dr. Sherif Zaki of the U.S. Centres for Disease Control and Prevention told a meeting of flu experts, including damage to the alveoli -- the structures in the lung that deliver oxygen to the blood.
Beyond the gastro-intestinal and pulmonary symptoms, we’ve also seen a number of neurological complications, particularly in children.
These sorts of complications have been seen in seasonal flu as well.
The first hint of this came last summer when the CDC’s MMWR (July 23rd issue) reported on 4 pediatric patients with the novel H1N1 virus who presented with neurological symptoms including unexplained seizures and altered mental status.
Neurologic Complications Associated with Novel Influenza A (H1N1) Virus Infection in Children --- Dallas, Texas, May 2009
. . . On May 28, 2009, the Dallas County Department of Health and Human Services (DCHHS) notified CDC of four children with neurologic complications associated with novel influenza A (H1N1) virus infection admitted to hospitals in Dallas County, Texas, during May 18--28.
In November, in a blog entitled Japan: Influenza Related Encephalopathy we looked at this report in the Yomiuri Shimbun.
132 flu patients hit with brain disorders since July
The Yomiuri Shimbun
A total of 132 influenza patients in Tokyo and 27 prefectures have developed encephalopathy, or swelling of the brain, since July, according to the National Institute of Infectious Diseases.
Normally, only about 40 to 50 seasonal flu sufferers develop encephalopathy each year, meaning the latest figure has already more than doubled in four months since the new strain of flu began spreading.
And in February of this year, we saw an expedited letter that appeared in the CDC’s Journal of Emerging Infectious Diseases .
DOI: 10.3201/eid1603.091699
Kitcharoen S, Pattapongsin M, Sawanyawisuth K, Angela V, Tiamkao S. Neurologic manifestations of pandemic (H1N1) 2009 virus infection [letter]. Emerg Infect Dis. 2010 Mar; [Epub ahead of print]
Neurologic Manifestations of Pandemic (H1N1) 2009 Virus Infection
This four page letter goes on to describe the presentation, testing, and course of illness of a 34-year-old man, previously healthy, who was admitted to Chaiyaphum Hospital in Chaiyaphum, Thailand, back in August with flu-like symptoms.
Novel H1N1 is obviously a somewhat different brand of influenza from that which we’ve dealt with over the past few decades. In terms of absolute mortality, it may have spared lives, but in terms of years of life lost (YLL), it has had a serious impact.
It also presents in some patients differently than ordinary flu, and may differ somewhat in modes of transmission.
Since novel H1N1 appears poised to be the dominant flu strain for the immediate future, it is important that we recognize and accept the ways that this flu is different.
Brushing it off as `no worse than regular flu’, as some are inclined to do, invites complacency.
Something that could end up costing lives in the long run.
While the pandemic of 2009 could have been a lot worse, there is little comfort to be gained by being felled by a `mild’ virus. And of course, the whole H1N1 story has yet to be written.
It is still worthwhile to get the H1N1 vaccine if you have not done so, and of course, the seasonal shot this fall. Hand hygiene, staying home when you are sick, and cough and sneeze etiquette are healthy habits year round, not just during flu season.
Meanwhile, a year downrange, the pandemic of 2009 is proving to be a fascinating and informative real-life laboratory experiment conducted on a global scale.
One that, thankfully, hasn’t produced more carnage than it has.
The knowledge we gain from observing this pathogen, unraveling its secrets, and dissecting our response to it will hopefully pay big dividends when the next pandemic virus threatens humanity.
