A ProMED-mail post
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International Society for Infectious Diseases
Date: Thu 2 Jul 2009
Source: CHealth, The Canadian Press [edited]
Swine flu transmission studies suggest new virus is here to stay
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Swine flu viruses are missing at least 2 key features seen in all flu
viruses present and past that transmit well among people and yet the
viruses are spreading quite efficiently, 2 new studies suggest. The
research groups which produced the work differ slightly in their views of
the degree to which the novel H1N1 virus is spreading, with one finding
that transmission isn't yet as efficient as with human flu viruses while
the other finding transmission rates are in lockstep with those of seasonal
flu cousins.
There is no disputing the evidence, though; the virus is spreading around
the globe, claiming at least 332 lives so far. And it is doing this without
all the tools scientists would expect a flu virus to need to become a
successful human pathogen. "The take-home message is that a virus that does
not have some of the features that we have previously recognized as
hallmarks of adaptation of flu in humans was able to establish itself in
humans and cause disease," said Dr Daniel Perez, an influenza virologist
with the University of Maryland. "Regardless of what the virus might do, I
believe it is here to stay either as a whole virus or with some of its
gene. It may be able to outcompete and/or co-circulate with seasonal flu
strains."
Perez was not involved in the studies, both of which will be published on
Friday [3 Jul 2009] in the journal Science [summary available at:
however, familiar with the work; his lab has completed a similar study. The
transmission studies were done by research groups at the US Centers for
Disease Control and Prevention (CDC) with colleagues from the Massachusetts
Institute of Technology and at Erasmus Medical Center in Rotterdam, the
Netherlands. Both groups tested spread in ferrets, which are considered an
excellent model for flu infection in humans.
CDC's work suggests that the virus isn't yet completely adapted to spread
among humans. When healthy ferrets were housed in cages adjacent to and
sharing feeding dishes with experimentally infected animals, only two
thirds of the healthy animals became infected in the CDC research. By
contrast, the group in the Netherlands found all healthy ferrets caught the
new virus when housed next to animals infected with the virus. Perez's work
also saw this 100 per cent transmission rate. In both the CDC and Erasmus
studies, ferrets that were infected with human flu viruses transmitted
infection to all their healthy neighbours.
Dr Terrence Tumpey, senior author of the CDC study, said variation in the
air flow setups between the CDC's ferret cages and those used in the other
studies may explain the differing findings. But based on what they saw, his
team believes this virus may still be getting used to its new human host. A
key piece of evidence supporting their conclusion relates to the virus's
ability to infect cells in the human respiratory tract. The CDC-MIT
scientists showed the novel H1N1 virus's hemagglutinin -- the surface
protein that locks onto a cell it is about to invade -- currently makes a
connection that is weaker or less efficient than that made by regular flu
viruses. That suggests the virus has room for improvement. And if it
mutates to bind more efficiently, it would become even more adept at
spreading from person to person.
"I mean, it's transmitting. But we think it could potentially transmit even
better," Tumpey said from Atlanta. He suggested with better transmission
could come more severe disease -- not just in sheer numbers, but in the
proportion of infected people who develop serious illness. "A lot of cases
have been mild. But if it was adapted more towards humans, it could be more
severe. More consistently severe," Tumpey said. His counterpart on the
Dutch paper, Dr Ron Fouchier, shares his concerns. "I do agree that the
virus might still pick up mutations to improve infection and transmission
in humans. But in our opinion, it is already good enough to beat the
seasonal flu viruses," he said via email.
The CDC scientists also reported that the virus is missing a feature in an
internal gene called PB2 that is known to relate to transmissibility. All
seasonal flu viruses and the past 3 pandemic viruses -- in other words, all
flu viruses which have successfully made the jump from other species into
humans -- have had this feature. The swine H1N1 virus does not. They don't
know how the virus achieved transmissibility without this mutation or how
likely it is to acquire it. But Tumpey said the mutation is also linked to
increased virulence or disease severity and the flu community is watching
closely for this change.
Both groups studied tissues from the infected ferrets. They found swine flu
viruses triggered infections that went deep into the lungs of the animals.
Human flu strains infect the animals' upper airways. That ability of the
virus to spread to and proliferate deep in the lungs could help explain
what doctors caring for severely ill swine flu patients are seeing:
aggressive viral pneumonias that incapacitate the lungs.
"Certainly, the lesions we noted in our ferrets are consistent with the
disease in humans," Fouchier said. "It seems that due to more extensive
virus replication, the virus does more damage, and spreads deeper down the
airways as compared to seasonal viruses."
Those findings are concerning, suggested Dr Malik Peiris, a virologist and
flu expert at the University of Hong Kong. Peiris, who was not involved in
the studies, said that while the swine flu virus is not as virulent as H5N1
avian influenza or the virus that caused the 1918 Spanish flu, its ability
to infect the lower respiratory tract "is clearly cause for caution in
regard to the pathogenic potential of this virus in humans." Both H5N1 and
the Spanish flu virus infect tissues deep in the lungs. Fouchier said he is
concerned the novel H1N1's ability to invade deep lung tissue could lead to
more severe disease when the virus is spreading in true winter conditions,
which are better suited to spread of flu.
A study in guinea pigs -- also a good animal model for flu -- published a
couple of years ago found that at lower temperatures flu viruses replicate
more and for longer durations. If that holds true for humans and for this
virus, the severity of swine flu infections in winter could be greater,
Fouchier warned.
[byline: Helen Branswell]
--
communicated by:
ProMED-mail rapporteur Mary Marshall
[It remains to be seem to what extent these transmission studies with
captive ferrets will be reflected in the course of the 2009 (swine-origin)
A (H1N1) pandemic in the human population. - Mod.CP]
[see also:
Influenza A (H1N1) - worldwide (81): epidemic analysis 20090703.2391
Influenza A (H1N1) - worldwide (80): Argentina, human to pig 20090701.2376
Influenza A (H1N1) - worldwide (79): case count 20090701.2372
Influenza A (H1N1) - worldwide (78): Tamiflu resistance, DK 20090630.2359
Influenza A (H1N1) - worldwide (76): comments on 1918 virus (03) 20090625.2309
Influenza A (H1N1) - worldwide (74): susp. origin 20090624.2303
Influenza A (H1N1) - worldwide (73): case count, epidemiology 20090622.2288
Influenza A (H1N1) - worldwide (72): case count, epidemiology 20090619.2261
Influenza A (H1N1) - worldwide (70): risk factors 20090619.2260
Influenza A (H1N1) - worldwide (69): other viral infections 20090618.2254
Influenza A (H1N1) - worldwide (68): southern hemisphere 20090618.2253
Influenza A (H1N1) - worldwide (65): antivirals in pregnancy 20090616.2224
Influenza A (H1N1) - worldwide (64): case count, pandemic 20090616.2221
Influenza A (H1N1) - worldwide (62): Egypt, Lebanon 20090611.2150
Influenza A (H1N1) - worldwide (62): Egypt, Lebanon 20090611.2150
Influenza A (H1N1) - worldwide (60): Egypt (Cairo) 20090608.2117
Influenza A (H1N1) - worldwide (59): Worldwide 20060608.2117
Influenza A (H1N1) - worldwide (58): USA, Africa 20090607.2109
Influenza A (H1N1) - worldwide (57): Brazil, USA 20090605.2090
Influenza A (H1N1) - worldwide (55) 20090603.2056
Influenza A (H1N1) - worldwide (47): China, epidemiology 20090526.1962
Influenza A (H1N1) - worldwide (45) 20090525.1951
Influenza A (H1N1) - worldwide (42) 20090523.1929
Influenza A (H1N1) - worldwide (39) 20090521.1903
Influenza A (H1N1) - worldwide (37) 20090520.1893
Influenza A (H1N1) - worldwide (34) 20090518.1863
Influenza A (H1N1) - worldwide (31) 20090516.1835
Influenza A (H1N1) - worldwide (29) 20090515.1824
Influenza A (H1N1) - worldwide (26) 20090514.1798
Influenza A (H1N1) - worldwide (23) 20090511.1764
Influenza A (H1N1) - worldwide (21) 20090510.1749
Influenza A (H1N1) - worldwide (19) 20090509.1733
Influenza A (H1N1) - worldwide (17) 20090508.1722
Influenza (H1N1) - worldwide (15) 20090507.1709
Influenza A (H1N1) - worldwide (13) 20090506.1695
Influenza A (H1N1) - worldwide (11): coincident H3N2 variation 20090505.1679
Influenza A (H1N1) - worldwide (09) 20090504.1673
Influenza A (H1N1) - worldwide (07) 20090503.1658
Influenza A (H1N1) - worldwide (05) 20090503.1657
Influenza A (H1N1) - worldwide (03) 20090501.1646
Influenza A (H1N1) - worldwide (02): case counts 20090430.1638
Influenza A (H1N1) - worldwide 20090430.1636
Influenza A (H1N1) "swine flu": worldwide (07), update, pandemic 5
20090429.1622
Influenza A (H1N1) "swine flu": Worldwide 20090427.1583
Influenza A (H1N1) virus, human: worldwide 20090426.1577
Influenza A (H1N1) virus, human - New Zealand, susp 20090426.1574
Influenza A (H1N1) virus, human - N America (04) 20090426.1569
Influenza A (H1N1) virus, human - N America 20090425.1552
Acute respiratory disease - Mexico, swine virus susp 20090424.1546
Influenza A (H1N1) virus, swine, human - USA (02): (CA, TX) 20090424.1541
Influenza A (H1N1) virus, swine, human - USA: (CA) 20090422.1516
Influenza A (H1N1) virus, swine, human - Spain 20090220.0715]
....................cp/ejp/sh
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