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จันทร์, 27 เมษายน 2009
A ProMED-mail post
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International Society for Infectious Diseases

In this update:
[1] Brief history and terminology of swine flu
[2] Comments on swine, poultry prevention and risk
[3] National Pork Board Producer Guidelines

[1] Brief history and terminology of swine flu
Date: 25 Apr 2009
Source: Peter Cowen, ProMED-mail Animal Disease Moderator

Since we have a great deal of experience with confusing terminology when a
disease occurs at the human/animal interface, I thought a few comments on
the current H1N1 situation were warranted. For example, one might argue
that the zoonotic disease, eastern equine encephalitis is not very aptly
named. Horses are a dead end host, wild birds are the zoonotic reservoir,
and poultry flocks serve as sentinel sites for surveillance and indicators
of when to begin mosquito control. Moreover, the disease occurs in people
on both the East Coast and Gulf Coast of the United States. Very little of
this epidemiology could be deduced from the name, eastern equine
encephalitis. With that as a preamble, let me make some comments concerning
the terminology in common use -- swine flu -- for our present outbreak.

In the first place, the H1N1 virus is being called "swine flu" because of
the outbreak of a different, 1918 origin virus that caused significant
mortality in both swine and human populations and was known as the Spanish
flu. The virus probably has a wild bird origin but it definitive origin
remains unknown (see Taubenberg reference below).

The subsequent history of the swine influenza virus is nicely summarized:
"Influenza as a disease of pigs was 1st recognized during the Spanish
influenza pandemic of 1918­1919. Veterinarian J S Koen was the 1st to
describe the illness, observing frequent outbreaks of influenza in families
followed immediately by illness in their swine herds, and vice versa [1].
Influenza virus was 1st isolated from pigs in 1930 by Shope and Lewis [2],
with the virus isolated from humans several years later [3]. The 1st
isolation of a swine influenza virus from a human occurred in 1974 [4],
confirming speculation that swine-origin influenza viruses could infect
humans." See Myers below.

In the second place, influenza viruses regularly circulate in swine
populations and include H1N1, H3N2, H1N2, H1N3 most commonly, with almost
25 per cent of more than 114 000 swine serum samples in the US being
positive for one of the serotypes (see Choi et al. Arch Virol 2002 Jun;
147(6): 1209-20). As such, it is well controlled in swine populations, even
though it can cause concern on particular farms that don't manage well for
its occurrence or at particular times.

Then, thirdly, swine flu viruses have been known to infect humans -- 50
cases were turned up from an extensive review of the literature. Of the
non-military cases, 19 occurred in the United States, 6 in Czechoslovakia,
4 in the Netherlands, 3 in Russia, and 1 each in Canada and Hong Kong. Most
(61 per cent) of the cases studied had reported an exposure to swine and
the median age was 24 years (see Myers below). The most recognizable H1N1
transmission event involved 12 soldiers at Fort Dix in 1976, one of whom
died. Contact with swine was never established. Another important
transmission resulted in the death of a pregnant woman, who was exposed to
pigs, in 1988 in Wisconsin.

Finally, it appears as if no exposure to swine has occurred among people
who have come down with the current novel H1N1 virus. The virus has
elements of human, swine, and avian viruses normally found in Europe or
Asia. It is this genetic analysis of the virus which has really developed
the level of concern for this outbreak. If there wasn't a match with the US
virus the fact that it is being transmitted out of season and in young,
healthy adults might have even been overlooked.

So, in summary, the reason that we are calling this virus swine flu is the
history and evolution of the virus. It also rests on the fact that some of
the genetic analysis indicates that elements from viruses that have
traditionally been found in swine populations are incorporated. However
since we know nothing of how this particular virus has gotten into the
human population but there apparently is no history of swine exposure, it
probably makes more sense epidemiologically to refer to this simply as an
H1N1 influenza virus.

To some extent a similar nomenclatural history has occurred over time with
the H5N1 virus becoming known by its viral strain, rather than bird flu. At
least with the H5N1 it can most often be traced to exposure to avian
species. But in the case of this so called swine flu, there really does
appear to be no exposure to swine and some evidence (father, daughter pair
in the US) of transmission without exposure to animals. Realistically,
however, the name seems to have stuck in the popular media already and the
terms swine flu does reflect what we know about the history of some very
important H1N1 viruses. Unfortunately, this name will imply a simple,
zoonotic transmission between swine and people, when in reality is origin
and epidemiology is liking to be much more complex. Therefore, good
epidemiologic studies in swine in Mexico could be very helpful in
understanding this apparently new virus.

1. Taubenberger JK, Morens DM. 1918 influenza: the mother of all pandemics.
Emerg Infect Dis [serial on the Internet]. 2006 Jan [date cited]. Available
from <http://www.cdc.gov/ncidod/EID/vol12no01/05-0979.htm>.
2. Myers KP, Olsen CW, Gray GC. Cases of swine influenza in humans: a
review of the literature. Clin Infect Dis 2007; 44: 1084­8 doi: 10.1086/512813.

communicated by:
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[2] Comments on swine, poultry exposure
Date: 25 Apr 2009
Source: Barrett Slenning
Animal Biosecurity Risk Management Group
College of Veterinary Medicine, North Carolina State University
< >

[I find Barrett Slenning to be unusually thoughtful on matters of disaster
preparedness as it relates to agriculture and zoonotic diseases, so here
are some comments he put together as a summary explanation for animal
producers from last night (24 Apr 2009) here in North America. - Mod.PC]

We are dealing with an H1N1 swine-associated type A influenza that carries
both human and avian characteristics. It is a novel virus that had not been
described previously. It appears to be resistant to some antivirals, but
sensitive to Tamiflu and similar drugs.

Human cases from this virus emerged nearly simultaneously in the border
counties (San Diego and Imperial counties) of California (27 Mar 2009) and
in north-central Mexico (22 Mar 2009). The news reporting out of Mexico is
not consistent, and I am not privy to the official consultations between
the 2 countries, but it appears the Mexican outbreak is in the Mexico City
area (south central Mexico), the sectors in and around San Luis Potosi
(about 400 miles north of Mexico City), and in Mexicali (on the border with
Calexico in Imperial county California).

To date, 8 cases in the US have been documented, all of whom recovered/are
recovering (only one was hospitalized and that was an immunocompromised
individual with other disease issues). There are over 900 cases in Mexico,
with over 60 fatalities (approx 6 per cent case fatality rate). Why the
disparity in severity between the 2 countries is unknown, but with the US
numbers being so low, it could just be a luck of the draw kind of thing
right now: my back of the envelope calculations suggest that if the US had
the same base case fatality rate as in Mexico, the likelihood we could have
8 cases without a death is nearly 60 per cent.

None of the US cases have any known exposures to either swine or poultry.
To date, though epidemiologic trace backs are far from complete, none of
the Mexican cases have swine or poultry exposures either. There are a
couple of apparent family clusters in Mexico, so with that evidence and no
recognized pig/bird exposures, it appears that we are seeing human to human

We do not know if this virus is circulating in swine populations. We have
no evidence of it in domestic swine, although the industry does not have
the depth of influenza surveillance that we see in poultry, and there is
some doubt that this new H1N1 strain would be identified as unusual with
standard tests anyway. Groups are doing rapid assessments right now, so
that information may change momentarily as data come in. Feral swine are
another possible source/reservoir, but we have no information on them. I do
not know if anyone is currently chasing the potential of this virus to be
in birds.

Nobody knows right now where we are in the epidemic -- if this will burn
itself out in a few weeks or not. It is nearing the end of the influenza
season, so that is on our side. However, there are historical precedents
that should give us pause:

1. It was an H1N1 directly from birds that caused the 1918 pandemic that
killed more people than all the wars in the 20th century combined;
2. The 1st cases of that agent showed up in May 1918 in an army base in
Kansas -- it went quiet over the summer -- and started ravaging the globe
in early fall that year.

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[3] National Pork Board Producer Guidelines
Date: 25 Apr 2009
Source: National Pork Board [edited]

Swine flu heightens emphasis on biosecurity
Media reports on a new strain of the swine influenza virus type H1N1
different from any other ever reported in US swine herds serve as a
reminder of the need for strict and enforceable biosecurity measured on US
pork production operations.

The virus has not been reported to cause illness in pigs in the United
States, but it has been associated with illness in 8 people in the states
of California and Texas. The Centers for Disease Control and Prevention
also has reported that the same virus may be responsible for outbreaks of
influenza in humans in Mexico.

The Pork Checkoff is recommending that pork producers implement biosecurity
practices on their farms to prevent that this new strain of swine influenza
does not enter the US swine herd, and to protect the health and safety of
our industry's workers.

Consider including the following biosecurity practices for your farm:
- limit the access of people to essential personnel (farm employees,
veterinarians and essential service people);
- implement policies that prevent employees presenting signs of flu-like
illness from having contact with the pigs or other people on the operation;
- prevent access of international visitors or people who have recently
returned from international travel, particularly from travel to Mexico,
into your operation;
- implement a shower-in/shower-out procedure and the use of farm-specific
clothing and footwear for employees entering the barns. At minimum,
employees should don farm footwear and completely wash hands and arms
before having contact with the pigs;
- enforce heightened personal hygiene practices including frequent hand
washing for all people in contact with pigs;
- establish contact with the herd veterinarian to discuss other biosecurity
practices that are merited by this event.

The importance of keen observation of the health and behavior of your
animals cannot be understated and the Pork Checkoff recommends that you
establish immediate contact with a swine veterinarian if you suspect that a
disease may be present on your farm.

More information on influenza can be found in the fact sheet Influenza:
Pigs, people and public health. And, additional information on swine
influenza and an update on the outbreak reported by the CDC can be found at

communicated by:
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[ProMED-mail's veterinary and viral disease moderators have discussed the
nomenclature of this condition, and have agreed that we should refer to it
in the titles of postings as "Influenza A (H1N1) virus, human", omitting
the word "swine". For now, at least, that is what we will do, amending the
titles of earlier postings in this thread (as shown in the "see also"
section below). - Mod.SH]

[See also:
Influenza A (H1N1) virus, swine, human - N America (03) 20090426.1566
Acute respiratory disease - Mexico, swine virus susp 20090424.1546
Influenza A (H1N1) virus, human - USA (02): (CA, TX) 20090424.1541
Influenza A (H1N1) virus, human - USA: (CA) 20090422.1516
Influenza A (H1N1) virus, swine, human - Spain 20090220.0715
Influenza A (H1N1) virus, swine, human - USA (TX) 20081125.3715
Influenza A (H2N3) virus, swine - USA 20071219.4079
Influenza, swine, human - USA (IA): November 2006 20070108.0077]



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