Q
& A with Rob Dorit: the Lowdown on Swine Flu
Rob Dorit (on right) examines cells with a student in
the lab. |
With the news about the
swine flu first reported in Mexico spreading as quickly
as the virus, Robert L. Dorit, Smith associate professor
of biological sciences, shares his knowledge
about what is known and unknown at this point.
“The influenza virus
is a notorious shape-shifter,” says
Dorit, who is an expert on molecular evolution, antibiotic
design and resistance. “This
strain has picked the lock of human-to-human transmission
quickly.”
Gate: What
do we know about the swine flu?
Rob Dorit: In
addition to humans, many species can be infected by the influenza
virus. We refer to the other species that
can harbor influenza as animal reservoirs. As a general
rule, influenza viruses are specifically adapted to infect
and thrive within a single host species--thus,
pig influenza viruses are best able to infect pigs, and bird
influenza strains do best in birds. Occasionally, however,
influenza viruses can make a cross-species jump, and infect
humans. There have been previous examples of a swine
flu infecting humans, but it has generally been confined
to individuals that come into extensive and frequent contact
with pigs. Once inside a human host, the swine strain
can be reshaped by evolution into a strain that can thrive
in the new host.
Gate: What don't we
know about the flu? How does that impact the public health
response?
RD: The initial
reports from Mexico suggest that this influenza A (H1N1)
strain of swine flu has infected a large number of individuals
(>1400), most of whom have not been in
recent contact with pigs. This suggests the possibility
that this strain was either already capable of infecting
humans (a fortuitous consequence of the similarity between
the human and pig immune systems), or else has quickly evolved
the capacity for human-to-human transmission. This
strain appears to be responsible for more than 100 deaths
in Mexico so far. There are now sporadic reports of possible
infection with this strain in the U.S. and Canada, as well
as in Spain, France, Israel, Brazil and the South Pacific.
But this situation is
in rapid flux, and there are many things we do not yet
know. We do not, for instance,
have confirmation that the deaths in Mexico are all the result
of this strain, or may represent an amalgamation of several
lethal respiratory infections. Similarly, the strain
responsible for the cases being reported outside of the
Americas is still unconfirmed. The confirmed cases in the
U.S. are characterized by quite mild symptoms, raising important
questions about the differences in disease severity between
the cases in Mexico and those in the U.S. Finally,
the infectivity of this strain is still not well understood,
and the possibility that the enormous density and environmental
degradation in Mexico City may be contributing to the spread
of the virus (and the severity of symptoms) cannot be dismissed.
The public health response
to date has been impressive. In
Mexico, public health authorities have responded quickly
and effectively, largely by engaging citizens in a series
of measures meant to reduce the transmission of this virus:
schools are closed, large gatherings discouraged, and individuals
exhibiting symptoms are being encouraged to report to clinic
and minimize contact with uninfected individuals. Outside
of Mexico, the response, for now, involves additional surveillance
for possible unexpected influenza outbreaks, as well as reminders
of some common sense measures, such as the importance of
washing hands, avoiding contact with infected individuals
and reporting to your health care practitioner if you suspect
you may have the flu.
Gate: How does a new
flu arise?
RD: New influenza
strains arise all of the time, both as a result of the biology
of the virus and the ecology of hosts. The
influenza virus is a notorious shape-shifter, and it accomplishes
this feat by virtue of its high rate of mutation and its
ability to occasionally sequester versions of the genes that
encode its surface proteins from other species. The
influenza virus is, in effect, constantly sending out a swarm
of variants, or versions, to test the defenses of actual
and potential hosts. Most of these scouts fail in their
mission. But occasionally, a variant strain will enter
a host whose immune system cannot recognize the surface proteins
of the virus. Although the immune system of the host
will mount a defense, this takes time, and the virus takes
advantage of this lull to establish a foothold in the new
host.
The ecology of hosts also
plays a role. In the case
of human influenza, the high population density in certain
urban areas, the limited access to primary care, and the
frequent burden of co-infection with other infectious diseases
may all be playing a role in the spread of this strain in
Mexico. For non-human hosts, the spread and intensification
of industrial agriculture with the consequent confinement
of very large numbers of animals (pigs) or birds (chickens,
ducks) in small spaces, may also be promoting the emergence
of new strains.
Gate: What
makes this one more or less scary than any other flu?
RD: Three aspects
of this outbreak are causing significant concern:
1) This strain of swine
flu appears capable from the outset of human-to-human transmission.
While other swine flu strains do occasionally enter the human
population, the outbreaks tend to be self-limiting, precisely
because the individuals that first acquire the infection
cannot easily pass it on to other humans. This strain
has picked the lock of human-to-human transmission quickly.
2) The deaths being attributed
to this H1N1 swine flu strain are occurring among otherwise
healthy children and young adults. While other flu strains
can be lethal, they usually fell individuals from immunologically
vulnerable groups, including the very young and the elderly.
This strain is killing individuals that would normally be able
to fend off the infection. The deaths early in the
most devastating pandemic of influenza (the 1918 pandemic)
also occurred among healthy young adults.
3) The world has become
a smaller and more deeply interconnected place, and the
flow of people through Mexico is very high. As
a result, a strain emerging in a cosmopolitan and well-connected
world capital such as Mexico City can be spread around the
world by travelers in a matter of days. All of the
confirmed cases reported outside the U.S. involve individuals
who had traveled in Mexico in the last two weeks.
There are, however, some reassuring aspects to this outbreak:
1) The strain appears
to be sensitive to currently available antivirals, provided
these are administered early in the course of infection.
Strains that are resistant to antivirals have been reported
in the past, and an outbreak of such a strain would significantly
complicate control and treatment. Extensive stockpiles
of these antivirals exist and are being effectively distributed
in the U.S. and around the world should they be needed.
2) The international response
appears to be well-coordinated and transparent. Over the
past decade, a number of mechanisms have been put in place
to allow for the rapid dissemination of information about
and the coordinated response to disease outbreaks. National
and international organizations appear to be collaborating
effectively to monitor the progress of the outbreak, to mount
an effective response, and to remind the public of some common-sense
measures that can reduce viral transmission.
3) In part as the result of the concern surrounding possible
bird flu outbreaks over the past three years, the public
appears to be aware of the risk of transmission of influenza,
and is reacting calmly and reasonably.
4) While this virus appears to be highly transmissible,
the milder symptoms being reported in cases outside of Mexico
suggest it may be somewhat less dangerous than initially
feared.
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