David Quammen -- Spillover: animal infections and the next human pandemic
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Quammen's meta subject is the human species and its relationship to
the ecosystem we live in and to specific species of plants and
animals in that ecosystem.  His subject also revolves around how
humans (and other forces, too, I suppose) have modified that
ecosystem and what effects those modifications have on us.

Actions have consequences, often unintended ones.  And, when our
behavior as a species modifies our environment and our surroundings
as significantly as we do, we can expect that some of those
consequences will be "interesting" and some might be negative.  Add
to this that we have modified our own behavior and have changed
*where* we live, in some cases building our homes out into nature
and in some cases moving out into natures because of population
pressure.  See "Nature wars", by Jim Sterba for more on that.  In
other cases as we expand our agricultural efforts the plants we grow
and the animals we raise come into closer contact with nature and
some of the species "out there".

Quammen says that bats are a serious disease vector.  I'm a fan of
bats.  I believe that insect eating bats, which is mostly what we
have in the U.S., are beneficial.  So, I'd try to argue that whether
bats are harmful or beneficial (to humans, of course; we're always
going to be human-centric, aren't we), depends on what species of
bat you are thinking about.  But, certainly, Quammen is right to
claim that bat colonies, where thousands and sometimes millions of
bats live close together, are very effective environments for
breeding and transmitting some diseases.  You (a human) do not want
to spend very much time there, and you likely want to avoid contact
with those who do, that is, with the bats themselves.  Quammen makes
clear that the specific cases of pathogens transmitted by bats that
he is talking about are transmitted by fruit eating bats.  The
reason for that, in one case at least, is that the fruit eating bats
that he is discussing roost in trees.  Domestic animals (horses, in
one case) stood under those same trees during the day and acquired
the pathogens there, possibly from bat feces.

Quammen treats each disease and its outbreak like a detective story;
he traces the occurrences from a reservoir host, through vectors
(e.g. the species that transmit it), possibly to other amplifying
hosts, and then to humans.  It's puzzle solving and it's similar to
the scientific method: hypothesis, evidence (or counter evidence),
reject or modify the hypothesis based on evidence and arguments,
then repeat.  I found some of these "detective stories" to be
fascinating.

But, it's not all just good fun, because Quammen is also *very* good
at making each outbreak seem fearful and threatening, and at helping
us to worry about how many more? ... and when will be the next one?
... and how bad will that one be?

Here is one such scary thought: An outbreak is defined as an
explosive increase in the population of a species in a relatively
short period of time.  Outbreaks are almost always followed by a
crash in the population size.  We, humans are an outbreak: our
population has grown explosively in the last century.  So, should we
not expect a crash in our population?

Quammen gives us good reasons to suspect that this is true and to
expect that the cause will be influenza.  There are the animal
reservoirs (chickens, pigs, bats); we're in close contact with those
species; and we (humans) travel fast and widely, so we're an
especially effective vector.  What's even worse is that influenza
viruses are constantly trying new genetic combinations.  Someday,
that virus will hit on the combination that is both lethal and can
be transmitted from human to human.

Quammen gives us some comfort and reassurance by telling us about
how effective researchers are at tracking down an outbreak, what
caused it, and how it is transmitted.  Scientists now understand
viruses and their genes mutate, about how errors are introduced in
genetic copying, and about how chunks/sequences of genetic code
break apart and are recombined, how chunks of DNA are swept along
with neighboring genes, and so on.  This advanced scientific
understanding gives us some reason to be hopeful, doesn't it? ...

Some of these stories are very complicated problems.  Transmission
is ecology and ecology is complicated.  For example, Lyme disease --
The tick that transmits it (the bacteria) has several life stages.
The tick lives off and is supported by several host mammals.
Whitetail deer, yes, but mice and shrews are much better at
infecting and supporting the ticks.  It's significant that the tick
is not born with the bacteria and must acquire it from a host.  Some
hosts are better at infecting the tick than others.  So, Quammen's
claim is that a forest with greater biodiversity and a reduced mouse
and shrew population (which seem to be especially good at infecting
the ticks they carry) has less likelihood of passing Lyme disease
along.

Quammen's meta-subject is the human species and its relationship to
the ecosystem we live in and how we have modified that ecosystem and
those relationships.  He describes the world we have created that
enables zoonosis, which is the ability of a pathogen to jump from
non-human animals to humans.  There have been serious human die-offs
in the past, for example the Black Death in the Middle Ages.  It
could happen again.  And, we've created the conditions that would
enable it.  Some of those enablers: (1) Reservoir hosts in the wild
that can carry and maintain a supply of the pathogen and that cannot
be eliminated.  (2) Hosts that we raise, for example pigs, chickens
and other fowl, and that we live close to.  (3) Humans that live in
dense populations and that move frequently from one population
center to another.  And, humans are living in closer contact with
nature and animals as we build out into the forests and (in the
southwest U.S.) into the desert and into the hills (around Los
Angeles, California).  Again, for more about that, see "Nature
wars", by Jim Sterba.

You will want to learn (from "Spillover") about zoonosis -- the
ability of a pathogen to jump from one species to another.
Surprisingly to me, we care about pathogens that can be transmitted
from one non-human species to another non-human species.  The
ability of a pathogen to do so means that there is the possibility
of a "reservoir" host that can maintain the existence of the
pathogen even if we were able to eliminate the host (vector) that
passes it to humans.  Again, the complex life cycle of some
pathogens, their ability to pass between multiple hosts is what
makes some of the detective work in Quammen's book fascinating.  See
Wikipedia for more on zoonosis: http://en.wikipedia.org/wiki/Zoonosis.

06/24/2013

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