Humans emerged from a soup of bacteria, viruses, and parasites. These entities have been, and always will be,
accompanying us as evolution continues to mold us. Since we have coevolved, these other
creatures generally coexist with us, sometimes providing benefits, often neutral
in effect, but occasionally harmful.
It is important to realize that evolution is a continuous
process—for all the participants. While
human evolution seems to operate on a time schedule where change is difficult
to notice, other creatures can change much more rapidly. Within our lifetime, bacteria have
encountered our antibiotics and developed defenses against them. Plants have encountered our herbicides and
tamed them. Some forms of virus are
relatively stable and have been countered with vaccines. Others are fundamentally unstable and are
continuously mutating into different forms.
Included among these are the familiar influenza viruses and those
responsible for the common cold.
Every few years, it seems, a new and dangerous form of
virus makes the headlines. The latest to
arise is the Zika virus. This is not really
a new species, but it seems to have developed the frightening capability to
cause birth defects in the infants of women who are infected while pregnant. The probability of such an occurrence and the
possible damage done are still being assessed.
Michael T. Osterholm, an infectious disease specialist, provided an
interesting perspective on the virus in the New
York Times article How Scared Should You Be About Zika?
“….the mosquito that transmits this disease, the species Aedes aegypti, has
never been more numerous or lived in more locations. Think of Aedes aegypti as
the Norway rat of mosquitoes; it has evolved to live in close quarters with
humans, and the trash that humans create. This is quite different from most
other species of mosquitoes, like the ones that transmit West Nile virus, which
tend to lay their eggs in marshes, rice fields, ditches, the edges of streams
and small, temporary rain pools.”
“The world has
changed dramatically in the past 40 years with regard to increasing the habitat
for Aedes aegypti breeding. An explosion of plastic and rubber solid waste now
litters virtually all parts of the globe, particularly in the developing world.
Non-biodegradable containers, used tires and discarded plastic bags and
wrappers — whether in the backyard, a roadside ditch or an abandoned lot — make
ideal habitats for these mosquitoes to lay their eggs. All they need is a
little rainfall.”
“Now we’ve got an outbreak on
our hands, and although the symptoms of Zika itself are absent to mild for
most, for some there can be devastating consequences to infection. An
increasing number of infected women have given birth to babies with
microcephaly, which causes small heads and brain damage. We’re learning that
Zika can lead to Guillain-Barré syndrome, a dangerous autoimmune disorder that
can cause paralysis. Some believe we need more scientific data to confirm these
more severe manifestations. I don’t agree; I believe the evidence is already
compelling.”
Osterholm
tells us that this mosquito is already found in twelve US states, mostly in the
southeast, and that it is also capable of transmitting other nasty diseases
such as Dengue fever. It is
disconcerting to learn that the common name for Aedes aegypti is the “yellow fever mosquito.”
Note the suggestion
that we have invited this plague upon ourselves by changing our environment in
a way that generated an increased mosquito population. In other words, our own actions are
increasing the dangers we face from infectious threats. Is this an exceptional situation, or is it
inevitable as we continue to impose change upon the planet we live on?
While humans will require multiple generations to evolve
different characteristics in response to a changing environment, they are currently
changing the environment in which they exist on a very rapid time scale. In addition, the human population has grown
dramatically in just a few generations.
Increasing population density increases the probability of infectious
epidemics. We are also expanding into
all areas of the globe, including places with microbial life we have yet to
encounter. As we eliminate animal
species, we force bacteria and viruses to seek new hosts in order to survive—and
we are often the most abundant option.
David Quammen addressed the issue of human activity as a
path to more frequent and more dangerous infections in his book Spillover: Animal Infections and the Next Human Pandemic. The term spillover refers to the transfers of
viruses from their normal animal hosts to humans. The correct technical term for this transfer
is zoonosis. Both AIDS and Ebola came to
humans from animal infections and were restricted to minor outbreaks in limited
regions until the pressures of population growth and increased mobility led to major
epidemics. One can add Machupo
(1959), Marburg (1967), Lassa (1969), Sin Nombre (1993), Hendra (1994), avian
flu (1997), Nipah (1998), West Nile (1999), SARS (2003) and MERS (2012) to the
list. Quammen believes that human
activity is the reason for the increased frequency.
“We have increased our
population to the level of 7 billion and beyond....We live at high densities in
many cities. We have penetrated, and we
continue to penetrate, the last great forests and other wild ecosystems of the
planet, disrupting the physical structures and ecological communities of such
places. We cut our way through the
Congo. We cut our way through the
Amazon. We cut our way through
Borneo. We cut our way through
Madagascar. We cut our way through New
Guinea and northeastern Australia. We
shake the trees, figuratively and literally, and things fall out. We kill and butcher and eat many of the wild
animals found there.”
“Make no mistake, they are
connected, these disease outbreaks coming one after another. And they are not simply happening to
us; they represent the unintended results of things we are doing. They reflect the convergence of two forms of
crisis on our planet. The first is
ecological, the second is medical. As
the two intersect, their joint consequences appear as a pattern of weird and
terrible new diseases, emerging from unexpected sources and raising deep
concern, deep foreboding, among the scientists who study them.”
Quammen uses the term outbreak both as a flare up of an
infectious disease, and as an exceptionally large increase in the population of
any species. This latter use of the term
is often associated with the populations of certain types of insects. Quammen refers to the cycles of population
growth and collapse endured by the gypsy moth.
Its population density will start low for an extended period until some environmental
change occurs that favors increased rates of reproduction and leads to rapid
growth in the population density. This
growth was always observed to be followed by a dramatic collapse in the
population. It was only recently that
the cause of this behavior was learned.
There is a virus classification called nucleopolyhedroviruses (NPV). These viruses lie in wait for the gypsy
moths, but it requires a population level large enough for an epidemic to
occur. At lower levels individual moths
will be infected but will not spread the disease sufficiently for it to propagate
throughout the entire population. Once
that species density is attained the virus takes over and decimates the
population.
Quammen suggests that we, with our rapid population
growth and our ham-handed treatment of the environment, are a species
undergoing outbreak.
“....we are hungry. We are prodigious, we are unprecedented. We are phenomenal. No other primate has ever weighed upon the
planet to anything like this degree. In
ecological terms, we are almost paradoxical: large-bodied and long-lived but
grotesquely abundant. We are an
outbreak.”
“And here’s the thing about outbreaks: they
end. In some cases they end after many
years, in other cases they end rather soon.
In some cases they end gradually, in other cases they end with a crash.”
Could there come a virus that was so deadly and so easily
transmitted that a collapse of the human population could occur? The
SARS virus had that potential. It had a
high degree of lethality and it was easily spread from human to human. What limited its effect was the fact that it
showed signs of illness before it became highly contagious. This drove most sick people off the streets
and often into controlled medical settings before they could spread the
virus. It also helped that the virus was
spread by air travel to locations that possessed modern medical
facilities. If the virus had emerged in
different locations, or, if it became contagious before severe symptoms were
exhibited, the result could have been disastrous.
Quammen and other experts point to influenza as a
possible source for such a virus. Influenza
is a zoonosis. This class of virus is
ultimately transferred from wild aquatic birds although it often reaches humans
after being passed through an intermediary host such as a pig. Influenza viruses know how to infect humans,
they mutate continuously, and can vary from mild to deadly in effect.
The flu pandemic of 1918 provides an example of the
potential for harm. According to Wikipedia
50 to 100 million people died (3-5% of the world’s population at the time) and
500 million were infected (about 30% of the world’s population). The 10% fatality rate is not unusual (Ebola
kills about 70% of those it infects), but the transmissibility is what made
this pandemic so deadly.
What seems to be worrying scientists now is the existence
of bird flu. This flu emerged in Hong
Kong in 1997. It was the first occasion
where a virus with the H5 designation was observed to infect a human. The virus resides in duck species. Some die from it, others don’t, like the
mallard and pintail, and they have spread it across the world. It is particularly prevalent in Egypt where
duck and poultry populations are infected, and about a quarter of all known
human infections have occurred. Most
human infections come from transmission from an infected bird rather than via
human-to-human transmission.
The virus is not going away and it continues to create
new versions of itself. It is widespread
and it has a high fatality rate of about 33%.
If, or when, it emerges in a form that allows humans to infect one
another, it could be catastrophic.
Let us return to Osterholm and his article. He is irate at the lack of preparation for
the next virus. We know that one will
come, and we knew that Aedes aegypti
resided in the US and was capable of spreading the dangerous Dengue viruses. That should have alerted us to the need to
take measures to control the mosquito population before the Zika crisis arose.
“The point is, we should have anticipated that the large increase in
mosquitoes would create a major health crisis. Just as we should have
anticipated that a deadly hemorrhagic disease caused by the Ebola virus would
emerge one day from the remote forests and threaten the vast slums of the
rapidly growing megacities of Africa. We should now anticipate that the MERS
virus will result in more deadly outbreaks outside of the Arabian Peninsula, as
it did in Seoul, South Korea. We should anticipate that viruses such as
Venezuelan equine encephalitis may spread from their jungle homes and be even
more deadly than Zika.”
Osterholm is
also concerned that “the big one” will emerge as a form of influenza.
“Even more than these viruses, we should be afraid of a planet-wide
catastrophe caused by influenza. The best way to avert a pandemic is to develop
a game-changing universal influenza vaccine. All these crises are largely
predictable and we can do much in advance to lessen the effects and diminish
the spread. And believe me, the cost of acting now will be infinitely less than
the cost of not acting in the long run.”
It might be a good idea to make the
effort to get a flu shot each year.
The interested reader might find these articles
informative:
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