Zika, Human Population Growth, and Epidemics

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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?

“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.”

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 Wikipedia50 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.

It might be a good idea to make the effort to get a flu shot each year.

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