Tuesday, May 21, 2013

Disease and the Human Outbreak

With reports coming out of China and the Middle East of human deaths being caused by new and mysterious viruses, what better time to encounter a book that delves into the issues associated with the emergence of new viruses. David Quammen has provided a survey of recent disease outbreaks and explores the paths by which viruses came to infect humans in Spillover: Animal Infections and the Next Human Pandemic. The theme that emerges repeatedly throughout Quammen’s work is that human’s are not merely the unlucky recipient of some random infection; rather, humans, as they multiply and modify or destroy ecosystems, are presenting themselves as an obvious target for disturbed or threatened life forms such as viruses and bacteria.

The term zoonosis is used to describe the transfer of a pathogen from a nonhuman animal species to humans. Quammen points out that the frequency of zoonosis seems to have accelerated in recent decades.

"The drumbeat has been sounding ever more loudly, more insistently, more rapidly over the past fifty years."

He provides this partial list of emerged viral diseases.

"If you assembled a short list of the highlights and high anxieties of that saga within recent decades, it could include....Machupo [1959]....Marburg (1967), Lassa (1969), Ebola (1976)....HIV-1 (inferred in 1981, first isolated in 1983), HIV-2 (1986), Sin Nombre (1993), Hendra (1994), avian flu (1997), Nipah (1998), West Nile (1999), SARS (2003), and the much feared but anticlimactic swine flu of 2009."

Quammen concludes that these outbreaks are a direct result of changes produced by human activity.

"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."

In fact, Quammen suggests we should think of ourselves as a species in a condition of "outbreak." This term can refer to the emergence of a rapidly spreading disease, but it also has a more general usage.

"Outbreak in the broader sense applies to any vast, sudden population increase by a single species."

"....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."

A fraction of the species on earth are capable of generating vast increases in population, but these expansions do not end well.

"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."

Species that increase in number and become more densely packed become greater targets for infectious diseases such as viruses. Quammen speaks in detail of how the gypsy moth population grows until a specific virus becomes active and lowers the number to near zero; at which point the growth and decimation process begins all over.

Quammen details the actions humans take that increase the probability of zoonosis.

"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."

We use and abuse animals in ways that are unnatural. We are reminded that the SARS virus was transferred to a human by a civet cat, a member of the mongoose family. However, the civet was similarly infected by another animal, a horseshoe bat. This type bat is the reservoir for the virus where it has permanent residence. How did the bat and the civet come together in the first place? Most likely in the live markets of Guangdong province in China where both animals were caged and sold as food.

"We multiply our livestock as we’ve multiplied ourselves, operating huge factory-scale operations involving thousands of cattle, pigs, chickens, ducks, sheep, and goats, not to mention hundreds of bamboo rats and palm civets, all confined en masse within pens and corrals, under conditions that allow these domestics and semidomestics to acquire infectious pathogens from external sources (such as bats roosting over pig pens), to share those infections with one another, and to provide abundant opportunities for the pathogens to evolve new forms, some of which are capable of infecting a human as well as a cow or a duck."

Is it possible that the human outbreak could be limited by a disease so lethal that our population could be significantly reduced? 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 the scientists who worry about such matters suspect that the "next big one" is likely to emerge as a form of influenza. 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 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.

While much progress has been made, we are still pretty much at the mercy of whatever mutations this class of virus chooses to produce.

"....there’s still no magical defense, no universal vaccine, no foolproof and widely available treatment, to guarantee that such death and misery don’t occur again. Even during an average year, seasonal flu causes at least 3 million cases and more than 250,000 fatalities worldwide. So influenza is hugely dangerous, at best. At worst, it would be apocalyptic."

Just to make sure we are paying attention, Quammen reminds us that bird flu, H5N1, is still out there. 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.

Quammen quotes one scientist, Robert Webster:

"’As long as H5N1 is out there in the world,’ Webster said, ‘there is the possibility of disaster. That’s really the bottom line with H5N1. So long as it’s out there in the human population, there is the theoretical possibility that it can acquire the ability to transmit human-to-human.’ He paused. ‘And then God help us’."

And don’t forget, there are two new and mysterious viruses out there that scientists are worrying over.

David Quammen has produced an excellent book. Not only is the topic timely and important, but it is presented in an extremely readable format. Quammen is a formidable writer.

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