Spillover: Animal Infections and the Next Human Pandemic

by David Quammen

Identifying the new virus was only step one

Step two would involve tracking that virus to its hiding place.

Step three would entail asking a further cluster of questions: How did the virus emerge from its secret refuge, and why here, and why now?

60 percent of all human infectious diseases currently known either cross routinely or have recently crossed between other animals and us.

Smallpox, to take one counterexample, is not a zoonosis. It’s caused by variola virus, which under natural conditions infects only humans.

Monkeypox differs from smallpox in one crucial way: the ability of its virus to infect nonhuman primates

A reservoir host (some scientists prefer “natural host”) is a living organism that carries the pathogen, harbors it chronically, while suffering little or no illness.

Ecological disturbance causes diseases to emerge. Shake a tree, and things fall out.

Nearly all zoonotic diseases result from infection by one of six kinds of pathogen: viruses, bacteria, fungi, protists (a group of small, complex creatures such as amoebae, formerly but misleadingly known as protozoans), prions, and worms.

Virus particles are so tiny they can’t be seen, except by electron microscopy, which involves killing them, so their presence during isolation must be detected indirectly.

the percentage of sampled individuals showing some history of infection, either present or past—is called seroprevalence. It constitutes an estimate, based on finite sampling, of what the percentage throughout an entire population might be.

An amplifier host is a creature in which a virus or other pathogen replicates—and from which it spews—with extraordinary abundance.

The amplifier host becomes an intermediate link between a reservoir host and some other unfortunate animal, some other sort of victim—a victim requiring higher doses or closer contact before the infection can take hold.

The amplifier host has a relatively low threshold for becoming infected, yet it produces a vast output of virus, vast enough to overcome the higher threshold in another animal.

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.

Human-caused ecological pressures and disruptions are bringing animal pathogens ever more into contact with human populations, while human technology and behavior are spreading those pathogens ever more widely and quickly.

Mankind’s activities are causing the disintegration (a word chosen carefully) of natural ecosystems at a cataclysmic rate.

Two: Those millions of unknown creatures include viruses, bacteria, fungi, protists, and other organisms, many of which are parasitic.

Viruses can only replicate inside the living cells of some other organism.

Three: But now the disruption of natural ecosystems seems more and more to be unloosing such microbes into a wider world.

Viruses, especially those of a certain sort—those whose genomes consist of RNA rather than DNA, leaving them more prone to mutation—are highly and rapidly adaptive.

an emerging disease is “an infectious disease whose incidence is increasing following its first introduction into a new host population.”

A re-emerging disease is one “whose incidence is increasing in an existing host population as a result of long-term changes in its underlying epidemiology.”

“Spillover” is the term used by disease ecologists (it has a different use for economists) to denote the moment when a pathogen passes from members of one species, as host, into members of another.

Spillover leads to emergence when an alien bug, having infected some members of a new host species, thrives in that species and spreads among it.

Any such spillover in the reverse direction—from humans to a nonhuman species—is known as an anthroponosis.

Ebola and Marburg, would be classified within a new family, Filoviridae: the filoviruses.

Many factors contribute to the case fatality rate during an outbreak, including diet, economic conditions, public health in general, and the medical care available in the location where an outbreak occurs.

Neighbors or acquaintances, envious of the wealth or power someone has amassed, could send ezanga to gnaw at the person’s internal organs, making him sick unto death. That’s why gold miners and timber-company employees suffered such high risk of Ebola, Hewlett was told. They were envied and they didn’t share.

Four teachers within one nearby village were members, or were thought to be members—and these teachers had been telling children about Ebola virus before the outbreak occurred. That led some traditional healers to suspect that the teachers had advance knowledge—supernatural knowledge—of the outbreak. Something had to be done, yes? On the day before Barry Hewlett and his wife arrived in Mbomo, the four teachers were murdered with machetes while they worked in their crop fields.

You can’t catch one by breathing shared air, but if a smidgen of the virus gets through a break in your skin (and there are always tiny breaks), God help you. In the terms used by the scientists: It’s not very contagious but it’s highly infectious.

viruslike particles (VLPs),

VLPs are essentially the outer shells of viruses, capable of inducing antibody production (immune readiness) but empty of functional innards, and therefore incapable of replicating or causing disease.

Rheumatoid arthritis is an immune dysfunction, and the medicine used to control it can potentially suppress normal immune responses.

One test, using the PCR (polymerase chain reaction) technique that’s familiar to all molecular biologists, looked for sections of Ebola RNA (the virus’s genetic molecule, equivalent to human DNA) in her blood.

measles virus, as with other pathogens, there’s a critical minimum size of the host population, below which it can’t persist indefinitely as an endemic, circulating infection. This is known as the critical community size (CCS),

Another crucial aspect of measles is that the virus is not zoonotic. If it were—if it circulated also in animals living near or among human communities—then the question of critical community size would be moot.

Continuation of the outbreak depended on the likelihood of encounters between people who were infectious and people who could be infected. This idea became known as the “mass action principle.”

By a strict definition, zoonotic pathogens (accounting for about 60 percent of our infectious diseases, as I’ve mentioned) are those that presently and repeatedly pass between humans and other animals,

polyphyletic. What that suggests, besides the diversity of their genus, is that each of them must have made the leap to humans independently.

Mathematics to me is like a language I don’t speak though I admire its literature in translation.

the three classes of living individuals: the susceptible, the infected, and the recovered. During an epidemic, one class flows into another in a simple schema, S → I → R, with mortalities falling out of the picture because they no longer belong to the population dynamic.

dR/dt = γI merely means that the number of recovered individuals in the population, at a given moment, reflects the number of infected individuals times the average recovery rate.

So you have density, infectivity, mortality, and recovery—four factors interrelated as fundamentally as heat, tinder, spark, and fuel. Brought together in the critical measure of each, the critical balance, they produce fire: epidemic.

basic reproduction rate. That rate represented, in his words, “the number of infections distributed in a community as the direct result of the presence in it of a single primary non-immune case.” More precisely, it was the average number of secondary infections produced, at the beginning of an outbreak, when one infected individual enters a population

(It’s a little confusing, I concede, that they use R0 as the symbol for basic reproduction rate and plain R as the symbol for recovered in an SIR model.

P. knowlesi occupies a strange place in medical annals, involving the treatment of neurosyphilis (syphilis of the central nervous system), which for a time in the early twentieth century was done using induced malarial fevers.

PCR amplification of DNA fragments, followed by sequencing (reading out the genetic spelling) of those fragments, plumbs far deeper than microscopy.

nucleotides, which are components of the DNA and RNA molecules. Each nucleotide consists of a nitrogenous base linked with a sugar molecule and one or more bits of phosphate. If DNA resembles a spiral staircase supported by two helical strands, those nitrogenous bases are the stair steps connecting the strands.

In the RNA molecule, which serves for translating DNA into proteins (and has other roles, as we’ll see), a different piece called uracil substitutes for thymine, and the Scrabble pieces are therefore A, C, G, and U.