The Floating Zoo

The air teems with viruses, bacteria, fungi, and other microscopic creatures. They can cross oceans on a gust of wind. Some can cause crop failures, disease, and death. Some can be used as invisible weapons, and we know next to nothing about them.

By Kevin Krajick|Saturday, February 01, 1997
In late 1991 at Lincoln Hospital, a huge red-brick building in New York City, something began killing patients on Ward 8C. First the victims, all of whom were already battling aids, came down with a fever and a nasty cough. Antibiotics prescribed by the doctors did nothing. Within weeks patients were eaten alive from inside; holes appeared in their lungs, muscles evaporated. Medical technician Bertha Doctor remembers their eyes, how they looked at her from deep within taut-skinned heads as the victims drowned in pain and fever. Toward the end, as blood filtered into dissolving lungs, the victims’ breathing came hard. They’re coughing as much as they can, they’re gasping, gasping, and then they stop, says Doctor. The sickness moved from room to room, hall to hall, in a pattern that suggested it was caused by an airborne microbe. Doctor and the rest of the 8C staff were so scared that they secretly gathered in a back room to join hands, cry, and pray. Then they went back to work.

Eventually the sickness was diagnosed and named: Strain W, a new drug-resistant strain of tuberculosis. Science once thought it had conquered this airborne scourge. Now tb is again a global pandemic, taking almost 3 million lives in 1995. Highly contagious forms that ignore antibiotics are becoming more prevalent. tb was implicated in the deaths of 70 people at Lincoln; 90 others (including Doctor) picked up the bacteria before it finished its wanderings.

The Lincoln Hospital outbreak was only one of many recent reminders that the air around us is full of life--life that we know little about and which sometimes views us as prey. A cubic yard of the atmosphere can contain hundreds of thousands of bacteria, viruses, fungal spores, pollen grains, lichens, algae, and protozoa. A good sneeze expels over 10 million germs. Certainly it comes as no surprise that diseases like tb, influenza, and chicken pox can spread through the air. But many people may find it shocking to learn how little we know about the airborne habits of the microbes that cause those familiar diseases; we know even less about the habits of the microbes that cause rarer, more lethal, plagues. As if threats from nature were not enough, airborne germ weapons are proliferating--and the Pentagon admits it has been poorly equipped to spot an attack. Aerobiologists, those who study life in the atmosphere, are scrambling to catch up.

Aerobiologists face a fundamental problem: unlike organisms in blood, food, and water, airborne pathogens are still mostly beyond our powers to track. We greatly underestimate the organisms in the air, says University of Nevada, Las Vegas, microbiologist Linda Stetzenbach. We can find maybe 10 to 30 percent. There are a lot we don’t know anything about.

In recent decades aerobiology has been burdened by its reputation as a charming but somewhat obsolete science. Actually, it is a cornerstone of microbiology. Well over a century ago, in the 1860s, Louis Pasteur proved that food rotted because ubiquitous organized corpuscles in the air quickly colonized any organic matter that wasn’t sealed off. Decades of experiments through the middle of this century showed that diseases like tb, polio, measles, pneumonic plague, diphtheria, and flu move through the air from host to host. But time and again, drugs and vaccines were so effective in halting the spread of these diseases that aerobiology was pushed to medicine’s back burner. If you can fight germs once they reach human bodies, why worry about how they get there?

Biologists and ecologists remained interested in life in the air, even if doctors felt they could safely ignore it. From the 1930s to the 1960s, aircraft- and balloon-mounted instruments found vast armies of seeds and bacteria floating miles above Earth. A Russian rocket sampled the scant air at 40 miles up and found bacteria. In a column of air one square mile in area and 14,000 feet high, researchers calculated that there were 25 million insects. Spores of fungus were found over the Pacific and Atlantic, far from the lands where they started their journeys. The ecology of this air started to make sense. It became apparent that living things routinely go aloft to migrate, to mate, and to find new hosts. Thus in a few weeks ragweed plants produce billions of pollen grains, casting them to the winds and to runny noses. The Pallas’s wallflower of the Arctic is a particularly exquisite example of airborne engineering: it has evolved seed heads that protrude just above the snow in winter; at this height they can be dislodged by winds, which then sweep them over the crusted surface for miles, even to islands across frozen seas.

While much of this traveling is dedicated to reproduction and finding new land to colonize, much of it, unfortunately, is aimed at infecting land-bound organisms. Like invisible swarms of locusts, fungi can travel from Mexico to Canada, wiping out crops of wheat and corn along the way. Researchers are only beginning to chart the course these spores take. During the day, rising thermals and gusts of wind carry hundreds of thousands of them in each cubic yard, lofting them up to 10,000 feet. At that height, smooth, interstate-like airstreams can carry them along for hundreds of miles at speeds up to 40 miles an hour until rain or other disturbances drive them back to Earth.

Aerobiologists have had good luck studying grains of pollen and fungus spores, in part because they are big and sturdy enough to survive the violence of being collected on an airplane-mounted screen, brought to a lab, and counted under a microscope. But detecting microbes in the vastness of the atmosphere is far more difficult--bacteria are 100 to 300 times smaller than fungi; viruses are 100 times smaller yet. Thanks to the endless eddying and mixing of air, huge numbers of microbes may occupy a small space for just a second before scattering, all the while remaining undetected. Instead of trying to spot germs directly, therefore, aerobiologists have until recently tried to vacuum up large volumes of air, blast it into a growth medium, and wait for something to multiply. It’s a poor method at best: the microbes are sometimes so battered from their long journeys that they can’t reproduce. Those that do manage to grow may take days or weeks to turn up in detectable numbers. And many microorganisms simply refuse to grow unless they are feeding on their particular living target--an apple leaf, say, or a human lung.

As a result of all these factors, estimates of bacteria and virus numbers vary wildly. According to Environmental Protection Agency microbiologist Bruce Lighthart, a cubic meter of air over a single Oregon farm field can harbor anywhere from 900 to 600,000 bacteria depending on the location and time of day. Viruses are the biggest mystery of all. Unlike bacteria, which are content simply to suck up nutrients from a culture dish, viruses must infect a cell before they can reproduce. They have rarely been directly detected in air except in tightly contained lab experiments. But they are certainly around, spreading flu and doing other jobs of which we are unaware. Maybe we’re picking up a hundredth or a thousandth of the viruses, says Lighthart. I don’t know.

Recently aerobiologists have at least begun to get a better catch, thanks to the revolutionary technique known as polymerase chain reaction amplification, or pcr. pcr typically involves using a genetic probe that locks onto snippets of DNA and reproduces them many times over until they are so numerous that they can be easily detected. Last year plant pathologist John Castello of the State University of New York at Syracuse collected cloud and fog particles from the Adirondack Mountains and from treeless islands off the coast of Maine and used pcr to pluck out a plant virus--called tomato mosaic Tobamovirus. He subsequently infected red spruce trees with it, proving that it was alive. Crop plants and timber are the hosts of the hardy Tobamovirus, but Castello found it far from either, offering up the first good evidence that viruses can probably travel on their own, without a host, for many miles.

Chances are the viruses we’re most familiar with won’t find us on a mountain but inside a home or office. Most of us human hosts spend 80 to 90 percent of our time inside, inhaling, exhaling, and building up concentrations of organisms that would dissipate quickly in the great outdoors. Sneezes and coughs expel germs in mucousy droplets, but most are unwieldy gobs over four-thousandths of an inch across--big enough to fall onto the floor or other surfaces within seconds. If they happen to end up in your nose, they will probably be filtered out by protective hairs. But some drops dry and shrink to droplet nuclei--measuring two ten- thousandths of an inch across and carrying a few bacteria or hundreds of viruses. These colonies are well-designed for finding a host to infect: they can float for hours or even days on the tiniest currents; the clumped germs help one another maintain critical moisture and temperature. They are small enough to whisk through nasal passages, yet big enough to get jostled out of respiratory-passage air streams by turbulence, gravity, or sudden rehydration and find purchase in the throat or lung.

Mycobacterium tuberculosis is the quintessential airborne germ. It may float alive for hours, protected from dehydration and damaging ultraviolet rays by its waxy coat. But although the bacterium has been studied exhaustively in humans, it has never been isolated from air outside laboratories. Most victims are thought to produce relatively few infectious droplets, and Mycobacterium reproduces so slowly in culture--three to six weeks to become apparent--that other airborne bugs that happen to be sampled with it can crowd it out. So our knowledge of its behavior in air comes in large part from inferences, such as those made from experiments in the 1950s and 1960s in which the air from tb wards was pumped into guinea pig chambers, and from studies of infection rates of humans accidentally exposed to known carriers. Theoretically, it takes only a single tb bacterium to infect a person, but no one knows for sure.

Researchers are now designing a pcr probe to detect tb in air directly, much as Castello has done with tomato mosaic Tobamovirus. They are encouraged by the work of Mark Sawyer, a pediatric infectious-diseases specialist at the University of California at San Diego, who succeeded in detecting chicken pox virus in hospital air. Less encouraging, though, is what Sawyer also discovered: a full day after a patient was discharged he could detect the virus’s DNA still drifting around the room--and as far as 50 feet down the hall. Even more disturbing was Sawyer’s detection of airborne viral DNA in the rooms of patients with the form of chicken pox that adults suffer from, known as shingles. Unlike the childhood form, shingles creates painful blisters on the skin but does not spread infectious droplets by coughing. Sawyer’s results suggest that even scaling skin is a suitable launching pad for the virus.

Epidemics of chicken pox, gastric infections, and tb can rage through hospital wards, apparently through the air, which suggests that institutions are doing a poor job of containing the germs. Investigations of the Lincoln Hospital tb outbreak have revealed that even in rooms that were equipped with pumps to vent air from the rooms outdoors, air was spewed in the wrong direction, letting germs slip under doors, sidle down corridors, and circle the nurses’ station. According to a report by the New York State Department of Health, this was typical. Many hospitals, lulled by declines in serious airborne disease since the sixties, were found with either poor protective systems or none whatsoever. The outbreak at Lincoln, along with others in New Jersey and Florida, brought improvements at many institutions.

Lincoln itself now has a revamped, state-of-the-art tb unit. The surge of fans is constantly audible; they change the air in each room 6 to 12 times an hour. Outside each patient’s door is a fist-size air-pressure gauge. Engineers periodically check it by opening the doors and blowing puffs of chalk in to make sure none blows back into the hall. Inside the room, a silvery venting tunnel recessed into the ceiling emits ultraviolet light to destroy bacteria that pass through it. When Celia Alfalla, the director of the tb unit, enters a room, she dons a tight-fitting respirator to keep out germs. If a patient needs to leave his room for tests or X- rays, he pulls a surgical mask over his face to keep germs in. Except for these brief forays, patients are permitted to leave their rooms only if drugs have succeeded in rendering them noncontagious. Alfalla obtains court orders to imprison patients who won’t comply, and she is backed up by a surly looking police officer at the entrance to the unit.

Little about these measures is particularly new--hospitals are simply using them more often now. No one at this point knows what else to do. Despite all the precautions, Alfalla has skin tests every three to six months to check for exposure. I do get scared every time I get a cough or wake up with a night sweat, she says. Bertha Doctor already carries the germ--noncontagious as long as she doesn’t show symptoms--and has regular checkups. She has a one-in-ten chance of getting sick, but she has decided to stay on the ward. We love our patients and we want to see them doing well, she says. Besides, you go on the bus, tb is there. You go on the street, it’s there. You can’t not breathe.

Technology has not only failed to curb airborne pathogens; it has inadvertently succeeded in creating new forms. Legionella bacteria are a case in point. For millennia, Legionella, which lives in ponds and lakes, left humans unmolested. If you drank Legionella-laden water, you usually suffered no ill effect. But then we invented giant institutional air conditioners that use reservoirs of water to remove heat, and these reservoirs have proved friendly to these microbes. Powerful vents sweep air over the water and pick up droplets that they carry throughout buildings; in some systems the reservoirs are put on roofs, and the droplets are then sprayed down onto streets. Suddenly, for the first time, it is possible to breathe the bacteria--and in this form Legionella can cause the fatal pneumonia known as Legionnaires’ disease.

In recent years epidemiologists have discovered that air conditioners are not the only way to spread Legionnaires’ disease. The bacteria can travel through sprays created by supermarket vegetable misters, whirlpools in spas, decorative fountains, and hotel showers. pcr and other new methods are showing more clearly how we unwittingly turn everyday appliances into weapons of germ warfare. One recent study showed that dentists’ offices are Legionella hot spots, with the bacteria building up inside water-cooling lines for dental drills and shooting out as cavities are filled.

A puzzle about Legionella is why it produces such a range of effects--some people get just a sniffle (sometimes referred to as Pontiac fever) and others drop dead. Part of the reason may be found in amoebas, large single-celled organisms that live in soil and water--including the water in cooling systems--and that are sometimes found floating in the air. Legionella and other kinds of bacteria are often eaten by amoebas. But according to Tim Rowbotham, a water microbiologist at Leeds Public Health Laboratory in England, the devoured bacteria can turn the tables and feed on the amoebas from the inside and reproduce. A hapless amoeba may end up carrying as many as 1,800 bacteria. If you’re unlucky enough to inhale one of those, you could get quite a dose, he says.

More frightening to many is the emerging evidence that the once- obscure and now-famous virus Ebola may also be capable of airborne transmission. One of the deadliest pathogens on Earth, Ebola spreads mainly through body fluids, which has limited its periodic rampages through Africa. But in 1989 and 1990 an outbreak of Ebola may have been spread by air from monkeys in a quarantine facility in Reston, Virginia. This event was popularized in the 1994 best-seller The Hot Zone, and became fictional grist for the 1995 movie Outbreak. While a fair amount of the accompanying publicity was flashy and dubious, the threat of airborne Ebola is being taken seriously by some experts. In an analysis of recent outbreaks of Ebola in Africa performed by virologist Peter Jahrling, principal scientific adviser to the U.S. Army Medical Research Institute of Infectious Diseases, and his co-workers, the researchers found clusters of deaths--in which victims had no known contact with one another--that made it, in Jahrling’s words, difficult to exclude a component of aerosol transmissibility.

Meanwhile, further research on monkeys has added more evidence to Ebola’s possible airborne danger. When the Reston monkeys contracted Ebola, they came down with an unusual pneumonia not normally associated with the bleeding that is the hallmark of the disease. Researchers injected the Ebola from these monkeys into healthy ones, and they too came down with the pneumonia. It’s possible, judging from this evidence, that there is actually a rare separate strain of pneumonia-causing Ebola that prefers to travel by air. In addition, Jahrling has published a long-neglected experiment in which he and his colleagues killed monkeys by putting them into Plexiglas chambers and spraying in small amounts of the germ. According to Jahrling, this shows that while nature may or may not make Ebola airborne, it’s no problem for us to do it. You can make this stuff into a weapon.

This is not just conjecture. According to military intelligence sources, someone--they won’t reveal who--is already experimenting with Ebola as a form of biological warfare. This gruesome research is only part of a recent surge of work on bioweapons going on in nations scattered around the world. The official list is classified, but military sources say it includes Iraq, China, Iran, Syria, Egypt, Taiwan, Libya, some countries of the former Soviet Union, and various terrorist groups. Virtually all biological weapons are designed for air delivery, but they can just as easily be spread by putting a spray tank of anthrax bacteria, say, into the trunk of a New York taxi and politely driving around town. Anthrax in fact is a serious threat, since it’s stable in air and easily made; the U.S. Defense Department is sufficiently concerned that it has proposed vaccinating its 2.4 million personnel against the disease--the first program of germ-warfare inoculation the military has ever considered undertaking. Cholera and Venezuelan equine encephalitis (a mosquito-born virus that can paralyze or kill) are also designated as agents of concern. Animal experiments suggest that diseases with effects similar to Ebola--such as Rift Valley fever, Lassa fever, Bolivian hemorrhagic fever, Marburg virus and Congo-Crimean hemorrhagic fever--can become infectious if made into aerosols. But Army researchers are running vaccine trials for only three of these diseases, and they may have only limited use. Recently the Army made the horrifying discovery that its vaccine for Rift Valley fever--normally transmitted by mosquitoes--doesn’t work if the germ is inhaled. A new one has been developed, but it’s still being reviewed by the Food and Drug Administration.

According to Brigadier General John Doesburg, head of the Pentagon’s Joint Program Office for Biological Defense, the Gulf War made the military realize how poorly prepared it was to detect a bioweapons attack. At the time, the Iraqis were stockpiling thousands of gallons of agents, including anthrax, pneumonic plague, and Clostridium perfringens (which leads to gangrene)--and UN inspectors suspect they may still have much of it on hand. The gigantic Desert Storm army fielded only a few outdated biological weapons detectors. We did not have a substantive low- level air-sampling system, admits Doesburg.

As far as the Army can tell, the Iraqis didn’t use bioweapons, but if its experience with chemical weapons is any guide, its soldiers are vulnerable. After years of denial and obfuscation, the Pentagon conceded this summer that some Gulf War veterans’ complaints of devastating neurological and intestinal problems could be related to airborne Iraqi sarin and mustard gas; as many as 20,000 soldiers might have been exposed to these substances during the explosion of a chemical-weapons stockpile or other incidents.

Doesburg says the Pentagon now has a very accelerated schedule of long-term research to develop biological air probes--in 1996 alone it allocated $66 million for this purpose. The military has already come up with an array of gizmos. The first line of defense is a detection system consisting of a 1,100-pound device that bolts into a Blackhawk helicopter and sweeps a pulsing infrared laser beam as far as 20 miles, analyzing the photons that bounce back from airborne particles. A man-made cloud looks different from a natural one, says Bruce Jezek, the program director of biodefense at the Chemical and Biological Defense Command at Aberdeen Proving Ground in Maryland. Any cloud that seems unnaturally shaped, he says, will show up on a computer screen. The researchers are working on a second, short-range system to complement this device: an ultraviolet laser that can analyze clouds two miles away, making most biological particles fluoresce. It will, in theory, let its operators know if they are looking at mere smoke and dust or an oncoming cloud of plague.

Then, in development at Aberdeen, there is the Biological Integrated Detection System (bids)--a windowless, airtight eight-by-ten- foot Army shelter bolted into the back of a Humvee and crammed with computer screens and other equipment. The shelter, too cramped to stand up in, has a stack that continually sucks air into a device that measures the size of particles. If particle concentrations change suddenly, an alarm goes off; the two operators locked inside the box don masks and switch on another powerful vacuum that quickly distills thousands of gallons of outdoor air into small test tubes of liquid. One operator pulls out some of the tubes and uses a laser to detect the presence of adenosine triphosphate--the molecule that fuels all life. Meanwhile the other operator adds a stain to additional tubes to make any DNA present fluoresce, and counts the glowing cells by running them past an ultraviolet laser. Then the operators put the samples on sticks impregnated with antibodies or other compounds and insert them into analyzer slots that look for reactions indicating the presence of certain bacteria including pneumonic plague, tularemia, and anthrax.

This first generation of bids has to be operated by hand, using off-the-shelf equipment found in hospitals or university labs. And to detect bacteria, it takes 25 or 30 minutes. That’s not acceptable, says A. Jeff Mohr, chief of the aerosol and environmental technology branch of the Dugway Proving Ground in Utah. If you’re downwind, you can’t get protective gear on. Nor are the detectors terribly reliable. At Dugway, where scientists test devices with simulated outdoor germ attacks using harmless microbes as stand-ins, dust clouds trigger false alarms--an experience that could play havoc on the nerves of soldiers worrying about clouds of deadly germs.

The Army is working on a new generation of detectors, which need 15 to 20 minutes to detect bacteria and boast a false alarm rate of only 5 percent. Mohr hopes to start field tests on the new equipment next month. Even more sensitive detectors are on the drawing board--at Aberdeen, for example, aerobiologist Charles Wick is working on a portable virus detector that will ultimately be able to tease particles through filters made of lipids, shoot them through a laser and give a readout of the quantity and identity of viruses in a matter of minutes. Pretty neat, huh? he says. If Wick can make it sensitive to bacteria as well as viruses, such a machine could be useful for civilian life as well. You would be able to go into a hospital or school and say, ‘Your virus count is such and such, and here’s what bacteria you have,’ and do something about it.

Other scientists agree that such devices would be a great advance, whether in a tb ward or on the battlefield. But airborne life is wildly diverse, and always on the move. We still can’t always predict even which way the wind is going to blow. With aerobiology, as soon as you solve one problem, another comes up, says Mohr. This is not as precise a science as we’d like.
Comment on this article
ADVERTISEMENT

Discover's Newsletter

Sign up to get the latest science news delivered weekly right to your inbox!

ADVERTISEMENT
ADVERTISEMENT
Collapse bottom bar
DSCOctCover
+

Log in to your account

X
Email address:
Password:
Remember me
Forgot your password?
No problem. Click here to have it emailed to you.

Not registered yet?

Register now for FREE. It takes only a few seconds to complete. Register now »