Deadly viruses

The deadly viruses that threaten human survival

Could the next big animal-human disease wipe us out?

The Marburg virus: 'If tourists were tripping in and out of some python-infested Marburg repository, unprotected, and then boarding their return flights to other continents… it was an international threat.' Photograph: Science Photo Library

The Guardian,

28 September, 2012

Astrid Joosten was a 41-year-old Dutch woman who, in June 2008, went to Uganda with her husband. At home in Noord-Brabant, she worked as a business analyst. Both she and her husband, Jaap Taal, a financial manager, enjoyed annual adventures, especially to Africa. The journey in 2008, booked through an adventure-travel outfitter, took them to theBwindi Impenetrable Forest, home to mountain gorillas. While there, the operators offered an optional trip, to a place called the Maramagambo Forest, where the chief attraction was a peculiar site known as Python Cave. African rock pythons lived there, languid and content, grown large and fat on a diet of bats.

Most of the other travellers didn't fancy this trip, Taal told me. "But Astrid and I always said, maybe you come here only once in your life, and you have to do everything you can."

Inside the cave, the footing was bad: rocky, uneven, slick with guano. The ceiling was thick with bats, big ones, many thousands of them, agitated at the presence of human intruders. Astrid and Jaap kept their heads low and watched their step, trying not to slip, ready to put a hand down if needed. "I think that's how Astrid got infected," he told me. "I think she put her hand on a piece of rock [covered with bat droppings]. And so she had it on her hand."

No one had warned Joosten and Taal about the potential hazards of an African bat cave. They knew nothing of a virus called Marburg, first identified in 1967 and thought to be carried by bats (though they had heard of Ebola, another filovirus). They stayed in the cave for only about 10 minutes. Then they left, visited the mountain gorillas, did a boat trip, and flew back to Amsterdam. Thirteen days after the cave visit, at home in Noord-Brabant, Astrid fell ill.

At first it seemed no worse than flu. Then her temperature went higher and higher. After a few days she began suffering organ failure. Her doctors suspectedLassa fever and moved her to a hospital in Leiden, where she developed a rash and conjunctivitis; she haemorrhaged. She was put into an induced coma, a move dictated by the need to dose her more aggressively with antiviral medicine. Before she lost consciousness, Taal went back into the isolation room, kissed his wife and said to her, "Well, we'll see you in a few days." Blood samples, sent to a lab in Hamburg, confirmed the diagnosis: Marburg. Astrid worsened. As her organs shut down, she lacked oxygen to the brain, suffered cerebral oedema, and before long she was declared brain-dead. "They kept her alive for a few more hours, until the family arrived," Taal told me. "Then they pulled out the plug and she died within a few minutes." The doctors, appalled by his recklessness in kissing her goodbye, had prepared an isolation room for Taal himself, but that was never needed.

The news of Astrid Joosten's death carried far. She was the first person known to have left Africa with an active filovirus infection and died. Back in 1994, a Swiss graduate student from Ivory Coast had recovered. Did any other person, apart from those two, ever pass through an international airport and depart the continent with Ebola or Marburg virus incubating in his or her body? No one of whom the experts were aware. Astrid Joosten's case proved that Marburg could travel in a human, though it didn't travel as well as Sars or influenza or HIV-1. Five thousand miles away, in Colorado, another woman heard the news with a shudder of recognition. She had visited Python Cave, too.

 

Astrid Joosten fell ill after visiting a bat cave in Uganda and died of Marburg within weeks. Photograph: Karlijn Bastiaansen/ED.nl

Infectious disease is all around us. It's one of the basic processes that ecologists study, along with predation and competition. Predators are big beasts that eat their prey from outside. Pathogens (disease-causing agents, such as viruses) are small beasts that eat their prey from within. Although infectious disease can seem grisly and dreadful, under ordinary conditions, it's every bit as natural as what lions do to wildebeests and zebras. But conditions aren't always ordinary.

Just as predators have their accustomed prey, so do pathogens. And just as a lion might occasionally depart from its normal behaviour – to kill a cow instead of a wildebeest, or a human instead of a zebra – so a pathogen can shift to a new target. Aberrations occur. When a pathogen leaps from an animal into a person, and succeeds in establishing itself as an infectious presence, sometimes causing illness or death, the result is a zoonosis.

It's a mildly technical term, zoonosis, unfamiliar to most people, but it helps clarify the biological complexities behind the ominous headlines about swine flu, bird flu, Sars, emerging diseases in general, and the threat of a global pandemic. It's a word of the future, destined for heavy use in the 21st century.

Ebola and Marburg are zoonoses. So is bubonic plague. So was the so-called Spanish influenza of 1918–1919, which had its source in a wild aquatic bird and emerged to kill as many as 50 million people. All of the human influenzas are zoonoses. As are monkeypox, bovine tuberculosis, Lyme disease, West Nile fever, rabies and a strange new affliction called Nipah encephalitis, which has killed pigs and pig farmers in Malaysia. Each of these zoonoses reflects the action of a pathogen that can "spillover", crossing into people from other animals.

Aids is a disease of zoonotic origin caused by a virus that, having reached humans through a few accidental events in western and central Africa, now passes human-to-human. This form of interspecies leap is not rare; about 60% of all human infectious diseases currently known either cross routinely or have recently crossed between other animals and us. Some of those – notably rabies – are familiar, widespread and still horrendously lethal, killing humans by the thousands despite centuries of efforts at coping with their effects. Others are new and inexplicably sporadic, claiming a few victims or a few hundred, and then disappearing for years.

 

Zoonotic pathogens can hide. The least conspicuous strategy is to lurk within what's called a reservoir host: a living organism that carries the pathogen while suffering little or no illness. When a disease seems to disappear between outbreaks, it's often still lingering nearby, within some reservoir host. A rodent? A bird? A butterfly? A bat? To reside undetected is probably easiest wherever biological diversity is high and the ecosystem is relatively undisturbed. The converse is also true: ecological disturbance causes diseases to emerge. Shake a tree and things fall out.

Michelle Barnes is an energetic, late 40s-ish woman, an avid rock climber and cyclist. Her auburn hair, she told me cheerily, came from a bottle. It approximates the original colour, but the original is gone. In 2008, her hair started falling out; the rest went grey "pretty much overnight". This was among the lesser effects of a mystery illness that had nearly killed her during January that year, just after she'd returned from Uganda.

Her story paralleled the one Jaap Taal had told me about Astrid, with several key differences – the main one being that Michelle Barnes was still alive. Michelle and her husband, Rick Taylor, had wanted to see mountain gorillas, too. Their guide had taken them through Maramagambo Forest and into Python Cave. They, too, had to clamber across those slippery boulders. As a rock climber, Barnes said, she tends to be very conscious of where she places her hands. No, she didn't touch any guano. No, she was not bumped by a bat. By late afternoon they were back, watching the sunset. It was Christmas evening 2007.

They arrived home on New Year's Day. On 4 January, Barnes woke up feeling as if someone had driven a needle into her skull. She was achy all over, feverish. "And then, as the day went on, I started developing a rash across my stomach." The rash spread. "Over the next 48 hours, I just went down really fast."

By the time Barnes turned up at a hospital in suburban Denver, she was dehydrated; her white blood count was imperceptible; her kidneys and liver had begun shutting down. An infectious disease specialist, Dr Norman K Fujita, arranged for her to be tested for a range of infections that might be contracted in Africa. All came back negative, including the test for Marburg.

Gradually her body regained strength and her organs began to recover. After 12 days, she left hospital, still weak and anaemic, still undiagnosed. In March she saw Fujita on a follow-up visit and he had her serum tested again for Marburg. Again, negative. Three more months passed, and Barnes, now grey haired, lacking her old energy, suffering abdominal pain, unable to focus, got an email from a journalist she and Taylor had met on the Uganda trip, who had just seen a news article. In the Netherlands, a woman had died of Marburg after a Ugandan holiday during which she had visited a cave full of bats.

 

Michelle Barnes and her husband, Rick Taylor, in Uganda, where she caught Marburg

Barnes spent the next 24 hours Googling every article on the case she could find. Early the following Monday morning, she was back at Dr Fujita's door. He agreed to test her a third time for Marburg. This time a lab technician crosschecked the third sample, and then the first sample.

The new results went to Fujita, who called Barnes: "You're now an honorary infectious disease doctor. You've self-diagnosed, and the Marburg test came back positive."

The Marburg virus had reappeared in Uganda in 2007. It was a small outbreak, affecting four miners, one of whom died, working at a site called Kitaka Cave. But Joosten's death, and Barnes's diagnosis, implied a change in the potential scope of the situation. That local Ugandans were dying of Marburg was a severe concern – sufficient to bring a response team of scientists in haste. But if tourists, too, were involved, tripping in and out of some python-infested Marburg repository, unprotected, and then boarding their return flights to other continents, the place was not just a peril for Ugandan miners and their families. It was also an international threat.

The first team of scientists had collected about 800 bats from Kitaka Cave for dissecting and sampling, and marked and released more than 1,000, using beaded collars coded with a number. That team, including scientist Brian Amman, had found live Marburg virus in five bats.

Entering Python Cave after Joosten's death, another team of scientists, again including Amman, came across one of the beaded collars they had placed on captured bats three months earlier and 30 miles away.

"It confirmed my suspicions that these bats are moving," Amman said – and moving not only through the forest but from one roosting site to another. Travel of individual bats between far-flung roosts implied circumstances whereby Marburg virus might ultimately be transmitted all across Africa, from one bat encampment to another. It voided the comforting assumption that this virus is strictly localised. And it highlighted the complementary question: why don't outbreaks of Marburg virus disease happen more often? Marburg is only one instance to which that question applies. Why not more Ebola? Why not more Sars?

In the case of Sars, the scenario could have been very much worse. Apart from the 2003 outbreak and the aftershock cases in early 2004, it hasn't recurred… so far. Eight thousand cases are relatively few for such an explosive infection; 774 people died, not 7 million. Several factors contributed to limiting the scope and impact of the outbreak, of which humanity's good luck was only one. Another was the speed and excellence of the laboratory diagnostics – finding the virus and identifying it. Still another was the brisk efficiency with which cases were isolated, contacts were traced and quarantine measures were instituted, first in southern China, then in Hong Kong, Singapore, Hanoi and Toronto. If the virus had arrived in a different sort of big city – more loosely governed, full of poor people, lacking first-rate medical institutions – it might have burned through a much larger segment of humanity.

One further factor, possibly the most crucial, was inherent in the way Sars affects the human body: symptoms tend to appear in a person before, rather than after, that person becomes highly infectious. That allowed many Sars cases to be recognised, hospitalised and placed in isolation before they hit their peak of infectivity. With influenza and many other diseases, the order is reversed. That probably helped account for the scale of worldwide misery and death during the 1918–1919 influenza. And that infamous global pandemic occurred in the era before globalisation. Everything nowadays moves around the planet faster, including viruses. When the Next Big One comes, it will likely conform to the same perverse pattern as the 1918 influenza: high infectivity preceding notable symptoms. That will help it move through cities and airports like an angel of death.

The Next Big One is a subject that disease scientists around the world often address. The most recent big one is Aids, of which the eventual total bigness cannot even be predicted – about 30 million deaths, 34 million living people infected, and with no end in sight. Fortunately, not every virus goes airborne from one host to another. If HIV-1 could, you and I might already be dead. If the rabies virus could, it would be the most horrific pathogen on the planet. The influenzas are well adapted for airborne transmission, which is why a new strain can circle the world within days. The Sars virus travels this route, too, or anyway by the respiratory droplets of sneezes and coughs – hanging in the air of a hotel corridor, moving through the cabin of an aeroplane – and that capacity, combined with its case fatality rate of almost 10%, is what made it so scary in 2003 to the people who understood it best.

 

A worker disinfects a poultry farm in China after an outbreak of the deadly H5N1 strain of bird flu in 2006. Photograph: China Photos/Getty Images

Human-to-human transmission is the crux. That capacity is what separates a bizarre, awful, localised, intermittent and mysterious disease (such as Ebola) from a global pandemic. Have you noticed the persistent, low-level buzz about avian influenza, the strain known as H5N1, among disease experts over the past 15 years? That's because avian flu worries them deeply, though it hasn't caused many human fatalities. Swine flu comes and goes periodically in the human population (as it came and went during 2009), sometimes causing a bad pandemic and sometimes (as in 2009) not so bad as expected; but avian flu resides in a different category of menacing possibility. It worries the flu scientists because they know that H5N1 influenza is extremely virulent in people, with a high lethality. As yet, there have been a relatively low number of cases, and it is poorly transmissible, so far, from human to human. It'll kill you if you catch it, very likely, but you're unlikely to catch it except by butchering an infected chicken. But if H5N1 mutates or reassembles itself in just the right way, if it adapts for human-to-human transmission, it could become the biggest and fastest killer disease since 1918.

It got to Egypt in 2006 and has been especially problematic for that country. As of August 2011, there were 151 confirmed cases, of which 52 were fatal. That represents more than a quarter of all the world's known human cases of bird flu since H5N1 emerged in 1997. But here's a critical fact: those unfortunate Egyptian patients all seem to have acquired the virus directly from birds. This indicates that the virus hasn't yet found an efficient way to pass from one person to another.

Two aspects of the situation are dangerous, according to biologist Robert Webster. The first is that Egypt, given its recent political upheavals, may be unable to staunch an outbreak of transmissible avian flu, if one occurs. His second concern is shared by influenza researchers and public health officials around the globe: with all that mutating, with all that contact between people and their infected birds, the virus could hit upon a genetic configuration making it highly transmissible among people.

"As long as H5N1 is out there in the world," Webster told me, "there is the possibility of disaster… There is the theoretical possibility that it can acquire the ability to transmit human-to-human." He paused. "And then God help us."

We're unique in the history of mammals. No other primate has ever weighed upon the planet to anything like the degree we do. 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 others they end rather soon. In some cases they end gradually, in others they end with a crash. In certain cases, they end and recur and end again. Populations of tent caterpillars, for example, seem to rise steeply and fall sharply on a cycle of anywhere from five to 11 years. The crash endings are dramatic, and for a long while they seemed mysterious. What could account for such sudden and recurrent collapses? One possible factor is infectious disease, and viruses in particular.

The dangers presented by zoonoses are real and severe, but the degree of uncertainties is also high. There's not a hope in hell, as Webster told me, of predicting the nature and timing of the next influenza pandemic. Too many factors vary randomly.

I have asked not just Webster, but many other eminent disease scientists the same two-part question: 1) will a new disease emerge, in the near future, sufficiently virulent and transmissible to cause a pandemic on the scale of Aids or the 1918 flu, killing tens of millions of people?; and 2) if so, what does it look like and whence does it come? Their answers to the first part have ranged from maybe to probably. Their answers to the second have focused on various viruses prone to mutation, especially those for which the reservoir host is some kind of primate.

But the difficulty of predicting precisely doesn't oblige us to remain blind, unprepared and fatalistic. We can at least be vigilant; we can be well prepared and quick to respond. The scientists are on alert. They are our sentries. But we, too, should understand in some measure the basic outlines and dynamics of the situation. We should appreciate that these recent outbreaks of new diseases, as well as the recurrence and spread of old ones, are part of a larger pattern, and that humanity is responsible for generating that pattern. We should recognise that they reflect things that we're doing, not just things that are happening to us.

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 continue to penetrate, the last great forests and other wild ecosystems of the planet. We cut our way through the Congo, through the Amazon, through Borneo. 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 settle in those places, bringing in our domesticated animals. We multiply our livestock as we've multiplied ourselves, under conditions that allow them to acquire infections, to share them with one another, and to infect humans. We export and import livestock across great distances and at high speeds.

We travel, moving between cities and continents even more quickly than our transported livestock. We stay in hotels where strangers sneeze and vomit. We eat in restaurants where the cook may have butchered a porcupine before working on our scallops. We visit monkey temples in Asia, live markets in India, picturesque villages in South America, bat caves in East Africa – breathing the air, feeding the animals, touching things, shaking hands with the friendly locals. And then we jump on our planes and fly home.