Fall 2006

Life and Health

A Modern Menace: Emerging Infectious Diseases

Rebecca Sweat

In Queens, New York, an 80-year-old man mowed his front lawn and began complaining of extreme fatigue to his wife. Unable to eat, he vomited, then went to bed. The next morning he woke up with a sweltering fever and struggled to utter even single-syllable words. A little later he collapsed in a chair and was rushed by ambulance to the emergency room. Before the end of the day his organs began to fail, and he suffered a heart attack and died.

This may sound like an episode from a television drama, but it’s an actual medical case from August 1999. Just a short time before his death, the man had been bitten by a mosquito carrying the West Nile virus, a pathogen that had never before been seen in the Western Hemisphere. He was the first North American to die from the disease, but in the weeks that followed, others in the New York metropolitan area succumbed to the same mysterious illness. Since then there have been over 20,000 reported human cases of West Nile virus in the United States, more than 800 of which ended in death. In addition, countless similarly infected crows, chickens, pelicans and other birds have died. The virus has proved to be an elusive enemy as mosquitoes carry it across the continent.

At first, of course, there was a media uproar over the West Nile virus, yet it is just one of a long list of emerging infectious diseases in the world today—caused by contagions that have only recently been discovered. Avian Flu has also grabbed the headlines, but others on the list include dengue, Ebola and Marburg hemorrhagic fevers; Nipah virus encephalitis; Hendra virus disease; Lassa fever; hantavirus pulmonary syndrome; monkeypox; Lyme disease; SARS; and drug-resistant forms of tuberculosis, malaria, staphylococcal infection and salmonellosis. All of these have the potential to wreak havoc on humanity, just as another virus on the list, the human immunodeficiency virus (HIV), has already done. June 5, 2006, marked the 25th anniversary of the first reported cases of the HIV-related disease that came to be known as acquired immunodeficiency syndrome, or AIDS. Since then an estimated 25 million people worldwide have died from it.

But, many ask, how can this be? After all, medical science has made incredible advances in the last century. “By the early 1970s, people were looking at progress with antibiotics and vaccines and thought the real problems of mankind were going to be related to old age rather than communicable diseases,” observes Klaus Stohr, director of the Influenza Task Force for the World Health Organization (WHO) in Geneva, Switzerland.

This optimism led to a widespread belief that humankind had won the war against infectious diseases. Medical researchers began to focus their efforts on chronic ailments such as cancer and heart disease rather than communicable diseases.

Yet today, Stohr continues, “there are far more virulent, difficult-to-treat infectious diseases than there were 20 or 30 years ago. Many new diseases have been emerging, and there has also been a resurgence of infections like malaria and tuberculosis.”

We’re a long way from eliminating the health threat posed by the avian flu, HIV, malaria, or a long list of other infectious diseases. It’s pretty scary, really. I don’t think there’s a lot of good news to report right now when it comes to infectious diseases.”

Thomas Monath, M.D., Chief Scientific Officer, Acambis, Inc.

According to WHO, at least 30 new infectious diseases have emerged in the last 20 years, many of which evade traditional therapies and have no cure. With so many deadly pathogens coming on the scene, notes Stohr, “infectious diseases are once again the leading cause of death in the world—something that hasn’t been the case since the pre-antibiotic era of the early 1900s.”

Three of a Kind

Emerging infectious diseases can be grouped into three categories of causation: viruses that have mutated or genetically recombined to become new strains or novel microbes; viruses that had previously existed only in one part of the world and started appearing in new regions; and viruses that may have existed for millennia but weren’t discovered until recent years.

A disease that fits into the first category is AIDS, which came into being through genetic recombination, says Roy Anderson, fellow of the Royal Society and professor of infectious disease epidemiology at the University of London’s Imperial College. “Recombination is when two different viruses infect the same cell, then the genomes get jumbled and something totally novel comes out,” he explains. HIV, for instance, is thought to be a fusion of the simian immunodeficiency virus (SIV), which infects monkeys and apes, and a similar virus that infects humans.

An example of the second category is the West Nile virus. This pathogen was first isolated in Uganda in 1937. It confined itself to Africa, the Middle East and Europe for about six decades before showing up in the United States. Migrating birds may have carried the virus to Europe, but how it got to North America is uncertain. One widely expressed theory is that an infected mosquito was inside someone’s luggage on a flight to New York.

Lyme disease is an example of the third category. “Lyme disease was first identified in 1976 in the Northeastern United States, but it was probably around long before that,” suggests Bruno Chomel, professor of zoonoses at the School of Veterinary Medicine at the University of California, Davis. “Mankind suddenly came in contact with the virus,” he theorizes, “when housing developments started being built closer and closer to the woodlands where a large number of white-tailed deer lived. The deer were the reservoir hosts of the Lyme disease virus” (see “The Zoonotic Connection”). 

All three types of emerging infectious diseases have one thing in common, scientists say, and that is the human role in facilitating the new pathogens’ emergence. “In almost every case humans are the most important single factor in the surge of new diseases, whether it’s feeding cow tissue to cattle, causing mad cow disease; people eating exotic animals, as in the case of Ebola; or air travel spreading dengue around the world,” claims Thomas Monath, chief scientific officer with Acambis, a vaccine development company in Cambridge, Massachusetts. He explains that people are not simply victims of emerging infections but are actually helping to cause or exacerbate them through changes they make to the natural world.

People Problems

One of the primary ways in which humans have facilitated the emergence of new diseases is by making alterations to animal ecosystems. “There are environments in the developing world that used to be quite remote but are now much less so as a result of human activities like deforestation, dam projects, irrigation, road construction and extensive agriculture,” says Jim Hughes, director of Global Infectious Disease Programs at Emory University and former director of the National Center for Infectious Diseases at the Centers for Disease Control (CDC) in Atlanta, Georgia.

Wilderness areas are often home to unique microbes—bacteria, parasites or viruses not found anywhere else. When people enter these ecosystems, they may encounter these pathogens for the first time. If they become infected, they take the pathogens with them wherever they go, thereby spreading disease.

Following the human disruption of ecosystems, animals themselves can also contribute to the spread of deadly microbes. “Once forests are cleared, the wildlife that used to live there have no choice but to migrate farther out in search of food and land in which to live,” says Stephen Corber, manager of Disease Prevention and Control at the Pan American Health Organization in Washington, D.C. “Often they end up in suburbs and farming communities, where they make contact with people. If they’re bringing diseases with them, that’s when you have problems.”     

A Change Is in the Air

Weather patterns can also come into play. In much of the world, average daily temperatures appear to be rising. Whether this warming is primarily an anthropomorphic effect—a result of automobile and truck exhaust, the use of fossil fuels, emissions from coal-powered generating plants, and other so-called greenhouse gases that have entered the atmosphere—or primarily the result of cyclical changes in the climate makes little difference to the pathogen. Either way, higher temperatures can greatly affect disease transmission.

As the environment becomes warmer, it becomes more hospitable to insect vectors [disease-transmitting organisms] such as mosquitoes,” says David Freedman, professor of medicine at the University of Alabama’s Division of Geographic Medicine. Mosquitoes don’t do well in very cold or very dry climates, he notes. Rising temperatures, along with increased rainfall (which can occur in association with global warming), make it possible for mosquitoes to survive in previously inhospitable climates, thus broadening their range. Some mountainous regions in Africa never had a malaria problem in the past because the higher altitudes were too cold for the mosquitoes to breed. But in recent years, Freedman says, “as average temperatures become warmer, we’re starting to get reports of malaria in some of those regions, because the mosquitoes are now able to survive at those higher altitudes.”

Worldwide, an estimated 200 million to 300 million malaria infections occur each year, with 2 million to 3 million deaths. . . . Urban migration, poverty, and poor sanitation have returned malaria to cities where it once was eliminated.” 

Malaraia Fact Sheet, Prepared by “The Directors of Health Promotion and Education,” Washington, D.C.

Warmer temperatures can also have a dramatic effect on the transmissibility of viruses carried by vectors. “When a mosquito feeds on an individual carrying a virus, that virus then has to replicate for a period of time before it can be transmitted by the mosquito as it feeds on another host,” Monath explains. This is the “extrinsic” incubation period of the virus. An increase of a single degree in average temperature will shorten that extrinsic period dramatically, he continues. “That means the interval between acquiring the infection and being able to transmit it shortens. Since mosquitoes live only a short time, that can have a dramatic effect on increasing transmission.”

Close Quarters

Other new viruses have emerged in the developing world, particularly Southeast Asia, where it’s common practice for people to keep their farm animals in their front yard or even inside their homes. “The cities are often overcrowded, and because of lack of space, people typically live in very close proximity to their livestock,” Chomel notes. “It’s not unusual for a family to sleep in the loft of a barn, while keeping their cows, goats and pigs downstairs.”

This close proximity of animals to people creates opportunities for an exchange of pathogens between animal and human hosts. “If a human infected with a virus comes in contact with an animal that has a similar type of virus, the genetic material of the two pathogens can get mixed up and recombine, which can result in the emergence of a new virus that infects both animals and people,” Anderson says. “This is not something that happens overnight or with one transmission event,” he adds. “In the beginning the animal pathogens may not be very transmissible, but slowly their transmissibility increases and they start to gain fitness in humans.”

The concern, though, is not just with diseases transmitted from domesticated animals. In China, exotic animals like civets, snakes, tree shrews, flying squirrels, badgers and pangolins are considered delicacies. “These animals are sold at the wet markets [markets that sell live animals], which are very crowded environments, with many different animal species and people crammed together,” Hughes says. “Any viruses carried by these animals can be transmitted to people via consumption, if people handle the animals, or sometimes if they just come into the same air space.” It appears that SARS got its start at a wet market in Hong Kong when infected masked palm civets transmitted the virus to people in the market.

Of course, the consumption of exotic animals is not limited to Asia. In Africa, for instance, monkeys, apes and other local animals are potential meat choices. Guinea pigs and their larger cousins, capybaras, are commonly eaten in Peru and Brazil. Armadillos are considered taste treats in Central America. The bottom line, Chomel says, is that “the practice of consuming wild species opens the door for a much wider variety of pathogens—those of wild animals—to come in contact with humans and develop transmissibility.”

Meting Resistance

Another factor that contributes to the development of bacterial pathogens in particular is the overuse and misuse of antibiotics. Widespread antibiotic use in the beef and dairy industry, for instance, is necessary to maintain animal health in unhygienic feedlots. Bacteria that are naturally resistant or immune to the antibiotic tend to multiply when the drug eliminates the harmless species. If this surviving strain later finds its way through the food supply to a human host, the disease it causes can be devastating because of its ability to resist treatment. Recent campaigns to encourage the complete cooking of hamburger and poultry have largely countered this danger.

Each year in the United States, doctors write an estimated 50 million antibiotic prescriptions for viral illnesses for which antibiotics offer no benefit.” 

Antibiotics: Too Much of a Good Thing,” MAYOCLINIC.COM, February 2006

This type of selection scenario has unfortunately operated within the medical field as well to create new antibiotic-resistant bacteria. “The public has the perception that you give them a pill and everything is fixed,” says Trish Perl, director of Hospital Epidemiology and Infection Control at Johns Hopkins University in Baltimore, Maryland. “People will often insist that they need an antibiotic when they have a cold or the flu, and sometimes doctors will give in to these demands. The problem is that colds and flu are caused by viruses, which are not treatable with antibiotics.”

The CDC acknowledged this problem and responded in the 1990s with an aggressive campaign to educate both doctors and patients on appropriate use of antibiotics. According to numerous studies published in the past five years, some overall decreases in such prescriptions have been seen, but the percentage of antibiotics prescribed in doctors’ offices for viral infections remains astonishingly high. This is not always due to patients demanding an antibiotic. Sometimes physicians prescribe them when they can’t make a definite diagnosis, or they may give them as a preventive measure.

With so many antibiotics in the environment, we’re pressuring these bacteria to evolve into resistant strains,” Perl asserts.

Hugh Pennington, president of the Society for General Microbiology in the United Kingdom and retired professor of medicine at the Institute of Medical Sciences at the University of Aberdeen, explains the process. When people take antibiotics, “the drug kills the defenseless bacteria, leaving behind—or ‘selecting’—those that can resist it. These renegade bacteria then multiply and become the predominant microorganism.”

Today there are drug-resistant forms of tuberculosis, malaria, and E. coli, Staphylococcus, Streptococcus and Salmonella infections, to name just a few of the superbug diseases that have emerged in recent years. Because they are resistant to antibiotics, some consider them to be genetically new organisms. “Some infections are now so resistant to the drugs we have available that they are virtually untreatable,” remarks Hughes of Emory University. 

The Search for Solutions

With so many infectious diseases emerging, it can all sound quite ominous. Still, “you don’t need to be terrified,” Corber assures. “You do, however, need to be aware of microbial threats. You need to understand what measures you can take to minimize your chances of becoming infected.” That includes hand-washing after using the toilet or handling raw meat; the appropriate use of antibiotics; and before traveling to developing countries, seeking input from health officials regarding what can be done to minimize risks of acquiring diseases like malaria. These are steps that individuals can and must take, says Corber.

Governments, too, have important roles to play. “Governments need to spend enough money on quality surveillance so that these problems can be picked up early,” remarks Pennington. “Without knowing what’s coming and what’s here already, it’s impossible to do anything to curb the problem, and we’re not going to be able to react as quickly as we need to. It’s also important that governments set aside enough money for basic research.”

Stohr sums up the situation this way: “Infectious diseases are not something we can just ignore. A constant investment in attention is necessary. The moment we become complacent, the moment we start thinking we’ve won the battle, infectious diseases will be back.”