A Modern Menace: Emerging Infectious Diseases

[Guest guest]

A Modern Menace: Emerging Infectious Diseases

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 (see " Preparing for the Next

Pandemic " ) 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. "

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 , 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, . " 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 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 , 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

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 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. "

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, " 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, " 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.

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 s Hopkins University in

Baltimore, land. " 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 of Emory University (see " Murderous Microbes " ).

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. "

REBECCA SWEAT

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