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Modern Menace: Emerging & Re-Emerging Infectious Diseases
The success of antibiotics after World War II led to the impression
that bacterial infections could be easily cured. "Infectious Diseases
are more easily prevented and more easily cured than any other major
group of disorders..." stated Harrison's Principles of Internal
Medicine 10th ed., 1983. Although many viral diseases remained unconquered,
effective vaccines prevented some of the most frightening ones.
Much of the industrialized world basked in a feeling of invulnerability,
assuming that heart disease and cancer were the major health problems
left to conquer.
Global microbial threats in the 1990s
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Today we view infectious disease with greater
respect. The victories of the past are seen in perspective
with the emergence of HIV/AIDS and at least 30 other new infections.
Old foes such as tuberculosis, rabies, malaria, and pneumonia
are evading traditional therapies and are now on the comeback.
Infectious diseases are the leading cause of death in the
world and the third leading cause of death in the United States.
What is happening to make it easier for these deadly microbes
to infect us? |
Changes in Land Use
Deforestation in South America
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Human activities, such as deforestation, irrigation,
extensive agriculture or building settlements, affect the
ecological conditions in which disease-causing microbes thrive. |
Urbanization - Megacities
Population density is increasing in countries that are not able
to provide adequate sewage systems, safe drinking water, housing
and medical facilities. People are living in overcrowded "megacities"
often million or more, many of which are in tropical or subtropical
regions where infectious diseases thrive. In the 1800s, London and
New York City approached that size. Today there are over 24 megacities,
mostly in developing countries. It is estimated that by 2010, 50%
of the world's population will be living in urban areas.
Urban slum
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Economic conditions encourage the mass movement
of workers from rural areas to cities. Rural urbanization
allows infections that may once have remained obscure and
localized in isolated rural areas to reach larger populations.
Urban slums are breeding-grounds for physical disease and
social ills, ranging from tuberculosis to drug abuse. |
Globalization of World Commerce
We eat out from the world's gardens, but not all are well-kept.
Raspberries from Guatemala made thousands of Americans ill from
a parasite called cyclospora, which was in the water used to spray
and irrigate the raspberries. Potential disease-carrying insects
and contaminated foods, plants, and other products cross U.S. borders
every day. Since the 1980's, food imports to the U.S. have doubled.
Increases in food imports strain the nation's food safety system.
While we rely on the FDA, USDA and other government agencies to
protect our food supply, inspections have dropped to half of what
they were five years ago. As the world's nations become more intertwined,
interdependent and intensely competitive, will the rest of the world's
standards become more like those of the U.S. or will the U.S., despite
high standards, become more vulnerable to the rest of the world's
microbes?
Population Movement
In 1990 it was estimated that there were 20 million refugees and
30 million displaced persons worldwide. Human population movements
due to political, economic or climatic events such as flooding,
earthquakes and drought, are important factors in disease emergence.
Such crises lead to interim living arrangements, such as refugee
camps and temporary shelters, that provide ideal conditions for
the spread of infections. Temporary living quarters often share
similarities with urban slums - crowding, inadequate sanitation,
limited access to medical care, lack of clean water and food, dislocation,
and inadequate barriers for disease-carrying agents. An example
is the movement of 500,000 - 800,000 Rwandan refugees into Zaire
in 1994. Almost 50,000 refugees died during the first month from
cholera and Shigella dysenteriae type 1 that swept through the refugee
camps.
Increased Air Travel
"Speed of transport is an unprecedented event in human history.
A million people a cross national boundaries by air transport alone.
So we have the opportunity for the mixing of gene pools and the
origins of these viruses, and for their very rapid spread literally
overnight from anyplace on the globe to any other."
Dr. Joshua Lederberg, Nobel Laureate (1958)
Lecture at the College of Physicians of Philadelphia
April 1, 1997
Modern air travel makes it possible to spread infectious diseases
to different parts of the world. People are traveling to areas where
they can get infected and bring new diseases home with them. How
many more victims could a lethal strain of influenza, similar to
the 1918 epidemic, claim today with half a billion passengers traveling
via jet planes? The speed of travel enables a person carrying a
disease such as Ebola to travel 12,000 miles, pass unnoticed through
customs and immigration before developing symptoms several days
later, thus infecting many other people before getting ill.
Microbial Adaptation: Tuberculosis
Tuberculosis is an acute or chronic infection caused by Mycobacterium
tuberculosis, which usually involves the lungs but may involve any
organ or tissue of the body. A worldwide health problem that reached
a peak in the l9th century, it was thought to have been brought
under control by the l960s due to active public health measures
and the use of modern drug therapies. However, this complacency
led to reduced funding for the diagnosis and treatment of TB at
the same time that the bacterium was developing resistance to the
drugs used to treat it. The problem has been compounded since the
1980s by the emergence of a new population of vulnerable individuals
- those infected with HIV.
The most effective weapon against regular TB today, and the best
insurance against the development of drug-resistant forms, is Directly
Observed Treatment -Short-course, or DOTS, which requires patients
to come to clinics for their medication. This ensures compliance
with the complex but standard six-month, four-drug regimen. However,
it is estimated that only 10% of the world's TB patients have access
to DOTS. The cost difference between standard therapy and that for
multi-drug- resistant TB (MDR-TB) is considerable, placing a further
burden on the health system of every country in which it occurs.
The cost of drugs for a six-month regimen of DOTS therapy is $15
to $25, while treatment for MDR-TB can reach $100,000 to $250,000
because of related hospital care, medications, and sometimes surgery.
Long-term control of TB may be achieved through the development
of more effective vaccines. But until that occurs, TB remains one
of the major infectious diseases in the world today, infecting perhaps
one out of every three people on earth, and killing approximately
three million people annually. It is the leading cause of death
among those infected with HIV.
Sputum cup
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Traditional therapies for TB included rest,
fresh air, and good food to build up the patient's resistance,
and isolation of the patient in special hospital wards or
sanitaria to prevent the spread of the disease. Respiration
masks and disposable sputum cups were used to control transmission
through sneezing and spitting. |
Respirator for nose and mouth
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Finally, in the mid-1940s, the first antibiotic
was found that was effective against TB-streptomycin. It was
often combined with para-amino-salicylic acid to prevent the
development of resistance. It was joined by isoniazid in 1953
and later by rifampin and several other drugs. There are now
ten main front-line drugs used against the disease. and the
original two-year course of treatment is usually accomplished
in six months.
Chemotherapy drastically reduced the need for surgical interventions
such as lung collapse and excision of infected tissue. A post-WW
II explosion of TB cases was effectively treated with drugs,
proving that prolonged rest and isolation in sanitaria were
no longer necessary. TB was thought to be all but extinct;
in 1962, the International Union against Tuberculosis declared
TB "conquered." The sanitaria had closed and public funding
for diagnosis and treatment was cut back severely. |
Scrofuloderma, draining from tuberculosis-infected
lymph nodes
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There are two forms of tuberculosis: primary,
which develops in individuals exposed for the first time;
and secondary, which develops in previously exposed individuals.
In primary TB, the organism usually enters through the lungs
and creates a characteristic focus of infection, the tubercle,
which is usually walled off by progressive fibrosis if the
individual is healthy. At this point the disease does not
progress, though active bacilli may persist in the body for
months and even years. This is also called latent TB.
Secondary, or reinfection TB, occurs when primary TB is reactivated
because the individual's immune system has become weakened
due to age, malnutrition, HIV infection, or other causes.
It is usually limited to the upper part of one or both lungs,
where it can undergo healing, scarring, and calciflcation
to become arrested TB. Alternately, it may spread to other
areas of the body through the lymphatics or bloodstream. It
is then called miliary TB, because the sites of infection
resemble millet seeds. This wax model shows a form of tuberculosis
of the skin once called lupus vulgaris, which commonly affected
the face. |
Child with tuberculosis of the hip
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Humans are susceptible only to the human and
bovine (cow) strains of tuberculosis. Bovine TB, spread through
contaminated milk, often affected the bones, and children
with tubercular inflammation of the hip - TB coxitis - were
once a familiar sight to physicians. With the virtual eradication
of tuberculous milk in the United States due to pasteurization
and vaccination of cows, the greatest source of infection
today is from infected people who cough up or sneeze bacilli
into the air. The bacilli may be inhaled directly or transmitted
indirectly from infected dust or clothing, and can remain
viable for days under ordinary conditions or for months when
kept in the dark. |
A most frightening development is the emergence of new strains
of TB bacilli that are resistant to existing drug therapies. These
multi-drug-resistant (MDR) strains often appear when patients failed
to complete the prescribed course of drug treatment. The susceptible
bacteria are killed, but the resistant mutant bacilli proliferate
and become dominant. The detection of multi-drug-resistant TB requires
sophisticated laboratory procedures, beyond the sputum smear microscopy
used initially to detect TB. These procedures are unavailable in
developing countries, with the result that all patients there are
treated with the standard drug therapy. When this is given to MDR
patients, it can make them even more drug-resistant, worsening the
problem.
A 1997 CDC survey of 35 countries found drug-resistant TB in each,
with "hot spots" in India, Latvia, Estonia, the Dominican Republic,
Argentina, and Africa's Ivory Coast. Immigration and the lack of
screening for TB at borders, means that even though MDR cases are
presently decreasing in the U.S., all Americans are in jeopardy
as long as drug-resistant TB remains present in the world population.
The relaxed vigilance against the disease set the stage for a dramatic
national upsurge in TB cases in the U.S., beginning in 1986. These
new cases were the result of many factors. Political and economic
unrest elsewhere in the world brought increasing numbers of infected
immigrants into the country. The continuing problem of poverty sent
many people to homeless shelters where transmission was widespread.
Reduced access to health care among the poor and substance abusers
meant delays in diagnosis and often interrupted or unsatisfactory
therapy. Growing numbers of immune-suppressed individuals, primarily
HIV-infected, easily caught the disease and, once infected, rapidly
progressed to active contagious TB.
Living In A World With HIV/Aids
In 1979, young men in the prime of life, particularly clustered
in urban areas, began dying from unusual and unexplained infections.
The tale unfolded slowly revealing a devastating illness that destroys
the immune system of its victims. When HIV/AIDS appeared in the
mid-1980s, many people thought a cure or vaccine would quickly be
developed. In November 1997, Peter Piot , Executive Director UNAIDS,
the United Nations' special program on the AIDS epidemic, reported
that some 16,000 people worldwide were being infected with the HIV
virus each day, nearly twice as many as previously thought. "The
AIDS epidemic is not over," Piot said. "It is more serious than
we ever imagined." Most of the projected estimates were on target
except in sub-Saharan Africa. The new estimates indicated that one
in every eight adults was infected in South Africa, while in Botswana
and Zimbabwe, infection rates had reached at least 25%. HIV/AIDS
makes us realize how much we did not know about microbes and the
way that they cause disease. Far from being invulnerable, we are
constantly on the defense from attacks by versatile microscopic
organisms who can erupt at any time, mutate (change) to a deadlier
form, or escape from an environmental niche.
Youngster gives himself fix of heroin
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Sharing needles for drug injection is a well-known
route of HIV transmission as well as many blood-borne infections.
Injection drug use contributes to the spread of infectious
diseases far beyond the circle of those who inject. People
who have sex with an injection drug user (IDU) are also at
risk for infection through sexual HIV transmission. Children
born to HIV-infected mothers may become infected as well.
Since the HIV/AIDS epidemic began, injection drug use has
directly and indirectly accounted for more than one-third
(36%) of AIDS cases in the United States. This disturbing
trend appears to be continuing. Of the 60,000+ new cases of
AIDS reported in 1997, nearly 20,000 (32%) were IDU-associated.
Racial and ethnic minority populations in the United States
bear the heaviest burden of HIV disease related to drug injection.
IDU-associated AIDS has a greater impact on women than on
men. Since 1981, at least 61% of all AIDS cases among women
have been attributed to injection drug use or sex with partners
who inject drugs, compared with 31% of cases among men. |
Use of noninjection drugs such as crack cocaine also contributes
to the spread of the epidemic when users trade sex for drugs or
money, or when they engage in risky sexual behaviors. One study
of over 1,000 young adults in three inner-city neighborhoods found
that crack smokers were three times more likely to be infected with
HIV than nonsmokers.
One of the biggest hazards of the 21st century to children will
be the continuing spread of HIV/AIDS. In 1997, 590,000 children
under the age of 15 became infected with HIV in the world. The disease
threatens to reverse major gains in child health over the last 50
years. HIV/AIDS also strikes at people in the prime of life. In
the United States, HIV -related deaths have the greatest impact
on young and middle-aged adults, particularly racial and ethnic
minorities. HIV/AIDS is the second leading cause of death for Americans
between the ages of 25 and 44. It is the leading cause of death
for African-American men and women in this age group. It is estimated
that half of all new HIV infections in the United States are among
people under 25, and the majority of young people are infected sexually.
Among 13- to 24-year-old males, 52% of all HIV/AIDS cases reported
in 1997 were among young men who had sex with men; 10% were among
injection-drug users and 7% were among young men infected heterosexually.
In 1997, among young women the same age, 49% were infected heterosexually
and 13 % were injection-drug users.
In some countries, about one young adult in five has HIV/AIDS.
Millions of children are also being orphaned, leaving grandparents
to bring up their grandchildren. HIV positive but symptomless people
are becoming more prevalent due to advances in the treatment of
AIDS. For the majority of those with HIV/AIDS outside the U.S. and
Europe, the cost of the "cocktail" treatments is unaffordable. The
average Kenyan would exhaust his annual income in less than one
week on this therapeutic regimen. |
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