Emerging Infectious Diseases Volume 1 Issue 2

34 Pages·1995·1.3 MB·English

by  CDC

Emerging
Vol. 1, No. 2, April–June 1995
Infectious Diseases
Travel and the Emergence
of Infectious Diseases Mary E. Wilson
Escherichia coli
Serotype O157:H7 Peter Feng
Epidemic-Associated
Neisseria meningitidis Michael W. Reeves
Dengue/Dengue Duane J. Gubler and
Hemorrhagic Fever Gary G. Clark
Eradication of Dracunculiasis Ernesto Ruiz-Tiben
Escherichia coli
O169:H41 in Japan Yoshikazu Nishikawa
The GAP Project in
Southeastern Turkey Serap Aksoy
Drug-Resistant
Streptococcus pneumoniae Martin S. Cetron
WHONET: Monitoring Thomas F. O’Brien and
Antimicrobial Resistance John M. Stelling
Preventing the Spread of Hospital Infection Control
Vancomycin Resistance Practices Advisory
Committee
Waterborne Cryptosporidiosis Daniel G. Colley
U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES
Public Health Service
Emerging Infectious Diseases
Editors Liaison Representatives
Editor Anthony I. Adams, M.D. Joseph Losos, M.D.
Joseph E. McDade, Ph.D. Chief Medical Adviser Director General
National Center for Infectious Diseases Commonwealth Department of Laboratory Center for Disease Control
Centers for Disease Control Human Services and Health Ontario, Canada
and Prevention (CDC) Canberra, Australia
Atlanta, Georgia, USA Gerald L. Mandell, M.D.
David Brandling-Bennett, M.D. Liaison to Infectious Diseases Society
Perspectives Editor Deputy Director of America
Stephen S. Morse, Ph.D. Pan American Health Organization University of Virginia Medical Center
The Rockefeller University World Health Organization Charlottesville, Virginia, USA
New York, New York, USA Washington, D.C., USA
Robert Shope, M.D.
Synopses Editor Gail Cassell, Ph.D. Director, Yale Arbovirus Research Unit
Phillip J. Baker, Ph.D. Liaison to American Society for Yale University School of Medicine
Division of Microbiology and Microbiology New Haven, Connecticut, USA
Infectious Diseases University of Alabama at Birmingham
National Institute of Allergy and Birmingham, Alabama, USA Bonnie Smoak, M.D.
Infectious Diseases Chief, Dept of Epidemiology
National Institutes of Health (NIH) Richard A. Goodman, M.D., M.P.H. Division of Preventive Medicine
Bethesda, Maryland, USA Editor, MMWR Walter Reed Army Institute of Research
Centers for Disease Control Washington, D.C., USA
Dispatches Editor and Prevention (CDC)
Stephen Ostroff, M.D. Atlanta, Georgia, USA Robert Swanepoel, B.V.Sc., Ph.D.
National Center for Infectious Diseases Head, Special Pathogens Unit
Centers for Disease Control William Hueston, D.V.M., Ph.D National Institute for Virology
and Prevention (CDC) Acting Leader, Center for Animal Sandrinham 2131, South Africa
Atlanta, Georgia, USA Health Monitoring
Centers for Epidemiology and Roberto Tapia, M.D.
Managing Editor Animal Health Director General de Epidemiología
Polyxeni Potter, M.A. Veterinary Services, Animal and Plant Dirección General de Epidemiología
National Center for Infectious Diseases Health Inspection Service Secretaría de Salud
Centers for Disease Control U.S. Department of Agriculture México
and Prevention (CDC) Fort Collins, Colorado, USA
Atlanta, Georgia, USA
Morag Timbury, M.D.
James LeDuc, Ph.D. Director
Advisor for Arboviral Diseases Central Public Health Laboratory
Division of Communicable Diseases London, United Kingdom
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Contents
Emerging Infectious Diseases
Volume 1 • Number 2 April–June 1995
Perspective
Travel and the Emergence of Infectious Diseases 39 Mary E. Wilson
Synopsis
Escherichia coli Serotype O157:H7: Novel Vehicles of Infection and 47 Peter Feng
Emergence of Phenotypic Variants
Dispatches
Epidemic-Associated Neisseria meningitidis Detected by Multilocus 53 Michael W. Reeves, Bradley A.
Enzyme Electrophoresis Perkins, Marion Diermayer, and
Jay D. Wenger
Dengue/Dengue Hemorrhagic Fever: 55 Duane J. Gubler and Gary G. Clark
The Emergence of a Global Health Problem
Progress Toward the Eradication of Dracunculiasis 58 Ernesto Ruiz-Tiben, Donald R.
(Guinea Worm Disease): 1994 Hopkins, Trenton K. Ruebush, and
Robert L. Kaiser
Letters
Heat-Stable Enterotoxin-Producing Escherichia coli O169:H41 in Japan 61 Yoshikazu Nishikawa, Masaki
Hanaoka, Jun Ogasawara,
Nelson P. Moyer, and Teruo Kimura
The GAP Project in Southeastern Turkey: The Potential for Emergence 62 Serap Aksoy, Sedat Ariturk,
of Diseases Martine Y. X. Armstrong, K. P.
Chang, Zeynep Dörtbudak, Michael
Gottlieb, M. Ali Ozcel, Frank F.
Richards, and Karl Western
Commentary
Action Plan for Drug-Resistant Streptococcus pneumoniae 64 Martin S. Cetron, Daniel B.
Jernigan, Robert F. Breiman, and
the DRSP Working Group
News and Notes
WHONET: An Information System for Monitoring Antimicrobial 66 Thomas F. O’Brien and John M.
Resistance Stelling
Recommendations for Preventing the Spread of Vancomycin Resistance 66 Hospital Infection Control Practices
Advisory Committee
Waterborne Cryptosporidiosis Threat Addressed 67 Daniel G. Colley
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Perspective
Travel and the Emergence of
Infectious Diseases
Mary E. Wilson, M.D.
Harvard School of Public Health and Harvard Medical School, Boston, Massachusetts, USA
Member: Harvard Working Group on New and Resurgent Infectious Diseases
Travel is a potent force in the emergence of disease. Migration of humans has been
the pathway for disseminating infectious diseases throughout recorded history and
will continue to shape the emergence, frequency, and spread of infections in geographic
areas and populations. The current volume, speed, and reach of travel are unprece-
dented. The consequences of travel extend beyond the traveler to the population visited
and the ecosystem. When they travel, humans carry their genetic makeup, immu-
nologic sequelae of past infections, cultural preferences, customs, and behavioral
patterns. Microbes, animals, and other biologic life also accompany them. Today’s
massive movement of humans and materials sets the stage for mixing diverse genetic
pools at rates and in combinations previously unknown. Concomitant changes in the
environment, climate, technology, land use, human behavior, and demographics
converge to favor the emergence of infectious diseases caused by a broad range of
organisms in humans, as well as in plants and animals.
Many factors contribute to the emergence of in- survive, proliferate, and find a way to enter a sus-
fectious diseases. Those frequently identified in- ceptible host. Any analysis of emergence must look
clude microbial adaptation and change, human at a dynamic process, a sequence of events, a milieu,
demographics and behavior, environmental or ecosystem.
changes, technology and economic development, Movement, changing patterns of resistance and
breakdown in public health measures and surveil- vulnerability, and the emergence of infectious dis-
lance, and international travel and commerce (1-4). eases also affect plants, animals, and insect vectors.
This paper will examine the pivotal role of global
travel and movement of biologic life in the emer-
gence of infectious diseases. It will also examine the
Table 1. Basic concepts in disease
ways in which travel and movement are inextricably
emergence*
tied at multiple levels to other processes that influ-
ence the emergence of disease. Emergence of infectious diseases is complex.
Travel is a potent force in disease emergence and Infectious diseases are dynamic.
spread (5). The current volume, speed, and reach of
Most new infections are not caused by genuinely
travel are unprecedented. The consequences of mi-
new pathogens.
gration extend beyond the traveler to the population
visited and the ecosystem (6). Travel and trade set Agents involved in new and reemergent
the stage for mixing diverse genetic pools at rates infections cross taxonomic lines to include
and in combinations previously unknown. Massive viruses, bacteria, fungi, protozoa, and helminths.
movement and other concomitant changes in social, The concept of the microbe as the cause of disease
political, climatic, environmental, and technologic is inadequate and incomplete.
factors converge to favor the emergence of infectious
Human activities are the most potent factors
diseases.
driving disease emergence.
Disease emergence is complex. Often several
Social, economic, political, climatic, technologic,
events must occur simultaneously or sequentially
and environmental factors shape disease
for a disease to emerge or reemerge (Table 1) (6).
patterns and influence emergence.
Travel allows a potentially pathogenic microbe to be
introduced into a new geographic area; however, to Understanding and responding to disease
be established and cause disease a microbe must emergence require a global perspective,
conceptually and geographically.
Address for correspondence: Mary E. Wilson, Division of The current global situation favors disease
Infectious Diseases, Mt. Auburn Hospital, 330 Mt. Auburn emergence.
Street, Cambridge, MA 02238 USA, fax [phone];
e-mail [email protected]. *Adapted from Wilson ME (6).
39
Vol. 1, No. 2 — April-June 1995 Emerging Infectious Diseases
Perspective
Analysis of these species can hold important lessons tients remained infectious for about 3 weeks, many
about the dynamics of human disease. opportunities for transmission were available. Even
To assess the impact of travel on disease emer- in this century, until the 1970s, smallpox continued
gence, it is necessary to consider the receptivity of a to cause epidemics. A pilgrim returning from Mecca
geographic area and its population to microbial in- was the source of a large outbreak in Yugoslavia in
troduction. Most introductions do not lead to dis- the early 1970s that resulted in 174 Yugoslav cases
ease. Organisms that survive primarily or entirely and 35 deaths (9). The pilgrim apparently con-
in the human host and are spread through sexual tracted the infection in Baghdad while visiting a
contact, droplet nuclei, and close physical contact religious site. Because his symptoms were mild, he
can be readily carried to any part of the world. For was never confined to bed and was able to continue
example, AIDS, tuberculosis, measles, pertussis, his travels and return home.
diphtheria, and hepatitis B are easily carried by For most of history, human populations were rela-
travelers and can spread in a new geographic area; tively isolated. Only in recent centuries has there
however, populations protected by vaccines resist been extensive contact between the flora and fauna
introduction. Organisms that have animal hosts, of the Old and New Worlds. Schoolchildren hear the
environmental limitations, arthropod vectors, or rhyme “Columbus sailed the ocean blue, in fourteen
complicated life cycles become successively more hundred ninety-two,” but may learn little about the
difficult to “transplant” to another geographic area disaster brought upon the native populations of the
or population. Epidemics of dengue fever and yellow Americas by the arriving explorers. By the end of the
fever cannot appear in a geographic area unless fifteenth century, measles, influenza, mumps, small-
competent mosquito vectors are present. Schis- pox, tuberculosis, and other infections had become
tosomiasis cannot spread in an environment unless common in Europe. Explorers from the crowded
a suitable snail intermediate host exists in that urban centers of Europe brought infectious diseases
region. Organisms that survive only under carefully to the New World (10), where isolated populations
tuned local conditions are less likely to be success- had evolved from a relatively small gene pool and
fully introduced. Even if an introduced parasite per- had no previous experience with many infections
sists in a new geographic area, it does not (11). The first epidemics following the arrival of
necessarily cause human disease. In the United Europeans were often the most severe. By 1518 or
States, humans infected with Taenia solium, the 1519, smallpox appeared in Santo Domingo, where
parasite that causes cysticercosis, infrequently it killed one-third to half of the local population and
transmit the infection because sanitary disposal of spread to other areas of the Caribbean and the
feces, the source of the eggs, is generally available. Americas (10). The population of central Mexico is
In short, the likelihood of transmission involves estimated to have dropped by one-third in the single
many biological, social, and environmental vari- decade following contact with the Europeans.
ables. Travel across the Atlantic Ocean transformed the
flora and fauna of the New World as well. Some of
the transported materials became important
Historical Perspective
sources of food (plants), clothing, and transportation
Human migration has been the main source of (animals). Other transfers were less welcome: Japa-
epidemics throughout recorded history. William nese beetles, Dutch elm disease, and chestnut tree
McNeill (7), in his book Plagues and Peoples, de- fungus. A.W. Crosby, exploring these exchanges be-
scribes the central role of infectious disease in the tween the Old and the New Worlds, sounds a pessi-
history of the world. Patterns of disease circulation mistic note: “The Columbian exchange has left us
have influenced the outcome of wars and have with not a richer but a more impoverished genetic
shaped the location, nature, and development of pool” (10).
human societies.
The explorers also paid a price in loss of lives from
Trade caravans, religious pilgrimages, and mili- disease. Philip Curtin (12) provides a quantitative
tary maneuvers facilitated the spread of many dis- study of “relocation costs,” the excess illness and
eases, including plague and smallpox. A map in death among European soldiers in the nineteenth
Donald Hopkins’ book, Princes and Peasants: Small- century when they lived or worked in the tropics.
pox in History (8), traces the presumed spread of Until the most recent armed conflicts, infectious
smallpox from Egypt or India, where it was first diseases claimed more lives than injuries during
thought to have become adapted to humans some- wars.
time before 1000 B.C. Smallpox spread easily from
Plague holds a prominent place in history and
person to person through close contact with respira-
remains with us today. A bacterial infection caused
tory discharges and, less commonly, through contact
by Yersinia pestis, it is primarily an infection of
with skin lesions, linens, clothing, and other mate-
rodents, spread by their fleas. Human infection is
rial in direct contact with the patient. Because pa-
incidental to the maintenance of Y. pestis in animal
40
Emerging Infectious Diseases Vol. 1, No. 2 — April-June 1995
Perspective
reservoirs. Yet plague periodically has erupted in Another type of travel relevant to disease emer-
human populations, wreaking great devastation, gence is the shift of populations to urban areas. It is
killing millions and causing infection that can be estimated that by the year 2010, 50% of the world’s
spread directly from person to person by the respi- population will be living in urban areas. It is pro-
ratory route. Human population movement has been jected that by the year 2000, the world will comprise
essential in the spread of plague and the dispersal 24 “megacities”—sprawling metropolitan areas with
of rodents and their fleas to new areas. For centuries populations exceeding 10 million (World Bank,
plague spread along trade routes. It reached Califor- UNDP, World Health Organization, unpublished
nia by boat around the turn of this century, caused data). These areas will have the population density
epidemic infection in San Francisco, and then to support persistence of some infections and con-
spread to wildlife, where it persists today in a large tribute to the emergence of others. Many of these
enzootic focus. areas are located in tropical or subtropical regions,
where the environment can support a diverse array
of pathogens and vectors. Also developing are huge
Movement of People
periurban slums, populated with persons from many
Travel for business and pleasure constitutes a geographic origins. Poor sanitation allows breeding
small fraction of total human movement (5,13). Peo- of arthropod vectors, rodents, and other disease-car-
ple migrating individually or in groups, may be rying animals. Crowded conditions favor the spread
immigrants, refugees, missionaries, merchant ma- of diseases that pass from person to person, includ-
rines, students, temporary workers, pilgrims, or ing sexually transmitted infections. Travel between
Peace Corps workers. Travel may involve short dis- periurban slum areas and rural areas is common,
tances or the crossing of international borders. Its paving the route for the transfer of microbes and
volume, however, is huge. In the early 1990s more disease. Transfer of resistance genes and genetic
than 500 million persons annually crossed interna- recombination may also occur in and spread from
tional borders on commercial airplane flights (World crowded environments of transients.
Tourism Organization, Madrid, unpublished data). Acute disturbances, whether climatic or political,
An estimated 70 million persons, mostly from devel- lead to interim living arrangements, such as refugee
oping countries, work either legally or illegally in camps and temporary shelters, that provide ideal
other countries (14). Movement may be temporary conditions for the emergence and spread of infec-
or seasonal, as with nomadic populations and mi- tions. Temporary living quarters often share simi-
grant workers who follow the crops. Military maneu- larities with periurban slums: crowding, inadequate
vers worldwide employ and move huge populations. sanitation, limited access to medical care, lack of
The consequences of armed conflict and political clean water and food, dislocation, multiethnic com-
unrest displace millions. In the early 1990s, there position, and inadequate barriers from vectors and
were an estimated 20 million refugees and 30 million animals. An example is the movement of 500,000-
displaced persons worldwide (International Organi- 800,000 Rwandan refugees into Zaire in 1994. Al-
zation for Migration, personal communication). most 50,000 refugees died during the first month as
Grubler and Nakicenovic estimated and plotted epidemics of cholera and Shigella dysenteriae type 1
the average kilometers traveled daily for the French swept through the refugee camps (17).
population over a 200-year period (1800-2000) and Movement into a rural environment poses differ-
found that spatial mobility has increased more than ent risks and often places new rural populations in
1000-fold (15). In the last 40 years, the size of Aus- contact with pathogens that are in the soil and water
tralia’s population has doubled and the number of or are carried by animals or arthropods (18). Some
persons moving into and out of Australia has in- of these pathogens such as Guanarito (19) and Sabià
creased nearly 100-fold (16). viruses (20) in South America, were only recently
Although social, economic, and political factors recognized as capable of infecting humans.
push people from an area or draw them to another,
environmental resources and their impact on food
and water supplies are behind many conflicts lead- Consequences of Movement
ing to displacement of populations. Acute disasters, Human migration favors the emergence of infec-
such as flooding, earthquakes, and hurricanes often tious diseases through many mechanisms. When
force populations to seek shelter and sustenance in people migrate, they carry their genetic makeup,
new lands. Chronic changes, such as drought, deple- their accumulated immunologic experience, and
tion of soil, and disappearance of fish from streams, much more (Table 2). They may carry pathogens in
lakes, and oceans, draw people to new territories, or, or on their bodies and may also transport disease
more frequently, to the fringes of large urban cen- vectors, such as lice. Their technology (agricultural
ters. and industrial), methods for treating disease, cul-
tural traditions, and behavioral patterns may influ-
41
Vol. 1, No. 2 — April-June 1995 Emerging Infectious Diseases
Perspective
ence their risk for infection in a new environment permissiveness can pertain to human behavior, the
and their capacity to introduce disease into the new environment, or the presence of appropriate vectors
region. Their social standing and resources may or intermediate hosts. For example, the ease with
affect their exposure to local infections and their which HIV spreads in a population depends on sex-
access to adequate nutrition and treatment. People ual practices, condom use, the number of sex part-
also change the environment in many ways when ners, and intravenous drug use, among other
they travel or migrate—they plant, clear land, build, factors. Malaria requires specific mosquito vectors
and consume. Travel is relevant in the emergence of (with access to susceptible humans) to spread to new
disease if it changes an ecosystem. The following geographic regions. Schistosomiasis can be intro-
examples show the many ways in which migration duced into a new region only if the appropriate snail
can influence the emergence of disease in a new area. host is present and if the eggs excreted (in urine or
1. Humans may carry a pathogen in a form that feces, from an infected person), reach the snails in
can be transmitted, then or later, directly or indi- an appropriate environment.
rectly to another person. The pathogen may be silent 5. Humans may carry a strain of microbe that
(during the incubation period, chronic carriage, or has an unusual resistance pattern or virulence
latent infection) or clinically evident. Examples in- genes. A multiple-drug-resistant strain of Klebsiella
clude hepatitis B virus, human immunodeficiency pneumoniae appears to have been transferred by an
virus (HIV), Mycobacterium tuberculosis, M. leprae, asymptomatic woman from a hospital in Bahrain to
Salmonella typhi, and other salmonella. Disease Oxford, where it caused outbreaks in two British
may be especially severe when a pathogen is intro- hospitals (23). People also carry their background
duced into a population that has no previous expo- flora, in the intestinal tract, for example, which may
sure to the infection. How long the consequences of contain plasmids and resistance genes that can in-
migration persist varies with the specific infection. teract with microbes in a new area. It is not just the
The two most critical characteristics are the dura- classic pathogens that may be relevant to the emer-
tion of survival of the pathogen in a potentially gence of a new disease but the individual traveler’s
infective form and its means of transmission. total microbiologic “baggage.”
2. Epidemic cholera in Africa spread along the 6. Visitors to a region may lack immunity to
West African coast and, when the disease moved locally endemic infections, such as hepatitis A and
inland, followed fishing and trading routes. Mar- sand-fly fever. Visitors may suffer severe or different
kets, funerals, refugee camps—events that involved manifestations of infection or disease at an age when
migration of persons and large gatherings with close the local population is immune to it. Resettlement
contact—helped spread the infection. With El Tor of populations into malaria-endemic regions can
cholera, asymptomatic and mild infections can out- lead to a high death rate from falciparum malaria.
number severe disease by 100 to 1 (21), thus permit- 7. Kala-azar caused a deadly outbreak in remote
ting those infected to continue to move and work. villages in southern Sudan in 1994. The origin was
3. Pilgrims carried an epidemic strain of group A thought to be the villagers’ exposure to the sand-fly
Neisseria meningitidis from southern Asia to Mecca vector during migration to a food distribution center
in 1987. Other pilgrims who became colonized with that had been established by a relief organization
the epidemic strain introduced it into sub-Saharan (24). The migration took a malnourished population
Africa, where it caused a wave of epidemics in 1988 from a nonendemic zone into the southern part of
and 1989 (22). the kala-azar–endemic zone. Unfamiliarity with the
4. Humans may carry a pathogen that can be disease and the poor nutritional status of the popu-
transmitted only if conditions are permissive. This lation probably contributed to a high death rate (24).
8. Behavioral patterns in a new region may place
visitors at risk for infection, while the local popula-
Table 2. What is carried by humans into new
tion, possibly because of their knowledge of disease
regions?
risks, may not be at risk. Behavior patterns may
Pathogens in or on body involve food preparation (such as eating some foods
Microbiologic flora raw), clothing (or lack of it), (for example, going
barefooted), sleeping arrangements (sleeping on the
Vectors on body
ground or out of doors in an unscreened area), and
Immunologic sequelae of past infections contact with animals.
Vulnerability to infections 9. Susceptibility of a population may vary be-
cause of genetic differences. A microbe introduced
Genetic makeup
into a new region may have a greater or lesser
Cultural preferences, customs, behavioral
impact, depending on the host population. Genetic
patterns, technology
factors influence susceptibility to and expression of
Luggage and whatever it contains several infectious diseases. Although these interac-
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Emerging Infectious Diseases Vol. 1, No. 2 — April-June 1995
Perspective
tions are not yet well defined for most infections, gens and vectors. The globalization of markets
genetic factors influence infections caused by differ- brings fresh fruits and vegetables to dinner tables
ent classes of organisms, including cholera (25,26), thousands of miles from where they were grown,
parvovirus infection (27), malaria, and Helicobacter fertilized, and picked. Tunnels, bridges, and ferries
pylori infection (28). form means to traverse natural barriers to species
spread. The roads built to transport people often
To determine the consequences of travel both the
speed the movement of diseases from one area to
traveler and the population visited must be consid-
another. Mass processing and wide distribution net-
ered. Migration may be in only one direction, though
works allow for the amplification and wide dissemi-
travel often involves returning to the point of origin,
nation of potential human microbes.
perhaps after the traveler has made many stops
Examples of introduced species include plants
along the way. The changes in the various ecosys-
and animals—insects, microbes, and marine organ-
tems as a consequence of the migration guide the
isms.
emergence of diseases; any study that simply focuses
on the traveler is too narrow. 1. Ships convey marine organisms on their hulls
and in their ballast water. For example, 367 differ-
The distance traversed is less important than the
ent species were identified in ballast water of ships
differences in biological life in different areas and
traveling between Japan and Coos Bay, Oregon (32).
differences in receptivity and vulnerability. In think-
Introductions have had devastating effects in some
ing about disease emergence, what matters is the
areas, for example such as the Black and Azov seas,
potential of a disease to appear in a place, popula-
where newly introduced jellyfishlike creatures
tion, or extent not previously reported.
called ctenophores have ruined local fishing (33).
What is the long-term impact of migration and
2. Vibrio cholerae may have been introduced to
travel on human disease? Carriage of pathogens is
South America by shipping (34). Researchers iso-
only part of the influence on disease emergence.
lated the organism in samples of ballast, bilge, and
Introduced technology, farming methods, treatment
sewage from 3 of 14 cargo ships docked at Gulf of
and drugs, chemicals, and pesticides may have a far
Mexico ports. The ships had last ports of call in
greater and longer impact on disease patterns in a
Brazil, Colombia, and Chile (35). V. cholerae O1,
region than the life of a person. Deforestation, build-
serotype Inaba, biotype El Tor, indistinguishable
ing of dams, and opening of roads into previously
from the Latin American epidemic strain, was found
inaccessible areas have all been associated with
in oysters and oyster-eating fish from closed oyster
population movements and changes in distribution
beds in Mobile Bay, Alabama (36). V. cholerae O139
and frequency of a variety of infections in humans
has spread along waterways in Asia, although the
(such as malaria, schistosomiasis, Rift Valley fever,
people carried on the boats doubtless played a role
and sexually transmitted diseases).
(37,38).
Increasingly the vehicle of transportation is the
3. Aedes albopictus was introduced into the
site or even the source of outbreaks. During travel,
United States inside used tires shipped from Asia
people from diverse origins are enclosed in close
(39,40). The mosquito’s introduction causes concern
proximity for a hours or days and then discharged to
because it is an aggressive biter, survives in both
move on to many distant places. These temporary
forest and suburban habitats, and appears to be a
new habitats, jumbo jets or huge ocean liners, can
competent vector for several human pathogens. It
be the sites for dissemination of the microbes (as
has been associated with epidemic dengue fever
happens, for example with Legionella pneumophila
transmission in Asia and is a competent laboratory
infections (29), foodborne infections, and cholera) or
vector of La Crosse, yellow fever, and other viruses
provide a milieu for person-to-person transmission
(41). In Florida, 14 strains of eastern equine en-
(influenza, tuberculosis (30,31)).
cephalitis virus have been isolated from A. albopic-
tus (42). The mosquito is now established in at least
Shipping and Commerce 21 of the contiguous states in United States and in
The biomass of humans constitutes only a frac- Hawaii.
tion of the matter moved about the earth. Humans 4. The African anopheles mosquitoes arrived in
carry and send a huge volume of plants, animals and Brazil in about 1929. This vector could breed under
other materials all over the face of the globe. Much conditions other New World mosquitoes could not.
of this movement results from the planned transport Although the malaria parasite was already found in
of goods from one place to another, but some is an Brazil, this new vector expanded the range of trans-
unintended consequence of shipping and travel. All mission. An estimated 20,000 persons died of ma-
has an impact on the juxtaposition of various species laria before the introduced anopheles mosquitoes
in different ecosystems. “Hitchhikers” include all were eliminated.
manner of biologic life, both microscopic and macro- 5. It has been repeatedly demonstrated that mos-
scopic. Animals can carry potential human patho- quitoes are present—and survive—on international
43
Vol. 1, No. 2 — April-June 1995 Emerging Infectious Diseases
Perspective
flights. In random searches of airplanes in London, As noted already, factors that can influence recep-
mosquitoes were found on 12 of 67 airplanes from tivity include climate and environmental conditions,
tropical countries (43). Arthropods can survive even sanitation, socioeconomic conditions (50), behavior,
more extreme environments. In one study, mosqui- nutrition, and genetics. V. cholerae persists in an
toes, house flies, and beetles placed in wheel bays of aquatic reservoir off the Gulf Coast of the United
Boeing 747B aircraft survived flights of 6-9 hours States, yet epidemic cholera has not been a problem
with external temperatures of -42° C (43). Airplanes in the United States. Where poverty and poor sani-
have also carried infective mosquitoes that caused tation prevail, the presence of V. cholerae can be a
human infection outside malaria-endemic areas (in source of endemic disease and periodic epidemics.
Europe, for example). Disease emergence is often complex. An outbreak
6. Vehicles can transport vectors over land. of malaria in San Diego, California, occurred when
Glossina palpalis, a vector for African trypanosomi- parasitemic migrant workers were employed in an
asis (sleeping sickness), can fly up to 21 km but can area where mosquitoes capable of transmitting ma-
be transported much longer distances on animals laria had access to the workers and to a susceptible
and in land vehicles. human population (51). Many conditions had to be
7. Seven persons in Marburg, Germany, died met to allow transmission.
after handling blood and tissues from African green Migration may introduce parasites into an area
monkeys from Uganda. The tissues contained an where a different intermediate host or vector could
organism later named Marburg virus (44). change the incidence of disease. Cycling through a
8. Exotic animals transported from their usual different host can lead to different transmission
habitats are clustered in zoos; others are used in rates, different infectivity, and even different clinical
research laboratories where they have occasionally expression. A parasite may be more successful in a
caused severe disease in humans. Two examples are new site because of a larger susceptible population
B virus from primates (45) and hemorrhagic fever or the absence of predators.
with renal syndrome from rodents (46).
9. The world trade and globalization of organs,
Confluence of Events
tissues, blood, and blood products is growing. Re-
Massive global travel is taking place simultane-
searchers are considering animals as sources for
ously with many other processes that favor the
tissues and organs for transplantation (47).
emergence of disease. For example, the human
10. Plants may not directly cause human dis-
population is more vulnerable because of aging, im-
ease. But they can alter an ecosystem and facilitate
munosuppression from medical treatment and dis-
the breeding of a vector for human disease. This can
ease (such as AIDS), the presence of prostheses (e.g.,
also displace traditional crops that provide essential
artificial heart valves and joints), exposure to chemi-
nutrition. Vertical transmission of plant pathogens
cals and environmental pollutants that may act
(and spread of plant diseases) can result from seed
synergistically with microbes to increase the risk of
movement (48). Carriage of seeds into new areas can
diseases, increased poverty, crowding and stress,
introduce plant pathogens.
and increased exposure to UV radiation. Technologic
11. Migration and altered environments have in-
changes, while providing many benefits, can also
creased the so-called weedy species. These species
promote disease dissemination. Resistance of mi-
migrate easily and have high rates of reproduction.
crobes and insects to antimicrobial drugs and pesti-
If they lack local predators, they can displace other
cides interferes with the control of infections and
species and often upset local ecology.
allows transmission to continue. Changes in land
use can alter the presence and abundance of vectors
Introduction of Species into New Areas and intermediate hosts.
Microbes are enormously resilient and adaptable.
Introducing species into new geographic areas is
They have short life spans, which allow rapid genetic
not new, but the current volume and frequency of
change. Humans, by comparison, are slow to change
introductions are unprecedented. A pathogen’s sur-
genetically but can change their behavior. People
vival and spread in a new environment are deter-
move and construct barriers to prevent contact with
mined by its basic reproductive rate, which is the
microparasites, macroparasites, and the extremes of
average number of successful offspring a parasite
the environment. Technology fosters a perception of
can produce (49). To invade and establish itself in a
human invincibility but actually creates new vulner-
host population, a parasitic species must have a
abilities, as it enables us to go deeper, higher, and
basic reproductive rate exceeding one (49). The sim-
into more remote and hostile environments. Studies
plicity of this statement belies the complexity of
show that no place on earth is devoid of microbes.
circumstances that influence invasion and persist-
Their range and resiliency are truly phenomenal.
ence. These circumstances encompass biological, so-
Only a fraction of the existing microbes have been
cial, and environmental factors.
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