Definition
The increased ease and frequency of international trade, travel, and
migration have renewed concerns about the spread of infectious diseases.
Infectious diseases such as cholera, yellow fever,
and communicable meningococcal illnesses reemerged over the last quarter of the
twentieth century, primarily because of the speed of international travel.
Twenty-first century outbreaks of H1N1 influenza and severe acute respiratory syndrome (SARS) have demonstrated the pace at which epidemics can become globalized and the potential global economic impact. Newly identified infectious diseases must also be contained to prevent them from becoming pandemics. According to the World Health Organization (WHO), one new infectious disease is being identified each year (eleven hundred epidemics occurred between 2002 and 2007 alone). Control of infectious diseases and prevention of pandemics require diligent monitoring of disease patterns, adherence to international regulations, reporting to world health authorities, and international response coordination when potential epidemics arise.
Background
The spread of infectious diseases through travel and migration has presented a problem for public health as long as humans have been mobile. The best known of these diseases is perhaps the plague, caused by the bacterium Yersinia pestis. Outbreaks of plague occurred numerous times in history, and already in the fourteenth century it was recognized as an "imported" disease. Although the germ theory of disease and modes of disease transmission were as yet unknown, it was accepted that the disease was somehow transported by travelers and in goods arriving from plague-infected areas. This realization led to the establishment of a forty-day isolation period (or quarantine) to identify infected persons, which, however, did little to stop the spread of plague throughout Europe. In time, advances in medical knowledge, improvements in living conditions, and the development of vaccination programs helped reduce or eliminate some infectious diseases.
In modern times controlling the spread of infectious diseases has become much more complicated, and the speed and frequency of worldwide travel has made communication of potential disease events a high priority. Diseases considered regionally controlled or eradicated are being reintroduced into these areas; new infectious diseases such as viral hemorrhagic fevers have the potential to travel around the world in less than twenty-four hours.
Migration
The extent of recent human migration has been a significant factor in the global spread of communicable diseases. In 2015 the United Nations estimated that there were 244 million international migrants, while in 2013 Gallup estimated there were 381 million adult within-country migrants and the number continued to grow. In addition, there are an unknown number of undocumented migrants throughout the world. Migrant workers, refugees, and nomadic groups can transport endemic diseases as they travel, often exacerbated by the poor living conditions commonly afforded these groups.
Migration not only has spread infectious diseases (such as dengue fever) into areas where they were not previously seen but also has reintroduced diseases (such as tuberculosis) into areas where they had previously been controlled. Additionally, permanent migrants change the complexion of infectious disease patterns in the host country. Migration patterns are typically from developing nations to more prosperous areas, and residents of these poor regions generally lack adequate health care, nutrition, and sanitation, making them more susceptible to contracting infectious diseases and harboring pathogenic microorganisms.
Migrants who enter host countries through official means may be subjected to medical histories and physical screening to identify infectious and sexually transmitted diseases. Their movements may be tracked through travel documents and passports. Immigrants who have entered a country by unofficial (or illegal) means have not undergone such screening and may therefore be principal vectors for infectious diseases. Illegal (undocumented) immigrants are of particular concern because they are not screened, may fear seeking treatment for any diseases they already have, and may have contracted additional pathogens during their journey into the host country.
Chagas’ disease, caused by Trypanosoma cruzi, provides an example of an infectious disease that has been globally spread to nonendemic areas. This organism is mainly transmitted by insects; however, it can also be spread through blood transfusions, organ transplants, contaminated food, and vertical transmission. Historically, Chagas’ disease has been endemic to Mexico, Central America, and South America. After the year 2000, however, it has been identified and documented in the United States, Canada, Europe, Australia, Japan, and regions of South America not previously affected. An estimated 2.9 percent and 2 percent, respectively, of documented Latin American immigrants to Europe and the United States were infected with T. cruzi, exemplifying a tropical disease that is now a global health concern.
The rate of tuberculosis (caused by Mycobacterium tuberculosis) cases among residents in the United Kingdom who are native born has been consistently controlled at a low rate. The number of tuberculosis cases among immigrants living in the United Kingdom, however, is high (from 2000 to 2008) because of migration. While the tuberculosis rate remained consistent at approximately 4 cases per 100,000 persons for the British-born population during this interval, the rate for migrants in the United Kingdom ranged from 80 to 102 cases per 100,000 persons (approximately 72 percent of total cases). A similar, though not as drastic, pattern was observed in the United States in 2008, in which the tuberculosis rates were 2 and 20 cases per 100,000 persons, respectively, for U.S.-born and foreign-born residents.
Although there are real disease risks caused by migration, it is also important to recognize that the majority of immigrants are not substantial public health threats. Despite the claims of some politicians that immigrants, especially undocumented ones, are a major cause of infectious disease in the United States, there is no data proving such allegations. The US Centers for Disease Control and Prevention (CDC), which monitors disease related to border crossings, has reported very few outbreaks directly connected to immigrants. Most health experts agree that cross-border travel, illegal or otherwise, shows no evidence of causing epidemics in the United States. Furthermore, the very nature of globalization means that immigrants are only one aspect of the increasingly interconnected global community, and other factors—such as food shipment or international travel—are just as risky in terms of infectious disease as immigration.
Global Travel
Air travel has become the fastest and most efficient means of infectious
disease movement across the globe. In 2009, 684 million passengers traveled
internationally by air. A trip from New York City to either Beijing, China, or
Mumbai, India, can be completed in about twenty-four hours. This is less time than
the incubation periods of many communicable diseases. Many infectious diseases of
concern are spread through airborne pathogens (such as influenza, SARS-associated
coronavirus, and M. tuberculosis); therefore, they are ideally
suited for transmission in a contained aircraft cabin.
The SARS epidemic provides a model of international airborne pathogen transmission within an aircraft cabin. Although it did not result in a catastrophic pandemic, this disease demonstrated how an outbreak can quickly spread to multiple countries (in this case thirty-seven) by international travelers. The disease itself has an incubation period of approximately seven days before symptoms appear, which allows it to unknowingly be transported anywhere around the world. The index case was a doctor from Guandong, China, who flew to Hong Kong in early 2003. The doctor had previously treated patients in Guandong with a respiratory illness that was, at the time, unidentified. While staying in Hong Kong, the doctor transmitted the disease to several other guests staying at the same hotel. One international business traveler subsequently transmitted the disease to twenty-two other passengers during air travel. All these transmissions occurred within days and before the onset of any symptoms of the disease in the persons who were infected.
Also of concern are diseases considered to be controlled or eradicated in a region that are imported back into the area. Measles was considered to be eliminated from the United States in 2000 because of a vaccination program that began in the 1960s; however, since this time, there have been periodic outbreaks of measles because of international travel. In the first six months of 2008, 131 cases of measles were reported to the CDC, which was about twice the number reported on average per year from 2000 to 2007. Most (89 percent) of the cases during the outbreak in 2008 could be linked to international outbreaks.
Measles is still very common in China, Southeast Asia, and, to a lesser extent, Europe, where occasional outbreaks still occur. Travel to these areas or contact with foreign visitors accounted for only 17 of the 131 US cases; association with these infected persons within the United States accounted for an additional 99 cases. The source of transmission for the remainder was unknown.
Most of the persons who contracted measles during this outbreak (91 percent) were either unvaccinated or had unknown vaccination statuses. In addition, 80 percent were less than twenty years of age, indicating that an increasing number of children were not being vaccinated because of religious objection or because of home schooling. (Children who are schooled at home are exempt from vaccination requirements.) Measles is highly contagious. It quickly resurfaces in the United States when vaccination rates decline because it is still so prevalent in the rest of the world. The 2008 measles outbreak, with the highest rate of infection in the United States since 1996, illustrates the potential reemergence of an infectious disease that is considered controlled, particularly in areas with low vaccination rates.
Infectious Disease Control
The spread of infectious diseases has been recognized as a threat to global health for several hundred years. Various health agencies have recognized this and endeavored to engage international cooperation. The first cooperative effort to control cholera, plague, and yellow fever began in Europe in 1851, when the first International Sanitary Conference (ISC) was convened. By the mid-nineteenth century, the international shipping trade and the growth of transcontinental railroads had successfully swept the cholera epidemic throughout Europe. National regulations and quarantine were inconsistent and ineffective; cooperation among nations was the only method of monitoring and controlling disease propagation with minimal interference in international trade. Over the next century, these efforts resulted in international, legally binding regulations for international travel and transport. The ISC issued a series of regulations and gave rise to four international health agencies: the Pan American Sanitary Bureau (1902), the Office International de l’Hygiène Publique (1907), the Health Organisation of the League of Nations (1923), and WHO (1948). By 1951, these agencies had converged into WHO and issued the International Sanitary Regulations by the World Health Assembly. These international regulations expanded on the treaties issued by the ISCs.
Since the early 1950s, WHO has played a pivotal role in monitoring and controlling the spread of infectious diseases around the world. The goal of the International Sanitary Regulations was to control smallpox, typhoid fever, relapsing fever, yellow fever, cholera, and the plague, establishing standards for reporting international travelers and goods possibly carrying infectious diseases. WHO subsequently revised the regulations to include the eradication of smallpox and to focus on yellow fever, cholera, and plague. These regulations were replaced by the International Health Regulations in 2005 (revised 2007), which took a broader, more fundamental approach to protecting public health. The focal points are on preventing the global spread of infectious diseases with minimal interference in international travel and trade, and on stringent reporting policies regarding any health situation that poses a threat to public health. WHO also has programs to help member states track and respond to outbreaks that have the potential of developing into large-scale epidemics.
Developed countries, especially those successful in the eradication and control of various infectious diseases, have attempted to institute health regulations on international travelers, migrants, and imports. In response to health threats posed by human migration, some receiving countries have implemented restrictions on incoming migrants from areas with endemic infectious diseases of concern. These may include health screening, proof of vaccination, and potentially, quarantine. However, border control policies are rarely effective because of the sheer volume of persons who cross international borders over any period and because of the undetermined number of undocumented immigrants around the world. Even if it were possible to examine each person at a border crossing, diseases in the incubation stage could not be detected.
Noncatastrophic epidemics such as the SARS outbreak in early 2003 provide health care agencies with models of the global spread of infectious diseases. Using these models and other techniques of epidemiology, the pattern of disease transmission can be predicted, allowing for the design of action plans in the event of a future infectious disease outbreak. SARS was the first example in the twenty-first century of an infectious disease outbreak that was tracked and contained by disease control measures. At the onset of the epidemic, little was known about the virus except that it could be spread by airborne transmission, that there was a fatality rate of about 10 percent, and that there were no vaccines or curative agents available. The release of public information and a global disease alert issued by WHO allowed potential travelers to be aware of the threat and to change their plans to avoid air travel or visiting areas in which SARS was identified. These actions had a substantial negative economic impact, but contact with infected persons could be minimized. Closely followed hygienic measures prevented the extensive spread of the disease among health care professionals. Furthermore, establishing the infectivity and incubation period of the virus facilitated tracking those persons who were infected and those who had been exposed.
In preparation for the Olympic Winter Games in Vancouver, Canada, in 2010, Canadian health authorities used known disease patterns from other mass human gatherings, such as the Hajj and the G-20 summit, to create a plan to identify and manage any potential infectious disease outbreaks. Before the Games, Canada had developed a Global Public Health Intelligence Network, a surveillance system that enabled them to acquire real-time information on potential disease threats (such as mumps, measles, and Norwalk-like virus) through the Internet. In conjunction with projected air-traffic patterns, health authorities developed models of potential patterns of infectious disease distribution. Had any infections be identified, alerts could have been issued to officials at the Games. Health officials in the native countries of visitors and athletes would also have been notified to prevent travelers from bringing any infectious diseases back from the Games to their home countries.
Impact
Migrants coming from poor countries typically require more health and social services and, therefore, can place a tremendous burden on health services in host countries. In some receiving countries, migrants make up a majority of the population; in the United Arab Emirates, the country that receives more migrants than any other in the world, migrants are 71 percent of the population. WHO recognizes that migration is not a temporary phenomenon, and that high migration rates have a significant impact on the nature of diseases and health care systems of the host countries. Therefore, it urges receiving countries to make migrant health a high priority, stressing that the health of the native population is affected by the health of migrants and their children. However, migrants have specific medical, genetic, and cultural health care requirements, and the cost of providing this migrant-specific care cannot be estimated.
In addition to the impact of infectious diseases on the cost of health services, they also have a surprising impact on the global economy. This is particularly true for infections in which the modes of transmission are not definitively known. In the case of SARS, which was a relatively mild outbreak, the cost to the global economy was estimated at $30 billion to $100 billion. Most of this cost was borne by the travel, tourism, and other industries. Most of this deficit was absorbed by countries that continue to rely primarily on travel and tourism for their income. On a microlevel, however, persons lost their jobs and their incomes. The long-term consequences of global disease outbreaks are often continued poverty and substandard living conditions, creating ideal conditions for further outbreaks.
To prevent a catastrophic pandemic, WHO has established regulations for the monitoring, reporting, and rapid response to any infectious disease events. The fundamental goal for WHO and other global health agencies is to provide access to health care for all, with the expectation that improved health in poor areas will result in less disease transmission and overall positive global health outcomes. The human population continues to grow, directing more focus on preventing the human-to-human transmission of communicable diseases.
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