Emerging Infectious Diseases
EMERGING INFECTIOUS DISEASES
The term "emerging infection," first widely used in the early 1990s, refers to newly identified and previously unknown infectious agents that cause public health problems either locally or internationally. Their impact, in terms of economic repercussions, goes well beyond the immediate costs to health systems. They may impede trade or travel or cause disproportionate alarm, especially if rumors of intentional use become widespread. During the last three decades of the twentieth century, over thirty emerging infections were identified in humans. They range from the Ebola, Marburg, and Nipah viruses to the more common hepatitis C virus and HIV (human immunodeficiency virus). Emergence of infectious agents has occurred throughout the world, causing many unexpected outbreaks (see Figure 1). Contributing factors for these outbreaks include widening development gaps, collapse of public health infrastructures, poverty, urbanization, civil strife, environmental change and degradation, and globalization of travel and trade.
NEWLY IDENTIFIED INFECTIOUS AGENTS
In 1976, the Ebola virus was identified for the first time during simultaneous outbreaks in Zaire (now the Democratic Republic of the Congo) and southern Sudan. It has since come to symbolize emerging diseases and their potential impact on populations without previous immunological experience. Ebola has caused at least four severe epidemics and numerous smaller outbreaks. In an outbreak that took place in Zaire in 1995 there were 315 cases, with a case-fatality rate of 77 percent. Approximately one-third of those infected were health care workers who came into contact with the blood or body fluids of infected patients. In a smaller outbreak in Gabon two years later, 61 cases occurred, with a case-fatality rate of 78 percent. During a recent outbreak, which was reported in Uganda in October 2000, nearly 425 cases and over 224 deaths had been reported by the end of the epidemic in February 2001.
The Marburg virus, a member of the same family of filoviruses as Ebola, was first recognized in 1967 when laboratory workers in Germany were infected by handling monkeys imported from Uganda. Since then, there have been reports of sporadic cases in 1975, 1980, and 1987. A recent outbreak took place in 1999 among gold miners in the Democratic Republic of the Congo.
In 1996, the occurrence in the United Kingdom of 10 cases of an apparently new variant of Creutzfeldt-Jakob disease (vCJD) was linked to an epidemic of bovine spongiform encephalopathy (BSE), also known as mad cow disease, among cattle. By September 2000, at least 84 people in the United Kingdom, 1 in Ireland, and 3 in France had contracted vCJD. Accurate prediction of the future number of vCJD cases is not possible, but the possibility of a significant and perhaps geographically diverse epidemic occurring over the next two decades cannot be excluded. The economic impact of this unexpected disease is being felt throughout the agricultural sector of all European countries, and costs continue to escalate.
Since first being recognized as a human pathogen in 1982, enterohaemorrhagic Escherichia coli has gained increasing importance as a human pathogen. The best known serotype, E. coli 0157:H7, has been responsible for recent large food-borne outbreaks in Japan, Scotland, and the United States, placing heavy demands on medical and public health response systems, while also causing major political concern about food safety.
In 1997, the World Health Organization global surveillance system for human influenza virus (FluNet) received reports of an isolated and fatal influenza infection in a three-year-old child in Hong Kong. The virus was identified as influenza A (H5N1), and was associated with epidemics of avian influenza with high fatality rates in live poultry markets. By the end of 1997, a total of 18 human infections had been confirmed, 6 of which were fatal. Thanks to prompt action on the part of public health authorities, the outbreak did not spread further. In 1999, FluNet received reports of another new influenza virus, A (H9N2), isolated from 2 human cases in Hong Kong, but no further spread is known to have occurred.
In the United States, Legionella infection was first identified in 1976 in an outbreak of fatal respiratory illness among war veterans. Legionellosis (Legionnaire's disease) is now known to occur worldwide and is a threat to travelers and others exposed to poorly maintained airconditioning systems. Cases of the disease contracted by European residents anywhere in the world are tracked by a specialized network. This has revealed that the number of cases reported in 1999 in Europe was the highest ever, with 2,136 cases in European residents, almost 700 more than were reported in 1998. One outbreak in Belgium and one in the Netherlands, both linked to trade shows, collectively gave rise to about 300 cases. At both trade fairs, whirlpool spas were on display and people became infected by breathing in contaminated aerosols after walking past them.
Although Rift Valley fever (RVF) had already been recognized in 1930 as the agent for a zoonotic disease in Kenya, outbreaks outside sub-Saharan Africa were first described in Egypt from 1977 to 1978 and in 1993. A large outbreak occurred in East Africa in 1998, and the disease has now extended its reach outside Africa. RVF virus transmission on the Arabian peninsula was documented for the first time in 2000, during a vast outbreak which has encompassed the border area between Saudi Arabia and Yemen. The disease, which affects both animals and humans, causes severe
Hepatitis C, first identified in 1989, had already spread worldwide with an estimated global prevalence of at least 3 percent in the mid-1990s. Meanwhile Hepatitis B, identified several decades earlier, continues an upward trend in many countries, reaching a prevalence exceeding 90 percent in populations at high risk, in countries ranging from the tropics to Eastern Europe.
Other newly identified viruses include Sin Nombre, which caused an outbreak of hantavirus pulmonary syndrome in the United States in 1993 (50 cases with a case-fatality rate over 75%); Hendra virus, which affects humans and horses, first identified in Australia in 1994; and Nipah virus (causing febrile encephalitis), first identified in 1999 in Malaysia, where it caused a severe epidemic in those that had close contact with pigs, leading to grave economic losses owing to the destruction of around 900,000 pigs.
One of the most important emerging infection is HIV. First identified in the early 1980s, it has rapidly spread worldwide, affecting over 36 million people by the end of 2000. Because working-age adults are the group most directly affected, it has become a significant impediment to economic development, especially in sub-Saharan Africa.
THE PUBLIC HEALTH THREAT
Emerging infectious diseases pose an international threat that can be countered through well-coordinated global surveillance and response. Whereas traditional approaches to containing outbreaks are defensive—trying to secure borders from the entry of infectious diseases—modern solutions are built on a combination of early warning, surveillance systems, speedy communications, and information sharing through networks to facilitate action.
World Health Organization (1999). Removing Obstacles to Healthy Development: Report on Infectious Diseases. Geneva: Author.
—— (2000). Communicable Diseases 2000: Highlights of Activities in 1999 and Major Challenges for the Future. Geneva: Author.