The traveller and emerging infections: sentinel, courier, transmitter

Summary, 1S Introduction, 1S Role of movement of populations in infectious diseases, 2S Historical events, 2S Current volume and range of movement, 2S Diversity of pathogens and mechanisms of transmission, 2S Trade, 3S Receptivity of places and populations, 3S Population size, density and vulnerability, 3S Location and physicochemical characteristics, 3S Competent vectors and intermediate hosts, 4S Socioeconomic, political factors; infrastructure and access to health care, 4S Populations and places affected, 4S Travel as a circuit, 4S En route transmission, 4S Process of travel; cruise ship as a new habitat, 5S Travellers, 6S Travellers as sentinels, 6S Travellers as couriers; key role of laboratory, 6S Travellers as transmitters/disseminators, 7S Time of consequences, 7S Specific disease examples, 7S Neisseria meningitidis, W135, 7S Dengue fever, 8S Microbial genetic material; resistance and virulence factors, 8S Conclusions, 8S Acknowledgements, 9S References, 9S The movement of populations shapes the patterns and distribution of infectious diseases globally. The consequences of travel are seen in the traveller and in places and populations visited and may persist long after travel. The traveller can be seen as an interactive biological unit who picks up, processes, carries and drops off microbial genetic material. A traveller can introduce potential pathogens in the absence of signs or symptoms of illness. Travellers can serve as a sentinel population; study of them can provide insights into the presence and level of risk of transmission of infections in other geographical regions. Travellers can also be seen as couriers who inadvertently ferry pathogens and microbial genetic material to regions where researchers can carry out detailed analyses that can help to map the location and movement of strains, genotypes and resistance patterns. The laboratory plays a key role in the identification and characterization of pathogens, which can inform management of individual patients and the public health response. The connectedness and mobility in the world today facilitate the emergence of infectious diseases in humans and also in animals and plants. Many traditional barriers have been breached by travel, roads and technology. Population size and density favour spread of many infections. The rapid generation time of microbes and their capacity to adapt to changes in the physico-chemical and immunological environment will pose continuing challenges. Travellers are interactive biological units that pick up, process and drop off microbial genetic material at different times and in different places. Travellers alter places and populations during and after travel. Changes in the biota may be sustained if a traveller carries a pathogen into a new environment where it is transmitted, and then survives, replicates, and persists in a new geographical area. Migrating humans and trade play a key role in the global dispersal of human pathogens and of microbial genetic material (Wilson 1995a). This paper will review the mechanisms through which the movement of populations affects the distribution and spread of infectious diseases and the factors that influence the vulnerability of places and populations to introductions of infections. The examples of dengue fever, influenza, meningococcus and multidrug-resistant pathogens will be used to illustrate key points. History books are full of vivid examples of the role of traders, travellers, pilgrims, missionaries, explorers, warriors and other migrating individuals or groups who introduced human pathogens into susceptible populations, often with devastating consequences. The New World was hard hit when European explorers brought with them measles, smallpox and tuberculosis, which caused high mortality in local non-immune populations (Crosby 1972). The major culprits were pathogens that were carried by humans and spread easily from person to person. Exploration of many parts of sub-Saharan Africa by Europeans was associated with high death rates from infections, such as malaria, yellow fever, and sleeping sickness – vector-borne infections that, in a sense, were embedded the landscape. In a book that explores the excess mortality or ‘relocation costs’ among European soldiers in the tropics between 1815 and 1914, Philip Curtin found that the death rates from disease were at least twice as high in soldiers during time spent in the tropics relative to that of soldiers who remained in temperate areas (Curtin 1989). Many of these vector-borne diseases persist today and pose a risk to local residents and visitors. Throughout most of history many more soldiers died from infectious diseases than directly from weapons of war. Population movements and displacements associated with war and violent conflicts also lead to outbreaks of infectious diseases that involve civilian populations. Infections, such as cholera, typhoid fever, and measles, in refugee and displaced populations are often a consequence of crowding, poor nutrition, lack of basic sanitation, and disruption of basic medical services, such as immunization programmes, during periods of social and political upheaval. Travellers form an important bridge to diverse geographical areas and populations. Infections that can be spread from person to person can be readily carried by travellers to any part of the globe. AIDS is a modern day example of an infection that, in a few short decades, has been carried to all parts of the world. The ease and volume of modern human traffic has facilitated its spread, assisted in many areas by sexual practices, sexual tourism, shared injection devices and the use of contaminated instruments or blood in the medical setting. An estimated 60 million persons have been infected since the mid-1980s and 40 million persons are estimated to be living with HIV today. The speed, volume and reach of travel today are unprecedented. The world has more and faster links than at any time in history. Today it is possible to travel to virtually any part of the world in a time period that is less than the incubation for many infectious diseases. The World Tourism Organization (WTO) estimated that world tourism grew by 7·4% in 2000 and the total number of international arrivals reached 699 million (World Tourism Organization 2001). More than 1·4 million persons cross-international borders on flights everyday. In the 1990s more than 5000 airports had regularly scheduled international flights, which means that dense urban centres and their surrounding periurban areas all over the world are linked. Of course, air travel is only one means of travel, as humans travel by boat, bus, train, car, truck and by foot. An estimated 400 million international travellers entered the US by air, land or ship in 1999. Travelling humans also carry or facilitate the transport of plants, animals, seeds and all matter of biological material. The movement of biota (including humans, animals and plants) impacts on the traveller and also on populations and places en route, at the destination, and where the traveller returns after travel. The traveller may be at risk of acquiring unfamiliar infections in a new environment, but the impact of travel encompasses a range of events that derive from microbes and other biota carried by the traveller, but also the traveller's behaviour, often based on cultural or societal traditions that may influence risk of exposure. The process of travel also changes the environment in ways that may make other populations more or less vulnerable to infections. The idea of emerging infections for many people conjures images of exotic or remote viruses that cause haemorrhagic fever, viruses such as Ebola, Marburg or Lassa fever. Although these remain a serious concern in some regions of the world, agents causing emerging microbial threats include common, familiar infections, such as influenza and tuberculosis. They come from all classes of organisms – viruses, bacteria, protozoa, fungi and helminths. They include infections spread from person to person as well as vector-borne infections and those acquired from contact with animals, soil or water. Although the focus in this paper is on emerging infections in humans, emerging infections also affect plants and animals, and these can have direct and indirect health and economic impacts on humans. Prominent examples include bovine spongiform encephalopathy (BSE), foot-and-mouth disease, avian influenza and West Nile virus infection. Among the agents causing emerging infections are microbes previously well characterized that have spread geographically (e.g. dengue and West Nile virus), increased in incidence (e.g. diphtheria, HIV and tuberculosis), changed in virulence (e.g. Neisseria meningitidis clones), or become more resistant to treatment (e.g. Mycobacterium tuberculosis, Streptococcus pneumoniae, Salmonella species, falciparum and vivax malaria, Staphylococcus aureus). For several established pathogens, the presence of HIV infection increases the risk of infection, alters the clinical presentation, or reduces the efficacy of standard treatment. Most of the infections perceived as being ‘new’ are not caused by novel organisms but by microbes long present but not well characterized or not known previously to cause human disease. Examples include Helicobacter pylori, ehrlichiae, cyclospora and Sin Nombre virus (one cause of hantavirus pulmonary syndrome). In some instances, humans on or changed a in ways that increased contact between humans and a This is of some of the viruses, such as the and the cause of haemorrhagic fever. New laboratory have it possible to at to of and that were causing infections in the US the disease was and the pathogen of of from and that from the that disease, was present in the US at least as as trade is dispersal that carries human pathogens, such as and intermediate the world. The is and may be and of from where it is and distribution also make it possible for a in one location and in to be to or For Salmonella outbreaks in in and the US in US and in were to from seeds from a are regularly carried the world by or other some can become established in a new environment and may spread to of these have the capacity to serve as vectors of human infections, to organisms from to to be an for the of to new regions. 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The physicochemical environment has direct and indirect on infectious diseases through its impact on pathogens, vectors and (Wilson changes can influence all of organisms and all of that a (e.g. or or intermediate may have a For be introduced into an are present that can part of the of the The presence of the of is but not for the transmission of the or with reach with other humans have contact with (e.g. in in for the to the human and and in the human The as more than a to carry a pathogen from one to The pathogen from one to and The period of time in the for the incubation is to for the of a in a is than the time period for the to in the transmission will not in that if a that is to that also affects time for from to it can affect the of species, such as Many areas are by vectors that are known to be to a infection may not The of presence of and and other factors may influence the contact between and humans, the of infection An or can be resistant to the and spread of a pathogen through several means – some are disease and are The may be of from infection (e.g. or immunization (e.g. measles, of and spread of many and infections (e.g. cholera, typhoid presence of and air and may vector-borne infections. 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Dengue is the most infection in the world and is and in and areas of dengue and humans. with one that but the individual for more disease if infected in the with a different The of disease, dengue haemorrhagic and dengue can be mortality can be by In a of dengue in of were in persons with a dengue infection. 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