Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Neglected tropical diseases wikipedia , lookup
Cross-species transmission wikipedia , lookup
Marburg virus disease wikipedia , lookup
Plasmodium falciparum wikipedia , lookup
Eradication of infectious diseases wikipedia , lookup
Leptospirosis wikipedia , lookup
Sarcocystis wikipedia , lookup
Aedes albopictus wikipedia , lookup
REVIEW ARTICLE Population Movements and Emerging Diseases Mary E. Wilson We are a world in motion. In 1996,2.5 billion people passed through airports. Travel today is unprecedented in its speed, volume, and reach. N o environment is too harsh and no destination too distant to discourage exploration. But much of the movement of populations today is unplanned and unwelcome, a consequence of environmental, economic, or political pressures. This brief paper will make four main points: individual by about one order of magnitude. Travel for the individual increased by a factor of more than 1000 during this time period. Figure 1 shows the exponential growth in daily travel range since 1800. Recent trends would bear this out.Air traffic volume has increased about 7% per year for the past 20 years.’ About 5000 airports have scheduled worldwide service. More than 500 million persons cross international borders on commercial flights each year. A greater range of travel means potential exposure to a larger variety of species, some of which may cause harm to us.’ Exploration and development of new lands may involve entry into new habitats and contact with microbes in soil and aninials not previously recognized as human pathogens. An example is the recent recognition of a form of hemorrhagic fever (Venezuelan hemorrhagic fever) caused by a newly characterized arenavirus (Guanarito ~ i r u s )When .~ we travel, we are both recipients and dispersers of microbes.We may complete a loop by picking~. up microbial flora in a new land and carrying it honie to friends and family, perhaps sharing it along the way with fellow travelers. A large outbreak of antibiotic-resistant shigellosis involving niore than 50% of the estimated 12,700 persons at a mass gathering in North Carolina was followed by the nationwide dissenlination of the organism and outbreaks in at least three other states.’ ’ Movement involves all biologic species; movement of nonhuman species is also relevant to hunian health. Migrating humans, while at risk for new infections, also aid in the global dispersal of infections. Human activities provide increased opportunities for genetic exchange among microbes and favor the selection and spread of resistant strains. Population growth, increasing urbanization, and (4) environmental change enhance the emergence and spread of infections. Travel continues to increase. In a book called The Rise and Fall oflnjrastructures, Grubler’ analyzed the average kilometers traveled daily per capita in France, starting with the year 1800 and continuing through the present. H e included horses, railways, buses, cars, and air travel, among others. What is notable is that each new transportation mode increased the average range of an Mary E. Wilson, MD:Assistant Professor, Departments of Population and International Health and Epidemiology, Harvard School of Public Health, Boston, Massachusetts; Assistant Clinical Professor, Harvard Medical School, Boston, Massachusetts; and Chief of Infectious Diseases and Director,Travel Resource Center, Mount Auburn Hospital, Cambridge, Massachusetts. lo.m 1.m This paper was presented in part at the Harvard School of Public Health 75th Anniversary Symposium, Gateway to World Health: New Science and Strategies in Public Health, 1997. IW I0 I Reprint requests: Mary E. Wilson, MD, Mount Auburn Hospital, 330 Mount Auburn Street, Cambridge, MA 02238, USA. Figure 1 Range covered (average km traveled daily) per capita in France by mode and total since 1800. Source: Grubler? JTravel Med 1997; 4:183-186. Reprinted with permission. 183 184 A century and a half ago,John Snow recognized the importance of travel and trade in the spread of cholera. H e noted:"Epidemics of cholera follow major routes of commerce. The disease always appears first at seaports when extending into islands or continents.""The role of travel and trade today persists and is expanding. We cannot talk about population movements without noting the massive population shifts that occur as a consequence of social and political unrest, economic and environmental pressures, and war.',' In the early 1990s, there were an estimated 20 million refugees and 30 million displaced persons.' Extreme weather events, such as floods, hurricanes, and droughts, displace persons from their homes. These semi to be occurring with increased frequency and have a growing impact as more people settle in coastal and vulnerable regions."' Population displacements are accompanied by conditions that favor disease emergence and spread.Witness the massive outbreaks of cholera, dysentery, measles, and other infectious diseases in refugee camps." In the late 1980s, hundreds of cases of visceral leishmaniasis were diagnosed in Khartoum among migrants to that area who had been displaced from their homeland by war and famine. Malnutrition and nomadic movement of populations into areas endemic for leishinaniasis were contributing factors in the outbreak." We are not the only species moving about the earth. When we travel, we carry our microbial flora in and on our bodies. HIV has spread throughout the world, transported by the human host.13We carry the microbes that cause tuberculosis, influenza, shigellosis, pneuniococcal pneumonia,14staphylococcal infections," and also microbial genetic material that through transfer, recombination, conjugation, reassortment and a variety of molecular maneuvers can confer virulence or resistance to other microbes." We also facilitate the dispersal of other species through shipping and commerce." We transport food, animals, insects, plants, seeds, and other materials about the globe on boats, planes, trucks, and trains.'>l8We have given wings to species that were previously confined to one geographic region. Our transport systems vastly amplie and extend the natural biologic movement of other species that has always occurred through migration, winds, ocean currents, and streams. Even if we do not travel, pathogens have multiple routes available to reach us.The globalization of markets nieans that fruits and vegetables harvested in one area may be consumed thousands of miles away." Alfalfa sprouts grown from contaminated seed sent to a Dutch shipper caused outbreaks of salmonella infections on two continents, in at least Arizona and Michigan in the United States and in Finland.'"The food chain has become very long.As it stretches around the globe, we have added new J o u r n a l o f T r a v e l M e d i c i n e , V o l u m e 4, N u m b e r 4 links.Analysis of another outbreak of salmonellosis identified a complex route that included wild animals, farm animals, animal feed, slaughterhouses, processing plants, retail outlets, and finally the human consumer.21 Old, familiar pathogens can enierge to cause new problems if they enter new populations, become resistant to drugs used to treat them, or if virulent clones emerge. Increasingly, Streptococcus pneumoniae is becoming resistant to penicillin and other commonly available antimicrobials. A group C, ET-15 strain of Neisseria rneningitidis, which was more virulent than were previous strains, spread in Canada and was associated with an increased case fatality rate and a higher proportion of cases in persons over the age of five." Commercial movement of fruits and vegetables redistributes resistance factors along with the microbes. In addition, antibiotics used in aquafarnling are the same ones used to treat human infections, and antibiotics used in farmed fish reach wild fish in nearby ponds. In one study,74-100% ofwild fish in close proximity to treated ponds had residues of quinolones." Bacteria have an impressive array of ways to acquire resistance.Many genes that determine resistance predate the use of antimicrobials. It now appears clear that horizontal as well as vertical transfer of resistance genes is common in the niicrobial kingdom. Bacteria can take up foreign DNA and incorporate it into their chromosomes. Resistance to multiple antibiotics can be transferred together. Bacteria can spread resistance to unrelated bacteria. Today we are providing bacteria more opportunities for genetic exchange through access to more populations; at the same time, broad use of antimicrobials applies selective pressure that favors survival of resistant organisms." Molecular markers make it possible to trace the spread of specific microbes. Pilgrims carried an epidemic strain of group A Neisseria meningitidis from southern Asia to Mecca in 1987.Other pilgrims who became colonized with the epidenlic strain introduced it into subSaharan Africa, where it caused a wave of epidemics in 1988 and 1989.'4 Multidrug-resistant strains of Streptococcus pneumoniac were introduced into Iceland.2s Many examples exist of the global spread of multidrug-resistant strains o r virulent strains of common human pathogens. Using molecular techniques coupled with epidemiologic data, investigators were able to show the emergence of multidrug-resistant tuberculosis clones in NewYork, and their dissemination in NewYork City and to at least four additional cities in the United States." Movement of insect species can also affect hunian health. In March 1930 in Brazil an entomologist on a Sunday stroll came upon Anopheles garnbiae larvae, to his great surprise." The usual habitat of this mosquito was Afiica.The postulated route of entry into South America Wilson, Population Movements and Emerging Diseases was on boats that made mail runs between Dakar, Senegal, and Natal, Brazil, covering the 3300 km in less than 100 hours. Over the ensuing years, the mosquito spread along the coastal region and moved inland, often traveling on river boats. In 1938 and 1939 there were devastating outbreaks of malaria that caused more than 20,000 deaths. Although malaria already existed in the area, the local mosquitoes were not efficient vectors. In contrast, the new mosquito, Anopheles gambiae, lived in close proximity to humans, entered houses, sought human blood, and was an efficient biter. In this case the simple introduction of a new mosquito vector caused severe problems. Fortunately, an intensive eradication campaign was effective." This event in Brazil is not just a historic anomaly. We regularly transport vectors around the globe. Aedes albopictcrs was introduced into the United States in used tires shipped from Asia and has since spread to at least 21 contiguous states and H a ~ a i i . ~This ~ - ~ mosquito " transmits dengue in Asia and is a competent laboratory vector for a number of human pathogens, including yellow fever, La Crosse, and other viruses. Insects are hardy creatures.When mosquitoes, houseflies, and beetles in special cages were placed in wheel bays of 747 aircraft and carried along on flights lasting up to 7 hours, they survived despite outside temperatures as low as -52°C.Temperatures in the wheel bays ranged from 8-25OC. More than 99% of the beetles, 84% of the mosquitoes, and 93% of the flies survived the trip.3' We occasionally see human disease as a direct result of these mosquito flights, so-called airport malaria: cases of malaria near airports in temperate regions, presumably from jetsetting mosquitoes that survived long enough in the new environment to take at least one blood meal? Introductions of species, whether microbes, plants, or animals, into new regions can have profound effects on local ecosystems.What is changing is the frequency with which invasions are occurring. Since 3 960, more than 40 new species have appeared in the Great Lakes; since 1970, more than 50 have appeared in San Francisco Bay? Most people think of a boat as a form of transportation, but it is also a floating biological island. In addition to passengers, crew, and cargo, ships transport biologic life around the globe in ballast, on the hull and in ship chests.Today 80% of the world trade volume is transported by ship. The increasing speed of travel increases the probability of the survival of species carried by sea. Ballast water, necessary to maintain the stability of ships, is taken on in one or multiple sites, transported, and discharged at another time and place. The volume is huge. In a large tanker, for example, the volume of ballast may exceed 200,000 cubic meters. It is a rich biological soup, containing viruses,bacteria, fungi 185 and basically all plants and animals less than 1 cm in size that happen to be adjacent to the boat at the time ballast water is taken in. It has been estimated that ballast water transports more than 3000 species around the world each day." In Coos Bay, Oregon, researchers studied ballast water from 159 cargo ships that had come from 25 Japanese ports.They were able to identify a minimum of 367 different t a ~ a ? ~ We can also note potential threats to human health related to release ofballast water. In studies ofballast and bilge of cargo ships in the U.S. Gulf of Mexico, researchers were able to identify Vibrio cholerae identical to the strains causing epidemic disease in Latin A m e r i ~ a . ~ ' In most instances, the emergence of an infection involves multiple factors often interacting in complex ways. For example, dengue fever, a viral infection spread by mosquitoes, has exploded in Latin America in the 1990~.~~Traveling humans have introduced the virus, but other conditions have helped it flourish. Aedes aegypti, an effective vector, has reinfested many areas.Widespread use of nonbiodegradable plastic containers provides good breeding conditions for the mosquito. Increasing urbanization, especially in tropical zones, means that more urban areas will reach the critical population size, perhaps somewhere between 150,000 and 1 million, to permit endemic transmission and to increase the risk of the severe forms of infection: dengue hemorrhagic fever and dengue shock ~yndrome.~' Recent studies suggest other reasons for concern. Over the past two centuries the number of dengue lineages has been increasing roughly in parallel with the increasing size of the human p~pulation.~~Will an expanding human population allow increasing rates of viral evolution and enhance the potential for the appearance of more virulent strains? Conclusions In summary, a few points bear emphasis. Global movement is massive, involves all species, and can be expected to expand in volume, speed and reach in the foreseeable future. Distributions of species are changing at an unprecedented rate. New juxtapositions of species allow exchange of genetic information that can affect human health through direct and indirect mechanisms. Population size and density can influence the spread and evolution of infectious diseases. Global movement of populations and evolution of microbes will continue and will present an ongoing challenge for practitioners of travel medicine. New infections will continue to emerge, and known infections will change in distribution, severity, and frequency. In today's world, socioeconomic, political, environmental, and demographic changes have converged to allow many infectious diseases to flourish. Any meaningful response 186 must integrate knowledge and skills from social, biological, and physical sciences and approach the problem at a systems level. Enhanced global surveillance and communication are essential to provide care for individual patients and to respond to outbreaks. References 1. World Tourism Organization. 2. Grubler A.The rise and fall of infrastructures. Dynaniics of evolution and technological change in transport. Heidelberg: Physica-Verlag, 1990:232. 3. Wilson ME.Travel and the emergence of infectious diseases. Enierg Infect I l i s 1995;1:39-45. 4. Tesh N3,Jahrling K, Salas K, Shope RE.Description of Guanarito virus (Arenaviridae:Arenavirus),the etiologic agent of Venezuelan hemorrhagic fever. Am J Trop Med Hyg 1994; 50:452-459. 5. Wharton M, Spiegel KA, Horan JM, et al. A large outbreak of antibiotic-resistant shigellosis at a iiiass gathering.J Infect Dis 1990;162:1324-1328. 6. Snow J. On the mode of comniunication of cholera. London:John Churchill, 1849.Cited in WWinkelstein Jr,A new perspective on John Snow’s communicable disease theory,Am J Epidemiol 1995;142:S3-9. 7. McMichael AJ, Martens WMJ.The health impacts of global climate change: grappling with scenarios, predictive models and multiple uncertainties. Ecosystem Health 1995;1:23-33. 8. Homer-Dixon TE Environrnental scarcities and violent coilflict: evidence from cases. Int Secur 1994;l 9:5-40. 9. Siem H , Uolliiii D, eds. Migration and health in the 1990s. Int Migration i992;30. 10. Wilson ME.Infectious diseases:an ecological perspective. BMJ 1995;311:1681-1684. 11. Gonia Epideiniology Group. Public health impact of Rwandan refugee crisis: what happened in Conia, Zaire, in July, 1904. Lancet 1995;345:339-344. 12. l l e Beer P, El Harith A, Deng LL, et a1.A killing disease epideinic among displaced Sudanese population identified as visceral leishinaniasis. Am J Trop Med Hyg 1991;44:283-289. 13. Ilecosas J, Kane E Anarfi J K , et al. Migration and AIIIS. Lancet 1995;346:826-828. 14. Munoz K, Coffey T, lhniels M, et al. Intercontinental rpread of inultiresistant clone of serotype 23F Streptococcus pnpumoniae. J Infect Dis 1991;164:302-306. 15. Ayliff GAJ.The progrersive intercontinental spread of rnethicillin-resistant Staphylococrrts a i < r r i / s .Clin Infect ])it 1 997;24(Suppl 1):S74-79. 16. Gold HS, Moellering KC. Antimicrobial drug resistance. N Engl J Med 1996;335:3445-1453. 17. TaylorJL,Tuttle J. Praniukul T, et al.An outbreak of cholera in Maryland arsociated w t h imported comniercial frozen kesh cocoiiut riiilk.J Infect Dis 1993;167:1330-1335. 18. Finelli L, Swerdlow D, Mertz K, et al. Outbreak of cholera associated with crab brought from an area with epidemic disease.J Infect Dis 1992:166: 1433-1 435. 19. Hedberg CW, MacDonald KL, Osterholni MT. Foodborne illness in the 1990s [Letter].JAMA 1993;269:2737-2738. J o u r n a l of Travel Medicine, Volume 4, N u m b e r 4 20. Mahon UE, I’onka A, Hall W N , et al.An international outbreak of salnionella infections caused by alfalfa sprouts grown from contaminated seeds.J Infect Dis 1997;175:876-882. 21. American Society for Microbiology. Report of the ASM task force on antibiotic reyistance.Washington DC: Ainerican Society for MicrobiolobT, 1995. 22. Whalen C M , Hockin JC, Ryan A, Ayhton F.The changing e p i d e i n i o l o ~of~ invasive nieningococcal disease in Canada, 1985 through 1992. Emergence of a virulent clone of h‘eisseria twnirgitidis. JAMA 1995;273:390-394. 23. Erivk A,Thorsen B, EriksenV, et al. Impact of administering antibacterial agents on wild fish and blue mussels Mytilrrs edirlis in the vicinity of fith farms. 13s Aquat O r g 1994;18:45-51. 24. Moore 1 5 , Keeves MW, Schwartz B, et al. Intercontinental spread of an epidemic group A Neiueria merziugitidis strain. Lancet 1989;2:260-263. 25. Soares S, Kristinsson KG, Musser JM,ToniaszA. Evidence for the introduction ofa multiresistant clone ofserotype 6 B Sfreptococcus pnerrrrioniac from Spain to Iceland in the late 1980s.J Infect Dis 1993;168:158-163. 26. Bifani PJ, Plikaytis BB, KapurV, et al. Origin and interstate spread of a New York City iultidrug-resistant Mycobacterium trrbrrculosis clone family. JAMA 1996;275:452-457, 27. Soper FL, Wilson DB. Anophrles gambiae in Brazil, 1930 to 1940. New York City:The Rockefeller Foundation, 1943. 28. Reiter P, Sprenger D.The used tire trade: a niechanisni for the worldwide dispersal of container-breeding mosquitoes. J Am Mosq Control Assoc 1987;3:494-501. 29. Craven RB, Eliason DA, Francy 1: et al. Importation ofAedcs albopicfur and other exotic mosquito species into the United States in used tires fr0mAsia.J Am Mosq Control Assoc 1988; 4: 138-1 42. 30. Moore CG, Francy DB, Ekiason DA, Monath TP. Aedes ahupictus in the United States: rapid spread ofa potential disease vector. J Am Mosq Control Assoc 1988;4:356-361. 31. Kusrell K C . Survival of insects in the wheel bays of a Boeing 747B aircraft o n flights between tropical and temperate airports. Bull World Health Organ i987;65:659-662. 32. Isaacson M . Airport malaria: a review. Bull World Health Organ 1989;67:737-743. 33. Coiiiniittee on Ships’ Ballast Operations, Marine Board, Commission on Engineering andTechnica1 System, National Research Council. Steniniing the tide. Controlling introductions of nonindigenous species by ships’ballast water.Wshington DC: National Academy Press, 1996. 34. Carlton JT, Geller JB. Ecological roulette: the global transport of nonindigenous rriariiie orgaiiisms. Science 1993;261: 78-82. 35. McCarthy SA, McPhearson RM, Guarino AM.Toxigenic Wbrio choierae 0 1 and cargo ships entering the Gulf of Mexico. Lancet 1992;339:624-625. 36. Wilson ME. Ureakbone basics: dengue fever in the 1990s. Infect Dir Clin Pract 1996;5:376-379. 37. Kuno G. Review of the factors modulating dengue transmission. Epideniiol Kev 1995;17:321-335. 38. Zanotto PM de A, Gould EA, Gao GF, et al. Population dynaniics of flaviviruses revealed by molecular phylogenies. I’roc Natl Acad Sci U S A 1996;93:548-553.