Modern global travel

The global population increased from 1.6 billion to 6.1 billion in the twentieth century, and reached 6.5 billion in 2006; everyone born before 1960, when the population was 3 billion, has lived through a doubling of the global population. Global travel has increased even more rapidly. Between 1950 and 2005, international tourist arrivals increased 32-fold, reaching 808 million in 2005 (Table 1.2; WTO, 2006a).

People travel for many reasons - tourism, work, research, study, humanitarian aid, religious purposes or missionary work, visits to friends and relatives, displacement due to catastrophic events, for economic incentives, and due to environmental disasters or sociopolitical upheaval. According to the data from the World Tourism Organization (WTO), international trips to visit friends and relatives and for health or religious purposes rose from 19.7 percent of 441 million in 1990 to 24.2 percent of 763 million in 2004; business and professional travel also increased, from 13.7 percent to 15.7 percent. In contrast, the proportion of visits for leisure and holiday travel declined from 55.4 percent to 51.8 percent in the same period of time (see Table 1.3).

The volume of travel has grown exponentially with an average annual growth of international tourist arrivals of 6.5 percent (WTO, 2006a). In the US, 18 airports receive more than 500,000 international arrivals by air annually (16 have a total of >25 million air travelers per year) and 14 ports each receive more than 150,000 maritime passengers (Sivitz et al., 2006: 127). Figure 1.2 displays the civilian global aviation network and the extensive interconnections. In total, the US has 19,500 airports, and at peak times 5000 airplanes are aloft in US airspace.

Table 1.2 Growth in world population and international tourist arrivals

Year

World population (millions)

International tourist arrivals

(millions)

1950

2557

25.3

2005

6451

808

Change

X2.5

X32

Data from the US Census Bureau (http://www.census.gov/ipc/www/worldpop.html) and World Tourism Organization (http://www.world-tourism.org/facts/menu.html).

Data from the US Census Bureau (http://www.census.gov/ipc/www/worldpop.html) and World Tourism Organization (http://www.world-tourism.org/facts/menu.html).

Table 1.3 Arrivals by purpose of visit

Reason for travel

1990 (international tourist arrivals = 441 million) % of total international arrivals

2004 (international tourist arrivals = 763 million) % of total international arrivals

Leisure, holiday

55.4

51.8

Business, professional

13.7

15.7

VFR, health, religion

19.7

24.2

Not specified

11.2

8.3

Data from World Tourism Organization: Tourism Indicators (available at http://www.world-tourism.org/facts/menu.html).

Data from World Tourism Organization: Tourism Indicators (available at http://www.world-tourism.org/facts/menu.html).

Figure 1.2 The global aviation network: civil aviation traffic among the 500 largest airports in >100 different countries, accounting for ^ >95 percent of international civil aviation traffic. Each line represents a direct connection between airports, and its shade encodes the number ¡2 of passengers per day (see shaded bar at the bottom) traveling between two airports. From Figure 1 in Hufnagel etal. (2004). l

Figure 1.2 The global aviation network: civil aviation traffic among the 500 largest airports in >100 different countries, accounting for ^ >95 percent of international civil aviation traffic. Each line represents a direct connection between airports, and its shade encodes the number ¡2 of passengers per day (see shaded bar at the bottom) traveling between two airports. From Figure 1 in Hufnagel etal. (2004). l

Travel by cruise ship is also growing rapidly. In 2003, 184 cruise ships served the US cruise market and 7.4 million cruise passengers went through US ports (Sivitz et al, 2006).

Today, approximately 2 percent of the world's population, or >200 million people, reside outside their country of birth (Gushulak and MacPherson, 2004). These include immigrants, migrant workers, refugees, asylum seekers, and international students. Those who return to their home country to visit friends and relatives (VFR) are more likely to visit rural areas or to stay in accommodations that lack good sanitary facilities and safe water. Because they are "going home," they may be unaware of risks to themselves and their children (who may have been born in a developed country) and thus have inadequate preparation (e.g. vaccines, chemoprophylaxis). VFRs are at higher risk for malaria, typhoid fever, and other infectious diseases than are people traveling for other purposes (Bacaner et al., 2004; Leder et al., 2007) - for example, 43 percent of imported malaria cases in Europe during the period January 1991 to September 2001 occurred in VFRs and other migrant populations (Schlagenhauf et al., 2003). The population of foreign-born in the US is estimated to be 34.2 million, with more than half from Latin America and about a quarter from Asia.

In 2004, international tourist arrivals traveled mainly by air or land, with sea travel contributing only 7.4 percent of the total (WTO, 2006b; Table.1.4). For Africa, America, and the Asia-Pacific regions, air travel surpassed land travel. This indicates a greater proportion of long-haul travel, likely associated with the use of large aircraft, and possibly greater potential for dispersal of organisms into and out of these regions.

Patterns of world travel today

A person with SARS in 2003 illustrates the potential consequences of today's travel patterns (Breugelmans et al., 2004). A 48-year-old businessman flew from

Table 1.4 Arrivals by mode of transport, 2004

Air (%)

Land (%)

Water (%)

Unspecified route (%)

World

43

49.3

7.4

0.3

Africa

48

43.9

7.8

0.3

America

53.1

41.3

5.5

0.1

Asia and Pacific

46.2

31.9

10.7

1.2

Europe

38.1

54.8

7.0

0.1

Middle East

45.3

49.5

5.3

0.0

Data from World Tourism Organization: Tourism Indicators (at http://www.world-tourism.org/ facts/menu.html).

Data from World Tourism Organization: Tourism Indicators (at http://www.world-tourism.org/ facts/menu.html).

Hong Kong to Frankfurt, Germany, on 30 March 2003. He traveled on seven flights throughout Europe during a five-day period from 31 March to 4 April 2003, with stops in Barcelona, London, Munich, and Hong Kong. He was admitted to a hospital in Hong Kong on 8 April with possible SARS, which was confirmed on April 10. His potential contacts spanned many countries.

The "sphere of travel" has enlarged over the years. Bradley described the spatial range of travel in sequential generations of his own family, demonstrating a 10fold increase with each generation (Bradley, 1989). As travel has become faster, cheaper, and safer, people take more trips and travel longer distances. The average daily distance that an individual in France travels has increased over 1000-fold over the past 200 years, and presumably is similar in other populations in industrialized countries (Cliff and Haggett, 2004). Social and demographic changes have affected travel patterns, and global immigration, migration, and changes in family structure today lead to frequent long-distance travel, often by air, to visit family.

Aircraft and cruise ships are also increasing in size, which expands the population of fellow passengers, often from multiple geographic regions, who come into close contact for a period of hours to days. Assuming homogeneous mixing of passengers, the risk of being exposed to a person with a communicable disease increases four-fold when an aircraft doubles in size - for example, from 200 to 400 passengers (Bradley, 1989). The WTO forecast of growth in long-haul travel between regions over intraregional travel also illustrates the concept of an enlarging sphere of travel (WTO, 2006b). In addition, regional shares for international tourist arrivals are shifting. The WTO predicts an average global growth of 4.1 percent in international tourist arrivals per year, but greater than average growth for Africa, and even greater growth of >6 percent for East Asia and the Pacific, South Asia, and the Middle East (see Figure 1.3). Although Europe is projected to remain the most popular destination, its overall share in the market is predicted to decline.

International tourist arrivals by region (millions)

2000 1500 1000 500 0

International tourist arrivals by region (millions)

2000 1500 1000 500 0

I - I I I r 1990 1995 2000 2005 2010 2015 2020 Year

Figure 1.3 International tourist arrivals by region (millions) with forecast. Data from WTO (2006a).

World

-m-

Africa

Americas

-x-

East Asia/Pacific

-*-

Europe

-•-

Middle East

-•—

South Asia

I - I I I r 1990 1995 2000 2005 2010 2015 2020 Year

Figure 1.3 International tourist arrivals by region (millions) with forecast. Data from WTO (2006a).

While Europe's and America's combined share in world tourist arrivals was >95 percent in 1950, it declined to 82 percent in 1990 and 76 percent in 2000. The shift of international travel to developing regions in tropical and subtropical regions also increases potential exposure to microbes and vectors endemic in those regions.

Big international airports are typically situated near large metropolitan areas. In many of the developing, low-latitude regions (most of them tropical or subtropical), large periurban slums surround large cities, often populated by people with families in rural areas. Regularly scheduled international flights and their passengers bring these populations into potential contact, and link urban and rural biota of the world.

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