“Travel is a potent force in disease emergence and spread,” says Mary E. Wilson in her paper for the US Centers for Disease Control and Prevention (CDC). In the 21st century, traveling has taken on a velocity, magnitude, and penetration the like of which has never been seen before. So great are the numbers of people traveling, and so deep the destinations, that entire habitats are falling prey to the tourist. Human and non-human genes are mixing at high rates and in diverse combinations. The effects of these factors are, as of yet, unknown.
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Background
Movement is associated with the spread of disease in many ways. One way in which travel helps infectious diseases to spread is by the introduction of a new microorganism into a new geographic area.
A novel pathogen entering a population with no previous experience of this microbe causes the most severe disease. Microbes that cause mostly asymptomatic and mild infections spread the infection widely and thus still manage to cause severe outbreaks.
Organisms that survive in the human host primarily, or almost completely, such as the measles virus, readily spread in a new area. However, if one or more intermediate hosts or vectors are involved, or the microbe has a convoluted life cycle, introductions rarely lead to outbreaks in the new geographic area.
Permissive conditions may exist in a new location that allows the introduced pathogen to infect and spread, such as the human immunodeficiency virus (HIV) in a population which has greater sexual freedoms, low use of barrier contraceptives, high frequency of anal sex, intravenous drug use, and so on.
Some parasites require the right intermediate host and environment to complete their life cycle. The dengue virus would be unable to establish itself without the concomitant presence of the right mosquito vectors. Tapeworms rarely cause human transmission in developed countries like the USA because the eggs, found in feces, which are disposed of hygienically, do not come in contact with other hosts.
Sometimes, antimicrobial resistance characteristics, or atypical virulence genes, are introduced into another population by travelers. They may also carry in disease vectors.
Visitors to another area may experience infections to which the local populace is immune, such as hepatitis A, either due to the lack of immunity or because they do not take the precautions that the locals do.
Historical Examples
History furnishes many examples of such successful spread. Plague is a bacterial infection caused by Yersinia pestis spread by fleas hopping off infected rats onto humans, transmitting the bacillus by their bites. The huge outbreaks of plague that have killed millions were possible only because of the migration of whole populations, often along trade routes.
Another instance is the decimation of several indigenous peoples in the Americas by the introduction of smallpox, measles, tuberculosis, and mumps, among other infections. On the Caribbean island of Santo Domingo, up to one in two are estimated to have died of smallpox, which then spread to the other islands as well as to the Americas.
Central Mexico is thought to have lost one in three of its people in just the first ten years after the arrival of the Europeans. The reverse also occurred, with foreigners falling prey to local diseases. One researcher estimates that until the world wars of the 20th century, more deaths in colonized countries in the tropics were caused by infectious disease than by war wounds.
Non-commercial travel includes leisure travel, business travel. immigration, refugee movements, missionaries, student travels, and Peace Corps work. The volume of short-distance travel is very high, at about 500 million, mostly by international commercial flights.
In France, the population mobility went up over a thousand-fold over the last 200 years. Again, the number of travelers into and out of Australia has gone up a hundred-fold in just 40 years.
About 70 million people travel to other countries for work, as legal or illegal migrants.
Recent examples
The Spanish flu caused by the influenza A virus subtype H1N1 spread over two years to cause an estimated 50 million deaths worldwide.
In recent times, the best-known pathogens to cause pandemics have been severe acute respiratory syndrome coronavirus (SARS-CoV), influenza A (H1N1), and the Middle East respiratory syndrome coronavirus (MERS-CoV). The coronavirus infections caused thousands of infections before they died out. However, H1N1 led to up to an estimated 575,000 deaths in its first year.
The current pandemic of coronavirus disease 2019 (COVID-19) has been triggered by the emergence of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which spread via international and local travel to cause over 200 documented infections, with over four million deaths, making it the worst pandemic of this century – so far.
Urbanization and Disease Spread
Urbanization is another form of travel that spreads disease. This phenomenon has led to the emergence of huge cities harboring 10 million or more residents. Many will be surrounded by immense slum areas, with a diverse mix of people from multiple regions. The lack of sanitation gives pests and parasites a field day, while the overcrowding allows rapid disease transmission.
These slum areas often disgorge their inhabitants into the nearby countryside, spreading infections wider afield. This also allows genes that confer antimicrobial resistance to emerge and spread.
How pandemics spread
Displaced People and Refugees
About 50 million people were either displaced or refugees, due to wars or natural calamities, according to the International Organization for Migration. Refugee camps and shelters are fertile soil for an outbreak, being very like slums in their lack of basic hygiene facilities, accumulating people from multiple regions, overcrowding, poor medical care, and susceptibility to vectors as well as animals that spread disease.
For example, half a million refugees from Rwanda died in just one month due to cholera and dysentery in the Zaire camp – accounting for approximately one in ten of those who arrived at the camp.
Globalized Commerce and Disease
Humans are not the major movers on earth. Fresh produce, meat, fish, and other materials are today transported from continent to continent. Along with this, microbes and vectors get a free ticket.
As already mentioned, such commerce has its price in the form of immense disruption of ecosystems by new roads, tunnels, bridges, and canals. These allow germs to spread rapidly and widely. The large-scale processing and distribution of foods allow pathogens in a single animal to be spread far and wide.
Among vectors introduced by shipping of goods, the dengue virus vector Aedes albopictus and African Anopheles mosquito species stand out. The former has been found to harbor no less than 14 different strains of the eastern equine encephalitis virus, besides being capable of acting as a vector for yellow fever virus, La Crosse virus, and so on.
Mosquitoes are also introduced by international flights. Land vehicles can carry other vectors such as that responsible for African sleeping sickness over a wide range, much more than it could fly by itself. Imported exotic animals may transmit viruses from one region to another, such as the Marburg virus, which killed seven people in Germany when they handled the tissues of its host, the African green monkey.
Long-term Impacts of Travel on Disease Spread
While infectious pathogens are spread by travelers, the latter also introduce other types of disease factors, related to new farm technology, novel drugs, and medical treatments, and the use of chemicals and pesticides. These often produce a long-term effect on the region that lasts for generations.
Signs of development such as clearing old forest, building dams for power and irrigation, and cutting roads into remote locations, are all linked with large-scale movements of people, along with shifts in the range and prevalence of infectious diseases in those regions, including schistosomiasis, sexually transmitted disease, and malaria.
The means of transportation, such as ships or aircraft, provide space for microbes to spread, either on a large scale such as cholera and other foodborne infections or from one individual to another, such as tuberculosis or the flu.
Converging on Transmission
Multiple factors are currently acting together to allow diseases to spread internationally. The population worldwide is aging. Many environmental and medical therapies of today weaken immunity, as after organ transplant, valve replacement, pollutants in the air, and chemicals used in the manufacture of common household goods.
When coupled with poverty, crowding, and stress, the risk of disease is much higher. Injudicious antibiotic administration can also cause resistant strains of the pathogen to emerge and spread.
Conclusion
The combination of movement at many levels and profound change in the physical environment can lead to unanticipated diseases spread by multiple channels. Research and surveillance can map the global movement and evolution of microbes and guide interventions.”
Dr. Wilson
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Further Reading
Diagnostic toolkit aims to improve response to Marburg virus cases