Wanderlust and Super-pathogens: How our global community creates global infection
Global traffic is at an unprecedented high. In 2015 the UN World Tourism Organisation (UNWTO) declared an all time high in international tourists, with 1.184 billion travelling outside their countries’ borders for a minimum of one night. The trend is set to continue, with upcoming data for 2016 expected to come in with a 4% increase from 2015. [i] Travel is not limited to tourism; our society relies on transport of materials and foodstuff, whilst political upheaval has also lead to global dissemination of populations. Travellers have become ‘part of the dynamic global process of moving biota’. [ii] Consequentially, the movement causes ‘juxtaposition’ of species that may never have had such physical proximity. As immune systems are exposed to new pathogens from other regions, humans and animals alike may harbour quiet killers whose incubation periods may last until they return to their home country, diseases undetected.
During the cusp of rapid global travel: European countries’ conquests for empires, examples of such phenomena can already be found. In 1520, smallpox arrived in Mexico carried by a member of Panfilo de Narvaez’ army, killing an estimate of 25%-50% (hundreds of millions)of people in the Aztec Empire.[iii] Extrapolate forwards 500 years: in 2017, with the spatial and temporal barriers of travel largely eliminated by air travel, humans can reach almost anywhere on the planet, and back again within 72 hours.
Many recent pandemics have been primarily facilitated by human travel. Strikingly, Canadian flight attendant Gaétan Dugas, incorrectly dubbed ‘Patient 0’ in the HIV/AIDS pandemic in the early 1980s, had up to 750 sexual partners across North America in three years while infected with HIV [iv]. Although Dugas was not the cause of the pandemic, he is an exaggerated example of modern travel accelerating the movement of infectious diseases. (The specific strain was recently found to, in fact, originate from Haiti and quietly circulated the US in the 1970s, well before Dugas was infected.[v]) Tuberculosis[vi], a disease thought to be practically ‘cured’ in western nations, is commonly found in individuals who have recently travelled. Despite western vaccinations, new multidrug resistant and extensively resistant strains are infecting travellers and returning to their homelands with them.
International travel also gave the Ebola virus the opportunity to transition from an epidemic to a pandemic. In 2014, Ebola virus reached Nigeria on 20 July through a singular infected man landing Lagos airport, setting of a chain of transmission of 19 people. Simultaneously, a smattering of cases began appearing globally in Spain, the US and the UK in volunteers returning from West Africa. A more infectious disease would have had the potential to rapidly initiate transmission chains, however despite Ebola’s fearsome reputation, it is transmitted through direct human-human or human-fluid contact: once the international cases were isolated, transmission was controlled. However, this example demonstrates scope for future crises.
When a traveller sets out from their home country, they immediately begin to play a role in microbial movement. Non-immune travellers may pick up infections existing in tropical regions, despite vaccinations and drugs; they may also carry microbes with resistance genes. This presents an even greater problem: a person may be carrying a “super-pathogen”, resistant to the effects of medication, unknowingly returning to a vulnerable population. They can also become an integral stepping-stone in the spread of vector-borne infections such as Malaria.
Vector-borne diseases have the potential to be far more devastating than human-human contact spread diseases. For example, a man who is infected with Disease X lands in a new area with a large mosquito population, which is not typically found in that region. He is swiftly bitten by a mosquito (the vector), which now carries the infection. The mosquito flies 3 miles away and bites a local woman, who is now also infected. The man has never met the woman, yet a transmission chain is already in motion. Any mosquito who bites either individual will carry the disease and go on to infect any others that they may bite. The spread of disease X becomes explosive.
A real world example of a vector-borne infection is dengue fever, which illustrates the complex factors behind infection transmission, most notably an expansion in distribution due to travel.[vii] Main factors include urbanisation (especially the rising number of people living in tropical regions), increase in population and ‘rapid, frequent travel of viremic (carrying a virus in the bloodstream) humans to areas infested with competent vectors’. A more recent vector-borne virus is the Zika virus, transmitted by Aedes mosquitoes, which became an epidemic in 2015.
Infections may feel unstoppable, and with the ferocious transmission accelerated by modern travel, it might feel depressingly inevitable that the human race will meet its demise through an uncontrollable pandemic. However, developing our resources to improve disease control is an excellent way to prevent this (slightly exaggerated) hypothetical situation, or more realistically, the next widespread epidemic or pandemic. Controlling the spread of diseases is a multi-pronged attack. Scientists work continually on accelerated drug development, disease surveillance, diagnosis and control. Travellers with foreign diseases can have a silver lining by acting as ‘sentinels’ - ‘an indicator of the presence of disease.’ They may have access to medical facilities where health professionals can diagnose, isolate viruses, culture bacteria, sequence organisms and study molecular components of pathogens which yields insights that can aid the prevention, control or treatment of that disease. Infections in travellers also alert global authorities to new infection sites and potential epidemics. The outlook isn’t bleak; it’s exciting.
In our increasingly connected world, travellers play an incredibly important role in the international movement of micro-biotic materials. Either through starting transmission chains in new counties, or alerting international health organisations to unusual disease patterns, travellers have changed the nature of infectious disease and will continue to do so if the human race continues on its path to become a global community. Effort must be poured into the monitoring of diseases, drug development and increasing awareness in all international communities. Infectious disease is a matter that affects us all, and that’s why it is important to me.
Figure 1: UNWTO, International tourist arrivals by region (in millions); UNWTO (United Nations World Tourism Organization) 2008a. [accessed July 22, 2009]. http://www.unwto.org/facts/eng/highlights.htm.
[i] http://edition.cnn.com/2016/01/19/travel/international-tourists-2015/ Accessed 10.1.2016
[ii] https://www.ncbi.nlm.nih.gov/books/NBK45724/# Accessed 10.1.2016
[iii]http://latinamericanhistory.about.com/od/theconquestofmexico/fl/Consequences-of-the-Conquest-of-the-Aztecs.htm Accessed 10.1.2016
[iv] Perrin L, Kaiser L, Yerly S. Travel and the spread of HIV-1 genetic variants. Lancet Infectious Diseases. 2003;3(1):22–27. [PubMed] [Reference list]
[v]https://www.theguardian.com/science/2016/oct/26/patient-zero-gaetan-dugas-not-source-of-hivaids-outbreak-study-proves Accessed 10.1.2016
[vi] Oeltmann JE, Varma JK, Ortega L, Liu Y, O’Rourke T, Cano M, Harrington T, Toney S, Jones W, Aaruchit S, Diem L, Riethong D, Tappero JW, Ijaz K, Maloney SA. Multidrug-resistant tuberculosis outbreak among U.S.-bound Hmong refugees, Thailand, 2005. Emerging Infectious Diseases. 2008;14(11):1715–1721. [PMC free article] [PubMed] [Reference list]
[vii] Wilder-Smith A, Gubler DJ. Geographic expansion of dengue: the impact of international travel. Medical Clinics of North America. 2008;92(6):1377–1390. [PubMed] [Reference list]
Clara Portwood is a D4C Ambassador, and co-founder of the charitable initiative Liberation Libraries.