Just as we jetsetters and nomads wander the wide world’s winding roads and byways by foot, on horseback, atop a bicycle or packed into an automobile, so too do infectious diseases make use of our ever-improving networks of thoroughfares. They ride along in human bodies, their journeys fueled by our social mobility and contact, two factors unavoidably intensified anytime we embark upon a voyage. But as these pathogens travel new routes and encounter new bodies, they can change and mutate. Luckily for researchers, by examining commonly traveled routes where we once naively believed that only humans trod, the active evolution and epidemiology of a pathogen can be revealed, providing insight into the development and patterns of disease affecting mankind.
HIV is one pathogen with a particular penchant for mutation and evolution as it travels across a landscape. As a sexually transmitted disease – a pathogenic type that especially thrives on human movement and behavior – HIV has greatly benefited from the the processes that commonly constitute progress in underdeveloped regions. Specifically, the ability of HIV to infect remote populations once isolated by natural barriers such as mountains, deserts, and jungles has been greatly augmented as those populations become geographically exposed and accessible due to newly built road networks. In many cases, these populations are left especially vulnerable in the period between the arrival of efficient roads and the influx of medical services that often follows. For HIV, travel and sex drive the spread of the virus and its current status as an ongoing pandemic.
By following major roads and freeways, researchers can track both the spread and evolution of this highly mutable virus. Examining HIV genotypes along transport corridors and freeway networks shows that the virus undergoes Rubik-cube-like genetic modification, its genetic code constantly mutating and revising and transforming away in populations that reside along major road systems. An early article in 2000 looking at HIV heterogeneity in pregnant South Africans found that proximity to national and regional roads was most highly correlated with HIV prevalence in individuals (1). Villages and homesteads located closest to busy, well-traveled roads reported a significantly higher prevalence of HIV. One such road in this study was used as a major trucking route that connects South Africa’s major ports on the coast, such as Durban and Richards Bay, to cities in Mozambique and Swaziland.
The distance to roads increases not only one’s risk of HIV infection, but also the probability of infection with complexly evolving strains of the virus. For more than a decade, researchers have found that along major transportation routes throughout Uganda there is both a higher incidences of HIV and enhanced diversity of such strains in the population. In a 2006 study that tracked these two factors within a southern district that abuts Tanzania, researchers found a “complex evolving pandemic”; individuals residing in areas with roadside trading centers – composed of hotels, bars, and shops – were not only more likely to be infected with recombinant HIV strains in which one subtype has merged with another, but were also more likely to be infected with multiple strains (2). These included people residing near a major transportation corridor of the Trans-African highway network that leads directly south to Tanzania and South Africa.
Highway systems and roads create environments for “increased social and sexual interaction, higher HIV-1 incidence, increased importation and transmission of novel HIV-1 strains, and increased risk for multiple exposures to HIV-1, leading to dual infection and generation of … recombinant strains”(3). The accessibility paved by roads in parts of Africa magnify the social dynamics of HIV transmission, with travel and transactional sex along these highway networks driving the virus’s mutation, recombination, and evolution. These mutations may affect the virus’s infectivity, its ability to be transmitted, as well as the efficacy of antiviral treatments. It is for these reasons that documenting HIV diversity and variation along these routes is so vital, to shed light on how HIV treatment and prevention might be impacted (3).
Throughout the world, just by virtue of creating accessibility and opportunity, roads and highways have generated pockets of vulnerability in regions formerly too isolated to support the level or recurrent social interactions common in more central, more densely populated areas . Wherever humans go, we bring our unique stockpile of microbes – both pathogenic and commensal – seeding and disseminating our own brand of microorganisms like pollen from a blown dandelion. This is one important reason why the HIV/AIDS pandemic has been so difficult to control and manage: a country’s highway network transports more than just supplies and willing workers from city to city, from country to country. Microbes, in short, are along for the ride.
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1) F Tanser et al. (2000) HIV heterogeneity and proximity of homestead to roads in rural South Africa: an exploration using a geographical information system. Trop Med Int Health. 5(1): 40-6
2) MA Arroyo et al. (2005) HIV-1 diversity and prevalence differ between urban and rural areas in the Mbeya region of Tanzania. AIDS. 19: 1517–1524
3) MA Arroyo et al. (2006) Higher HIV-1 incidence and genetic complexity along main roads in Rakai District, Uganda. J Acquir Immune Defic Syndr. 43(4):440-5