Asia’s infrastructure push will involve building roads, railways, and ports across a vast expanse of land that spans many different ecosystems, among them tropical and subtropical areas and biodiversity hot spots. The region includes sleek modern cities, huge urban slums, and relatively isolated and often unstudied rural areas. These will be joined in a network that will allow the physical movement of humans – and all that accompanies them. Many benefits are likely to follow, but there are other possible consequences.
Consider first the building of roads, railways, and ports. Building roads can disrupt ecosystems, fragment landscapes, displace wildlife, and reduce biodiversity. Intruding on new environments and changing land use can be associated with exposure to previously unrecognized pathogens that can infect humans. Some are found in wildlife and can spill over into the human population, sometimes with dire consequences. Although HIV, for example, emerged from nonhuman primates in Africa, it spread throughout Asia (and the world), carried by travelers. Fragmentation and degradation of natural habitats are associated with increased infections acquired from animals.
Asia’s remarkable biodiversity, a great asset, could also pose challenges. At lower latitudes, there is greater species richness, which includes pathogens that cause infections in humans. By spanning rivers, mountains, deserts and other inhospitable barriers, roads and railroads can also eliminate natural barriers to the movement of pathogens or animals that carry them.
Recent history underscores the stakes. In recent decades, the Nipah virus, avian influenza viruses, and the SARS (severe acute respiratory syndrome) coronavirus have all originated in Asia. It is notable that all originated from animals, with bats playing a prominent role in two of the three. The dengue situation has worsened with all four dengue serotypes circulating in many areas. A new strain of vibrio (bacteria) that causes cholera emerged in Asia – and subsequently spread widely. A form of malaria, caused by Plasmodium knowlesi, a parasite found in nonhuman primates, has also increased in the region, especially in Malaysia.
Resistant bacteria could also arise. Growing populations of humans and food animals expand the interface between humans and animals – wild and domestic. Markets, where live chickens and other food animals are sold, are a rich source of new strains and combinations of influenza viruses. Antibiotic resistance genes, many able to spread horizontally among bacteria, have been documented in abundance in estuaries along China’s coastline, and bacteria that are resistant to most or all commonly used antibiotics are widely distributed throughout China and India. Intensive use of antibiotics for growth promotion in food animals in China has contributed to the selection of resistant bacteria. Humans can acquire, carry, and spread these resistance genes, which may be found in bacteria that live in the gut.
Roads and railroads provide mobility to hosts and mosquito vectors. Humans and animals can carry infections (with or without symptoms) that can be introduced into a new area. Infections that are spread from person-to-person, like tuberculosis, influenza, and measles, can be carried anywhere and introduced into new populations. Mosquito vectors have been dispersed globally through travel and trade. After the Asian tiger mosquito, Aedes albopictus, was introduced into the United States from Asia in used tires, it initially spread along major roadways. HIV/AIDS moved along truck routes. Plant pathogens, including ones that threaten food crops and hence food security, will be able to move more easily in food, vehicles, and with other materials.
Populations and places can be described by their receptivity or vulnerability either to the introduction of new pathogens or to the reintroduction of old ones. Tropical and subtropical areas that are infested with Aedes aegypti mosquitoes are vulnerable to the introduction of many viruses that they are competent to transmit, including dengue, chikungunya, yellow fever, and Zika viruses. The likelihood of introduction is higher when a high volume of travelers is arriving from an area with active transmission. The risk of local and sustained spread will depend in part on the housing (screens and air conditioning), the strength of mosquito control activities, and ecoclimatic conditions.
The new network will connect populations with widely different economic statuses. Diagnostic capacity and surveillance activities in many poor and rural areas may be limited – so little may be known about locally endemic infections in many of the areas. Undetected pathogens could move out of isolated niches when transport is possible.
But there are also health advantages to increased connectedness. New infrastructure could allow better access to the remaining populations in Asia with malaria, especially in Myanmar. There is urgency in the effort to eliminate malaria before parasite resistance to drugs and mosquito resistance to the insecticides used to treat bednets disable the current key control tools. Of course, those with infection may also become more mobile, transporting infection to other areas. Even after malaria elimination, many areas will remain susceptible to reintroduction by infected persons visiting the area, assuming competent mosquito vectors continue to infest the area.
A number of steps can be taken to reduce these risks. Good diagnostics and surveillance will be essential to identify and promptly treat any new cases. In fact, increased connectedness should be accompanied by better surveillance of mosquito vectors and infections in humans, animals, and plants.
Responses should be tailored to specific threats and locales. It is useful to consider the concept of the receptivity of a place and population to specific microbial threats. Receptivity will depend on the specific agent and its means of transmission. Infections that can spread directly from person-to-person, like influenza and sexually transmitted infections, including HIV, can be carried by mobile populations and introduced anywhere.
Local conditions matter greatly. Local spread may be affected by the living conditions and density of the population, ecoclimatic conditions (temperature and humidity), demographics of the population, behavior, host factors, such as nutrition, immunosuppression, and previous infections or immunizations. Measles and diphtheria, for example, cannot be introduced into a population that is completely immunized. Infections that require a specific vector, such as a mosquito, cannot be introduced into an area where the specific vector is absent.
One must consider the dynamics of known infectious diseases that are present in the area and acknowledge the likelihood that currently unknown or unrecognized infections are likely to emerge as new areas are explored and populations are linked by roads. New connections are just that – conduits for flows. To make the most of them, we must not only seize the positive opportunities they present but also reduce the health risks they could carry.
Dr. Mary E. Wilson is a Clinical Professor of Epidemiology and Biostatistics at the University of California and an Adjunct Professor of Global Health and Population at the Harvard T.H. Chan School of Public Health.