Zika is the latest in a long line of infectious zoonotic diseases to achieve pandemic status as a result of trade and travel. The reason it rings alarm bells is the potentially devastating effect it can have on unborn children. The World Health Organization and the Centers for Disease Control found a link between Zika infections in pregnant women and the birth defect microcephaly. Just how far and fast it has spread is, however, uncertain. Zika symptoms are similar to those of Chikungunya and dengue, also transmitted by the mosquito Aedes aegypti, which makes diagnosis difficult. In fact, since blood samples can react in tests for both Zika and dengue, mapping the spread of the disease is hard. Add to this the fact that Zika can be transmitted from person to person via sexual contact, and we have all the conditions for what is unfortunately becoming a classic disease response. To meet concern over the latest threat, public resources are diverted from other diseases.
The problem with this is that Zika is just one among many emerging and re-emerging infectious disease risks linked to recent global demographic and economic trends. Like Chikungunya before it, Zika is an arbovirus—a virus transmitted by arthropods—that has spread from a center of origin in Africa. As with Chikungunya, successive mutations of the virus have made it increasingly challenging for previously unexposed populations. And as with all other mosquito borne arboviruses, it is especially challenging in the world’s cities. Zika, like other mosquito-borne diseases, is primarily an urban phenomenon, spread through the growth of trade and travel among the world’s cities.
Two things make cities central to the problem. First, cities everywhere share microclimates that favor mosquitoes. Cities are typically warmer and more humid than surrounding areas. They also experience lower diurnal temperature ranges—less day-night variation. They have more impermeable surfaces, and so collect more water and have more places for mosquitos to reproduce. In fact the primary vector for Zika, Aedes aegypti, is currently spreading to many cities around the world that lie well beyond its natural range. Second, mosquito-borne diseases are generally spread more by the movement of infected people than by the movement of infected mosquitoes, and cities are critical nodes in a rapidly expanding global trade and travel network.
Reducing the risk of any one infectious zoonotic disease involves an integrated set of control and intervention measures, each operating at a different spatial scale. Typically, risk reduction includes the development of an effective vaccine. While vaccine development is notoriously difficult for rapidly mutating viruses, there are potentially promising lines of attack on Zika. Perhaps the most promising is a modification of a Sanofi dengue vaccine that is currently in clinical trials. Beyond vaccine development, risk mitigation is almost entirely a matter of changing the activities that bring susceptible people into contact with disease vectors, and that bring infected people into contact with susceptible people.
In the case of Zika, risk factors include the unplanned and uncontrolled growth of cities, the absence of effective vector control within cities, and the neglect of personal defensive measures against Aedes aegypti. The trade and travel that connect one city to the next means that infected people, often asymptomatic themselves, may transmit the virus from one mosquito population to another. And because Zika can be transmitted directly, risk factors include the same activities that affect the spread of all sexually transmitted diseases. People travelling to and from infected areas may be infected, and may infect others with whom they come into sexual contact. The probability of infection can be reduced through the adoption of precautionary measures, but it cannot be eliminated.
Zika is particularly frightening because of the danger it poses to future generations, but it is not unique. It is not the first, and it will not be the last frightening disease to spread globally. What is needed is a more systematic effort to manage infectious zoonotic disease risks at the scale at which those risks are realized. Globalization and the growth of the world trade system has delivered many benefits to people everywhere, but it has also imposed costs—the spread of infectious zoonotic diseases amongst them. The international transmission of disease is an externality of trade and travel. While the World Health Organization (WHO) is currently exploring ways to improve the international response to disease outbreaks, there is little scope for coordinated preemptive action to reduce trade- and travel-related risks of disease. The WHO’s hands are effectively tied by three international agreements that are notoriously hard to renegotiate: the General Agreement on Tariffs and Trade, the International Health Regulations, and the Sanitary and Phytosanitary Agreement.
There may, however, be another way to meet the international challenge posed by emerging zoonotic diseases. Corporations operating in countries affected by infectious diseases have a narrow, short-term interest in the financial consequences if customers defer or cancel trade and travel plans. Zika’s potential impact on the Brazilian Olympics is a case in point. But corporations are also responsible to their employees in infected areas, and to the wider communities within which they operate. They have a broad, longer-term interest in the disease risks of international engagement, and the scope for direct action to mitigate those risks.
Increasingly, corporations are taking responsibility for the international environmental consequences of their activities whether or not there is a legal obligation to do so. A number are engaged directly in the provision of environmental public goods. Indeed, the Gates Foundation’s work on child and maternal health has shown that corporations have the capacity to provide global public goods on a scale that the community of nation states finds hard to match. The Gates Foundation’s work has also shown that direct action in the most vulnerable countries can pay global health dividends worldwide. For the general problem of infectious zoonotic disease risks, targeted direct intervention by businesses in support of vaccine development, urban planning, vector control, and other preventive measures, has the potential both to serve the bottom line and to reduce the risks to all.
Charles Perrings is a professor of environmental economics at the School of Life Sciences at Arizona State University. Carlos Castillo-Chavez is Regent’s professor of mathematical biology in the School of Human Evolution and Social Change. Bertram Jacobs is a professor of virology and director of the School of Life Sciences.