Climate Change and the Migration of Infectious Disease

The Zika virus is all over the news. Zika is surely dangerous, but it has its limitations and is likely to be well contained. However, its greater significance extends beyond any current spread. Instead, it exemplifies the crucial emerging trend of a novel infectious agent that has swiftly become a global threat.

The common phrase, “this time is different,” is almost always wrong. Yet, our modern circumstances are distinctly unlike any previous era. Humans possess a unique ability for rapid travel, and many choose to journey with our favorite pets and plants. This unprecedented degree of mobility extends across every planetary habitat. Further yet, it now occurs during a phase of a rapidly shifting climate. Certainly, species migration or global climate change are not new, but it is only in this present moment that these factors can amplify through instantaneous global travel in a singular manner.

In fact, the results of this unusual conjunction are already apparent. For example, Zika’s advance across Europe and to the Americas has been extremely rapid. This is such an extraordinary event that at the beginning of this year, the World Health Organization declared Zika a global emergency in recognition of its rapid spread from continent to continent. Its rising incidence mirrors our prior concerns about the global scope of other recent epidemics such as Ebola or SARS.

The World Health Organization notes that infectious diseases are emerging at a greater pace and spreading faster than ever before. There are at least forty new infectious diseases that have emerged that were unknown only a generation or two ago. For example, Zika was first identified in the 1950s and was initially confined to a narrow band of equatorial Africa and Asia. Monkeys were its primary host. Abruptly, it appeared in humans in Polynesia in 2013. Thereafter, it is suspected to have arrived in the Americas by plane, appearing in Brazil in that same year. Now it demonstrates pandemic spread throughout the Americas and the Caribbean. It is a flexible virus, too. It can be transmitted by mosquito bites, from mother to child, by sexual contact, or blood transfusions.

Chikungunya virus is also a newly emerging threat. Chikungunya is characterized by the sudden onset of fever accompanied by joint pains. It has a low mortality rate but is associated with a high frequency of chronic medical disabilities, some of which can be devastating. First identified in 1952 and initially confined to small outbreaks in West Africa, it has spread worldwide with millions of cases across Africa, India, and Asia. Only recently, it has appeared in the Caribbean and South America, and hundreds of cases have been reported in the United States.

Each of these is an example of a new enemy. However, there are also consequential old players whose distribution can also be affected by climate and global travel. For example, dengue fever is a mosquito-borne virus first described in China in about 200 CE. For centuries, there were only sporadic epidemics. Yet, it surged dramatically after World War II and is now a major factor in childhood mortality in the Pacific and the Americas. Only recently, dengue fever achieved a toehold in the United States. Worldwide, it affects tens of millions on an annual basis. Even malaria, an old and persistent scourge, is demonstrating an increasing speed of transmission attributed to socioeconomic factors that include land use, population growth and urbanization, migration changes, and travel.

The spread of disease through travel is not itself new. For example, in the Middle Ages, bubonic plague originated in China and arrived by trading ships to Europe. That briskly led to a series of devastating epidemics in the fourteenth century that is estimated to have killed one-third of the European population.

Yet, even these horrific results are not the limit of epidemic disease. Twelve thousand years ago, there were abundant giant mammals on the North American continent such as mammoths, saber-toothed tigers, and giant sloths. Suddenly, they all disappeared. The exact cause is not known. However, “hyper-disease” is one prominent theory. It is believed by some scientists that migrating humans or their companion animals inadvertently spread novel diseases from Asia when they crossed to the Americas, and the indigenous megafauna had no natural immunity. In fact, extinction on the basis of infectious epidemics may not be rare. For example, the world’s amphibians are threatened by the chytrid fungus, and the extinction of several amphibian species has been attributed to this pathogen.

If this is our current reality, what is the appropriate response? As we continue attempts to mitigate the effects of climate changes we must also direct resources toward the remediation of infectious disease. This can be accomplished by mosquito nets, removal of standing water sources in endemic regions, research into improved treatments and vaccines, and a deeper understanding of the reproductive cycle of vector insects or other intermediary hosts. Clearly, our concerns about climate should emphasize coping with and conquering infectious disease as the truest common denominator of human health and welfare and proper planetary stewardship.