By Isobel Whitcomb ‘17
Nearly one year ago, the first cases of zika virus were confirmed in Brazil, causing the Pan American Health Organization (PAHO) to issue a public health alert. Since then, the virus has spread across South and Central America. After briefly being relegated by the US media as old news, last week, the zika outbreak made headlines once more when it was confirmed that the thousands of cases of microcephaly appearing in babies born in the past year are in fact linked to zika infections. Occurring only a year after the West African ebola outbreak of 2014, the zika virus is the latest disease in a long line of brand-new diseases to emerge in the past 20 years. So why are we suddenly seeing these outbreaks of new, deadly diseases? This question opens the door to an important area of cross-disciplinary study between medical science, public health, and environmental biology.
Recent studies on diseases ranging from ebola to malaria show that anthropogenic impacts on the environment may come haunt us-- by creating disease. According to the CDC, around 12% of all infectious diseases have emerged only in the past 20 years. Out of these emerging diseases, 70% are zoonotic, meaning they are passed to humans from animals, either directly or indirectly. Examples of emerging zoonotic infections include HIV (a mutated form of a non-pathogenic virus called SIV which jumped from monkeys to humans), and ebola, which is believed to come from bats. It’s important to note that today, both these example pathogens are passed human-to-human and are designated zoonotic based only on their origins. However, for some emerging zoonotic diseases, such as avian flu or swine flu, animal-to-human transmission is still a major concern. The increasing prevalence of zoonotic infections is incredibly important for anyone studying medicine, health, or the environment to be aware of because it tells us that the ways humans interact with other organisms and their habitat may be directly affecting our health.
One major reason for the increased prevalence of zoonotic diseases is pretty intuitive. More frequent contact with animals means increased risk of a disease jumping between the animal and the human. So why are modern humans so likely to come in contact with disease bearing animals? A major explanation is habitat fragmentation. Formerly large swaths of undisturbed habitat have experienced an unprecedented surge in development and urbanization in recent decades. The result is that instead of large patches of wilderness, we end up with many smaller, island-like patches. Basic math tells us that with less area, these small habitats have exponentially greater contact with human developments.
We see the impact of habitat fragmentation in the prevalence of lyme disease on the East Coast. Researchers have found that habitat fragmentation is also far worse in these areas than on the west coast- even compared to the urban sprawl of Southern California. The development of suburbs in New England has been extremely correlated with the rise of lyme disease, which is far more prevalent in these areas than on the west coast.
It’s important to note that that zoonotic diseases usually do not pass to humans directly (eg: through animal bites or consuming animal products). Most frequently, they are passed through a disease vector. A vector is a third species capable of transmitting the disease. The most notorious disease vectors are biting insects — mosquitoes, ticks, and fleas. Therefore, an increase in disease vectors also means an increase in the prevalence of zoonotic diseases. Recent research indicates that climate change may increase the prevalence of diseases like malaria and zika virus due to erratic weather patterns that allow for the growth of mosquito populations. Both drought and heavy rainfall are known to create conditions ideal for mosquito populations. Unusually heavy rainfall tends to alter the salt content of water, changing nutrient concentrations, while drought can create warm, stagnant pools. In addition, warmer weather in previously cool regions creates new habitats for these warm-weather disease vectors. These trends show us that in future we could see a rise in the prevalence of malaria, zika, and other mosquito borne diseases.
My intention in writing this article is not to create pessimism or guilt. Rather, I want to show how all areas of science are intertwined. Humans are often seen as separate from the environment, and relatively immune to its changing conditions. In fact, 20 years ago, public health specialists scoffed at the idea that disease outbreaks in the American Southwest were caused by unusually heavy rainfalls. However, the environment is inextricably linked to our health. For liberal arts students, especially if those of us who study science, it’s vital that we keep looking for these connections and challenge current assumptions within our fields of study.