Neglected Tropical Diseases in the Anthropocene: The Cases of Zika, Ebola, and Other Infections

While we advance through a geological epoch that increasingly reflects human intervention on a massive scale, we might expect to see the continued expansion of epidemic neglected tropical diseases, as we have recently seen for Zika and Ebola virus infections. Emerging evidence indicates that the Holocene, our most recent geological epoch that began at the end of the last ice age almost 12,000 years ago, has undergone some fundamental changes because of human activity. Since the origins of agriculture and deforestation and later accelerating with the industrial revolution, followed by rapid 20th century population growth extending into the nuclear age, our planet has undergone a fundamental and seemingly irreversible geological shift [1]. According to many (but not all) prominent Earth scientists, humans have profoundly altered the planet, thereby ushering in a new and so-called Anthropocene epoch (Fig 1). Fig 1 Geological epochs over the last 5 million years. In a January 2016 article in Science, Colin Waters from the British Geological Survey and his colleagues provide important geochemical evidence to support designating the end of the Holocene as the Anthropocene [1]. It includes data showing increasing lead levels after World War II, altered soil nitrogen and phosphorous levels because of increased fertilizer use, and the appearance of newly created radionuclides, beginning with the atomic bomb tests in the New Mexico desert at Los Alamos [1]. Alongside these human-induced geochemical signatures are elevated carbon dioxide and methane levels and sharp increases in average global temperatures [1]. Levels of concrete and plastic have also dramatically increased in recent years, while in parallel, there has been massive loss of animal and plant species [2]. Species extinctions have reached unprecedented levels [1,3]. In this late Anthropocene epoch, we have seen significant increases in the incidence or prevalence rates of several neglected tropical diseases (NTDs), due partly or mostly to human-induced changes to our planet. This is especially true for NTDs transmitted by invertebrate vectors, including mosquitoes, kissing bugs, and snails, as well as highly lethal zoonotic virus infections from bats and other mammals. For example, in the Americas, dengue fever reemerged in the 1980s, while chikungunya and Zika virus infections have aggressively spread across the Latin American and Caribbean region. Venezuela in particular has seen dramatic increases in malaria and most of its neglected tropical diseases (NTDs), including Chagas disease, schistosomiasis, and Zika virus infection, for which unprecedented urban foci are also occurring [4]. Across the Atlantic Ocean, Southern Europe has of late seen the emergence or reemergence of malaria in Greece, West Nile virus infection and chikungunya in Italy and Spain, dengue in Portugal, and schistosomiasis on the French island of Corsica [5]. The Middle East and North Africa (MENA) region is now considered one of the worst-affected global hotspots for NTDs and other emerging infections such as leishmaniasis, schistosomiasis, and MERS coronavirus infection; measles and polio have also returned [6]. Ebola caused thousands of deaths and overwhelmed the health systems of Guinea, Liberia, and Sierra Leone in West Africa in 2014–2015 [7], while East Africa and the Sahel are considered among the most important regions for kala-azar and multiple other NTDs [8]. Schistosomiasis continues to increase throughout Africa, where it is now a major cofactor in its AIDS epidemic [9]. Southeast Asia has seen the rise of Nipah and Hendra virus from bats, in addition to drug resistant malaria, enterovirus 71, melioidosis, and foodborne trematodiases transmitted by snails [10]. Several human activities that characterize the Anthropocene account for the increases in NTDs. It is instructive to see how some of these factors illustrated in Fig 2 helped to facilitate the emergence of two of the most devastating NTDs in 2014 and 2015—Ebola and Zika virus infections, respectively, as well as other high-disease-burden NTDs such as the cutaneous and visceral forms of leishmaniasis and schistosomiasis.