Land-use change, globalization of trade and travel, and social upheaval are driving the emergence of diseases in many regions, experts say
West Nile virus, Lyme disease, dengue fever, and plague are examples of "vector-borne zoonotic diseases," caused by pathogens that naturally infect wildlife and are transmitted to humans by vectors such as mosquitoes or ticks.
According to Marm Kilpatrick, who studies the ecology of infectious diseases at the University of California, Santa Cruz, a broad range of human activities can affect the spread of zoonotic diseases. In an article in the December 1 issue of the British medical journal Lancet, Kilpatrick and coauthor Sarah Randolph of the University of Oxford describe how widespread land-use change, globalization of trade and travel, and social upheaval are driving the emergence of zoonotic vector-borne diseases around the world. The article is part of a special series of papers focused on emerging zoonotic diseases.
"This collection of papers offers a bridge between ecologists and clinicians whose combined efforts are needed to address the ongoing challenges of emerging zoonotic diseases," said Kilpatrick, an assistant professor of ecology and evolutionary biology at UC Santa Cruz.
Emerging infectious diseases can be roughly split between introduced and locally emerging diseases. Introduced diseases arise from the spread of a pathogen to a new location, as when West Nile virus arrived in New York in 1999 and subsequently spread across North America. Locally emerging diseases increase in importance in areas where they are endemic, as with Lyme disease in the United States over the past three decades. These two types of emerging diseases can differ markedly with respect to infection dynamics, or the number of cases over time, Kilpatrick said.
"Introduced diseases often cause a big spike in infections and then decrease substantially. Locally emerging diseases often show a steady, sustained increase," he said.
The movement of pathogens by global trade and travel results in the emergence of diseases in new regions. Once established, introduced pathogens often evolve to take advantage of their new environment, including new hosts and vectors. With so much of the landscape shaped by human activities, pathogens may thrive by infecting hosts and vectors that do well in manmade environments.
"Increasing human population and the urbanization and agricultural intensification of landscapes puts strong selective pressure on vector-borne pathogens to infect humans and be transmitted by vectors and hosts that live around humans," Kilpatrick said.
Emergence of endemic vector-borne diseases can result from changes in land use, such as expansion of people into new habitats, or environmental changes affecting the wild animals that serve as natural hosts or the insect vectors that spread the disease to humans. Although vector-borne diseases are highly sensitive to climate, climate change does not appear to be a major driving force behind emerging diseases, the authors said.
"So far, climate change has been a relatively minor player compared to land use and socioeconomic factors in the emergence of vector-borne disease," Kilpatrick said.