Tropical Diseases Emerging in New Places
The northward progression of many tropical diseases is not just because of the hotter weather that comes with climate change. Multiple interacting biological, social, and technological forces are playing a role.
Scientists say disease-carrying mosquitoes and ticks once most populous in hot equatorial zones are are expanding their range into new northerly areas. The list of alarming bug-born illnesses that have traveled along with people and warming weather includes Lyme disease, West Nile virus, Chagas, dengue, chikungunya and Zika, a disease gaining new attention as a possible cause of thousands of deformed babies in Brazil.
Scientific American story by Dina Fine Maron reports that nearly 3,000 Brazilian babies were born in 2015 with microcephaly—an incurable condition in which the head and brain are abnormally small and the children tend to have serious cognitive and neurological disabilities as they grow. There were only 147 cases of the abnormality in Brazil in 2014.
A New York Times story by Donald McNeil Jr., explains that virologists say Zika is a certainly a factor in microcepahly though it may not be the sole cause; if a mother has previously been infected with dengue, a related virus, it may be the two viruses together that trigger the prenatal devastation. Zika, first identified in 1947 in Uganda, has not been extensively studied because there haven ‘t been large outbreaks until recently. Zika was not found in the northern hemisphere, except for Easter Island, 2,200 miles off the coast of Chile, until last spring. Now, the Times story says, it circulates in 14 Latin American and Caribbean countries and Puerto Rico.
Zika us carried by the Aedes aegypti mosquito, the same biting pest that carries yellow fever and dengue. Florida had two dengue outbreaks in 2013, which were halted with aggressive mosquito control measures. To date, the aegypti mosquito hasn’t been found much farther north than Washington D.C. and Kentucky. Epidemiologists say potential spread of Zika in the U.S. will depend on whether the Asian tiger mosquito, a cold-tolerant variety found as far north as New York and Chicago, carries the disease as efficiently as aegypti.
Throughout history human travel has spread some diseases and modern air travel increases the potential. Some scientists think Zika arrived in Brazil with the influx of tourists for the 2014 World Cup, or it could have arrived with French Polynesian paddlers who participated in a canoe race in Rio de Janeiro. The first Lymes cases in New York in 1999, came from a strain identical to an Israeli strain, McNeil reports, so virologists believe it likely that the disease arrived by airplane in the blood of someone on board. By 2005, West Nile had reached the Pacific Northwest.
Lyme disease is spread by ticks, which carry more than 30 viral and bacterial diseases. The tick population has been advancing northward for more than a decade, and a growing number of tick borne diseases are being found in nearly all regions in the U.S. Ticks are second only to mosquitos as pest-carried vectors of disease. As temperatures rise, scientists say, mosquitoes can multiply rapidly, potentially enhancing their collective ability to transmit diseases. Further, weather extremes have an impact on populations. Increased precipitation increased rain in some areas expands mosquitoes breeding locations, and droughts, like those that recently afflicted parts of Brazil, encourages people to save water in containers, providing additional mosquito habitats.
As Maria Diuk-Wasser, a scholar with expertise in entomology, zoology and parasitology at Columbia, reminds us, “The mosquito is exquisitely adapted to human hosts, living in close proximity to humans and feeding repeatedly.”