The last few weeks have been crazy for mosquito biologists like me. We’ve been asked by reporters to make pronouncements about how the mosquito-borne Zika virus will affect human health in Latin America and around the world.
The attention on Zika may be justified. Or the threat posed to human health by Zika may turn out to have been overhyped. Either way, there are other mosquito-borne viruses that should concern us — and that have largely been overlooked. And these viruses are far more likely to affect broader populations in the United States.
Around the world, two species of mosquito can efficiently transmit viruses that cause human illnesses such as yellow fever, dengue, chikungunya, and Zika. One is Aedes aegypti. A native of sub-Saharan Africa, it was brought to the New World by the slave trade in the 1500s. This mosquito can survive only in warmer climates, which is why the diseases it carries have traditionally been considered tropical diseases. (That may also explains why these diseases haven’t received much attention: They occur primarily in developing countries, and pharmaceutical companies can’t make much money developing medicines or vaccines for them.)
The second species is Aedes albopictus. Beginning in the 1960s, this mosquito burst out of its native range in Asia and began conquering the world. Its common name, Asian tiger mosquito, reflects the flashy stripes on its body. It can transmit all of the same viruses as Aedes aegypti. The Asian tiger mosquito, like its relative, thrives in human habitats and feeds aggressively on humans. But it can survive year-round in much cooler climates.
Permanent breeding populations now occur throughout much of Europe and in the United States, living as far north as New Jersey and perhaps Connecticut. In the northern part of their range, Asian tiger mosquitoes lay eggs in the fall that don’t develop until the weather warms in spring; then they hatch.
Keeping an eye on Asian tiger mosquitoes
Why might the Asian tiger mosquito become a scourge in the United States and Europe? In addition to expanding its range, this species has been adapting to its newly occupied habitats over the last 30 to 50 years. These adaptations let more and more mosquitoes survive. Larger populations spell trouble — for an epidemic of mosquito-borne disease to persist, a certain density of mosquitoes is needed to keep the virus quickly hopping from one host to another before the hosts’ immune systems kill off the virus. The Asian tiger mosquito is likely reaching a critical density for disease transmission in parts of its expanded range.
So far, Aedes aegypti is probably better at transmitting disease than the Asian tiger mosquito. But that may be changing. Dengue exists in parts of China that don’t have any Aedes aegypti mosquitoes, only Asian tiger mosquitoes, meaning this species is able to cause epidemics of dengue if the density is high enough.
More disturbing is what happened in 2005 and 2006 on the small island of Reunion in the Indian Ocean, an island with only Asian tiger mosquitoes. Among the 800,000 residents, about 250,000 became infected with chikungunya. Because the Asian tiger mosquito wasn’t very efficient at transmitting chikungunya to humans, it shouldn’t have been able to cause an epidemic of this disease. So how did it happen? Blame it on mutation and evolution. The virus strain responsible for the epidemic on Reunion was particularly good at replicating inside Asian tiger mosquitoes. Evolution of the virus turned a poor mosquito vector — a vector is any organism that can transmit a disease — into a good one.
Chikungunya is coming
Only a minority (20 percent to 25 percent) of humans infected with dengue or Zika develop symptoms, while 80 percent to 90 percent of those infected with chikungunya have a fever, rash, or severe joint pain. The symptoms often continue long after the body has cleared itself of the virus.
The first report of chikungunya in the Western hemisphere came in 2013, when it was found in St. Martin in the Caribbean. Since then it has spread throughout the Western tropics, transmitted mostly by tropical Aedes aegypti mosquitoes. The strain of the virus that caused the outbreak on Reunion island hasn’t yet been seen here. But it isn’t a stretch of the imagination to think it is only a matter of time until a strain of the chikungunya virus that can be efficiently transmitted by the Asian tiger mosquito will show up here, either introduced from afar or popping up as a new mutation.
If that happens, traditionally tropical diseases will begin affecting human populations in temperate climates. The silver lining in that cloud is that such an extension of these diseases should prompt pharmaceutical companies and the medical establishment to take more seriously the development of treatments for what had been until now largely neglected diseases.
Jeffrey R. Powell, PhD, a professor at Yale University, investigates ways to control mosquito-borne diseases.