Boasting more than 3,500 species inhabiting every continent except Antarctica, it’s no secret the mosquito is an annoying insect.
Though the majority of mosquito species are harmless to humans, feeding on flower nectar and animals, others have changed the course of history thanks to the punishing diseases they transmit. Some historians even say mosquito-borne illnesses contributed to the fall of Rome. As Bill Gates has pointed out, these whiny little insects are by far the world’s most deadly species.
“What would you say is the most dangerous animal on Earth? Sharks? Snakes? Humans?” wrote Gates in an April blog post. “Of course the answer depends on how you define dangerous. Personally I’ve had a thing about sharks since the first time I saw Jaws. But if you’re judging by how many people are killed by an animal every year, then the answer isn’t any of the above. It’s mosquitoes. When it comes to killing humans, no other animal even comes close.”
Alexander Raikhel, distinguished professor of entomology at the University of California, Riverside, also has strong feelings about the mosquito. “They are quite remarkable,” he says. “They adapted to sensing humans and any prey with an acute sense of odors and visual cues.”
The species of most concern transmit diseases like malaria, dengue and West Nile virus. Other illnesses include Japanese encephalitis, Rift Valley fever and lymphatic filariasis. And there are still new diseases gathering steam, such as the recent headline-grabbing chikungunya, now making landfall in the West, and the zika virus, beginning to creep across the Pacific.
Mosquitoes are a key source of food for a variety of fish, amphibians and birds. But they cause about a million deaths each year by transmitting pathogens to humans in two ways. In the enzootic cycle, they pass the virus back and forth between themselves and vertebrates. In this case, humans are somewhat incidental to their feeding and act as a dead end for the virus, as we don’t pass it back to mosquitoes.
With diseases like malaria, humans are the main meal. We incubate the virus, transferring the plasmodium parasite back to other mosquitoes – and other humans. While there have been major international campaigns to end malaria, which alone kills nearly 500,000 people per year, they’re typically not a corporate priority.
In 2013, the Bill and Melinda Gates Foundation launched Accelerate to Zero, an initiative focused on eradicating malaria through measures such as new drug regimens, new strategies, and local, community-based efforts. SC Johnson is involved in the Nimbus Project, which promotes the use of mosquito nets.
Though these entities as well as the research community and governments continue to fight mosquito-borne diseases – and have made significant progress in the last decade – bringing these killers to heel remains a huge task. Here’s what we’re up against.
Malaria
In terms of human mortality, this is the most destructive disease spread by mosquitoes. More than half the world’s population is considered at risk from malaria and annual death estimates range as high as a half-million. About 90% of malaria deaths occur in Africa; the majority of those affected are children under age five. All this despite the fact that increased intervention has reduced deaths from the disease in Africa by an estimated 54% since 2000.
The parasite that causes malaria multiplies in the human liver and can quickly become life-threatening if not identified and treated quickly. Anophelese gambiae is the principal carrier in Africa, but more than 45 of the 400 Anophelese species have been shown to carry it around the world.
“On most measures you’d say malaria is more dangerous overall,” says Harry Savage, chief of ecology entomology activity in the arboviral diseases branch of the US Centers for Disease Control and Prevention. “But in the actual number of cases per year, it’s not that far apart from dengue when you consider the number of infections.”
Dengue
Dengue, a virus-caused disease also known as “breakbone fever”, is thought to infect 390 million people per year, with 96m cases considered severe. Though deaths are thought to be less than 1% of infections, the disease has been spreading rapidly in recent years thanks to the invasive nature of the mosquitoes that carry it. Dengue is considered a leading cause of illness in Asia and Latin America, and has been making appearances in parts of Europe and the southern US over the last few years.
Mosquito species Aedes aegypti, the prime dengue carrier, also comes from Africa, but it masters new territory easily. Its eggs stay dormant for up to 30 weeks, allowing it to easily survive transport around the world.
“Basically it’s a mosquito rat that proliferates with our trash cans and abundance of humans,” says Raikhel. “While there is no vaccine for dengue fever, efforts to introduce male sterility and bring local populations down are some of the preventative efforts showing promise.”
West Nile
As a zoonotic illness, most cases of the West Nile virus occur after mosquitoes feed on infected birds and subsequently bite humans. While most people won’t develop any symptoms from exposure, West Nile can result in a range of illnesses, including fever, encephalitis and meningitis. It also has been recently been detected in an increasing number of countries, including Australia, the US and nations in Europe. The first outbreak in the US occurred in 1999, and it has since been detected in most of the lower 48 states and in Canada. Outbreaks occur along many routes of bird migration, with other serious outbreaks occurring most recently in Greece, Russia and Romania.
“West Nile has adapted to a temperate climate,” says Savage. “It’s going to persist [in the US] for as long as people inhabit North America. In places like Florida and south Texas, mosquito populations exist all winter long, so there’s a possibility of transmission all winter long. But in most temperate areas, once it starts getting cool the numbers really drop off.”
Chikungunya
Chikungunya is a relative newcomer on the mosquito-borne diseases list. It rose to public awareness thanks in large part to the hospitalization of actress Lindsay Lohan in King Edward VII’s Hospital in London, after a bout with the virus late last year left her unable to walk due to intense joint pain.
“With chikungunya, the mortality rate is really low,” says Savage. “It’s a severe threat, though, because not only do you get the fever presentation, but also severe arthritis or joint pain.”
The disease is likely to persist wherever the Aedes albopictus and aegypti settle. As it is transmitted through the same mosquito vector as dengue, misdiagnosis is fairly common, according to the World Health Organization. And while Lohan contracted the virus in French Polynesia, it has also been detected in Europe and the US.
Copy on this page is brought to you by SC Johnson, sponsor of An apple a day series and the values-led business hub.
