The world’s deadliest animal

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What is the most dangerous animal in the world? In the United States we seem to be obsessed with the great white shark, so much so that one television network has an annual “shark week” dedicated to nothing but stories about this fabled killer. The reality, though, is that sharks kill only about 10 or 20 people a year worldwide.

Africa has many really dangerous big animals. Take your pick from lions, elephants, cape buffalos, crocodiles, or hippos. None actually stalks people, but each of these species is responsible for up to a few hundred human deaths a year.

What about the poisonous creatures? Yes, there are all kinds of deadly exotic species, ranging from the box jellyfish, with enough toxin in each of its 60 tentacles to kill 60 people, to the poison dart frog, whose slimy neurotoxin has the power to kill 10 men. They don’t get around much, though, and are responsible for only a scattering of human deaths.

The animal that comes closest to fulfilling our nightmare expectations is the snake. Feared since antiquity and present in most of the world’s regions, snakes really do take a large human toll. When you sum the deaths from poisonous snakes such as the black mamba, the Asian cobra, and the boomslang, you get around 50 000 a year. This is at least double the number of deaths caused by dogs, man’s best friend (except when rabid).

But as pointed out recently in a blog post by the Microsoft founder and health philanthropist Bill Gates,1 all these numbers pale when compared with the havoc caused by the real deadliest animal in the world: the mosquito. Depending on who you ask and how you count, mosquitoes are responsible for between 700 000 and 2.5 million human deaths a year.

The big killer is, of course, malaria, with more than 600 000 deaths a year, the overwhelming majority of which are in Africa. Half the world’s population lives in areas at risk of malaria transmission, with young children, pregnant women, and travelers at greatest risk of infection.2 Mosquitoes also transmit yellow fever (30 000 deaths a year) and the viruses that cause dengue fever and several types of arboviral encephalitides.

Although we tend to think of the diseases transmitted by mosquito as being a problem only in the tropical areas of the world, in fact many are threats to parts of the US as well. Malaria itself, though not endemic to the US, is imported in ever greater numbers by tourists venturing to areas where it is endemic. It is occasionally transmitted within the US too, by transfusion, from mother to child, or even by domestic mosquitoes (the last outbreak was in Florida in 2003).

Dengue fever is endemic in all the US tropical territories: Puerto Rico, the Virgin Islands, Samoa, and Guam. The most recent epidemic occurred in Puerto Rico in 2007, with 10 000 cases. Recent outbreaks have also been reported in Hawaii, Texas, and in 2010 in the Florida Keys.3

The virus most commonly transmitted by mosquitoes to humans in the US is West Nile virus. Over 2300 cases and 114 deaths were reported in 2013. Most infected people have no symptoms; about 20% have a fever and other symptoms; and less than 1% develop neuroinvasive disease, which can be fatal. No specific treatments are available.4

Other arboviruses transmitted by mosquitoes to humans cause rare diseases such as eastern equine and western equine encephalitis, St Louis encephalitis, and LaCrosse encephalitis. In all these diseases many infected people have no symptoms and the infections are discovered only through antibody testing related to blood donations. People who do become ill, however, develop severe neuroinvasive disease, leading to seizures, coma, or paralysis. Again, no specific treatments are available.

Finally, a novel mosquito transmitted virus is apparently now heading towards the US: chikungunya virus, which leads to headache, fever, and muscle and joint pain. It is most often spread by the same mosquitoes that transmit dengue, which bite mostly during the daytime. Outbreaks have occurred throughout the world, and in 2013 it spread to the Caribbean. Most experts expect it to appear in the US soon, perhaps this year.5

All these diseases, which range from bothersome to deadly, are spread by this incredibly efficient blood injecting machine, the mosquito. We need to continue to support activities to control mosquitoes in endemic areas, such as providing bed nets treated with insecticide and indoor residual spraying, as well as supporting research to develop vaccines against malaria and other mosquito transmitted diseases. But my main message, as a (long overdue) spring and summer begin here in the US, is a much less daunting and expensive one: prevent mosquito bites.

The Centers for Disease Control and Prevention recommends a wide range of effective mosquito repellants.6 To help prevent infection with West Nile virus, the most common mosquito transmitted disease in the US, they need to be applied only around dusk and dawn. Or stay indoors. Or wear long sleeves. Or plug in a fan when you sit in your backyard or deck: mosquitoes, it turns out, are not very strong flyers. Simple.

The most dangerous animal in the world can, with a little preparation and care, be defeated.