Why Mosquitoes Trump Birds

As I write this, the media has been hammering away every day at the latest "confirmed cases" and rumors about avian flu, H5N1. Then there are the blogs that see every unexplained death anywhere in the world as a probable case of a virus that might, just might, one day mutate enough to be able to pass from human to human, killing us with frightening efficiency. Well, there's a virus that has already mutated to pass from human to human, and it kills around half its victims. It causes yellow fever, and all it needs is a certain mosquito to achieve that.

The mosquito's name is Aedes aegypti. Fortunately for half the world, it thrives only in the humid tropics and subtropics (Texas, for example), but that means that many who are headed out to warmer climes to escape rigors of the northern winter are at risk of exposure to it.

Consider the following. The same mosquito carries the viruses for both dengue and yellow fever, and dengue is found in most of the capital cities of tropical countries. Where there's dengue transmission, yellow fever transmission also can occur; it just needs an infected human to arrive and set it off.

Unlike H5N1, a human vaccine proven effective against yellow fever does exist; 10 days after getting your shot, you're protected for life. So why don't all travelers get the vaccine? Well, it's available only at certain specified clinics, and for some, it's too much trouble or expense to get there. Consider these three hapless travelers:

>> A 42-year-old American took a 10-day fishing trip to the Amazon in 1996. It was a 50-mile drive from his home to get the shot, which cost $50, so he didn't get it. He had a lovely time fishing, then fell ill with a fever, flew home to Tennessee, and died of yellow fever. The dread mosquito lives in Tennessee, where it is most active in August. We're darned lucky we didn't see an epidemic then.

>> On Sept. 23, 1999, a 48-year-old unvaccinated man traveling in Venezuela became ill and returned to California, where he died of yellow fever. Fortunately, no A. aegypti mosquitoes were present in California at that time.

>> On March 10, 2002, a 47-year-old unvaccinated traveler returned home to Texas from a fishing trip to the Amazon. He developed a hemorrhagic fever and later died of yellow fever. Texas has the mosquito and has had cases of dengue.

After being almost eradicated from the Americas by a hemispheric campaign, the A. aegypti mosquito has reappeared in many places along the Gulf coast of the United States, including Houston, Galveston, and New Orleans. A yellow fever epidemic in Galveston in 1867 killed 1,100 people; another epidemic could do so again.

Of course, if yellow fever were to get into India, China, and southeast Asia, thousands of deaths would occur. To give an idea of the risk, consider that in 2005, 17,000 dengue cases were reported in Jakarta, more than 800 in New Delhi, 5,000 in Kuala Lumpur, and 13,000 in Singapore. Again, if dengue is present, yellow fever transmission can occur, too.

The three US cases are probably just the tip of the proverbial iceberg. After all, what US clinician is going to suspect yellow fever rather than malaria in a traveler returning home from the tropics with fever and vomiting? So how many times will the United States dodge the bullet?

I realize that no one is going to take a blind bit of notice of what I've written here, because you're all suffering from information overload about bird flu (much of it wrong), and you can't take any more dire prophecies of doom and gloom. But I've written it anyway: Forget about bird flu, and lose sleep about Yellow Jack instead.

Jack Woodall, is the director of the Nucleus for the Investigation of Emerging Infectious Diseases in the department of medical biochemistry at the Federal University of Rio de Janeiro, Brazil.