One of the knocks on the alarms about bird flu is that it is just another in a series of false alarms like Y2K, West Nile and SARS. Not true. Pandemic influenza is indeed another in a series of alarms, but the only one that might conceivably be considered a false alarm (and this isn’t even sure) is Y2K. Let’s take them one at a time.
The investment in fixing the Y2K bug was substantial on the part of business and government world wide, extending over several years prior to 2000. It is difficult to say what the results might have been without that investment. In many respects it is similar to an investment in public health preparedness, where the perennial problem is expressed by the old adage, “When public health works, nothing happens.” Not a recipe to build a political constituency, but a good way to live a longer and happier life.
What about SARS? SARS was first thought to be H5N1 and then discovered to be a hitherto unknown virulent coronavirus. It spread to 29 countries via air travel in a short time and infected more than 8000 people (WHO). It had an unusually high case fatality ratio of just under 10%. In the 9 months of the outbreak, no one knew what to expect from this previously unknown disease. If you reflect on it, the concern by health and civil authorities and the public was well justified. We dodged a bullet with this one, and vigorous action, enabled by appropriate concern, may have played an important part.
What about West Nile Virus? When WNV first appeared in Queens, NY in 1999 it dominated the headlines in US newspapers. WNV was not a new disease, like SARS, but it was new and exotic for the US. Most infections are mild, but a certain percentage are serious and some are fatal. Health authorities warned it would become widespread and endemic throughout the US in a short time. They were correct. Like a stone in our shoe, we have become used to it. But it is as serious as first thought and should not be considered a poster child for public health false alarms.This disease is now the most common mosquito-borne encephalitis in the US. It is just about to start its annual assault on the US population.
Indeed, WNV infected mosquitoes have already been detected in 16 states this season. Like influenza, WNV is primarily a disease of birds, with humans (and horses) being dead-end hosts. We are collateral damage in the virus’s quest to make copies of itself, using birds as a factory. A few wild bird species (notably corvids like crows and ravens) are killed by the virus, but most seem to suffer little, again like influenza. The virus is passed from bird to bird by mosquitoes which bite an infected bird and then bite an uninfected one. It is not passed directly from bird to bird or human to human. Human to human spread via mosquitoes is considered unlikely since most people don’t have significant levels of the virus in their blood to make this happen.
There are some special circumstances that need to be fulfilled for humans to be infected, primarily being bitten by a “bridge vector,” that is, a mosquito that bites both birds and humans. Most mosquito species are fairly host specific and like to bite only one or a few species. Despite this requirement, WNV has managed to gain a foothold and now infects tens of thousands of US residents each year and causes serious illness in a small proportion of them. CDC estimates that in the seven years since its introduction, 1.2 to 1.3 million Americans have become infected, most subclinically or with mild symptoms. About 10% get hit hard. We don’t know why some people do and not others.
Still, West Nile has killed almost 800 people in the U.S. in that brief period, and caused severe neurologic illness, meningitis or encephalitis, in more than 8,300. Others are left with polio-like paralysis.
Even the less severe West Nile fever is “really quite a horrible kind of illness,” says [CDC's Dr. Lyle] Petersen. He caught the disease himself in 2003 — spending a week in bed and a month afterward battling bone-deep fatigue — and he worries that people don’t take the threat seriously enough.
“I guarantee it’ll ruin your summer.” (AP)
Health authorities in the Katrina-ravaged Gulf Coast are especially nervous, not because of the flooding but because of the botched clean-up. Flooding actually decreased mosquito prevalence by washing away larvae. But important bridge vectors for WNV are in the genus Culex, which includes common household mosquitoes. They breed in tiny pools of water, like puddles, lids of cans, bottoms of empty pots, etc. When the weather is hot and dry, birds and mosquitoes compete for scarce water and come together. Culex species also like to bite people.
As predicted, WNV is now nationwide from its original focus in a small section of Queens in 1999. Only two states (Maine and Washington) have yet to report a human case. All continental states have found the virus in mosquitoes. The US blood supply is routinely tested for WNV because it can be passed through the blood of an infected person and many people don’t know they are infected. WNV has lived up to its advance billing — and more.
The track record on warnings has been pretty good. And pandemic influenza is potentially the most consequential of all of them. We had no experience with SARS to calibrate the possibilities. We have experience with influenza.
That experience does not bring comfort.