22 Apr 2010 21:29
The popular press is beginning to report on an article that appeared in PLoS Pathogens today, and you can bet public alarm will spread incomparably faster than the “highly virulent fungus” discussed in the article.
The Los Angeles Times was one of the first to pick up on the scare. They interviewed the article’s lead author, Edmond J. Byrnes, III for their report. In PLoS Pathogens, Byrnes and his coauthors described an “increasingly fatal fungal outbreak” in their discussion, without providing statistical support or an inline reference. The Times knows what readers will glom onto, however, and they devoted a couple of paragraphs to this “more lethal” angle.
The spread is also a concern because the strain of the fungus that moved into the United States in 2004 has mutated to become more lethal than the original strain that invaded British Columbia in 1999.
Five of the 21 people who contracted the fungus in the United States have died (about 25%), compared with 8.7% of the 218 infected people in Canada. The fungus has also infected many different species of mammals.
These details aren’t in today’s PLoS Pathogens article. Today’s article focuses on the molecular biology of C. gattii, not its epidemiology. My admittedly cursory search for the source of the Times’s numbers turned up one mention of an “over 25%” U.S. fatality rate. That was in a previous article of Byrnes’s, where he referenced the figure as the “unpublished observations” of two other scientists. But let’s give Byrnes and the Times the benefit of the doubt and suppose the numbers are reliable.
Math time. If you know only a handful of small numbers about a disease, drawing any epidemiological conclusions — especially alarmist ones that might be misconstrued — is risky, but mathematics can still help us evaluate the numbers. There will be lots of ifs, but that’s to be expected when you have limited data. The standard way to proceed is to ask the following question: If in fact nothing scary is happening (meaning that the infections in the U.S. are not more deadly than they are in Canada), might we really see as many as five deaths in 21 cases?
Under the probabilistic assumptions of this kind of analysis, getting infected with C. gattii is treated like a crap shoot as to whether you die or not; you die with probability 8.7%, the Canadian fatality rate. If 21 people get infected and for each one God rolls the dice to see who dies, what’s the likelihood at least five of them will succumb? Well, it’s around 3%. To put that in context, imagine that tomorrow’s traffic is terrible, like once-a-month terrible. Would you chalk it up to bad luck or a real change in traffic congestion? I’d chalk it up to bad luck and assume a real change only if traffic was similarly terrible again tomorrow or maybe next week. (Even then I would probably look for other explanations, like an announced construction project or a visiting dignitary.)
Other factors should be considered when comparing fatality rates to test the hypothesis of increasing deadliness, particularly with such small numbers of cases. Were the U.S. persons infected by C. gattii diagnosed as promptly (or not) as the Canadians and given identical treatments? If not, the higher U.S. fatality rate could be due to late diagnosis or ineffective treatment. Were the U.S. persons infected or killed similar to the Canadians in age, general health, and other factors known to be independent predictors of mortality from disease? If not, the higher U.S. mortality might not be due to greater virulence.
PLoS Pathogens is a peer-reviewed journal, and that provides some assurance that the scientific conclusions were based on accepted scientific practices. Unfortunately, published science has the potential to affect society and policy via the popular press, and reviewers need to think about how a publication might be construed by a journalist.
To Byrnes’s credit and the Times’s, readers who ventured a few paragraphs past the alarming headline will read this more moderate assessment of the situation: “Overall, I don’t think it is a large threat at this time. But the fact that it is continuing to spread geographically and the number of cases is rising makes it a concern.”
Whether or not Byrnes et al. were more justified than I can tell in calling this a “increasingly fatal fungal outbreak,” sure, this could be the next black plague. So could any number of currently very rare or unknown pathogens. But based on the science I’ve seen, you shouldn’t be any more worried about C. gattii today than you were last week, when I’m guessing you’d never heard of it.
- PLoS: Emergence and Pathogenicity of Highly Virulent Cryptococcus gattii Genotypes in the Northwest United States
- LA Times: Potentially lethal fungus moves south from Canada
Updated at 2010-04-22 at 23:28. I added the words Deadly and Fungus to the post title hoping to get more play. A list of headlines on this story follows.
One rebuttal of the alarm: Ore. DHS questions article statements about deadly fungus
(DHS is Department of Human Services, not Homeland Security)
Alarm and alarm in all the other headlines, though.
- Airborne fungus claiming lives
- New strain of virulent airborne fungi, unique to Oregon, is set to spread
- New Deadly Fungus Found in US, Has Already Killed Six
- Potentially Lethal Airborne Fungus May Spread to California
- Deadly Oregon fungus may spread on West Coast
- Deadly airborne fungus in Oregon set to spread
- Killer fungus seen in Pacific Northwest
- Potentially deadly fungus spreading in US and Canada
- Deadly Oregon fungus may spread on West Coast
- Deadly Fungus In Oregon: New Strain Of Fungus Killed 6 in Oregon
- Fungus Cryptococcus gatti Threat to Healthy People
- New Likely Deadly Fungus Invading US & Canada – Signs & Symptoms
- Emerging Northwest fungal disease develops virulent Oregon strain
- Oregon Fungus Spreading South
- Toxic Airborne Fungus From Oregon Spreading Across West Coast
- New Concerns About Deadly Fungus Found in Oregon
- Deadly strain of airborne fungus spreading among healthy people and animals
- Life threatening tropical fungus seen in Pacific Northwest
- Fungus Spreading Throughout US, Canada
- Killer Fungus Migrates To The US
- Killer Lung Fungus Hits Northwest
- ‘Highly Virulent’ Strain of Killer Fungus Found in Ore