Ask Emily

24 05 2012

Will you explain the differences (and similarities) between endemic and epidemic diseases?

Yes, and I’ll throw in “outbreak” and “pandemic” for good measure.

First, anything that is endemic, whether a disease or an organism, occurs only in a specific group or area. “Endemic” implies “occurs only in.” So, the marine iguana, which occurs only on the Galapagos Islands, is an endemic species to those islands—it doesn’t live anywhere else. Substitute a disease for “marine iguana,” and you get the start of an idea of what an endemic disease is.

But there are a few extensions of that idea. First, an endemic disease can be one that occurs only in a certain area—and in this global society, that’s becoming increasingly rare. An example is Venezuelan equine encephalitis, which lurks in neotropical areas, usually in horses, but occasionally crops up among humans in these regions.

Endemic can also mean, however, that the disease has a constant presence in the population or area, perhaps at low levels, but always there—it never quite reaches zero in the defined population. Tuberculosis is an example of a disease that is endemic in many areas of the world, often carried around by people who don’t even know they’re infected.

Endemicity also comes in subtypes, depending on when infection occurs. If it occurs mostly in children in the population, the disease is holoendemic. Malaria is an example. A hyperendemic disease like influenza, on the other hand, is usually an equal-opportunity infector.

Is it possible for an endemic to become epidemic? Yes. If a disease that’s been lying low in a population suddenly shows sharp uptick in the population, that’s an outbreak. An epidemic is a burst of disease activity that spreads beyond the local population. So something that is endemic because it never quite hits zero cases—like measles—or something that’s endemic because it’s so localized, like Venezuelan equine encephalitis—could break out of its usual population bounds and spread across other populations.

Back to our endemic marine iguanas: If they suddenly kicked up their population levels on Galapagos alone but nowhere else, that would be an outbreak of iguanas. If they broke away from the Galapagos and established a claw-hold on the mainland, they’d be an epidemic of iguanas.

If the iguanas—or a disease—were to break the bounds of its immediate continental confines and spread to other continents or globally, that’s called a pandemic. A pandemic of iguanas is probably not a realistic concern, but the human population has already faced down a few pandemics in recent memory, including the Spanish flu, and still grapples with the pandemic of HIV/AIDS. While influenza remains the watchword for pandemic anxieties, no one can genuinely predict what the nature of the next pandemic will be.

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By Emily Willingham

Image courtesy of Wikimedia



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