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

Virus Slams Unvaccinated

7 07 2011

A deadly disease is marching its way across the United States and Canada. It’s a disease that infects about 20 million people every year and kills about 200,000. The United States once was a hotbed of infection, seeing almost 900,000 cases of this disease in 1941. But by the 1990s, that number had dropped to fewer than 150 cases annually. Why? Vaccinations.

The disease is measles. It sounds . . . childish, doesn’t it? And people often refer to it as a “childhood disease.” But make no mistake. It’s a virus, one that doesn’t care whom it infects or what tissues it targets, whether brain or lungs. A virus that has a 90% infection rate. A virus that kills children who seem perfectly healthy one day and are dead from lung complications or encephalitis the next. Roald Dahl’s daughter died of measles. Mark Twain almost did. Even though the descriptive “childhood” often accompanies it, there’s nothing remotely childish or casual about this virus. Hospitalization rates are high, and death is not uncommon. In 2005, for example, a total of 311,000 children worldwide died from measles.

And a couple of shots in the arm (or leg) can prevent all of it.

You might think that the outbreak in 2008 would’ve spurred some parents to ensure vaccinations for their children. After all, that year saw more measles cases in the United States than had happened in any year since 1997. Of the people infected, 90% had not been vaccinated or had an unknown vaccination status, according to the Centers for Disease Control and Prevention. Now, this year is well on its way to besting that record and then some.

Some notable facts about this year’s outbreak through May 20, 2011:

  • From 2001 to 2008, a median of 56 measles cases were reported annually to the CDC.
  • During the first 19 weeks of 2011, 118 were reported.
  • 89% of this year’s cases have been linked to import from other countries.
  • About 89% of those who have contracted measles so far have been unvaccinated.
  • 40% of those who have contracted measles in this outbreak have been hospitalized.
  • All but one of the hospitalized patients were unvaccinated (the one vaccinated patient was hospitalized for observation only).
  • Rates of hospitalization have been 52% for children under 5 years and 33% for children over age 5 and for adults.
  • Transmission has occurred in households, childcare centers, shelters, schools, emergency departments, and at a large community event.
  • One outbreak alone in Minnesota has encompassed 21 people so far, including seven infants too young to have been vaccinated.

This virus doesn’t care who you are, how old you are, how healthy you are, whether or not you were breastfed or organically fed or loved beyond all measure. It’s a virus. It kills, with pain and distress. And, it bears repeating, a couple of shots in the arm can stop it.

By Emily Willingham

Image courtesy Wellcome Library, London