This is part II of “Measles Week”; see Part I for an explanation of what this is about, and an outline of what’s to come.
Measles is a fairly young disease. Just how young is it?
One of the most characteristic features of measles is its epidemic nature. For example, look at the huge difference between the peaks and valleys here:
|Weekly measles cases, England and Wales, 1950-1965
(click for a larger version)
There are usually a couple of years (one to five years is typical) in between each epidemic. There are a couple of things that drive this pattern:
- Measles is incredibly contagious. R0 for measles — the number of new cases that can arise from a single case, in the absence of immunity — is around 15, more than ten times higher than that of the swine-origin H1N1 we recently have been seeing.
- Measles is very immunogenic and infection confers pretty much life-long immunity. As a corollary to this: Measles infection is short-lived. The immune system eliminates the virus fairly quickly — there’s no carrier stage.
|Part I: Introduction|
|Part II: Emerging disease|
|Part III: Not the answers|
|Part IV: Some of the answers|
|Part V: What about the vaccine?|
That means that when measles has enough susceptible hosts, it can explode and spread across a country almost overnight. But it leaves behind it a firebreak of people who are immune, who are no longer potential hosts. After the epidemic, just about everyone is immune; the virus smolders at some very low level, in the handful of people who are not yet immune. After a couple of years, there are enough new births that there’s a new population of susceptible hosts who can sustain a new epidemic, and so on.
If you think about it, those two factors mean that measles virus needs a pretty good-sized population to keep going. With no carrier stage, the virus has to meet up with a susceptible host in the brief period during which it sheds. In between epidemics, there aren’t many of those. If you’re talking about family groups or tribes or villages with a few hundred people, there aren’t enough newborns to keep the virus going in the epidemic valleys.
Given the observed facts on epidemic timing, spread, and so on, you can calculate out how many people you need to keep measles from going extinct: It’s somewhere around 250,000 to 500,000 people in contact with each other.1
In other words, measles needs cities. It’s a disease of urbanization, and it couldn’t have existed in its present form before good-sized city-states were around.
Even in our highly connected modern world, measles is still an urban disease. Epidemiologically, measles epidemics look like waves emanating from cities:
In the pre-vaccination era, conspicuous hierarchical waves of infection moved regionally from large cities to small towns; the introduction of measles vaccination restricted but did not eliminate this hierarchical contagion. A mechanistic stochastic model suggests a dynamical explanation for the waves-spread via infective ‘sparks’ from large ‘core’ cities to smaller ‘satellite’ towns.2
|Gozu Tenno, a Shinto god, punishes two gods of measles and offers advise on what to eat while suffering from measles. (1862)|
Populations of that size arose around 2000-3000 BC, in the Middle Eastern river valleys; so measles really can’t be older than 4000-5000 years. It’s an emerging disease that emerged some time in recorded history. (Also, it probably arose from the closely-related cattle disease rinderpeste, or some common ancestor; so again, it has to post-date domestication of cattle.)
I personally don’t read Confucian-era Chinese or medieval Arabic, but there are people who do, who say that measles may be described in various old writings. But apparently these earliest descriptions aren’t very clear, and are most likely referring to some other disease. The earliest description that is clearly of measles is around the 9th century, by the great Persian physician Abu Becr Mohammed Ibn Zacariya Ar-Razi (Rhazes) (Wikipedia link).
Rhazes’ description may well have been of a brand-new disease, because it turns out to fit pretty well with a recent analysis, 3 tracking back measles mutation rates to see when it arose, that found a likely origin of measles from Rinderpeste somewhere around 1000 AD — somewhere in the range of 500 – 1600 AD. Also:
Linguistic evidence suggests that the disease was recognized before the Germanic migrations but after the fragmentation of the Roman Empire, i.e., between 5th and 7th centuries … Epidemics identified as measles were recorded in the 11th and 12th centuries3
So, based on genomic information, written documentation, and linguistic evidence, measles isn’t an ancient disease; it’s a disease that jumped into humans some time in the first millennium A.D.
Tomorrow in Measles week: Some explanations for the drastic drop in measles deaths that are plausible, but probably not correct.
- Bartlet MS (1957) Measles periodicity and community size. Journal of the Royal Statistical Society 120:48–70.[↩]
- Grenfell, B., Bjørnstad, O., & Kappey, J. (2001). Travelling waves and spatial hierarchies in measles epidemics Nature, 414 (6865), 716-723 DOI: 10.1038/414716a[↩]
- Furuse, Y., Suzuki, A., & Oshitani, H. (2010). Origin of measles virus: divergence from rinderpest virus between the 11th and 12th centuries Virology Journal, 7 (1) DOI: 10.1186/1743-422X-7-52[↩][↩]