In putting together yesterday's post on Lyme disease prevalence I noticed something very strange with Massachusetts:

It's even more striking if you look at a map (2018):

There's no way Lyme follows state borders like this; it has to be some sort of data issue.

Looking over several years of CDC maps, the year that Massachusetts starts looking different matches up with the huge drop in the chart above. Compare 2015:

To 2016:

The only public explanation there seems to be is this short news article. In 2016 the MA Department of Public Health "stopped spending time and resources trying to track down the clinical information, instead relying solely on positive lab results to give a more accurate estimate of Lyme disease case numbers."

The CDC gets their information from the states, and in this case requires both a clinical diagnosis and a positive test. MA, apparently uniquely among the states, has decided that when compiling statistics confirming a clinical diagnosis is extra work that doesn't improve accuracy. So MA doesn't generally have the stats it would need to file with the CDC, and instead of marking MA as "no data", the CDC publishes the trickle of reports that it does get from MA.

I don't know whether the CDC or MA is being more unreasonable here, but the effect is pretty bad: user-facing websites like TickCheck show MA as now having low Lyme levels. The CDC does not even include a note in their FAQ to say MA data shouldn't be trusted.