The Centers for Disease Control and Prevention released an alarming report yesterday, declaring that cases of diseases spread by ticks, mosquitos, and fleas more than tripled in the US between 2004 and 2016. Unnerving headlines followed, emphasizing the tripling (e.g. The Washington Post) or some making claims that tick and mosquito infections are “spreading rapidly” (e.g. The New York Times).

But a look at the data tells a more nuanced, less alarming story.

The CDC’s data, published in the agency’s Morbidity and Mortality Weekly Report, clumps together cases of 16 different types of diseases spread by insects (called "vector-borne diseases"), which are each reported to the National Notifiable Disease Surveillance System (NNDSS). This system covers all US states as well as US territories. The agency noted in its press materials that this is the first time they’ve ever aggregated disease counts together like this for one analysis. (Why they chose to start this year is anyone’s guess. Perhaps because it allowed them to say things like “diseases tripled.” Who knows?)

Pack of pathogens

The 16 diseases include six tick-borne diseases; the big one is Lyme disease, but there's also spotted fever, and anaplasmosis and ehrlichiosis (counted together). There’s also the relatively new babesiosis, and the rare tularemia and Powassan virus. Those last two affected just 230 and 22 people, respectively, in 2016.

CDC researchers examined nine mosquito-borne diseases, including West Nile virus, which is endemic to the US. Its case counts fluctuated with periodic outbreaks during the timeframe but didn’t increase overall. CDC experts also netted counts of the newcomers in US-based mosquito-borne diseases—Zika, dengue, and chikungunya—as well as malaria and yellow fever, cases of which are entirely travel related. (There was no malaria or yellow fever transmission within the US during the 13-year period. And there was only one case of yellow fever in that time, which occurred in 2016.)

Researchers also looked at cases of rare mosquito-borne diseases, such as a collection of California serogroup viruses, St. Louis encephalitis virus, and eastern equine encephalitis virus. These did not significantly change over the 13-year period and caused 53, 8, and 7 illnesses, respectively, in 2016.

So what did increase?

Last, the researchers looked at one flea-borne disease— plague . Perhaps surprising to some, plague is entrenched in rural rodent populations in many areas of the Western US and has been since the early 20century (see map in gallery below). As such, there are a handful of cases in people each year, typically presenting as bubonic plague. Case counts usually range from one to fewer than 20. There were three cases in 2004 and four in 2016, with a high of 17 in 2006. There was no significant change in the rate of plague during the time frame, so the rest of the article will focus on the tick and mosquito-spread diseases.

There are a lot of diseases here, each with its own factors and quirks. What accounts for this scary “tripling”? The short answer is: mostly Zika and Lyme disease.

Between 2004 and 2016, the 16 vector-borne diseases caused in total 642,602 cases of illnesses. The grand total of diseases for 2004 was 27,388, but that jumped to 96,075 in 2016.That means that there were an extra 68,687 disease cases in 2016 compared with 2004.

About 61 percent—or 41,680 cases—of that increase came from Zika; all of the Zika cases occurred in 2016. That was the only year that the NNDSS system collected case counts on Zika. That means that there is not enough data to establish trends for this disease’s prevalence. Of those 41,680 cases in 2016, 36,512 were counted in US territories (Puerto Rico, US Virgin Islands, and American Samoa, which all had outbreaks). The remaining 5,168 were counted in US states, but 4,897 of them were travel-related. The last 271 cases were transmitted in US states, mainly in Florida and Texas.

Unexpected and explosive epidemics of emerging pathogens, such as Zika virus, are devastating and require resources to prepare for and control. That said, it’s a bit misleading to say that vector-borne diseases overall tripled throughout the US, when the biggest cause of that increase is one year of epidemic cases in specific territories—one that just happened to occur in the last year of the 13-year period analyzed.

Ars reached out to the CDC to ask why the data was framed this way. Dr. Ronald Rosenberg, a vector-borne disease expert and lead author of the new report got back to us saying: "Your point about Zika inflating the tally is well taken but we can say that about all the epidemic arboviruses [these types of insect-spread viruses], depending on the period you choose it could be up or down. What is significant is that we continue to have epidemics of not only those viruses known before 2004 but the two new ones [Zika and chikungunya]."

Uptick in tick-borne diseases

So, what about the rest of the increase? About 24 percent was from Lyme disease. But, that increase also requires an asterisk. Lyme disease case counts went from 19,804 in 2004 to 36,429 in 2016. But the CDC changed the way it counted Lyme disease cases back in 2008, adding “probable” cases into the “confirmed” case counts. Unsurprisingly, the number of Lyme disease cases peaked in 2009, with 38,468 cases. It has been below that level ever since (see graph in gallery above).

That said, there’s no doubt that the incidence of Lyme disease is increasing—albeit at a slower pace. This has been going on for decades, as is the case for some other tick-borne diseases. A study by CDC researchers in 2008 looking at Lyme disease surveillance between 1992 and 2006 concluded that the number of reported cases doubled in that timeframe. They attributed the rise to “multiple reasons… including a true increase in the number of infections, enhanced surveillance, increased awareness among health-care professionals and the public, misdiagnosis, and reporting errors.” The “true increase” in cases may be due to booms in tick populations, encroachment of human development into rural and suburban areas, and expansion of reservoir animal populations (that is, animals that harbor the pathogens before they’re transmitted to humans, such as mice and squirrels in the case of Lyme disease).

Overall, the CDC researchers concluded that the 2008 report’s findings “underscore the continued emergence of Lyme disease and the need for tick avoidance and early treatment interventions.”

The gradual, continued increase of Lyme disease has in turn upped detection and surveillance of other tick-borne diseases, such as babesiosis, spotted fever, and anaplasmosis and ehrlichiosis. Those also saw increases in the current study, albeit at much lower levels than Lyme.

Overall, the bigger bites of tick-borne diseases and the buzz of periodic and emerging mosquito-borne diseases are urgent threats to public health. The CDC and local health authorities need more resources to monitor, anticipate, prevent, and control their spread. And the public needs to remain vigilant, including packing that bug spray as tick and mosquito-season is upon us. But most of the US is not seeing a sudden surge in these diseases—it's just that these truths make for less exciting headlines.

This post has been updated to include a response from the CDC.