Range expansion of the Lyme disease vector

Since 1975, Lyme disease has been diagnosed in 49 US states, with about 300,000 cases a year, concentrated in the Northeast and Midwest. Recent years have seen a dramatic increase in diagnoses. In Pennsylvania, for example, diagnoses rose 25 percent between 2013 and 2014. Scientists attribute the spread to the fact that populations of blacklegged ticks (Ixodes scapularis), which carry the bacteria that causes the disease, now flourish in areas once thought to be devoid of ticks. A 2012 study confirmed a tight association between populations of blacklegged ticks and numbers of human cases and also documented increases in tick populations in areas of the Northeast that were previously thought to not harbor ticks.

In a new study, researchers used genetic and phylogeographic analyses to determine the origin and recent migratory history of newly discovered tick populations in the Northeastern United States. The researchers considered two hypotheses: either the "new" tick populations were actually not new but reflected growth in populations that were previously in the area at undetectable levels, or the populations could have arisen from ticks moving in from other areas, likely by hitching a ride on birds or mammals.

To determine the recent population history of blacklegged ticks in the Northeast, they analyzed data collected in four locations in New York's Hudson River Valley between 2004 and 2009. They sequenced portions of the mitochondrial DNA (16S rRNA, COII, Control region) to understand the relationships between tick populations and how they changed over time. Too bad that they did not include COI in their study to allow connection to existing DNA Barcode data. There are hundreds of sequences on BOLD which would perhaps have allowed for a much larger phylogeographic analysis.

In any event, their findings indicate that the ticks moved into new areas from established populations, mainly through short-distance, local moves. The phylogeographic analysis indicates that ticks mainly moved in progressive south-to-north migration events between neighboring locations, with occasional long-distance movements.

We know that migrating birds can transfer ticks over hundreds of miles during their annual movements. But since the majority of movements documented in this study took place over short distances, less than 100 kilometers, it is not clear under what circumstances animals were moving the ticks. The colleagues roughly estimated that the northernmost locations were colonized 14 years ago, the second most northern 31 years ago and the third most northern 40 years ago. These findings are supported by the fact that no ticks were found in the northernmost site when sampling was done in the early 2000s.

The reason for the population expansion remains unclear. It is possible that the ticks are adapting to new local environments, or that changes in land use and climate are making the new environments more suitable for them. While more work remains to be done to understand what is driving the movement and expansion of ticks, knowing more about their migrations could help inform efforts to protect the public from Lyme disease. Any knowledge on patterns of disease spread could have implications for strategies to control ticks in order to reduce disease. 

From a control perspective, if you know they are moving extremely easily, you could control them in your backyard but they might be back in a week. If we want to reduce tick populations over the long term, this means we have to start thinking about more sophisticated approaches.