N(orth)-S(outh) gradients in Lyme disease in US
Very interesting new paper on causes of the sharp N-S gradient in Lyme disease in US
There is a huge gradient in Lyme disease incidence in the eastern US, but no simple explanation. The main tick (I. scap.) is present from ME to FL, as are key reservoir hosts (mice, shrews).
Multiple hypotheses have been proposed for this N-S gradient, including:
-a gradient in host species diversity that results in fewer ticks feeding on the most infectious hosts (called "the dilution effect")
-a gradient in selective feeding by ticks on hosts
-A gradient in the questing behavior of ticks; ticks in the south stay below the leaf litter whereas ticks in the north climb on top of leaves or vegetation to attach to a passing host.
-Variation in tick abundance
-Variation in timing of 2 stages of ticks (nymphs, larvae) that is critical for transmission. If nymphal ticks seek hosts earlier in year & infect them & larvae then feed on those infected hosts, transmission is high. If larvae feed before hosts are infected, little transmission.
This paper assembles data from a multi-year project w/ data collected across the E US (blue circles are sites).
They find that the fraction of adult ticks infected shows huge drop coinciding w/ human incidence pattern, with prev dropping from 35-70% in N to <2% in S.
They argue this results from 2 patterns:
1) gradient in selective feeding patterns: ticks feed (orange bars; % of ticks) on common (blue: % of hosts) & highly infectious hosts in N (mice, voles, shrews) & poorly infectious hosts in S (skinks, lizards)
2) large gradient in tick abundance (L fig: ticks/host)
Even @ S sites where ticks feed on mice (prev tweet: TN), there are so few ticks that hosts rarely become infected (R fig).
Authors previously also showed (doi.org/10.1016/j.ttbd…) that tick questing behavior varies among tick pops by translocating ticks from all sites to MI & TN & measuring fraction questing above leaf litter. S ticks rarely quest above leaf litter; N ticks frequently do.
Hypotheses w/ less support:
-(doi: 10.1093/jme/tjy104) diffs in timing of larval (blue) & nymphal (red) ticks (prev paper, showed mixed patterns). Some S sites look same as N (TN, FL), some have larvae feeding 1st (NC, SC)
Hypotheses w/ little support:
-dilution effect (latitudinal prev doesn't match diversity; but I wish authors had just plotted prev vs diversity)
Obvious Q: why do (some) S ticks selectively feed on lizards instead of mice? Authors suggest it is partly due to latitudinal gradient in questing (S ticks forage under leaves, so encounter lizards more than mice), but more work needed.
Authors conclude w/ speculation about how climate change will affect N-S gradient of Lyme & suggest complexities of invasion by N ticks into S areas might temporarily elevate Lyme before warmer temps select for ticks to feed below leaves to avoid dessication.
Overall, a fantastic body of work by this group of researchers. Some obvious limitations (only 8-9 sites; no rigorous estimates of host density or tick density; no host infec prevalence) but still convincing to me.
A few authors:
please tag more if you know their twitter handles. I couldn't find Nick Ogden, or Howard Ginsberg

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