Hard ticks (Acari: Ixodidae) act as important vectors of zoonotic pathogens. For instance, Borrelia burgdorferi s.l. spirochetes pose a severe health risk as aetiological agents of Lyme borreliosis. Commonly, to study the abundance of questing (host-seeking) ticks, a 1 m2 piece of cloth is dragged over vegetation for a determined distance. Here, we designed a tick-sampling study to estimate the sampling efficiency of this standard method. We established 10 m dragging transects in a hemiboreal mixed forest patch in SW Finland for a 5-day monitoring period.
- Five of the transects were cloth-dragged 3× a day,
- whereas another five transects were dragged 6× a day in a manner that after each morning, midday and afternoon dragging, a second dragging was conducted on the same transect immediately.
Captured Ixodes ricinus ticks were subsequently analyzed for tick-borne pathogens. The initial population size of nymphal ticks on a transect was approximated by the accumulated nymph catch from the dragging sessions. The sampling efficiency of the cloth dragging was low, as
- a single dragging in a previously untouched vegetation strip always caught less than 12% (mean 6%) of the estimated population of active nymphs that were assumed to be questing during the study.
- Clear results were not found for daily activity rhythm, as ticks were caught in all daily dragging sessions.
- Approximately every third nymph and every second adult carried a pathogen, but nothing indicated that the occurrence of a pathogen affected the likelihood of the tick being caught by cloth dragging.
Our results suggest that only a minority of active ticks can be caught by a single cloth dragging. The abundance estimates in many tick investigations might thus be downward biased.
Cloth dragging to estimate tick prevalence has been used worldwide. Similarly to the CDC’s past vastly low estimates of those infected with Lyme disease, and then having to increase this number 10-fold (which is still probably low), this study reveals tick prevalence to be much higher than we’ve been told due to single cloth dragging missing many ticks. This in turn means there are more infected ticks that can transmit disease to animals and humans.