Small mammal trapping to determine tick burdens on Lyme disease reservoir species

     We have been setting nonlethal traps on sixteen plots throughout the property to catch small mammals.  Traps are set for three consecutive nights, and are checked early in the morning.  Each small mammal is weighed, measured, and tagged on the ear with a unique identification number that allows us to recognize individuals that have been caught more than once.  The main objective of our small mammal trapping project is to determine the tick burden the mammals are carrying. This late in the summer, we are most likely to encounter larvae, although we have found that some nymphs are still present.

Left: A tick larva (above my thumb) and nymph (above my thumb and slightly to the
right)  Right: A close-up of the larva (note: larvae have six legs while nymphs and
adults have eight)

     White-footed mice (Peromyscus leucopus) are the most common small mammals we catch, and they are also a major host species for black-legged or deer tick (Ixodes scapularis) larvae and a reservoir for Lyme disease (Borrelia burgdorfer).  In order to pass from one life stage to another, ticks require a blood meal, after which they drop off to molt and must find a new host.  Larvae often attach to small mammals or birds, and if a larva's host is carrying the bacterium B. burgdorfer, that larva will carry Lyme disease and spread it to future hosts.  For more information on the tick life cycle and how they spread Lyme, click here.
     When we catch a small mammal, we count and remove tick larvae and nymphs.  If we catch the same small mammal more than once in a three-day period, it allows us to see how quickly they are picking up new ticks  This can help us understand the abundance of ticks in each area.  Trapping will also give us a better understanding of small mammal subpopulations, demographics, and diversity throughout the property.

White-footed mouse (Peromyscus leucopus) being checked for ticks.

     Most of the ticks we find on small mammals are on the head, especially the ears.  It is estimated that ticks found on the head make up about 40% of the animal's entire body tick burden.  One unlucky individual had 31 tick larvae on one ear and 26 on the other, in addition to carrying a botfly larva.  This was an extreme case, and tick burdens vary from one site to the next.  

     One of the white-footed mice we've caught is a veteran of last summer's mouse over water experiment.  Last year he was caught, tagged, and kept for seven days in an enclosure over water until his ticks dropped off into the water and could be counted.  The mice in that experiment were well-fed and released near where they were originally caught.  This year, he was caught on three consecutive trapping days, and seems to have decided that having an easy meal is worth being handled.  

Trap-happy #46, calmly enduring being handled

     So far we have finished trapping at three of our sixteen sites.  Sadly, we had to say goodbye to #46 and leave him to forage for food like a real mouse.  The data we have gained from this project will be used along with the tick phenology data and earthworm data from the same plots to better understand the relationship between tick populations and survivorship and the presence or absence of invasive earthworms.