This week Deborah and I saw our first ticks of the season. One was an engorged tick that came off of one of our pets (we have since treated our cats and dog with a topical flea and tick repellant), and the two others were deer ticks that we found on ourselves following a walk in a nearby woods. My guess is that along with everything else that is bursting into activity with our “sudden” conversion into spring, the ticks are also going to start to be both plentiful and active.
Pasted below is a species page I wrote for the Virtual Nature Trail about deer ticks. I also came across an interesting article (2001 in The New England Journal of Medicine) concerning prophylactic treatment of tick bite patients with doxycycline to lower the incidence of Lyme Disease. Incidence of the development of the “bull’s eye” pattern of erythema and the progression into Lyme Disease went from 3.2% to 0.4% in patients given a single antibiotic dose. As always, talk to your doctor if you encounter a tick! Tick removal suggestions are included at the end of the species page.
The Deer Tick
(Information for this species page was gathered in part by Brittany Wetzel for Spring 2006, Biology 220W at Penn State New Kensington)
Deer ticks (Ixodes scapularis) are small, relatively hard-bodied, chelicerate arthropods that are found in some abundance in the northeast, midwest, and southeast sections of the United States. Adult deer ticks are approximately 3 mm in length and are dark brown to black in color (another common name for this species is the “black legged tick”). Immature life stages (larvae and nymphs) are even smaller than the adults and were at one time thought to comprise a separate tick species (“Ixodes dammini“). These life stages now, however, have been clearly shown to be part of the developmental sequence of I. scapularis.
The deer tick’s life cycle represents a repeating pattern of blood feeding, growth and moulting, and the selection of ever larger vertebrate host species. Eggs laid in the leaf litter early in the spring hatch in mid to late July into 6-legged larvae. These larvae quickly attach themselves to small vertebrate host species (like white footed mice or chipmunks) and feed on these hosts for 3 to 5 days. Dropping off of these hosts and falling back into the forest leaf litter, the engorged, larval ticks either directly moult into the larger nymphal life stage or delay moulting and overwinter until the next May. The nymphs have the characteristic 8 legs of chelicerate arthropods. They seek out larger hosts (like squirrels or opossums) for their next blood meal. These nymphal life stages are very commonly the form of the deer tick that opportunistically attach to and feed on humans. After a 3 or 4 day feeding period, the nymphs drop back into the forest litter and moult into the adult life forms. These adult deer ticks become active starting in October and may remain active through the winter if non-freezing microhabitats are available (see discussion of the “subnivia”). The adult ticks attach to white tailed deer (hence the name “deer tick”) and females feed on the deer for 5 to 7 days becoming massively engorged. The males only feed slightly on the deer but may use their attachment time to seek out females. Mating may occur on the deer host or in the leaf litter immediately following release from the host. Females lay between 1000 and 3000 eggs in the leaf litter typically in May. Both males and females die soon after mating and egg laying.
To successfully complete their life cycles, then, I. scapularis requires the presence of white tailed deer. It is not surprising, therefore, that any area with substantial populations of white tailed deer also frequently has high densities of I. scapularis. White tailed deer are also important dispersal agents in the spread of I. scapularis up and down the eastern seaboard, into the midwest, and across the southern sections of the United States. Within a forest ecosystem, I. scapularis is most abundantly found along deer trails and in feeding and bedding areas frequented by white tailed deer.
Ixodes scapularis is especially found in forested ecosystems but is also frequently abundant in shrub lands, leaf piles, and even in mowed fields and lawns. Ixodes scapularis is especially common in small forest plots (which by their geometry have a high percentage of edge ecotones which are typically rich in browse vegetation for white tailed deer). These ticks also seem to prefer mixed hardwood forests (especially mixes containing hickory, poplar, maple, and beech) and are also positively correlated with the presence of greenbrier, blueberries, pepperbush, snakeroot, and sassafras and with the abundance of the exotic, invasive plants like barberry and Japanese honeysuckle. The accumulation of leaf litter and brush is positively correlated with the presence and abundance of I. scapularis undoubtedly due to its contribution to the quality of the tick’s “off-host” habitat. Moderate temperatures and high humidity also favor the survival and abundance of I. scapularis.
An interesting behavior called “questing” has been observed in all three life stages of I. scapularis. When an individual is ready to find a host for a blood meal it climbs to the top of the surrounding vegetation (typically the herbaceous plants of the forest floor) and remains in place on the tips of these plants with its forelegs extended up from the plant’s surface. When a potential host brushes past the plant, the tick clamps onto it with its front legs and then quickly moves up and over the host to find a suitable feeding spot.
One of the significant human related problems associated with I. scapularis concerns the tick’s role in the transmission of the bacterium Borrelia burgdorferi. This bacterium is pathogen that causes Lyme disease in humans. The bacterium is picked up by I. scapularis when it takes a blood meal from an infected host. Inside the tick the bacterium goes into an inactive state until the tick begins to feed on its next life-cycle host. It takes 12 to 48 hours for the bacterium to become active enough to be transmitted by a feeding tick. This information is extremely important to anyone who picks up an I. scapularis. Prompt removal of the tick (and this is best accomplished by using forceps to gently pull the entire feeding structure (“head”) of the tick out from the skin) greatly reduces the possibility that the Lyme disease pathogen, if present, will be transmitted. It is vital, then, to do a careful “tick check” on anyone who has been out in the woods or in areas known to have I. scapularis populations. Of course, an even better way to prevent the possibility of getting Lyme disease is to prevent the tick attachment in the first place. Long sleeves and long pants, use of insect repellants with high concentrations of DEET, and avoidance of deer frequented areas are all excellent strategies by which one can deal with this species and still go out and enjoy hiking and exploring in our natural ecosystems.