As a blacklegged tick (Ixodes scapularis) expends energy, it burns fat content in its body, and the ratio of certain body measurements change accordingly. Researchers at Fordham University say the fat content is a reliable predictor of the tick’s remaining energy, and thus life expectancy, but the body measurements are not reliably predictive. (Photo credit: Susan Ellis, Bugwood.org)

The winter of 2011-2012 was a mild one in the New York City area, and the following summer saw the lowest population density of nymphal blacklegged ticks (Ixodes scapularis) in the region since the Vector Ecology Laboratory at Fordham University had begun measuring it in 1987. And the ticks that were collected did not survive long in the lab. Researchers at the lab wondered why, and the work to answer that question eventually led to the doctoral dissertation work of Fordham Ph.D. candidate Justin Pool.

Part of that work was published this month in the Journal of Medical Entomology. Pool and colleagues tested methods for determining the physiological age—or, essentially, the life expectancy—of blacklegged ticks. They found that a simple measure of physical aspects of tick nymphs was not sufficiently predictive of aging but that the extraction of a tick’s fat content was.

“These findings are important because we can analyze ticks from the field and determine their physiological age, or roughly how much energy they have to survive and seek a host, known as questing behavior,” says Pool. “This type of data can be tracked over multiple years to look for associations with key environmental drivers affecting the blacklegged tick population.”

I. scapularis nymphs develop by feeding on a host, requiring one “bloodmeal” per instar stage. Between bloodmeals, the nymphs survive on stored lipids (fats), which they can only replenish with another feeding. Seeking a host, however, requires energy, as the ticks climb up on plants to get closer to potential hosts passing by and return to the ground to restore moisture, repeating this travel once a day. Because of this energy expenditure, a tick’s lipid content is understood to be an indicator of its age and life expectancy.

Previous research has also suggested that a tick’s body dimensions, specifically the ratio of the size of its scutum to its alloscutum, could indicate physiological age, as well. However, Pool’s team found the physical, or “morphometric,” measurement to correlate significantly with lipid content but not enough to be reliably predictive. That’s unfortunate news for researchers hoping that field measurements of tick body size could be as effective as more complicated lab procedures.

“We were a little surprised that the morphometric age ratio did not have good predictive value,” Pool says. “If that ratio could reasonably predict how much total lipid a blacklegged tick had, it would be possible to measure a tick and then place it back in the field or laboratory setting and follow how the tick was utilizing its energy reserves. So, while the pattern of energy usage is clear and now well-documented, it appears that other factors also affect morphometric measurements and that measuring lipid content is needed to accurately gauge energy reserves.”

The lab study of the lipid extraction method shows that an I. scapularis nymph’s lipid reserves can indeed be used as a reliable predictive index for ticks’ aging. The method’s potential implications for management of ticks aren’t yet clear, Pool says, but a better understanding of ticks’ energy usage and how that is affected by environmental factors will inform the usage of various control methods.

“Now that we have a system for measuring lipid content in nymphal ticks, there is an opportunity to bring this protocol into our long-term monitoring efforts by collecting ticks throughout the year and determining how they use energy,” says Thomas Daniels, Ph.D., co-director of the Fordham Vector Ecology Lab and co-author of the study. “We anticipate that with ongoing changes in local climate patterns—a result of global warming—ticks will respond in predictable ways to a warmer world over the following years. Ticks may turn out to be a good model organism for investigations of the effects of climate change on parasitic arthropods. The fact that they are also vectors of several disease agents makes the range of questions that can be asked even more interesting.”