Mighty health threats from little acorns grow

By Richard S. Ostfeld, Ph.D.

2012 could be a terrible year for Lyme disease. To understand why, we need to go back in time to the autumn of 2010.

Over vast parts of the northeastern USA the oak trees that dominate many forests let loose with a bumper crop of acorns. Oaks are notorious for producing highly variable seed crops, from a trickle of one or two acorns per square meter in some years to several dozen per square meter in others. When protein- and lipid-rich acorns are superabundant, white-footed mice are able to cache large numbers and feast all winter, surviving well and breeding early and often. Consequently, their populations can reach peaks of up to 200 individuals per hectare the summer following a good acorn year. Legions of mice scampering around on the forest floor spell good news for blacklegged ticks, the vector responsible for transmitting Lyme disease and other tick-borne infections.

New baby (larval) ticks, which emerge in midsummer and look for almost any mammal or bird to feed on, survive best when they feed on the blood of white-footed mice (mice appear to be lax groomers). To make matters worse, these larval ticks are much more likely to become infected with Lyme disease bacteria (Borrelia burgdorferi) if they feed on mice than if they feed on some other host. (Why mice are so permissive to ticks and pathogens is not yet understood.) Consequently, the more mice in the forest, the more larval ticks get infected and survive to the next stage, the nymph. And it is these nymphs that are responsible for the vast majority of Lyme disease cases.

If that weren’t bad enough, the autumn of 2011 saw an apparently large-scale failure of the acorn crop in the Northeast. As we have observed in prior acorn busts, this can cause a strong decline in the mouse population over winter, and we are currently (spring 2012) seeing mouse populations only about 20% of their abundance last autumn. If mice remain scarce for the next few months, it is likely that the tick nymphs that emerge in May, June, and July will find very few mice to feed on. Some might die for lack of a host. But others will latch onto some other host, including humans. Mice set the stage for a really bad year then ducked out leaving us to pay the consequences.

This past winter and the current spring have been among the warmest on record. Conventional wisdom suggests that these conditions will foster tick survival and replication, but there is little evidence for this. Ticks that are normally dormant in the cold months have certainly been more active than usual during this mild winter and spring, but the evidence suggests that temperatures are affecting their behavior more than their abundance. We expect that the peak in host-seeking activity of tick nymphs might begin as soon as late April or early May instead of the more typical late May to July peak.

Based on 20 years of monitoring acorns, rodents, ticks and tick-borne pathogens, we have sounded the alarm about the expected high risk in 2012. We’re acting much like Smokey Bear in that we are judging high-risk conditions and publicizing them for public protection. However, just like fire danger (and like the cliché for investing in stocks, that past performance is no guarantee of future results), unforeseen events can affect actual risk. For instance, if current drought conditions persist or worsen, or lead to massive wildfires, then the anticipated tick population boom might not materialize. That’s the frustration and beauty of ecological systems — contingencies are integral. But absent large anomalies like drought and fire, prepare for a really bad year. It’s not too soon to start using repellents, doing daily tick checks of oneself and loved ones, and familiarizing oneself with symptoms of Lyme and other tick-borne diseases. Early detection, removal, and if necessary treatment, are key.

Richard S. Ostfeld is the author of Lyme Disease: The Ecology of a Complex System and Senior Scientist at the Cary Institute of Ecosystem Studies, a not-for-profit research institution in Millbrook, New York, dedicated to providing the science behind environmental solutions. He is also Adjunct Professor at Rutgers University and the University of Connecticut. He has published more than 160 peer-reviewed articles and co-edited 4 books, including a Princeton University Press volume on Disease Ecology (2008). His research focuses on ecological determinants of human risk of exposure to infectious diseases, emphasizing Lyme and other tick-borne diseases as well as West Nile Virus. He sits on the editorial boards of Ecology Letters and Vector-borne and Zoonotic Diseases.

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