White-footed mouse. Sadly no one is certain whether this little cutie has been able to find enough acorns to survive the winter. The black-legged tick larvae she carried in her coat though are probably still alive and well and now wandering the forest, in search of their second blood meal. That could be you. Nothing too harmful if it wasn’t for that nasty Lyme bacterium a bite may bring along. [Photo credit: Rick Ostfeld.]

Pests and plagues are often related to climatic changes. The pine beetle outbreaks in the warming Canadian boreal forests are a modern-day example. Yesterday we learned such an example can also be found in the southern US, where the Chagas’ disease-spreading kissing bugs seem to be on the march as the climate warms.

Outbreaks of natural diseases and plagues can however also be stimulated by climatic swings on shorter timescales, seasonal anomalies, like a wet summer can increase mosquitos and malaria risk – or a mild winter may be followed by higher wasp numbers in the following summer.

If you live in the northeastern US and you like to spend time in anything from meadows and forests, you may encounter another such example, scientists of the Cary Institute of Ecosystem Studies warn: a high number of tick larvae – which could well be carriers of Borrelia burgdorferi, the bacterium that causes Lyme disease.

The expected high concentration of ticks (for the months May, June and July 2012) is not a result of the mild US winter of 2011-2012, but has to do with the two previous US summers, so 2011 and 2010 [we recall the US summer of 2011 was hot in the south and southwest, but not exceptionally warm in the North Atlantic states]. The real connection here is not so much with temperature, but with precipitation.

However the local meteorology may have affected this [usually when trees think they may not outlive the season – dry that is – they put all their strength in seeding] it has been noted that oak trees in the northeastern US have produced exceptionally large amounts of acorns in the summer of 2010, followed by very low amounts of acorns in the summer of 2011.

As a result first there was an explosion of white-footed mice, which peaked during the summer of 2011. As the acorn harvest of last autumn was very disappointing to them many of the mice have sadly starved to death last winter.

The initial mice explosion has however also led to a boom in ticks – for which the white-footed mouse is the preferred host. These mice can also carry B. burgdorferi, and thereby infect an entire new generation of ticks.

The ticks need just three blood meals to complete their lifecycle. As larvae they clang on to the mice. After that meal they let go – and underwent a growth transformation to the (still very tiny) nymph stage. [A second blood meal will allow them to become an adult tick and a third to lay tick eggs.]

The large quantities of mice may have gone, but the ticks they helped raise are still around, and so could that Lyme bacterium, the researchers think. Don’t stay indoors all spring [the indoors’ disease is far more prevalent]. But do check your legs. If you manage to carefully remove a tick within 24 hours after the bite, chances of Lyme transmission are still small.

© Rolf Schuttenhelm | www.bitsofscience.org