Lyme disease, Lyme borreliosis, has been reported by the Centers for Disease Control as existing, or probably existing, in many of the United States. The Lyme Disease Association also reports it in over 80 countries worldwide.
The disease is caused almost exclusively by a bite from a tick in the Ixodes family. According to the Centers for Disease Control in America, in the Eastern United States it's the blacklegged deer tick, Ixodes scapulari, and in the Mid-Western and Western United States, it's the blacklegged tick Ixodes pacificus. The European Centre for Disease Prevention and Control attributes much of Europe's Lyme to the Ixodes ricinus.
I live in Massachusetts, Northeastern United States, which is one of the major hot spots. From 2005 to 2015, the CDC consistently ranked us among the 14 states with the highest reported incidences. In 2015, we were number 5, a dubious honor indeed!
The tables and data all show changes in the number of reported cases over the years. Also, a large number of cases worldwide go unreported.
Still, it's obvious that the Lyme-carrying tick population varies, sometimes greatly, from year to year.
What are some causal factors for the fluctuating populations of ticks which carry Lyme Disease? I'm most interested in the larger fluctuations.