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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.

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The Lyme Disease is not transferred from ticks to their larvae. This is really uncommon to say the least, so most ticks are "clean" at "birth".

To carry Lyme Disease, the ticks have to become infected themselves. They usually get infected by infected hosts, like mice or hedgehogs. The bacteria now reside in the tick's digestive system. Ticks now develop into small nymphs, and can start to spread the disease already. After successfully feeding again, the nymphs develop into adult ticks.

Usually after infecting a host as an adult, ticks copulate on the host. The male tick dies; the female lays up to 3.000 eggs and also dies. This cycle repeats in the next spring.

This entire cycle can last 3 to 6 years. -- source (in german). This Centers for Disease Control article also explains it well in English.

What we can take away from these information is that there are not too many factors that influence the population of infected ticks. One factor is obviously the amount of infected hosts, from which ticks can get the disease. One could argue that the better term would be "availability", since a large number of infected hosts among a larger number of "clean" hosts might not be beneficial at all.

The availability of infected hosts, again, is somewhat dependent from the amount of infected ticks that can spread the disease. A second but less important factor is the amount of infected ticks (or larvae of nymphs) that survive from previous generations.

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