B. burgdorferi is transmitted by the Ixodes ("deer tick") family of "3 host" ticks. These ticks require 2 years to complete their life cycle and must feed on 3 independent hosts during this cycle. (Click here for a photo of the larvae, nymph and adult Ixodes scapularis ticks)

• There is minimal transovarial transmission of B. burgdorferi in ticks, so each new generation of ticks must be infected de novo by feeding on an infected host. The organism can, however, be transmitted transstadially from larvae to nymph to adult.
• In general, the tick larvae first become infected by feeding on rodents that are competent hosts for B. burgdorferi (i.e., these rodent hosts replicate B. burgdorferi to a sufficient level to be infectious for subsequently feeding ticks).
  • Robins have also been shown to be competent hosts for B. burgdorferi.
  • In the southeastern and western U.S., reptiles may also serve as competent hosts, although, conversely, some lizards contain a borreliacidal substance in their blood that eliminates Borrelia from feeding ticks.
• The tick nymphs and especially adults obtain their blood meals by feeding on larger mammals. Deer are the preferred enzootic feeding hosts- dogs, horses, cows, people etc. are accidental victims of a hungry tick!
  • Deer are not competent hosts for B. burgdorferi - their role is to maintain the ticks, not the Borrelia.
• Nymphs are the life stage most commonly involved in transmitting B. burgdorferi to dogs and humans.

There are 3 major foci of LD in the U.S.:

• The Northeast was historically the initial hotbed of LD, although LD is now known to occur widely throughout the Mid-Atlantic and Southern regions of the eastern U.S. as well. (~85+% of human cases; incidence = 110/100,000 persons in CT)
• The upper Midwest (Wisconsin and Minnesota in particular) (~10% of human cases)
  • In both the Northeast and Midwest, the white-footed mouse is the major reservoir species and Ixodes scapularis is the primary tick vector.
• The Pacific coast (northern California and Oregon in particular) (~4% of human cases)
  • In this region, the dusky-footed wood rat is the major reservoir species and Ixodes pacificus is the major tick vector responsible for transmission of B. burgdorferi to humans and dogs. However, a different tick (I. spinipalpis) is the primary vector that maintains B. burgdorferi in the wood rat reservoir.

(Click here for a map describing the epidemiology of Lyme disease in the U.S.)

* Ticks must be attached to a host for 24-48 hours before transmission of B. burgdorferi can occur. (Time is required for the bacteria to migrate from the tick midgut to the salivary glands.) Therefore, regular examination for and removal of ticks is an important and effective preventive measure in endemic areas!

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