The Lyme-related Borrelia species are collectively known as Borrelia burgdorferi sensu lato, and show a great deal of genetic diversity.B. burgdorferi sensu lato is a species complex made up of 20 accepted and three proposed genospecies. Eight species are known to cause Lyme disease: B. mayonii (found in North America), B. burgdorferi sensu stricto, (found in North America and Europe), B. afzelii, B. garinii, B. spielmanii, and B. lusitaniae (all found in Eurasia). Some studies have also proposed that B. valaisiana may sometimes infect humans, but this species does not seem to be important causes of disease. Tick life cycle
There are three stages in the life cycle of a tick. Larva, nymph, and adult. During the nymph stage, ticks most frequently transmit Lyme disease and are usually most active in late spring and early summer in regions where the climate is mild. During the adult stage, Lyme disease transmission is less common due to the fact that it is less possible for adult ticks to bite humans and because they tend to be larger in size and therefore can be easily seen and removed. Transmission
Lyme disease is classified as a zoonosis, as it is transmitted to humans from a natural reservoir among small mammals and birds by ticks that feed on both sets of hosts. Hard-bodied ticks of the genus Ixodes are the vectors of Lyme disease (also the vector for Babesia).
In the ten states where Lyme disease is most common, the average was 31.6 cases for every 100,000 persons for the year 2005.Although Lyme disease has been reported in all states about 99% of all reported cases are confined to just five geographic areas (New England, Mid-Atlantic, East-North Central, South Atlantic, and West North-Central). New 2011 CDC Lyme case definition guidelines are used to determine confirmed CDC surveillance cases.Effective January 2008, the CDC gives equal weight to laboratory evidence from 1 a positive culture for B. burgdorferi; 2) two-tier testing (ELISA screening and Western blot confirming); or 3) single-tier IgG (old infection) Western blot. Previously, the CDC only included laboratory evidence based on (1) and (2) in their surveillance case definition. The case definition now includes the use of Western blot without prior ELISA screen.The number of reported cases of the disease has been increasing, as are endemic regions in North America. For example, B. burgdorferi sensu lato was previously thought to be hindered in its ability to be maintained in an enzootic cycle in California, because it was assumed the large lizard population would dilute the number of people affected by B. burgdorferi in local tick populations; this has since been brought into question, as some evidence has suggested lizards can become infected.Except for one study in Europe, much of the data implicating lizards is based on DNA detection of the spirochete and has not demonstrated that lizards are able to infect ticks feeding upon them.