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Finding a tick on the skin does not equate to an infection. Once a tick crawls onto a human it will take at least 24 hours to find an appropriate site to feed. The most common sites are warm and moist areas, such as the genital and axillary areas, behind the knees, and on the neck or midriff. Once the tick has chosen a site, it inserts its barbed mouthparts. This usually goes undetected by the host. Next, the tick injects its saliva, which helps promote an optimal feeding opportunity for the tick. The saliva contains many active anti-inflammatory agents that render the body’s natural response to the bite useless, but in some cases an allergic reaction occurs. This may become an advantage to the host who becomes aware of the tick at that time and is therefore able to remove it prior to engorgement. However, in most cases the bite remains unnoticed due to the minute size of the nymphal tick that has been likened to a tiny freckle. Upon attachment, the tick secretes a compound called cementum, a substance that allows the tick to adhere without likelihood for easy dislodgement. Once these steps are accomplished the tick will begin to feed. While it is commonly stated that the Ixodes tick must feed an average of 48-72 hours in order to become sufficiently engorged to transmit Borrelia burgdorferi, this may not necessarily be the case (24). As with many scientific statements that become dogma, the concept that transmission of Borrelia burgdorferi requires a minimum of 72 hours came from animal studies which are now criticized as being flawed (25). In all probability, it is likely that some Ixodes nymph forms require 72 hours or more to transmit Borrelia burgdorferi, while other nymph ticks do so in less time. In the end, we learn that one should not take comfort in a limited exposure to the Ixodes scapularis tick and related species.
The likelihood of contracting LD is dependent on many factors. As mentioned above, the duration of engorgement and the stage of the tick involved may be major factors. Only a minority percentage of ticks are infected with Borrelia burgdorferi (or other pathogens such as Babesiosis, Ehrlichiosis, or Bartonella), but the consensus among epidemiologists is that the absolute numbers are increasing, likely in part due to a succession of mild winters on the East coast, which have lead to increased survival rates for adult ticks (26). In addition, the recently described theory of dilution is an explanation by which Borrelia Burgdorferi may proliferate (27). In short, this theory points out that the destruction of forested areas leads to elimination of some of the native species due to loss of habitat. Less discriminating and more enduring species like the white tailed mouse, a favorite host for Ixodes scapularis, survive preferentially as they can exist almost anywhere. With less “competition” for a host, Ixodes mediated infection with Bb naturally increases. In addition, it turns out that deforested sites which leave five acres or less, a common occurrence in urban America, also tend to favor species such as the white-tailed mouse, simply because other species require larger territories for survival and/or optimal propagation (28).
The description of “coinfections” with Borrelia Burgdorferi by the aforementioned microorganisms is also becoming better recognized, and has important clinical implications for diagnosis and treatment. Unfortunately, the scope of this report does not allow for detailed descriptions of the epidemiology, clinical features and treatment of these important tick-borne infections. In future updates, we hope to provide this information. Until then, the reader is referred to the following sites (29,30,31,32,33,34,35).
In highly endemic areas for Lyme Disease, such as certain counties in Connecticut, New York, and New Jersey, territories known as “hot pockets” have been identified, where the incidence of tick infection with Borrelia Burgdorferi exceeds 25% of the population studied (36). Those living in these areas obviously have a greater risk of contracting Lyme Disease, and communities now recognize this and therefore are responding with awareness campaigns (37). Unfortunately, no such information about tick infestation currently exists for the Carolinas or surrounding regions. We speculate that the number of cases of Lyme Disease will increase over time in the Southeast, and we can only hope that this will lead to identification of high risk areas so that public officials, once they accumulate necessary information, will make the necessary changes for surveillance and referral.
Debate about the best approach to limit the epidemic is being conducted regularly (38). Broad based methods have been proposed, such as those aimed at limiting the acknowledged vectors, whether it be reduction of deer, rodent, or tick populations through an assortment of measures. Most of these suggestions appear doomed to failure, as they are intuitively impractical. Some of the more creative approaches, such as rodent traps which coat the animal’s fur with a long lasting insecticide, and thereby reduce the tick infestation, appear promising as a rational, targeted, and affordable (39).
Those living in areas of lower incidence for Borrelia Burgdorferi infection are at increased risk during travel to endemic areas. These individuals and groups tend to be more vulnerable since they are not aware of the dangers of Bb and the necessary protective measures that are recommended to avoid tick bites. Please take note of the precautionary measures discussed later in our article in the section “An Ounce of Prevention“.
Finally, the issue of transmission of Borrelia Burgdorferi that is not tick-borne must be acknowledged. Regrettably, it is proven that human vertical transmission, i.e. mother to child, exists and is more likely to occur when the mother is infected in the first trimester (40). Fortunately, the incidence of transmission is believed to be quite low. Additional risk factors for transmission, which are routinely identified in other models of infection, have not been studied in vertical Borrelia Burgdorferi transmission. For example, we know nothing about the relative risk or characteristics of intrauterine transmission versus exposure in the birth channel, breast feeding, etc. However, we recognize that perinatal transmission of Bb can result in a multisytemic illness which can prove fatal (18), although we do not have evidence for the converse situation, i.e. asymptomatic infection for a prolonged period. Even less well studied is the issue of sexual transmission. Borrelia species, including Bb, have been found in genital secretions (41). A close relative of Bb, Treponema pallidum, was perhaps, until HIV happened, the most notorious sexually transmitted disease known to man.
It is of interest that one of our colleagues, Dr. Bill Harvey in Houston, has begun to study his population of chronic fatigue patients, many of whom have proven to be serologically positive for Borrelia Burgdorferi infection. In noting various demographic and social characteristics among his patients, he has begun to question whether all Bb infections represent a zoonoses (personal communication). He considers routine sexual transmission of Borrelia Burgdorferi a distinct possibility, certainly one worthy of a longitudinal study. Again, such epidemiological information will be virtually impossible to gather without more verifiable means of recognition and diagnosis than are currently available. The implications Dr. Harvey’s observation and theory are profound and deserve an assiduous scientific effort.