Transmission of the Lyme Disease Spirochete
By Louis A. Magnarelli
Department of Entomology
The Connecticut Agricultural Experiment Station
123 Huntington Street
P.O. Box 1106
New Haven, CT 06504-1106
Telephone: (203) 974-8441 Fax: (203) 974-8502
Lyme disease is the most prevalent tick-associated disease in the United States. Thousands of human cases are reported annually, mainly in northeastern and upper midwestern regions of the country. The blacklegged tick (Ixodes scapularis), formerly known as Ixodes dammini and sometimes referred to as the deer tick, is the most important species that transmits the bacterial (spirochete) disease agent.
The rise in deer populations over several decades in and near forests is correlated with substantial increases in blacklegged ticks and corresponding amplification of the disease organism in nature. Although deer are important hosts for adult blacklegged ticks, they do not serve to infect ticks. White-footed mice are considered the chief reservoirs for the Lyme disease agent. Larval and nymphal blacklegged ticks acquire the pathogen when they feed on these rodents and possibly other hosts, such as chipmunks and some birds. The disease organism can then be passed from larvae to nymphs to adults during the developmental process. There is occasional passage of the disease agent from infected females to larvae (via the eggs), but this form of pathogen transmission is not considered to be epidemiologically significant.1
The length of time that a nymphal or adult female tick is attached to a host is an important factor in pathogen transmission. The nymphal tick, which is about the size of a poppy seed, is the most important stage for transmitting the disease organism. Most human infections are acquired during the months of May, June, and July when nymphs are most abundant. Not all ticks are infected. Depending on the site and other factors, infection rates are highly variable and usually range between 10% and 35%. After several hours of attachment, the tick begins to ingest blood. It is in this phase that the disease agent is transmitted to the host.
Several laboratory studies,2-4 using mice as experimental hosts, have shown that infected ticks need to be attached for more than 24 hours to effectively transmit the pathogen. The probability of transmission increases with the duration of attachment; maximal transmission usually occurs between 48 and 72 hours post attachment. In summary, an infected, flat (unengorged) tick does not transmit the disease agent until host blood is ingested.
Early detection and prompt removal of attached ticks is of paramount importance in reducing risk of infection. A pair of tweezers or forceps can be used to grasp the tick’s mouthparts at the skin surface. Gentle pulling or twisting of the tick with steady pressure will dislodge it. The area of attachment can be cleaned with an antiseptic or rubbing alcohol.
1. Magnarelli, L.A., J. F. Anderson, and D. Fish. 1987. Transovarial transmission of Borrelia burgdorferi in Ixodes dammini (Acari: Ixodidae). Journal of Infectious Diseases 156:234-236.
2. Piesman, J., T. N. Mather, R. J. Sinsky, et al. 1987. Duration of tick attachment and Borrelia burgdorferi transmission. Journal of Clinical Microbiology 25:557-558.
3. des Vignes, F., J. Piesman, R. Heffernan, et al. 2001. Effect of tick removal on transmission of Borrelia burgdorferi and Ehrlichia phagocytophila by Ixodes scapularis nymphs. Journal of Infectious Diseases 183:773-778.
4. Piesman, J. and M. C. Dolan. 2002. Protection against Lyme disease spirochete transmission provided by prompt removal of nymphal Ixodes scapularis (Acari: Ixodidae). Journal of Medical Entomology 39:509-512.