Ixodes scapularis in: Tick Pickin'

My last Ixodes blog discussed some of the interesting biology that hard-bodied ticks use to attach themselves to their hosts. Also discussed were some implications of limiting tick pathogen transmission by genetically interfering with salivary anticoagulants and adhesives. But an obviously simpler preventive measure exists: manual tick removal. A paper published in 2001 by the CDC and Yale’s department of Epidemiology and Public Health reported on the efficacy of tick removal on pathogen transmission in tick vectors. By analyzing transmission of Lyme disease between mice and ticks, computer models were used to predict the benefits of simple tick removal in preventing transmission of Borellia burgdorferi (Lyme) spirochetes. 

Examining both laboratory and wild strains of B. burgdorferi in Ixodes scapularis, infected ticks were collected from three endemic sites in the Northeast: Lyme, Connecticut, Colts Neck, New Jersey, and Armonk, New York. The researchers tested specimens for infection with both B. burgdorferi and Ehrlichia phagocytophilia, two common zoonotic pathogens. Of those collected, 21% were infected with E. phagocytophilia, 26% were infected with B. burgdorferi, and 6% were infected with both (des Vignes et al., 2001). The wild Lyme strains were isolated and cultured in I. scapularis, but only 30% of field-collected ticks were infected with B. burgdorferi were usable - good news for humans.


To measure Lyme transmission, 4-week-old mice were fed on by nymphal ticks infected with each strain. The nymphs were removed after 24, 48, 72, and ≤ 96 hours (or until replete) of feeding. One month later, the same mice were fed on by uninfected nymphal ticks, and the transmission rates of B. burgdorferi from mouse to tick was determined via dark microscopy. Mice that were fed on by infected ticks were held for approximately a month before biopsies of ear, urinary bladder, and heart tissues were performed. These tissues were tested for the presence of spirochetes to confirm infection.


Des Vignes, et al. (2001) reported 0% transmission within 24 hours, and only 12.5% within 48 hours of feeding. By the third day, however, transmission rates climbed to 78.9% and peaked at 93.8% infection within 96 hours (or until the ticks fed to full engorgement). Based on their experimental results, and those previously reported in the literature, the researchers used statistical algorithms to estimate the probability of infection per hour of feeding. The models estimate that only 4.6% of all nymphal I. scapularis in Lyme endemic regions will transmit the spirochetes if removed before the ticks can feed to repletion. The researchers note that this frequency is largely based off of the 30% transmission capacity of field-collected nymphs, though the model’s estimates are similar to those previously published (des Vignes et al., 2001). 

It is important to note that these frequencies are specific to I. scapularis, and that Eurasian, Pacific, and European species are well known to infect hosts within 24 hours of feeding. Nevertheless, this research supports the idea that mechanical removal of larval I. scapularis is effective in preventing transmission of Lyme spirochetes to humans in the Northeastern United States. Additionally, this report emphasizes the importance of regular tick checks, though simple mechanical removal alone is not  completely effective in combatting all tick-transmitted disease. 

des Vignes, F., Piesman, J., Heffernan, R., Schulze, T., Stafford, K., Fish, D. 2001. Effect of Tick Removal on Transmission of Borrelia burgdorferi and Ehrlichia phagocytophilia by Ixodes scapularis Nymphs. Journal of Infectious Diseases. Vol. 183(5): 773-778.