During the warmer months here in the Midwest, ticks are abundant and our microbiology lab receives several tick submissions per day for identification. When possible, we provide species level identification as well as sex for any tick submitted. While this is common practice in most microbiology laboratories, our molecular laboratory accidently received a tick specimen and, in the process of routing it to the microbiology lab, was curious as to why the tick identification matters—what does that tell us clinically? This led to an impromptu plate rounds with both labs and prompted me to write this post.
How do we determine tick identity?
A tick is submitted in a cup and sent to the laboratory. Ideally the tick would be submitted whole without missing appendages or damaged in any way. The tick is placed in ethanol to kill the organism and to allow for examination under a microscope. The mouth parts, scutum, and festoons are examined for defining features. Thorough examination is challenging when the tick arrives damaged or only partially intact.
Why do we provide tick identification?
Certain ticks carry specific pathogens. For instance, Amblyomma americanum (lone star tick) can transmit ehrlichiosis, Francisella tularensis, Heartland virus, Bourbon virus, and Southern tick-associated rash illness, while Ixodes scapularis can transmit Borrelia burgdorferi & Borrelia mayonii (both are causative agents of Lyme disease), Anaplasma phagocytophilum, and Erhlicia muris as well as Powassan virus. Knowing which tick that the patient was bitten by can allow providers to understand what potential pathogens they may or may not have been exposed to. If Amblyomma americanum is submitted, for example, that tick does not carry Borrelia burgdorferi. However, it is important to note that the majority of patients who develop tick-borne illness have no recollection of a tick bite! So while one tick may be discovered and sent to the lab, the patient could still have been unknowingly bitten by a different tick, which could carry other pathogens. When a patient exhibits clinical symptoms that are consistent with a tick-borne disease, such as Lyme Disease, the patient should be tested for that disease regardless of their tick history.
The patient has an Ixodes tick! They are worried about Lyme Disease. Should we send the tick out for molecular testing?
We discourage the use of molecular testing on the ticks themselves because ticks carry a variety of pathogens and there is a high likelihood of carrying a particular pathogen in a high prevalence area. For Ixodes ticks in Lyme Disease endemic areas, 15-70% of ticks may carry the causative agent, Borrelia burgdorferi. However, just because a tick carries a particular pathogen, it does not mean that the patient is now infected. This can lead to unnecessary treatment and misdiagnosis. Moreover, ticks must feed for a certain amount of time before pathogens can be transmitted. For example, Ixodes ticks must typically feed for more than 24 hours before it can transmit Lyme Disease or other pathogens.
In summary, tick identification can provide a glimpse into what the patient was potentially exposed to and if symptoms do arise days to weeks later, the tick identification may offer additional clues. However, just because a person was bitten by a tick does not mean that they are infected. Identification is just a piece of the puzzle!
References
- Blaine A. Mathison and Sam R. Telford III, 2019. Arthropods of Medical Importance, In: Carroll KC, Pfaller MA Manual of Clinical Microbiology, 12th Edition. ASM Press, Washington, DC. doi: 10.1128/9781683670438.MCM.ch151
- Bobbi Pritt, and J. Stephen Dumler, 2019. Ehrlichia, Anaplasma, and Related Intracellular Bacteria, In: Carroll KC, Pfaller MA Manual of Clinical Microbiology, 12th Edition. ASM Press, Washington, DC. doi: 10.1128/9781683670438.MCM.ch67
- Blaine A Mathison, 2021. 9.11 Gross Examination of Helminths and Arthropods, Leber ALClinical Microbiology Procedures Handbook, 4th Edition. ASM Press, Washington, DC. doi: 10.1128/9781683670438.CMPH.ch9.11
- https://www.cdc.gov/ticks/tickbornediseases/tickID.html
- https://www.mayoclinic.org/tick-species/sls-20147911
- https://dph.illinois.gov/topics-services/environmental-health-protection/structural-pest-control/common-ticks/identification.html
-Paige M.K. Larkin, PhD, D(ABMM), M(ASCP)CM is the Director of Molecular Microbiology and Associate Director of Clinical Microbiology at NorthShore University HealthSystem in Evanston, IL. Her interests include mycology, mycobacteriology, point-of-care testing, and molecular diagnostics, especially next generation sequencing.
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