Every Thursday, Laboratory Equipment features a Scientist of the Week, chosen from the science industry’s latest headlines. This week’s scientist is Graham Hickling from the Univ. of Tennessee. He and his team found that tick-borne diseases vary by geographical location.
Q. What made you interested in studying the different diseases ticks can carry?
A: My previous faculty position was in Michigan, which is a state that had very few Lyme disease ticks in the Lower Peninsula. Then, 10 years ago, it was discovered that blacklegged ticks from northern Indiana were invading the southwest corner of the state, with a consequent increase in Lyme disease cases. One of my students, Sarah Hamer, was able to track their spread up the Lake Michigan coastline during her PhD studies. From this we began to see how quickly tick distributions and disease risk were changing in the eastern U.S. I then moved to Tennessee and was confronted with a different assortment of ticks and disease agents of public health concern. I saw a need for more basic research in the Southeast, because there is so much debate here presently about Lyme and other tick-borne diseases.
Q. What are the future implications of your research and findings?
A: For the public to be at risk from Lyme disease, the bacteria, the “disease agent,” must occur in human-biting ticks, the “disease vector.” In Michigan, we found that the disease agent was already present in natural transmission cycles between wildlife hosts such as birds and rabbits. However, the tick species involved in these cycles never attack humans. So it was only when blacklegged ticks invaded southwest Michigan, and started acting as that "bridge vector," that we saw increasing cases of human disease.
In southeastern states such as Tennessee, there are similar findings of the Lyme disease agent within wildlife cycles – but we lack a robust population of bridging vectors. Lone star ticks would seem to be the obvious suspects, but all the studies that I am aware of have concluded that they don't transmit Lyme disease. Blacklegged ticks are found throughout the Southeast, but their numbers are much lower than in the North, and their behavior is altered so that the nymphal stage rarely bites people. The future implications, however, are that if northern strains of blacklegged ticks expand southwards (which we are seeing presently in Indiana, Ohio, Kentucky and Virginia) then there is the possibility of a surge in southern Lyme disease in coming years.
Q. What was the most surprising thing you found in your research?
A: We spent a year collecting and testing more than 1,000 blacklegged ticks from Tennessee and never found any evidence of the Lyme disease bacterium. The same was true when we tested lone star ticks. However, in these lone stars we immediately found several species of Ehrlichia bacteria. Most tick bites in southeastern states are from lone stars – because they are so abundant and so aggressive – yet I don't meet many people who know about the ehrlichiosis infection that they can spread. It's an emerging disease in humans that is increasing in this region and we need to be watchful for it.
Q. What is the take home message of your research and results?
A: Pay attention to which species of tick bites you, as that can be helpful information if you later become sick. One web resource I recommend for identifying ticks–particularly the immature life stages which can be tricky to tell apart–is the identification page on tickencounter.org. I would also emphasize the importance of taking basic precautions with your family – do a tick check at the end of each day spent out-of-doors, tuck your pants into your socks when you are in the woods and use repellent spray. Our research team spends hundreds of hours wandering through tick habitat each year, and by taking these precautions we have yet to encounter any health problems.
Q. What new technologies did you use in your lab during your research?
A: We have a modest lab at UT Knoxville's Center for Wildlife Health. We do real-time polymerase chain reaction (PCR) to screen ticks for pathogens, followed by nested PCR, sequencing and BLASTing to obtain species-level identifications for any Borrelia bacteria that we detect. We also use PCR methods for molecular identification of certain tick species that are difficult to distinguish under a microscope. Presently we are developing a Reverse Line Blot (RLB) assay to identify which wildlife hosts ticks are feeding on. This RLB project is a challenge because only minute traces of the host's DNA remain in the tick by the time we get to test it.
Q. What is next for you and your research?
A: Our next field project is focused on gathering female blacklegged ticks from 15-20 different states, measuring differences in the questing behavior of their offspring, and then seeking genetic markers that correlate with those behavior differences. If we are successful, the project will give us the tools we need to better track the geographic spread of northern ticks moving south. We are also investigating the role lizards play as potential reservoir hosts for the Lyme disease bacterium–given the choice it seems that blacklegged ticks would rather feed on a lizard than a mouse–but in the Northeast they don't usually have that option.