Research

The overarching goal of our research is to better understand tick-borne diseases in the southern United States. Current research is primarily focused on the emerging zoonotic pathogen Rickettsia parkeri, transmitted by Amblyomma maculatum. Our research depends on invaluable collaborations with several key colleagues. Most significantly, these include Dr. Jerome Goddard in the Department of Biochemistry, Molecular Biology, Entomology and Plant Pathology at MSU as well as Dr. Chris Paddock at the CDC. Current and past projects are summarized below.

Amblyomma maculatum (Gulf Coast tick)

Amblyomma maculatum is the primary vector for Rickettsia parkeri, a spotted fever group rickettsia first isolated over 70 years ago and first associated with a human disease less than a decade ago.  Initial studies in our lab focused on the potential for cattle to become infected with R. parkeri as they are common hosts for adult stages of the tick. We are now looking at the role of vertebrate hosts for immature A. maculatum in R. parkeri maintenance and transmission using both experimental and field studies (G. Moraru, PhD candidate). In addition, we are investigating population heterogeneity of the Gulf Coast tick and R. parkeri in Mississippi, as well as the biology of another recently described rickettsia, “Candidatus R. andeanae” found in the Gulf Coast tick (F. Girao, PhD candidate). Other projects have included ecological studies of Gulf Coast tick questing behavior and vegetation associations, as well as studies of R. parkeri prevalence in areas such as North Carolina. Recent results in our lab have prompted us to target future studies that evaluate potential interactions between R. parkeri and “Ca. R. andeanae” in A. maculatum and the contributions of these rickettsiae to spotted fever rickettsioses in the United States. Human disease due to R. parkeri infection may be misdiagnosed as the more fatal rickettsial disease, Rocky Mountain spotted fever (RMSF) caused by R. rickettsii. Thus, we anticipate that understanding the natural history of A. maculatum rickettsiae may help unravel and clarify the complex epidemiology of spotted fever rickettsioses, including RMSF, in the United States. 

Amblyomma americanum (Lone Star tick)

Amblyomma americanum serves as a competent vector for Ehrlichia chaffeensis, the agent of human monocytic ehrlichiosis. Most of our research on this organism was performed as part of Dr. Varela-Stokes’ dissertation, however we have performed some collaborative work in Dr. Pinchuk’s lab investigating differential gene expression from E. chaffeensis infection in monocytic cells. Our ultimate goal is to investigate the mechanisms behind differences in host species susceptibility to disease from E. chaffeensis.

The spirochete, Borrelia lonestari, is also found in A. americanum.  Although it has been speculated to be a potential agent of “southern tick-associated rash illness” (STARI), a Lyme disease-like illness in the southern United States, data either implicating or vindicating B. lonestari are somewhat equivocal and the cause of STARI remains unknown. In our past research, we were able to isolate B. lonestari for the first time and demonstrate that white-tailed deer could be infected by the organism. Current research efforts on this species are being revived to investigate the biology of this spirochete.

Other tick-borne disease research

Using Ixodes scapularis collected from Mississippi sites, we are evaluating the prevalence of a variety of infectious agents transmitted by these ticks (L. Goltz, MS student). We also continue to collaborate with the University of Georgia (Drs. Fred Quinn and Russ Karls) on a tularemia project investigating differential gene expression in Fransicella tularensis LVS in the vector compared to a mammalian host.

Research and diagnostics

Through collaborations with clinicians and pathologists at MSU CVM, the Varela-Stokes lab has helped diagnose and confirm several cases of canine hepatozoonosis using PCR of clinical samples from affected dogs. We have also been investigating leucocytozoonosis in backyard poultry flocks and other affected birds in Mississippi that have been clinically affected by seasonal black fly outbreaks. Ongoing efforts include identification of Leucocytozoon spp. and other hemosporidians in these birds using molecular techniques and investigating the potential use of flow cytometry to enrich diagnostic samples for hemosporidians.

Finally, Dr. Varela-Stokes serves as one of the veterinary parasitology consultants here at CVM and is often asked to identify all sorts of odd parasites that come her way.  There is rarely a dull moment around the lab!

Funding

While most of her research has been supported by intramural grants while at MSU, Dr. Varela-Stokes has also been funded extramurally by the Southeastern Center for Emerging Biologic Threats and by NIH (K08) when she was at UGA. She is currently pursuing funding from NIH, NSF, and other sources.