Cattle and the Natural History of Rickettsia parkeri in Mississippi

Cattle have been recognized as hosts for Amblyomma maculatum, the Gulf Coast tick, for over 100 years. For nearly as long, A. maculatum have been known to harbor the spotted fever group Rickettsia (SFGR), now known as Rickettsia parkeri. However, human infection with R. parkeri was not documented until 2004. Results presented herein describe a laboratory and a field study evaluating cattle and the natural history of A. maculatum and R. parkeri in Mississippi. In the laboratory study, seroconversion to R. parkeri antigen occurred in calves exposed to R. parkeri by injection or by feeding R. parkeri-infected A. maculatum, and two out of six animals were transiently rickettsemic. All calves remained clinically normal during the study, except for gotch ear-like lesions in all tick-infested calves, regardless of infection status of ticks, suggesting that R. parkeri is not involved in the condition. In the field study, A. maculatum (n = 34) removed from Mississippi sale barn cattle (n = 183) and the cattle hosts were tested for R. parkeri. Cattle were not rickettsemic by polymerase chain reaction, but 49.7% demonstrated low titers to R. parkeri antigen when tested by indirect fluorescent antibody for SFGR. Of ticks removed from cattle, 11.8% were hemolymph positive and 8.7% were indirect fluorescent antibody positive. Approximately 22% (5/23) and 4% (1/23) of harvested tick extracts were positive for R. parkeri by polymerase chain reaction of the 17 kDa antigen gene and ompA gene, respectively. An amplicon for the ompA gene from one tick was successfully sequenced and showed 100% similarity with the homologous sequence of R. parkeri. Thus, cattle may harbor R. parkeri-infected A. maculatum and produce antibodies to SFGR. Cattle may play a role in the natural history of R. parkeri infection by expanding populations of A. maculatum and transporting R. parkeri-infected ticks to various locations, rather than as a reservoir for R. parkeri.