Isolation of Rickettsia parkeri and identification of a novel spotted fever group Rickettsia sp. from Gulf Coast ticks (Amblyomma maculatum) in the United States

Abstract

Until recently, Amblyomma maculatum (the Gulf Coast tick) had garnered little attention compared to other species of human-biting ticks in the United States. A. maculatum is now recognized as the principal vector of Rickettsia parkeri, a pathogenic spotted fever group rickettsia (SFGR) that causes an eschar-associated illness in humans that resembles Rocky Mountain spotted fever. A novel SFGR, distinct from other recognized Rickettsia spp., has also been detected recently in A. maculatum specimens collected in several regions of the southeastern United States. In this study, 198 questing adult Gulf Coast ticks were collected at 4 locations in Florida and Mississippi; 28% of these ticks were infected with R. parkeri, and 2% of these were infected with a novel SFGR. Seventeen isolates of R. parkeri from individual specimens of A. maculatum were cultivated in Vero E6 cells; however, all attempts to isolate the novel SFGR were unsuccessful. Partial genetic characterization of the novel SFGR revealed identity with several recently described, incompletely characterized, and noncultivated SFGR, including "Candidatus Rickettsia andeanae" and Rickettsia sp. Argentina detected in several species of Neotropical ticks from Argentina and Peru. These findings suggest that each of these "novel" rickettsiae represent the same species. This study considerably expanded the number of low-passage, A. maculatum-derived isolates of R. parkeri and characterized a second, sympatric Rickettsia sp. found in Gulf Coast ticks.

FIG. 1.

Electron microscopy of the R. parkeri Moss Point isolate in Vero E6 cells stained with uranyl acetate and lead citrate. (A) Multiple intracellular rickettsiae free in the cytoplasm of an infected cell. Bar = 2 μm. (B) Bacillary form of R. parkeri, showing an outer electron-lucent halo or “slime” layer adjacent to the central beaded microcapsular layer and the internal trilaminar cell wall. Bar = 100 nm.

FIG. 2.

Unrooted dendrogram showing the phylogenetic position of a novel spotted fever group Rickettsia sp. detected in A. maculatum ticks, inferred from comparison of 725 nucleotides in the outer membrane protein B gene (ompB) sequence aligned with complete or partial ompB sequences available in the GenBank database by using the neighbor-joining method. Bootstrap values of >70% for 1,000 replicates are indicated at the nodes. Bar = 2% nucleotide sequence divergence.

FIG. 3.

Unrooted dendrogram showing the phylogenetic position of a novel spotted fever group Rickettsia sp. detected in A. maculatum ticks, inferred from comparison of 1,095 nucleotides of the citrate synthase gene (gltA) sequence aligned with complete or partial gltA sequences available in GenBank by using the neighbor-joining method. Bootstrap values of >70% for 1,000 replicates are indicated at the nodes. Bar = 1% nucleotide sequence divergence.