text: Genomic comparison of strains R. rickettsii has a relatively small genome; however, variances in gene expression between different strains can lead to various functions of the bacteria. For instance, there are two major strains of R. rickettsii called the Iowa and the Sheila Smith strains. The Sheila Smith strain is a virulent strain, while the Iowa strain is an avirulent strain. Microarrays revealed that there were only four distinct differences in the gene expression of R. rickettsii; however, these four changes lead to complete differences in virulence, and thus the niche of the bacteria. A key feature allowing for differentiation is the rickettsial outer membrane protein, rOmpA and rOmpB  which contributes to the identification of R. rickettsii strains as virulent. The detection of single nucleotide polymorphisms (SNPs) are used to differentiate these strains.
  Clinical manifestations
  The Centers for Disease Control and Prevention states that the diagnosis of Rocky Mountain Spotted Fever (RMSF) must be made based on the clinical signs and symptoms of the patient and then later be confirmed using specialized laboratory tests. However, the diagnosis of Rocky Mountain Spotted Fever is often misdiagnosed due to its non-specific onset. The majority of infections from R. rickettsii occur during the warmer months between April and September. Symptoms can take 1–2 days to 2 weeks to present themselves within the host. The diagnosis of RMSF is easier when there is a known history of a tick bite or if the rash is already apparent in the affected individual. If not treated properly, the illness may become serious, leading to hospitalization and possible fatality.