Onset of disease is sudden in about half of the cases. Fever and headache are the most commonly reported symptoms, but chills, myalgias, arthralgias, and anorexia are also noted. Mild pulmonary involvement, manifested by cough and infiltrates, is found in about one-third of patients with Rocky Mountain spotted fever. Rash is a hallmark of infection, but it usually follows systemic symptoms and its absence should not rule out a possible rickettsial etiology. Appearance of an IgM antibody response normally occurs 7-14 days after the onset of disease.
Testing immediately post-exposure is of no value without a later convalescent specimen.
While the presence of IgM antibodies suggest current or recent infection, low levels of IgM antibodies may occasionally persist for more than 12 months post infection. Such a residual IgM response may be distinguished from early IgM response to infection by testing sera from patients 2-3 weeks later for changing levels of specific IgM antibodies. A low positive suggests past exposure or infection, while high positive results may indicate recent or current infection but are inconclusive for diagnosis. Seroconversion between acute and convalescent sera is considered strong evidence of recent infection. The best evidence for infection is a significant change on two appropriately timed specimens where both tests are done in the same laboratory at the same time.