Differentials
Other rickettsial diseases
SIGNS / SYMPTOMS
Less severe illness with maculopapular/papulovesicular rash rather than petechial rash. No rash with species 364D.
Eschar present at site of tick bite.
INVESTIGATIONS
Serology: positive.
Biopsy and polymerase chain reaction (PCR) of serum or eschar: a positive PCR is diagnostic of rickettsial infection.
Meningococcaemia
SIGNS / SYMPTOMS
More prevalent in winter and early spring. Rash characteristically begins earlier than in Rocky Mountain spotted fever (RMSF), starting centrally and spreading peripherally.[1] Septic shock, disseminated intravascular coagulation, and digital necrosis more likely than in RMSF.
Rarely, other pyogenic bacteria, such as Streptococcus pneumoniae or Staphylococcus aureus, can produce bloodstream infections with similar signs to meningococcaemia.
INVESTIGATIONS
Blood cultures (and sometimes cerebrospinal fluid culture or multiplex PCR) positive for Neisseria meningitidis.
Ehrlichiosis
SIGNS / SYMPTOMS
Less frequent rash (present in one third of adults, two-thirds of children).
INVESTIGATIONS
More frequent leukopenia (60%), thrombocytopenia (90%), and aminotransferase elevations (70% to 90%). Intracellular morulae may be visible in cytoplasm of monocytes.[1]
Diagnosed by positive PCR, rising serum antibody titres, or isolation in culture of Ehrlichia chaffeensis.
Anaplasmosis
SIGNS / SYMPTOMS
Occurs in regions endemic to Ixodes scapularis (black-legged tick).
Rash is rare.
Toxic shock syndrome (TSS)
SIGNS / SYMPTOMS
Presents with macular erythroderma that subsequently desquamates.[1] Clinical criteria for TSS also include fever, low BP, and abnormalities in three or more other organ systems.
INVESTIGATIONS
Fifty percent or greater of Group A beta haemolytic streptococcal TSS will have the organism isolated from a normally sterile site.
Scarlet fever
SIGNS / SYMPTOMS
Pharyngitis more pronounced than in RMSF.
INVESTIGATIONS
Throat culture or antigen detection test positive for Streptococcus pyogenes.
Leptospirosis
SIGNS / SYMPTOMS
Typically follows biphasic course; rash is only petechial in the second phase.
INVESTIGATIONS
Serological diagnosis with microagglutination assay for leptospirosis.
Viral exanthems
SIGNS / SYMPTOMS
Rash may or may not involve palms or soles in viral illnesses.[1] Rapidly improving clinical course without antirickettsial therapy suggests a viral illness.
INVESTIGATIONS
Viral exanthems are usually diagnosed clinically. Sometimes confirmed with viral cultures or serology.
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