This group includes several species that cause arthropod-borne diseases. They are obligate intracellular microorganisms, so they can’t grow in cell-free cultures.

Rickettsia are Gram-negative coccobacilli, but they don’t stain well with the Gram stain. Instead, they’re typically identified using special stains such as Giemsa, Gimenez, Castaneda, or Macchiavello stains.

Transmission occurs through specific arthropod vectors. A common underlying pathology in rickettsial infections is vasculitis.

Rickettsial diseases and vectors

Bacteria	Disease	Vector
R.rickettsii	Rocky Mountain Spotted Fever	Ticks
R.akari	Rickettsial Pox	Mites
R.prowazekii	Epidemic typhus	Lice
R.typhi	Endemic / Murine typhus	Fleas
R.tsutsugamushi	Scrub typhus	Mites / Chiggers

Rocky mountain spotted fever: This disease is endemic in southeastern U.S. states such as Georgia, North Carolina, and Virginia. Most cases occur in spring and summer, often in children, after a tick bite. The typical vector is the dog tick (Dermacentor). These ticks are different from the ticks that transmit Lyme disease.

Clinical features usually begin with fever, myalgia, and headache, followed by a macular rash. The rash typically starts on the wrists and ankles (including the palms and soles) and then spreads centripetally toward the trunk. In severe cases, encephalitis and DIC can occur.

Typhus: There are three types of typhus fever.

• Epidemic typhus is spread by infected lice in overcrowded areas. It presents with fever, myalgia, headache, rash, and confusion. It may relapse several years later as Brill Zinsser disease, which is a milder variant.
• Endemic typhus is spread by infected fleas. It presents with fever, rash, stomach ache, muscle aches, cough, and vomiting, with rare organ damage.
• Scrub typhus is spread by the bite of infected chiggers. It presents with fever, headaches, myalgia, a scar-like black rash at the bite site, lymphadenopathy, and mental changes. Severe cases can include bleeding.

For diagnosis of Rickettsial diseases, samples include blood and, when needed, a rash biopsy. These can be stained with Giemsa or Gimenez stain. Direct immunofluorescence can also be used for antigen detection.

Antibody detection is the easiest way to diagnose. This can be done by latex agglutination, ELISA, complement fixation test, and indirect fluorescent antibody test. In the olden days, the Weil-Felix test was used. PCR can also be employed.

In the rare case that culture is needed, it can be done in chick embryo yolk sac or in HeLa, HEP 2, and mouse fibroblast cell cultures, or it can be isolated by animal inoculation in mice and guinea pigs.