Rickettsia

This includes several species that cause arthropod borne diseases. They are obligate intracellular microorganisms that cannot grow in cell free cultures. Rickettsia are Gram negative coccobacilli but do not stain well with the Gram stain. Instead special stains like Giemsa, Gimenez, Castaneda or Macchiavello stains are used. Disease is transmitted by specific arthropod vectors. Vasculitis is a common underlying pathology seen in Rickettsial diseases.

Rickettsial diseases and vectors

Rocky mountain spotted fever: It is endemic in Southeastern states of Georgia, North Carolina, Virginia etc. Most cases are seen in spring and summer in children, following tick bite typically dog ticks called Dermacentor. Note that these ticks are different from Lyme disease ticks. The infection starts as fever, myalgia, headache with macular rash that begins on the wrist and ankles including the palms and soles and then spreads centripetally towards 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 as 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 and presents as 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, scar like black rash at the site of bite, lymphadenopathy and mental changes with bleeding in severe cases.

For diagnosis of Rickettsial diseases, samples are blood and even rash biopsy stained with Giemsa or Gimenez stain. Direct immunofluorescence can also be done 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 be employed. In the rare case that culture is needed, it can be done on chick embryo yolk sac or HeLa, HEP 2 and mouse fibroblast cell cultures or can be isolated by animal inoculation in mice and guinea pigs.