Rickettsiae: Cultivation, Animal Inoculation and Laboratory Diagnosis

In this article we will discuss about Rickettsiae:- 1. Cultivation of Rickettsiae 2. Animal Inoculation of Rickettsiae 3. Laboratory Diagnosis  4. Resistance.

Cultivation of Rickettsiae:

All rickettsiae cannot be grown on cell free media, except R. Quintana which will grow on blood agar medium. They can be cultivated in yolk sac of chick embryo, grow on mouse fibroblast, HeLa, Hep2 and on continuous cell lines.

Pure preparations of rickettsiae can be obtained by differential centrifugation of yolk sac suspension. For primary isolation, tissue cultures are not satisfactory. Rickettsiae of Typhus group grow in cell cytoplasm, whereas those of spotted fever in nucleus and coxiellae in cytoplasmic vacuoles.

Animal Inoculation of Rickettsiae:

Isolation of Rickettsiae from clinical materials can be done by inoculating into mice and guinea pigs. Q fever is an acute systemic infection usually characterised by an interstitial pneumonia clinically resembling influenza; unlike other rickettsial infections, it has no rash.

There is only high fever. The name of the disease is derived from the letter “Q” in Query as its aetiology was uncertain, though it was recognised in Queensland (Australia).

The causal agent, named Coxiella burnetii, was first recognised by Cox and Burnett. Q fever is quite widespread in Europe and Egypt. It is an obligate intracellular ,parasite. It can be cultivated in animals and chick embryo. It remains viable for several days in water or milk or butter. Pasteurization at temperature 145°F or 62.7°C kills C. burnetti.

C. burnetti differs from other members of rickettsiae by the following properties: it is filterable, highly resistant to heat and disinfectants, does not produce rash in infected patients and does not elicit the agglutination to the non-motile Proteus strains which is characteristic to typhus fever (Weil Felix test is negative for C. burnetti); hence it is not considered as the true member of rickettsiae and has been designated as a new member Coxiella.

Q fever is spread throughout the world mostly in animals. Infected animals may excrete coxiella in the milk and also during parturition; hence plenty of the organisms are present in the placenta and birth canal. Man may be infected by drinking, contaminated milk or by inhalation of infected dust from the straw or dried placenta.

C. burnetti is conveyed from animal to animal by the bite of tick. Ticks rarely transmit the infection to man. The risk of person to person transmission is small; so strict isolation procedures are not needed.

Laboratory Diagnosis of Rickettsiae:

Isolation of causative agent is done in animals or chick embryo. Laboratory workers may easily get infected, so they should be careful.

Resistance of Rickettsiae:

Rickettsiae are destroyed at 56°C and at room temperature when they are free from host components.

Serology:

Some non-motile strain of proteus and rickettsiae are antigenically similar. Proteus organisms can be grown artificially in plenty. Therefore antigens from Proteus OX2, OXK, OX19 are used in Weil-Felix test which is positive for Typhus fever, except Q fever. This test is useful but not reliable as it is strongly positive in patients with Proteus infection than Typhus fever. CFT is accurate and specific.

Treatment:

Tetracyclines, Chloramphenicol and paraaminobenzoic acid exhibit a rickettsiostat effect; but, in Q fever, chloramphenicol and tetracycline are effective therapeuti­cally.