RICKETTSIAE

             Rickettsiae are small, Gram-negative bacilli adapted to obligate intracellular parasitism, and transmitted by arthropod vectors. They are primary parasites of arthropods such as lice, fleas, ticks and mites in which they are found in the alimentary canal. In vertebrates, including human, they infect the vascular endothelium and reticulo-endothecial cells. The family Rickettsiaceae is named after Howard Taylor Ricketts who discovered spotted fever rickettsia (1906) and died of typhus fever contracted during his studies.

            The family currently comprises three genera – Rickettsia, Orientia and Ehrlichia which appear to have descended from the common ancestor.

1. GENUS: RICKETTSIA

            The genus Rickettsia consists of the causative agents of two groups of diseases – typhus fevers and spotted fevers.

            Morphology – In smears from infected tissues, Rickettsiae appear as pleuromorphic coccobacilli, 0.3-0.6 x 0.8-2µm in size. They are non motile and non-capsulated. They are Gram-negative, though they do not take the stain well. They stain bluish purple with Giemsa and Castaneda and deep red with the Machiavello and Gimenez stains.

            Under the electron microscope, Rickettsiae are seen to have the three layered cell wall, a trilaminar plasma membrane and the outer slime layer.

            Cultivation – Rickettsiae are unable to grow in cell free media. Growth generally occurs in the cytoplasm of infected cells, but in case of spotted fever rickettsiae, growth may take place in nucleus as well. The optimum temperature for growth is 32-25⁰C.

            In laboratory they can be cultivated in the yolk sac of developing chick embryos, mice fibroblast, guinea pigs, etc.

            Resistance – Rickettsiae are readily inactivated by physical and chemical agents. They are rapidly destroyed at 560C and at room temperature when separated from host components, unless preserved in skimmed milk or a suspending medium containing sucrose, potassium phosphate and glutamate.

            Rickettsiae are susceptible to tetracycline, chloramphenicol and ciprofloxacin. Penicillin and sulphonamides are infective but para-aminobenzoic acid has an inhibitory action on Rickettsiae. Sulphonamides may actually enhance the growth of Rickettsiae and worsen the condition if administered to patients.

            Typhus Fever Group – This group of disease consists of epidemic typhus and endemic typhus.

            Epidemic typhus – It has been one of the great scourges of mankind, occurring in devastating epidemics during times of war and famine. The disease has been reported from all parts of the world but is particularly common in Russia and Eastern Europe. During 1917-1922, there were some 25 million cases in Russia, with about three million deaths.

            The causative agent of epidemic typhus is R. prowazekii, named after von Prowazek, who died of typhus fever while investigating the disease. Humans are the only natural vertebrate host. Several animals, guinea pigs, mice, cotton rats, etc. may be infected experimentally.

            The human body louse Pediculus humanus corporis is the vector. The lice become infected by feeding on rickettsiamic patient. The Rickettsiae multiply in the gut of the lice and appear on the feces in 3-5 days. Infection is transmitted when the contaminated louse feces is rubbed through the minute abrasions caused by scratching.

            Endemic typhus – Endemic typhus is worldwide in prevalence but is not of much public health importance as the disease is mild and sporadic and can be easily controlled.

            The causative agent of this disease is R. typhi which is maintained in nature as mild infection of rats, transmitted by the rat flea Xenopsylla cheopis. The Rickettsia multiplies in the gut of the flea and is shed in its feces. Human get the disease usually through the bite of the infected fleas, when their saliva or feces is rubbed in through the aerosols of dried feces.

            Spotted fever Group – This group of disease consists of two types based on vector.

            Tick typhus – This type of spotted fever is transmitted by different types of ticks. The infection may be transmitted through vertebrate host by any of the larval stages or by adult ticks. The Rickettsiae are shed in tick feces but transmission to human beings is primarily by bite.

            Many species have been recognized in this group – R. rickettsii causing Rocky Mountain Spotted fever, R. siberica causing Siberian Tick Typhus, R. conori causing Indian, Mediterranean, Kenyan, South African Tick Typhus and R. japonica causing Oriental Spotted Fever, etc.

            Rickettsial Pox – This is the mildest rickettsial disease of humans. The name Rickettsial Pox is derived from the resemblance of the disease to chicken pox.

            The causative agent is R. akari, the reservoir of which is the domestic mouse, and are transmitted by mite Liponyssoides sanguineus.

2. GENUS: ORIENTIA

            Scrub Typhus – Scrub Typhus is caused by Orientia tsutsugamushi. T occurs all along east Asia, from Korea to Indonesia and in the Pacific islands including Australia.

            The vectors are trombiculid mites belonging to the genus Leptotrombidium. Human beings are infected when they treepass into these mite islands and are bitten by the mite larvae. Various rodents and birds act as reservoirs and also help in spreading the bacteria.

            The disease is not so serious problem in civilian practice but assumes great importance in military medicine, especially during jungle warfare.   

3. GENUS: EHRLICIA

            Ehrlichiae are small, Gram negative, obligately intracellular bacteria which have an affinity towards blood cells. In the cytoplasm of infected phagocytic cells, they grow within phagosomes as mulberry-like clusters called morula. They are tick borne.

            Three human infections caused by this group of organisms have been identified.

            The first type was reported from Japan in 1954, the case resembling glandular fever, showing serological response against the agent of canine ehrlichiosis. The causative agent has been named Ehrlichia sennetsu. It is endemic in Japan and Southeast Asis, and causes lymphoid hyperplasia and a typical lymphocytosis. Human infection is suspected to be caused by ingestion of fish carrying infected flukes.

            The second type of infection is ‘human minocytic ehrlichiosis’ caused by E. chaffeensis. It is transmitted by Amblyomma ticks. Deer and rodents are believed to be reservoir hosts. Human disease is associated with leucopenia, thrombocytopenia and elevated liver enzymes.

            The third is ‘human granulocytic ehrlichiosis’ caused by an organism either identical with or closely related to the equine pathogen E. equi. It is transmitted by ixodes ticks. Deer, cattle and sheep are the suspected reservoir. Lecuopenia and thrombocytopenia are seen in patient.

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