By definition, Rickettsiae are a heterogeneous group of small obligately intercellular, gram negative, coccobacili and short bacilli; most of which are transmitted by a tick, mite, flea, or louse vector.
Properties
Rickettsiae are pleoplasmic coccobacili, appearing either as short rods (0.3 micrometer to 0.6 micrometer) or as cocci (0.3 micrometer in diameter). They have small genome composed of 1-1.5 million base pairs. These organisms, because of their small size were once thought to be virus.
Nevertheless, there organisms are bacteria because they show following characters:
1. They have typical; gram negative cells
2. They contain both DNA and RNA, enzymes for the kreb’s cycle and ribosomes for protein synthesis.
3. They multiply by binary fission.
4. They are susceptible to antibiotics.
Classification of Rickettsiae
Rickettsiae are a group of bacteria that photo-genetically occupy a position between bacteria and viruses.
These pathogens are included in order Rickettsiales, tribe
rickettsiae and family rickettsiae.
The family of rickettsiae consists of four major groups namely Typhus fevers, Spotted fevers, Q-fever and Trench fever.
Growth of Rickettsia
Though rickettsiae do not grow in cell free media with the exception of Rochalimaea Quintana which can be grown on blood sugar, they readily grow in yolk sacs of embryonated eggs.
Many strains of Rikettsiae also grow in cell culture, where the generation time is 8-10 hours at 34 degree centigrade.
They also grow in continuous cell lines such as Hela, Hep 2, and Detroit 6 etc of developing chick embryo and also in many laboratory animals.
Rickettsiae grow in different
parts of the cell. Those of typhus groups are usually found in the cytoplasm
and those of the spotted fever groups are usually found in the nucleus.
Their growth is
enhanced in the presence of sulphonamides.
Importance of Rickettsia
Rickettsiae Helvetica, a spotted fever Rickettsiae and emerging pathogen with ixodes ricinus ticks as the main vector, is an agent of human disease and may cause febrile illness as well as Meningitis.
In three parallel series the isolated standard type of R. Helvetica obtained from a PCR-positive I ricimus tick, was high passasged and propagated in a vero cell line.
By using quantitive real time PCR, the generation time from inoculation to stationary phase was obtained degradation of R. Helvetica DNA during the 14 days studied.
Microscopy showed that the organisms invaded the host cells rapidly and primarily found free in the cytoplasm and only occasionally located in the nucleus.
Four days after inoculation some of the host cells were broken and many indifferent stages of cytoplasmic organic decomposition were seen.
However the R.Helvetica organisms did not show any morphologic alterations and the number of organisms was stable after the replication peak which may indicate the R.helvetica is adapted to growth in a vero-cell line that the phase of degradation occurs in later than the 24 days studied.
The findings
differ from what has been reported for other rickettsiae of the spotted fever
group may be importance and virulence of R. Helvetica.
Vector and Reservoir:
Ticks primarily dermacenter speices are the main vector in US.
Ticks
are also agent reservoir, given that there are tick-totick mechanisms of
transmission, allowing infections to all four life-cycle stages (eggs, larvae,
nymphs and adults).
Usually
only 1-5% of ticks are infected by Rickettsiae rickettsii, even in high
incident areas.
The main tick species are involved in transmission are:
# Dermacentor Variabillis (American dog tick) – eastern, and far west US. Dogs and medium sized animals are preferred hosts.
# Dermacenter andersoni ( Rocky mountain wood tick) – western US states and south western Canada. Small Rodents and large mammals are preferred hosts.
# Rhiphalus sanguineus (brown dog tick) – Mexico, US (Arizona) and Europe. This is mainly recognized vector in US.
# Amblyomma cajennense – South America
#Other reservoirs include wild Rodents (e.g. capybaras) dogs, horses and donkeys.
Diseases from Rickettsiae
Rickettsial infections are characterized by fever, headache, malaise, prostration, suin rash and enlargement of the spleen and liver.
However, they are mainly divided in two groups;
A. Typhus group B.
A. Typhus group: It is divided by some other groups: It is again divided by three groups
1. Epedimeic Typus ( Rickettsia prowazekii) group:
This disease is transmitted by the body louse in human long cycle. In epidemic typhus, systemic infection and prostration are severe, and fever lasts about 2 weeks.
The disease is more severe and more often fatal in patients older than 40 years of age. During epidemics, the case fatality rate has been 6-30%.
2. Endemic typhus or murine typhus
(rickettsia typhi) group:
Infected flee feces rubbed into bite wound is he method of transmission.
The clinical picture of epidemic typhus, but the disease is milder and is rarely fatal in elderly patients.
3. Spotted fever group:
The spotted fever resembles typhus clinically; unlike the
rash in other Rickettisial diseases, the rash of the spotted fever group
usually appears first on the extremities moves centripetally and involves the
palmes and soles.
Some such as Brazilian spotted fever may produce severe infections; others such as Meditation fever are mild.
The case of fatality rate varies greatly. In
untreated Rocky Mountain spotted fever, it is usually much greater is elderly
persons (up to 50%) than in young adults and children.
4. Traditional group:
Rickettsial pox (Rickettsiae akari)
is a mild disease with a rash resembling that of varicella about 1 week before
onset of fever, a frim red pupule appears at the site of the mite bite and
develops into a deep seated vesicle that in turn forms a black eshcar.
B. Scrub typhus group:
(Orientia tsutsugamushi) this disease resembles epidemic typhus clinically.
One feature is the eschar, the punched-out ulcer covered with a blackend scab that indicates the location of the mite bite.
Generalized lymphadenopathy and lymphocytocis are common; cardiac and cerebral involvements may severe.
Please see this video on Rickettssiae and other parasites
Abstract of Rickettsiae
1. Ridkettsia are pleomorphic cocobacili that are obligate intercellular pathogens are similar to gram-negative bacteria but they do not stain with gram stain.
2. Riskettsia can be cultivated in cell culture lines and yolk sacs, but immune-histo-chemical or immune-fluorescent stains, serology or molecular methods are usually used for their detection in clinical material.
3. The hallmark of infection with rickettsiais vasculitis.
4. Rickettsia can be divided into typhus, spotted fever, traditional and scurb typhus groups; vectors, clinical manifestations, and geopgraphic distributions vary by group.
5. Disease may be wild as in the case of Rickettsial pox severe as in rocky mountain spotted.
0 Comments