This document provides an overview of Rickettsiae, including their classification, characteristics, life cycle, transmission, and pathogenic species. Rickettsiae are obligate intracellular bacteria that parasitize arthropod and vertebrate cells. They are transmitted between hosts by arthropod vectors like ticks and mites. Major diseases discussed include Rocky Mountain spotted fever caused by Rickettsia rickettsii transmitted by ticks, and typhus caused by R. prowazekii transmitted between humans by body lice. The document outlines the classification, characteristics, life cycles and transmission mechanisms of the main Rickettsiae genera.

1. TOPIC: RICHETTSIAE
PRESENTED BY:
TEJASWINI PETKAR

2. CONTENTS
 INTRODUCTION
 CHARACTERISTICS
 CELL STRUCTURE AND STAINING
 METABOLISM
 CULTIVATION
 PATHOGENICITY
 RESISTANCE
 LIFE CYCLE AND TRANSMISSION
 CLASSIFICATION
 ROCKY MOUNTAIN SPOTTED FEVER
 Q FEVER
 LAB DIAGNOSIS
 IMMUNOPROPHYLAXIS
 CONCLUSION
 REFERENCES

3. INTRODUCTION
• Rickettsiae are considered to be a transitional group between
viruses and bacteria. They resemble viruses as they are
obligate parasites growing only on living tissues –generally
intracellularly ( except Coxiella ).
• The fact that they can not pass through bacteria retaining
filters,have a larger size, have both types of nucleic acids-
DNA and RNA, have a cell wall and are sensitive to antibiotics
ensure that they are not viruses.
• Since these microbes have a cell wall containing muramic acid
and peptidoglycan, divide by binary fission and have metabolic
enzymes independent of the host cell, they are true bacteria.
 Richettsiae tend to parastize cells lining the intestinal
tract of insects –only a few being pathogenic to their
insect hosts (arthropods).
 Transmission to animal host can be through the feces or
saliva of insect host.

4. HOWARD TAYLOR RICKETTS (1871- 1910)
 He was an American pathologist.
 The pathogen, Rickettsia rickettsii, was named after him.
 In his early part of career, he undertook research at
Northwestern University on blastomycosis. He later
worked on Rocky Mountain spotted fever at the
University of Chicago and Bitterroot Valley of Montana.
 He was devoted to his research and on several occasions,
injected himself with pathogens to study their effects.
 On account of the apparent similarity between Rocky
Mountain fever and typhus fever, he became occupied in
investigating the latter in Chicago where the disease was
epidemic. He became a victim of the epidemic in 1910.
 His investigations and discoveries added materially to the
sum of medical knowledge.

5. CLASSIFICATION
 SCIENTIFIC CLASSIFICATION
Domain: Bacteria
Phylum: Proteobacteria
Class: Alphaproteobacteria
Subclass: Rickettsidae
Order: Rickettsiales
Family: Rickettsiaceae
Genus: Rickettsia

6. CHARACTERISTICS
 Richettsiae consist of bacteria which are gram
negative, highly pleomorphic, non motile, non
capsulated and non sporing.
 The bacterial cells are surrounded by protein
microcapsular and slime layers.
 They can be present as cocci (0.2-0.5 μm in
diameter) or short rods (1–3 μm long) or thread like
(10 µm under poor or harsh conditions). They may at
times appear longer when cell division is impaired.
 They are mostly aerobic.
 Growth occurs in the cytoplasm, sometimes in the
nucleus of certain vertebrate and arthropod cells but
not in phagocytotic vacuoles.
 The mol percentage G+C of the DNA is 29-33%.

7. CELL STRUCTURE AND STAINING
 These bacteria stain poorly.
 In smears from yolk sacs, tissues or cell cultures,
rickettsiae are best visualized by the Gimenez stain where
they retain their basic fuchsin by appearing bright red
while the background is decolourised and stains a pale
greenish blue with the counterstain malachite green.
Tick hemolymph cells infected with R.rickettsii under
Gimenez stain. Cells of R.rickettsii are uniform in appearance.

8. METABOLISM
 Detailed experiments have been conducted with only a few
species and it is found that there are no major
differences in metabolic pathways within the genus.
 These bacteria derive energy from the metabolism of
glutamate via citric acid cycle . Glucose and glucose 6
phosphate are not utilized and hence most of the enzymes
commonly associated with glucose metabolism are absent.
 Rickettsiae also are seen to interact with exogenous
phosphorylated compounds. They depend on the mono
phosphate compounds of the host.

9. CULTIVATION
 Rickettsiae are unable to grow on lifeless or cell-free
media.
 R.quintana , the causative agent of trench fever is an
exception which has been found to grow on blood agar.
 They are readily cultivated in the yolk sac of developing
chick embryos as first shown by Cox in 1941.
 Chicken embryo fibroblasts and mouse L cells are
employed most frequently but a variety of other cells such
as monocytes and polymorphonuclear leukocytes have also
been used.
 Laboratory animals like guinea pigs and mice are useful to
isolate the bacteria from patients. They may also be
propagated in arthropods.

10. PATHOGENICITY
 The genus Rickettsia is notorious for its virulence in men.
 Although a toxin separable from rickettsial cell has not
been isolated, several viable species display toxin-like
action.
 The effect of injecting the bacteria is described as
damage to the endothelial cells accompanied by increased
capillary permeability, flow of plasma into tissues,
hemoconcentration and eventual collapse of blood
circulation making the animals die within a span of 1-8
hours.
 Natural cycle generally involves a vertebrate and an
invertebrate host.

11. RESISTANCE
 The bacteria are readily inactivated by physical and
chemical agents.
 They are rapidly destroyed at 56°C and room temperature
when separated from host components.
 They are best preserved by rapid freezing and storage
below -50°C.
 Most of the broad range antibiotics are efficacious
against Rickettsial infections.
 Rickettsiae are susceptible to tetracycline and
chloramphenicol. The tetracycline class of compounds has
some in vitro rickettsiacidal effect through metabolic
inhibition and is more effective than chloramphenicol.
 Penicillin, streptomycin, erythromycin and sulphonamides
are ineffective.

12. LIFE CYCLE AND TRANSMISSION
 Natural cycle generally involves a vertebrate and an
invertebrate host.
 The most striking aspect of the life cycle of Rickettsiae is
that they are maintained in nature in certain arthropods
such as ticks, lice, fleas, and mites , with one exception,
are transmitted to humans by the bite of the arthropod.
 In most cases, the arthropods excrete the bacteria with
their feces and ticks transmit them with their saliva while
sucking blood.
 An uninfected tick can become infected when feeding on
the blood of an infected mammalian host in the larval or
nymph stages. Once a tick becomes infected with this
pathogen, it is infected for life.
 Humans are considered to be accidental vectors and are
not essential in the rickettsial cycle.

13.  An uninfected mammal can become infected with
Rickettsia when eating food that contains the feces of the
infected tick. They can also be infected through the bite
of an infected tick. After getting bitten by an infected
tick, rickettsiae are transmitted to the bloodstream by
tick salivary secretions or by infected vector feces.
 C. burnetii is transmitted exclusively by inhalation of dust
containing the pathogens.
 These bacteria circulate widely in the bloodstream
(bacteremia), infecting primarily the endothelium of the
blood vessel walls.
 The organisms invade the host organism through skin
injuries. Once inside the body, rickettsiae reproduce
mainly in the vascular endothelial cells. These cells then
die, releasing increasing numbers of organisms into the
bloodstream. Numerous inflammatory lesions are caused
locally around the destroyed endothelia.

14.  Rickettsia normally multiplies by transverse binary fission.
Rickettsia enters via the skin and spreads through the
bloodstream to infect vascular endothelium in the skin,
brain, lungs, heart, kidneys, liver, gastrointestinal tract,
and other organs.
 Rickettsial attachment to the endothelial cell membrane
induces phagocytosis, soon followed by escape from the
phagosome into the cytosol. It then divides inside the cell
and remains inside the apparently healthy host cell until
massive quantities of intracellular rickettsiae accumulate
and the host cell bursts, releasing the organisms. This is
seen in the case of R. prowazekii.
 Alternately the bacteria leave the host cell via long, thin
cell projections (filopodia) after a few cycles of binary
fission as seen in case of R rickettsii.

15. Rickettsial infection in endothelial cells

16. Depending on the diseases they produce, vectors
transmitting them, antigenic relationships, growth
properties and resistance to physical and chemical agents,
the bacteria of this group belong to 3 genera-
 Rickettsia,
 Coxiella
 Bartonella (formerly called Rochalimaea)

17. RICKETTSIA
 The genus Rickettsia contains several species and consists
of two antigenically defined groups- spotted fever group
( SFG) and typhus group(TG) which are related.
 The members are obligate parasites of eukaryotic cells.
 They are more widespread.
 They do not multiply within vacuoles and do not parasitise
leucocytes.
 They are known to be associated with arthropods, leeches
and protists. Arthropod-inhabiting rickettsiae are
generally associated with reproductive manipulation (such
as parthenogenesis) to persist in host lineage.

18. COXIELLA
 The genus Coxiella consists of one species- C.burnetii. . It
is the causative agent for Q fever.
 It is named after Herald Rea Cox, an American
bacteriologist who first isolated the agent from ticks and
introduced the technique of yolk sac inoculation for its
cultivation.
 This obligate intracellular parasite grows preferentially in
the vacuoles in the host cells.
 It differs from Rickettsia in being smaller, more resistant
to physical and chemical agents and transmissible without
arthropod vectors.

19. BARTONELLA
 Genus Bartonella or Rochalimea includes emerging
opportunistic human pathogens.
 The recognized species like B.quintana cause trench fever,
B.henselae cause bartonellosis or cat-scratch disease and
B.bacilliformis cause Carrion’s disease.
 Bartonella species are transmitted by vectors such as
ticks, fleas, sand flies and mosquitoes. Immediately after
infection, the bacteria colonise endothelial cells as primary
niche where they infect erythrocytes. The bacteria then
invade phagosomal membrane in the erythrocytes where
they multiply till they are taken up by a blood sucking
arthropod.

20.  Although distinguished by fundamental phenotypic
differences, Rickettsia and Bartonella are genotypically
related, as indicated by 25-33 % hybridization between
their DNAs. An evolutionary relationship between these
two genera can be explained on the basis of a common
niche in man and ectoparasites.
 There is however no genetypic relationship but superficial
phenotypic similarity between Rickettsia and Coxiella.

21. CLASSIFICATION BASED ON
DNA SEQUENCING AND SEROLOGY
The bacteria are classified as the following groups-
• spotted fever
• typhus
• scrub typhus

22. SCRUB OR BUSH TYPHUS GROUP
 This group includes a single species- Orientia tsutsugamushi
with 3 major serovars (distinct variations within species).
 Orientia tsutsugamushii is a Gram-negative, α-
proteobacterium of family Rickettsiaceae. It was first
observed in Japan in 1930 where it was believed to be
transmitted via mites.
 The disease was thus named ( from tsutsuga meaning
dangerous and mushi meaning insect or mite).
 It is found in areas with a suitable climate, plenty of moisture
and scrub vegetation.
 Rickettsiae are transmitted transovarially in mites. Various
rodents and birds act as reservoirs and also help in spreading
the Rickettsiae to fresh areas.
 The incubation period is 1 to 3 weeks.
 Most patients develop a characteristic eschar ( dry scar or
falling off of dry skin) at the site of the mite bite. The
disease sets in with fever, headache and conjunctival
infection. A red macular rash appears on the fifth day.

23. SPOTTED FEVER GROUP:
 Rickettsiae of this group possess a common soluble antigen
and multiply in the nucleus as well as the cytoplasm of host
cells.
 6 species have been identified – Rickettsia rickettsii causing
Rocky Mountain spotted fever, R.conori causing
Indian/Mediterranean/South African tick typhus
,R.australis causing Queensland tick typhus ,R.parkeri
infecting guinea pigs , R.siberi causing Siberian tick typhus
,and R.akari causing rickettsial pox.
 They are transmitted via ticks except for R.akari which is
mite- borne.
 These vectors act as reservoirs as well. The ticks are not
harmed by the Rickettsiae and remain infected for life.
 Rickettsiae are shed in tick feces but transmission to men is
mainly by bite as the bacteria invade the salivary glands of
the ticks as well.
 Rocky Mountain Spotted fever is the most serious type and is
the first to have been described by Howard Ricketts.

24. TYPHUS GROUP:
 This group consists of epidemic typhus, recrudescent
typhus( Brill-Zinsser disease) and endemic typhus disease-
causing Richettsiae.
 The causative agent of epidemic typhus is R.prowazekii ,
named after Von Prowazek, who had contracted fatal
typhus fever while investigating the disease. Man is the
only natural vertebrate host while the human body louse-
Pediculus humanus corporis is the vector.
 The incubation period is 5-21 days. The disease starts
with fever and chills and a characteristic rash on the fifth
day. Rickettsiae may remain latent in the lymphoid tissues
or organs for years.
 Such latent infection may at times be reactivated leading
to recrudescent typhus ( Brill –Zinsser disease). This
disease explains the manner in which the Rickettsia is able
to survive without extrahuman reservoirs. The disease is
not much significant but in cases of louse-ridden
communities may initiate epidemics of typhus fever.

25.  Endemic typhus (murine or flea borne typhus) is a milder
disease than epidemic typhus. It is caused by R.mooseri (
R.typhi) and transmitted by the rat flea, Xenopsylla
cheopis. The flea is unaffected but remains infectious
throughout its lifecycle. Man acquires the disease usually
through the flea bite. Human infection is a dead end. Man
to man transmission does not occur.
 R.prowazekii and R.mooseri are closely similar but may be
differentiated by biological and immunological tests such
as complement fixation tests and rickettsial agglutination
tests.

26. ROCKY MOUNTAIN SPOTTED
FEVER(RMSF)
 This disease is the most serious type and caused by
R.rickettsii. It was first described by Howard
Ricketts.
 It was originally identified in Montana and Idaho,
America.
 The case fatality is variable.
 The bacterium is a gram negative, intracellular and
coccobacillus. The cells are slightly smaller and more
uniform. Strains of the species vary considerably in
virulence. Virulence for these two hosts appears to
vary independently.
 This disease is characterized by high fever and
damage to small blood vessels resulting in skin rash,
intravascular thrombosis with consequent lesions in
various organs and necrosis of extremities.The rash
first appears on the wrist and ankles then spreads
throughout the body including buccal mucosa.

27.  Only members of the tick family Ixodidae (hard ticks) are
naturally infected with Rickettsia rickettsii. The most
common vector is the wood tick-Dermacentor andersoni.
 Transmission to man occurs through bite of an infected
tick that remains attatched to skin for a number of hours.
 Before antibiotics 20-25% of infected people would die
from Rocky Mountain spotted fever and even today 3-5%
still die due to late diagnosis.
 Dogs are susceptible to natural infection while mice are
quite resistant ( Lissman and Benach., 1980).
Rashes of RMSF

28. Q FEVER
 Q fever or the Query fever, is a bacterial infection
caused by the bacteria Coxiella burnetii.
 It is the most widely distributed of all Rickettsial diseases
and has been reported from virtually all countries.
 It is an aerosol borne disease.
 It is primarily zoonotic, solidly established in domestic
livestock worldwide.
 Transovarial transmission has been demonstrated in ticks.
Rickettsiae are abundant in tick feces and survive in them
for long periods in dry state. Ticks transmit the disease to
cattle, sheep and poultry.
 Rickettsiae are shed in milk of infected cattle. Ticks do
not play any significant role in transmission to men. Human
infection is acquired by inhaling dust contaminated with
Rickettsia derived from animals and their products.
Infection may also occur by ingesting infected milk.

29.  Clinical picture is variable as asymptomatic infections are
common. No rashes occur.
 The Rickettsiae spread through all organs especially heart
and liver lead to hepatitis. Recovery may occur
spontaneously. Man to man transmission is rare.
 Coxiella may remain latent in tissues for 2-3 years.
 C.burnetii is pleomorphic occurring as small rods 0.2-0.4µ
by 0.4 to 1µ. It is filterable. It grows preferentially in the
vacuoles of the host cell.

30. LAB DIAGNOSIS
 Rickettsial diseases may be diagnosed in the laboratory
either by the isolation of the bacteria or by serological
methods.
 As Rickettsiae are very infectious and have caused serious
and fatal infections among laboratory workers, their
isolation should be attempted only with utmost care.
 Smears from peritoneum, and spleen of infected animals
may be stained by Giemsa method to demonstrate
Rickettsia.
 Serological diagnosis may be Weil-felix reaction or
specific tests using Rickettsial antigens. The most
frequently used serological method using Rickettsial
antigens is complement fixation test. This may be done
using group specific soluble antigen or type specific
washed rickettsial antigen. The former is used for routine
test while the latter is used to differentiate between
epidemic and endemic typhus.

31.  Microimmunofluorescence( micro-IF) provides a high
satisfactory antigenic analysis. The results obtained by
micro-IF are in agreement with resuts of other tests used
for species differentitation.
 Other serological tests include passive haemagglutination
of red blood cells sensitized by erythrocyte sensitizing
substance ( ESS), toxin neutralization and radioisotope
precipitation.
 A skin test has been developed to demonstrate delayed
hypersensitivity to C. burnetii.

32. IMMUNOPROPHYLAXIS OF RICKETTSIAL
DISEASES
 Rickettsial diseases may be prevented by general
measures like controlling vectors and animal reservoirs.
 Control of Q fever requires adequate pasteurization of
milk and care in handling animals and their products.
 Killed and live vaccines have been prepared against
epidemic typhus.
 Cox type vaccine confers effective protection against
Rocky Mountain Spotted fever.
 Immunoprophylaxis in scrub typhus is rendered difficult
by the antigenic heterogenecity of O.tsutsugamushi.

33. CONCLUSION
 The genus Rickettsia is notorious for its virulence in men.
 Richettsiae consist of bacteria which are gram negative,
highly pleomorphic, non motile, non capsulated and non
sporing. They can be present as cocci (0.2-0.5 μm in
diameter) or short rods (1–3 μm long).
 Rickettsiae are considered to be a transitional group
between viruses and bacteria.
 They are true bacteria since these microbes have a cell wall
containing muramic acid and peptidoglycan, divide by binary
fission and have metabolic enzymes independent of the host
cell.
 The bacteria of this group belong to 3 genera- Rickettsia,
Coxiella and Bordetella (formerly called Rochalimaea)
depending on the diseases they produce, vectors
transmitting them, antigenic relationships, growth
properties and resistance to physical and chemical agents.

34.  Based on DNA sequencing and serology, the bacteria are
classified as- spotted fever, typhus, and scrub typhus
groups.
 Rickettsial diseases may be prevented by general
measures like controlling vectors and animal reservoirs.

35. REFERENCES
 R.Ananthanarayan and CK Paniker, Textbook of
Microbiology, edition 1979.
 Noel R Krieg et al ,Bergey’s manual of systematic
bacteriology, volume 1
 Warren Levinson, Review of medical microbiology and
immunology, 10th edition,2008.
 Kayser and Thieme, Colour atlas of medical microbiology,
revised edition 2005
 CE Clifton, Introduction to the Bacteria, 1st edt, 3rd
impression ( 1950).
 https://www.ncbi.nlm.nih.gov/books/NBK7624/
 http://eol.org/pages/976598/details