Subject: Medical Microbiology and Virology
Disease causing microorganisms and their detail information
1. RICKETTSIA
2. MYCOPLASMA
3. CHLAMYDIA
4. FUNGI
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,1. RICKETTSIA
Introduction
Rickettsiae are small gram-negative bacilli adapted to obligate intracellular parasitism. They are
primary parasites of arthropods such as lice, fleas, ticks and mites, in which they are found in the
alimentary canal. In humans, they infect the vascular endothelial and reticuloendothelial cells. Many
rickettsiae are transmitted transovarially in the arthropod, which serves as both vector and reservoir.
The family Rickettsiaceae is named after Howard Taylor Ricketts who discovered the spotted fever
rickettsia (1906) and died of typhus fever contracted during his studies. Rickettsiae are easily inactivated
by physical and chemical agents.
The genus Rickettsia consists of the causative agents of two groups of diseases Typhus fevers caused
by R. prowazekii Spotted fevers caused by R. rickettsii.
Table – Human diseases caused by Rickettsia
Rickettsia rickettsii
R. rickettsii is most common rickettsial pathogen.
Epidemiology
Disease is worldwide. Hard ticks (e.g., dog tick, wood tick) are the primary reservoirs and vectors. Ticks
serve as vector as well as reservoir. Other mammals like dogs, small rodents can also act as reservoir.
Transmission requires prolonged contact. Disease is most common April through September.
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, Clinical Symptoms
Rocky Mountain spotted fever is an acute potentially fatal disease characterized by fever, headache
and rash and myalgia and anorexia. Rashes appear typically on extremities (wrist and ankles) and trunk.
Initially they are maculopapular, later on become hemorrhagic. Incubation period ranges from 4 days
to 14 days. Other symptoms include vascular damage, increased permeability, edema, hemorrhage,
disseminated intravascular coagulation, interstitial pneumonitis (central nervous system) CNS
involvement, myocarditis and renal failure. It is most fatal rickettsial disease and is associated with
higher mortality rate.
Rickettsia prowazekii
Epidemic Typhus (Louse-borne) Epidemic typhus is caused by infection with R. prowazekii.
Epidemiology
Disease is worldwide, with most infections in Central and South America and Africa. Sporadic disease is
seen in the eastern United States. Humans are the primary reservoir, with person-to-person
transmission by louse vector. Human body louse, Pediculus humanus corporis acquires the organism
while taking the blood meal from an infected patient. It is believed that sporadic disease is spread from
squirrels to humans via squirrel fleas. Recrudescent disease can develop years after initial infection.
People are at greatest risk those living in crowded, unsanitary conditions.
Clinical Symptoms
Epidemic typhus is an acute febrile disease; accompanied by headache, myalgia, eye discharge and
rashes occurring after an incubation period of 1- 2 weeks. Recrudescent typhus (Brill-Zinsser disease) is a
milder form of the disease. Complications include interstitial pneumonitis, CNS involvement like mental
confusion and coma, myocarditis and acute renal failure.
Diagnosis and treatments are mostly similar for both species.
Diagnosis
1. Microscopy
Although rickettsiae stain poorly with Gram stain, they can be stained with Giemsa or Gimenez stains.
Specific fluorescein-labeled antibodies can also be used to stain the intracellular bacteria in biopsy
tissue specimens.
2. Nucleic Acid–Based Tests
Specific nucleic acid amplification tests are now used in many reference laboratories for the diagnosis of
rickettsial diseases. Unfortunately, these assays are relatively insensitive when blood samples are used.
3. Cultivation
Rickettsiae don’t grow in cell free media. They are easily cultivated in the yolk sac of developing
chick embryo. They also grow on continuous cell lines such as Hela, Hep2, Detroit 6, etc. but the
growth is not satisfactory. Laboratory animals such as guinea pigs and mice are useful for the isolation of
rickettsiae from patients.
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Disease causing microorganisms and their detail information
1. RICKETTSIA
2. MYCOPLASMA
3. CHLAMYDIA
4. FUNGI
1|Page
,1. RICKETTSIA
Introduction
Rickettsiae are small gram-negative bacilli adapted to obligate intracellular parasitism. They are
primary parasites of arthropods such as lice, fleas, ticks and mites, in which they are found in the
alimentary canal. In humans, they infect the vascular endothelial and reticuloendothelial cells. Many
rickettsiae are transmitted transovarially in the arthropod, which serves as both vector and reservoir.
The family Rickettsiaceae is named after Howard Taylor Ricketts who discovered the spotted fever
rickettsia (1906) and died of typhus fever contracted during his studies. Rickettsiae are easily inactivated
by physical and chemical agents.
The genus Rickettsia consists of the causative agents of two groups of diseases Typhus fevers caused
by R. prowazekii Spotted fevers caused by R. rickettsii.
Table – Human diseases caused by Rickettsia
Rickettsia rickettsii
R. rickettsii is most common rickettsial pathogen.
Epidemiology
Disease is worldwide. Hard ticks (e.g., dog tick, wood tick) are the primary reservoirs and vectors. Ticks
serve as vector as well as reservoir. Other mammals like dogs, small rodents can also act as reservoir.
Transmission requires prolonged contact. Disease is most common April through September.
2|Page
, Clinical Symptoms
Rocky Mountain spotted fever is an acute potentially fatal disease characterized by fever, headache
and rash and myalgia and anorexia. Rashes appear typically on extremities (wrist and ankles) and trunk.
Initially they are maculopapular, later on become hemorrhagic. Incubation period ranges from 4 days
to 14 days. Other symptoms include vascular damage, increased permeability, edema, hemorrhage,
disseminated intravascular coagulation, interstitial pneumonitis (central nervous system) CNS
involvement, myocarditis and renal failure. It is most fatal rickettsial disease and is associated with
higher mortality rate.
Rickettsia prowazekii
Epidemic Typhus (Louse-borne) Epidemic typhus is caused by infection with R. prowazekii.
Epidemiology
Disease is worldwide, with most infections in Central and South America and Africa. Sporadic disease is
seen in the eastern United States. Humans are the primary reservoir, with person-to-person
transmission by louse vector. Human body louse, Pediculus humanus corporis acquires the organism
while taking the blood meal from an infected patient. It is believed that sporadic disease is spread from
squirrels to humans via squirrel fleas. Recrudescent disease can develop years after initial infection.
People are at greatest risk those living in crowded, unsanitary conditions.
Clinical Symptoms
Epidemic typhus is an acute febrile disease; accompanied by headache, myalgia, eye discharge and
rashes occurring after an incubation period of 1- 2 weeks. Recrudescent typhus (Brill-Zinsser disease) is a
milder form of the disease. Complications include interstitial pneumonitis, CNS involvement like mental
confusion and coma, myocarditis and acute renal failure.
Diagnosis and treatments are mostly similar for both species.
Diagnosis
1. Microscopy
Although rickettsiae stain poorly with Gram stain, they can be stained with Giemsa or Gimenez stains.
Specific fluorescein-labeled antibodies can also be used to stain the intracellular bacteria in biopsy
tissue specimens.
2. Nucleic Acid–Based Tests
Specific nucleic acid amplification tests are now used in many reference laboratories for the diagnosis of
rickettsial diseases. Unfortunately, these assays are relatively insensitive when blood samples are used.
3. Cultivation
Rickettsiae don’t grow in cell free media. They are easily cultivated in the yolk sac of developing
chick embryo. They also grow on continuous cell lines such as Hela, Hep2, Detroit 6, etc. but the
growth is not satisfactory. Laboratory animals such as guinea pigs and mice are useful for the isolation of
rickettsiae from patients.
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