Notes
Rickettsia, Q Fever, & Related Viruses
Sections
Rickettsiaceae, Anaplasmataceae, Coxiellaceae
Small, intracellular pathogens
Obligate aerobes
Gram-negative rods that stain best with Giemsa or Gimenez stains.
Some infections caused by these species are self-limiting.
Doxycycline can be administered to shorten illness duration and prevent complications.
Species overview
Be aware that all of these species were formerly categorized as members of Rickettsiaceae, and that intertextual variation exists.
Rickettsiaceae:
Rickettsiaceae pathogens damage endothelial cells of blood vessels
Rickettsia rickettsii causes Rocky Mountain Spotted Fever
Rickettsia akari causes rickettsial pox.
Rickettsia prowazekii and Rickettsia typhi cause different forms of typhus.
Orientia tsutsugamushi causes scrub typhus.
Anaplasmataceae:
Ehrlichia chaffeensis causes human monocytic ehrlichiosis.
Ehrlichia ewingii causes human ewingii ehrlichiosis.
Anaplasma phagocytophilum causes human granulocytic anaplasmosis.
Coxiellaceae:
Coxiella burnetti causes Q fever.
Rickettsia rickettsii
Rocky Mountain Spotted Fever
Fever, headache, myalgias, and, sometimes, confusion (as a result of CNS involvement).
Patients commonly develop a macular rash that can progress to petechiae.
The rash characteristically begins on the wrists and ankles, then spreads to the palms, soles, and trunk. This pattern of spread is a helpful distinguisher.
Gastrointestinal involvement is possible, and can cause abdominal pain, nausea, vomiting, and/or diarrhea.
Left untreated, disseminated vasculitis can lead to multi-organ failure.
Ticks and rodents are key reservoirs; hard ticks are vectors.
Rickettsia akari
Rickettsialpox
Typically less severe than Rocky Mountain Spotted Fever.
Patients experience fever and papulovesicular rash with eschars. Headaches and myalgia are possible.
Rodents are the reservoirs; mites are the vectors.
Rickettsia prowazekii
Epidemic typhus
Brill-Zinsser disease is the result of latent infection that manifests years, even decades, later.
Patients experience fever, headache, chills, myalgia, and a macular rash that spreads from the trunk to the extremities.
CNS involvement is possible, and can present as confusion.
If left untreated, vasculitis can lead to multi-organ failure.
Humans are the main reservoir, and the human body louse is the vector; be aware that flying squirrels and their fleas have also been described as reservoirs and vectors.
Rickettsia typhi
Endemic typhus (aka, murine typhus)
Patients experience fever, headache, myalgia, and a maculopapular rash that spreads from the trunk to the extremities.
Gastrointestinal involvement is possible, and more common in children.
Cases are typically mild, but severe cases can lead to renal dysfunction or respiratory impairment (experienced as cough, dyspnea).
Small mammals, particularly cats and rodents, are key reservoirs; their fleas are the vectors.
Orientia tsutsugamushi
Scrub fever
Fever, intense headahces, mylagias, and a maculopapular rash that starts on the trunk; in some cases, eschars will form, especially at the site of inoculation.
Lymphadenopathy and pulmonary and neurologic involvement are common; gastrointestinal involvement may also occur.
Reservoirs include mites (chiggers) and rodents; mites are the vector.
Ehrlichia chaffeensis
Human monocytic ehrlichiosis
Fever, headache, and myalgia; Coughing is common in adults.
The rash associated with this infection varies, and is more common in children.
Central nervous system involvement is possible.
Leukopenia, thrombocytopenia, and elevated transanimases.
Deer, dogs, and other mammals are common reservoirs; soft ticks are the vector.
Anaplasma phagocytophilum
Granulocytic anaplasmosis
Similar symptoms and signs to human monocytic ehrlichiosis
Rash is rare.
Small mammals are the reservoirs; soft ticks are the vector.
Coxiella burnettii
Q fever
Fever, headache, and myalgia, but no rash.
Chronic Q fever can lead to serious complications, including hepatitis, pneumonia, and subacute endocarditis.
Reservoirs include mammals, ticks, and birds; though ticks are a potential vector, most cases of Q fever are the result of the aerosol inhalation or consumption of the bacteria in contaminated milk.
Full-Length Text
Here we will learn about Rickettsiaceae and related species.
To begin, write that these bacteria are small, intracellular pathogens;
They are obligate aerobes,
And are Gram-negative rods that stain best with Giemsa or Gimenez stains.
Next, let's show an overview of these species according to current classification standards; be aware that all of these species were formerly categorized as members of Rickettsiaceae, and that intertextual variation exists.
First, show that the Rickettsiaceae family comprises two genera:
Rickettsia and Orientia.
Show that key Rickettsia species include:
Rickettsia rickettsii, which causes Rocky Mountain Spotted Fever;
Rickettsia akari, which causes rickettsial pox;
And, Rickettsia prowazekii and Rickettsia typhi, which cause different forms of typhus.
Then, show that the Orientia family includes the causative agent of scrub typhus.
Indicate that Rickettsiaceae infection leads to damage to the endothelial cells of blood vessels.
Next, show that the Anaplasmataceae family includes:
Ehrlichia chaffeensis, which causes human monocytic ehrlichiosis,
Ehrlichia ewingii, which causes human ewingii ehrlichiosis;
And, Anaplasma phagocytophilum, which causes human granulocytic anaplasmosis.
Finally, show that Coxiella burnetti causes Q fever.
Some infections caused by these species are self-limiting; however, write that doxycycline can be administered to shorten illness duration and prevent complications.
Now, create a table to organize these species, their diseases and associated clinical features, key reservoirs and vectors.
Write that Rickettsia rickettsii causes Rocky Mountain Spotted Fever, which presents with fever, headache, myalgias, and, sometimes, confusion (as a result of CNS involvement).
Patients commonly develop a macular rash that can progress to petechiae; the rash characteristically begins on the wrists and ankles, then spreads to the palms, soles, and trunk. This pattern of spread is a helpful distinguisher.
Gastrointestinal involvement is possible, and can cause abdominal pain, nausea, vomiting, and/or diarrhea.
Write that, left untreated, disseminated vasculitis can lead to multi-organ failure.
Ticks and rodents are key reservoirs; hard ticks are vectors.
Next, write that Rickettsia akari causes rickettsialpox, which is typically less severe than Rocky Mountain Spotted Fever.
Write that patients experience fever and papulovesicular rash with eschars. Headaches and myalgias are possible.
Rodents are the reservoirs; mites are the vectors.
Write that Rickettsia prowazekii causes epidemic typhus; Brill-Zinsser disease is the result of latent infection that manifests years, even decades, later.
Patients experience fever, headache, chills, myalgia, and a macular rash that spreads from the trunk to the extremities. CNS involvement is possible, and can present as confusion.
If left untreated, vasculitis can lead to multi-organ failure.
Show that humans are the main reservoir, and the human body louse is the vector; be aware that flying squirrels and their fleas have also been described as reservoirs and vectors.
Next, write that Rickettsii typhi causes endemic, aka, murine, typhus.
Patients experience fever, headache, myalgia, and a maculopapular rash that spreads from the trunk to the extremities.
Gastrointestinal involvement is possible, and more common in children.
Cases are typically mild, but severe cases can lead to renal dysfunction or respiratory impairment (experienced as cough, dyspnea).
Small mammals, particularly cats and rodents, are key reservoirs; their fleas are the vectors.
Write that Orientia tsutsugamushi causes scrub fever, which is characterized by fever, intense headahces, mylagias, and a maculopapular rash that starts on the trunk; in some cases, eschars will form, especially at the site of inoculation.
Lymphadenopathy and pulmonary and neurologic involvement are common; gastrointestinal involvement may also occur.
Write that reservoirs include mites (chiggers) and rodents; mites are the vector.
Now, write that Ehrlichia chaffeensis causes human monocytic ehrlichiosis.
Patients experience fever, headache, and myalgia; Coughing is common in adults.
The rash associated with this infection varies, and is more common in children.
Central nervous system involvement is possible.
Signs of monocytic ehrlichiosis include leukopenia, thrombocytopenia, and elevated transanimases.
Deer, dogs, and other mammals are common reservoirs; soft ticks are the vector.
Write that granulocytic anaplasmosis has similar symptoms and signs, and that rash is rare.
Small mammals are the reservoirs; soft ticks are the vector.
Finally, write that Coxiella burnettii causes Q fever, which presents with fever, headache, and myalgia, but no rash.
Chronic Q fever can lead to serious complications, including hepatitis, pneumonia, and subacute endocarditis.
Reservoirs include mammals, ticks, and birds; though ticks are a potential vector, most cases of Q fever are the result of the aerosol inhalation or consumption of the bacteria in contaminated milk.
References:
Murray, P. R., Rosenthal, K. S., & Pfaller, M. A. Medical microbiology. Philadelphia: Elsevier/Saunders. (2013).
Levinson, W. E. Review of Medical Microbiology and Immunology. 14th Ed. Lange (2016).
Images:
Eschar example (CDC/ James H. Steele)