Page 226 - Review of Medical Microbiology and Immunology ( PDFDrive )

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CHAPTER 26 Rickettsiae
Coxiella burnetii exists in two phases that differ in their
antigenicity and their virulence: phase I organisms are iso-
consists of the killed organism. Pasteurization of milk
kills C. burnetii.
lated from the patient, are virulent, and synthesize certain
surface antigens, whereas phase II organisms are produced
by repeated passage in culture, are nonvirulent, and have
ANAPLASMA PHAGOCYTOPHILUM
lost the ability to synthesize certain surface antigens. The
clinical importance of phase variation is that patients with
chronic Q fever have a much higher antibody titer to phase I
family that causes human granulocytic anaplasmosis
antigens than those with acute Q fever. Anaplasma phagocytophilum is a member of the Rickettsia
(HGA). Disease is endemic in northeastern and north cen-
mebooksfree.com mebooksfree.com mebooksfree.com HGA, granulocytes rather than mononuclear cells are mebooksfree.com
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tral states (e.g., Connecticut and Wisconsin). Distribution
Transmission
is similar to that of Lyme disease. Ixodes ticks are the main
vectors. Rodents and dogs are important reservoirs. In
Coxiella burnetii, the cause of Q fever, is transmitted by
aerosol and inhaled into the lungs. Q fever is the one rick-
infected, but the disease is clinically indistinguishable from
ettsial disease that is not transmitted to humans by the bite
that caused by Ehrlichia chaffeensis (see later). The organ-
of an arthropod. The important reservoirs for human infec-
ism forms an inclusion body called a morula in the cyto-
tion are cattle, sheep, and goats. Coxiella burnetii causes an
plasm of infected cells. The morula, which is shaped like a
inapparent infection in these reservoir hosts and is found
mulberry, is indistinguishable from that formed by
in high concentrations in the urine, feces, placental tissue,
and amniotic fluid of the animals. It is transmitted to
a rise in antibody titer. Doxycycline is the treatment of
humans by inhalation of aerosols of these materials.
choice. This organism was formerly known as Ehrlichia
equi, and the disease it caused was formerly known as
Clinical Findings & Epidemiology Ehrlichia. The diagnosis is made serologically by detecting
human granulocytic ehrlichiosis (HGE).
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Unlike other rickettsial diseases, the main organ involved
in Q fever is the lungs. It begins suddenly with fever, severe
headache, cough, and other influenzalike symptoms. This
is all that occurs in many patients, but pneumonia ensues
Ehrlichia chaffeensis is a member of the Rickettsia family
in about half. Hepatitis is frequent enough that the combi-
and causes human monocytic ehrlichiosis (HME). This
nation of pneumonia and hepatitis should suggest Q fever.
disease resembles Rocky Mountain spotted fever, except
A rash is rare, unlike in most of the other rickettsial dis-
that the typical rash usually does not occur. High fever,
eases. In general, Q fever is an acute disease, and recovery
severe headache, and myalgias are prominent symptoms.
is expected even in the absence of antibiotic therapy. Rarely,
The organism is endemic in dogs and is transmitted to
chronic Q fever characterized by life-threatening endocar-
ditis occurs.
the Lone Star tick, Amblyomma. Ticks of the genus Ixodes
The disease occurs worldwide, chiefly in individuals
are also vectors. Ehrlichia chaffeensis primarily infects
whose occupations expose them to livestock, such as shep- humans by ticks, especially the dog tick, Dermacentor, and
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mononuclear leukocytes and forms characteristic morulae
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herds, abattoir employees, and farm workers. Ingestion of
in the cytoplasm. (A morula is an inclusion body that
cow’s milk is usually responsible for subclinical infections
resembles a mulberry. It consists of many E. chaffeensis
rather than disease in humans. Pasteurization of milk kills
the organism.
liver enzyme values are seen. In the United States, the
disease occurs primarily in the southern states, especially
Laboratory Diagnosis
Arkansas. The diagnosis is usually made serologically by
detecting a rise in antibody titer. Doxycycline is the treat-
Serologic tests, such as the indirect immunofluorescence
ment of choice.
assay, are used rather than isolation of the organism. Coxi-
ella burnetii can be grown in cell culture or embryonated
eggs but this is a hazardous procedure that is not available
in the standard clinical laboratory.
SELF-ASSESSMENT QUESTIONS
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Treatment
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1. Your patient is a 40-year-old woman with the sudden onset of
The treatment of choice is doxycycline.
fever to 40°C, severe headache, and petechial rash over most of her
nately, despite antibiotics and other support, she died the follow-
Prevention
ing day. An autopsy was performed, and immunohistochemical
Persons at high risk of contracting Q fever, such as veterinar-
tests on her brain tissue revealed an infection by Rickettsia rickettsii.
ians, shepherds, abattoir workers, and laboratory personnel
Of the following, which one is the most accurate?
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