Welcome to CDC's training on rickettsial disease diagnostics.
This video provides information on rickettsial disease diagnostic methods
including what tests are available and when it is most appropriate to collect samples. It will
focus on the use of polymerase chain reaction or PCR tests and the indirect immunofluorescence
antibody IFA assay for rickettsial disease diagnosis. Rickettsial diseases are caused by
closely related bacterial species typically spread to people from infected fleas, ticks, and mites.
In the United States, rickettsial diseases of clinical importance include Rocky Mountain
spotted fever or RMSF and other spotted fever rickettsioses such as rickettsial pox and
Rickettsia parkeri rickettsiosis, ehrlichioses, anaplasmosis, and flea-borne typhus. Signs
and symptoms of rickettsial disease can vary, but most patients experience fever or chills,
fatigue, malaise, muscle or body aches, and headache within the first few days of illness.
Rash may occur and is most commonly seen in RMSF and other spotted fever rickettsioses.
Patients with diseases such as Rickettsia parkeri rickettsiosis typically develop a scab-covered
ulcer, or eschar at the site of tick attachment. Because early clinical symptoms are non-specific,
patients are often misdiagnosed with other conditions at their first visit
to a healthcare provider.
Knowing which testing options are available and recommended can help with rickettsial disease diagnosis.
If you suspect rickettsial infection in any person at any age, begin treatment with doxycycline. Do not wait for laboratory test results.
Doxycycline is the treatment of choice for all rickettsial diseases in patients of all ages.
Several of these infections, particularly RMSF, can be rapidly progressive and life-threatening.
Various laboratory methods exist for the diagnosis of rickettsial diseases. Methods differ in
availability, type and timing of specimen collection, and interpretation of results.
Molecular methods such as PCR and serologic techniques are the most commonly available
methods of testing for rickettsial diseases and will be discussed in further detail. Microscopic
visualization such as immunohistochemistry and peripheral blood smears as well as culture are
additional methods used for rickettsial disease diagnosis but are less commonly performed.
Rickettsial diseases can be diagnosed using PCR. PCR detects genetic material of the bacteria.
The ability to detect genetic material decreases after appropriate antibiotic treatment so
these methods are most sensitive at the time of, or within 48 hours of antibiotic therapy.
Spotted fever group rickettsiae, such as Rickettsia rickettsii, circulate in the blood at low levels
in the early stage of disease making PCR on early samples less sensitive. A positive PCR test result
is confirmatory, but a negative test result does not rule out the possibility of infection.
For PCR, collect samples within the first 14 days of illness while a patient is still symptomatic.
The quantity of bacteria in blood or tissue may decrease as the body fights infection
or following antibiotic therapy.
PCR can be performed on a wide range of sample types.
Whole blood and fresh tissue samples, such as skin biopsy of a rash,
are the preferred sample types. However, other clinical specimens, such as eschar swabs,
serum, bone marrow, or cerebrospinal fluid can be used in certain circumstances.
Bacterial genetic material may degrade under non-ideal sample storage conditions.
Contact your testing laboratory about acceptable sample types and how best to store and ship specimens
for molecular testing.
PCR for the diagnosis of anaplasmosis and ehrlichiosis, using whole blood is widely available
and the sensitivity is high.
Other rickettsial species, such as Rickettsia rickettsii, are more difficult to detect by PCR because bacteria circulate in low numbers,
especially in early stages of disease. PCR is not widely available for all rickettsial pathogens;
however, certain state public health laboratories and the Centers for Disease Control and Prevention
have the capacity to offer these diagnostic tests. Importantly, the specificity of PCR assays depends
on the assay type and targets. Contact your state or local health department to learn more.
Several types of serologic tests are available, but IFA is the most widely used serologic
test for rickettsial diseases and is preferred as it provides quantitative results.
Many commercial and state public health laboratories offer IFA testing for domestic rickettsial diseases.
As a note, for rickettsial diseases, we rely primarily on IgG antibody reactions.
IgM antibodies are generally less specific than IgG, so
IgM results alone should not be used to diagnose a rickettsial disease.
Closely related species, especially within the spotted fever group, frequently produce cross-reacting antibodies.
Cross-reactivity also occurs between spotted fever group and typhus group species. Because
of this cross-reactivity, IFA cannot provide species-specific results. Serologic tests,
performed on paired serum samples, can confirm a recent infection. The first sample should be
an acute serum, collected in the first 14 days of illness or while that patient is symptomatic.
For best interpretation, convalescent samples should be taken two to four weeks after the acute sample.
Look for a four-fold or greater increase in IgG antibody titer between the acute
and convalescent specimens to confirm a recent infection.
Serologic tests are an indirect detection method that measure antibody levels, rather than directly
detecting the presence of the pathogen.
Antibodies take an average of 7 to 10 days following illness onset to reach detectable levels, and a negative result during the first week of illness is common.
In fact, a measurable titer in the first week of illness is generally more reflective of a
past exposure rather than indicative of current illness.
In some patients, titers have been observed to be elevated for years after recovery from acute illness.
A single titer, whether positive or negative, is insufficient to confirm or rule out a rickettsial disease.
Microscopic visualization is much less commonly used than PCR and IFA assays.
A blood smear may show clusters of bacteria within white blood cells, called morulae.
Blood smears can support the diagnosis of ehrlichiosis or anaplasmosis, but do not allow for species identification.
Culture is uncommon and not widely available. Rickettsia are intracellular organisms
and do not grow on typical blood culture, rather requiring cell culture.
In summary, PCR and IFA are the most widely available and reliable means of diagnosing rickettsial diseases.
For healthcare providers considering a rickettsial disease diagnosis,
the ideal samples to collect are whole blood and serum during the acute phase of illness,
and a convalescent serum sample two to four weeks later. The acute and convalescent sera
are then run in parallel to assess antibody development looking for a four-fold or greater
change. Acute whole blood, tissue, or eschar swab specimens can be tested using PCR.
For more information on rickettsial diseases, please consult the following resources.