Rickettsial Infections
Description
Several species of Rickettsia can cause illnesses
in humans (Table 3–17). [3] These
agents are usually not transmissible directly from person to person.
Transmission occurs via an infected arthropod vector or through exposure
to an infected animal reservoir host. In addition, transmission has
been documented to occur via blood transfusion. Rickettsial agents
that cause human disease are typically categorized not by disease
manifestation but according to antigenic similarity. The clinical
severity and duration of illnesses associated with different rickettsial
infections vary considerably, even within a given antigenic group.
Rickettsioses range in severity from diseases that are relatively
mild (rickettsialpox and African tick-bite fever) to those that can
be life-threatening (Rocky Mountain spotted fever and Oroya fever),
and in duration from those that can be self-limiting (cat-scratch
disease) to chronic (Q fever and Brill-Zinsser disease). Most patients
with rickettsial infections recover with timely application of appropriate
antibiotic therapy.
Travelers may be at risk for exposure to agents
of rickettsial diseases if they engage in occupational or recreational
activities that bring them into contact with habitats that support
the vectors or animal reservoir species associated with these pathogens.
Occurrence and Risk for Traveler
The geographic distribution and the risks for exposure
to rickettsial agents are described below (by disease).
Epidemic Typhus and Trench Fever
Epidemic typhus and trench fever (caused by infection
with Rickettsia prowazkeii and Bartonella quintanta,
respectively) are transmitted from one person to another by the human
body louse. Contemporary outbreaks of both diseases are rare in most
developed countries and generally occur only in communities and populations
in which body louse infestations are frequent (typically seen in
refugee and prisoner populations, particularly during wars or famine).
These diseases also occur sporadically in cooler mountainous regions
of Africa, South America, Asia, and Mexico, especially during the
colder months when louse-infested clothing is not laundered and person-to-person
spread of lice is more frequent. Additional foci of trench fever
among homeless populations in urban centers of industrialized countries
have been recognized recently. Travelers who are not at risk of exposure
to lice or to persons with lice are unlikely to acquire these illnesses.
However, health-care workers who care for these patients may be at
risk of acquiring louse-borne illnesses through inhalation or inoculation
into the skin of infectious louse feces.
Murine Typhus
Murine typhus (caused
by infection with R. typhi) occurs worldwide and is transmitted
to humans by rat fleas. Flea-infested rats can be found throughout
the year in humid tropical environments, but in temperate regions
are most common during the warm summer months. Travelers who visit
in rat-infested buildings and homes, especially in harbor or riverine
environments, can be at risk for exposure to the agent of murine
typhus.
Scrub Typhus
Mites (“chiggers”)
transmit Orientia tsutsugamushi, the agent of scrub typhus,
to humans. These mites occur year round in a large area from the
Indian subcontinent to Australia and in much of Asia, including
Japan, China, Korea, and parts of Russia. Their prevalence, however,
fluctuates with temperature and rainfall. Humans typically encounter
the arthropod vector of scrub typhus in recently disturbed terrain
(e.g., forest clearings).
Tick-Borne Rickettsioses
Tick-borne rickettsial
diseases have a worldwide distribution, but are most apparent in
temperate and subtropical regions. These diseases include Rocky
Mountain spotted fever (caused by R. rickettsii), Mediterranean
spotted fever (R. conorii), African tick-bite fever (R.
africae), Queensland tick typhus (R. australis), and
North Asian tick fever (R. sibirica), and ehrlichiosis (Ehrlichia spp., Anaplasma
phagocytophilum, and Neorickettsia sennetsu). In general,
peak transmission of tick-borne rickettsial pathogens occurs seasonally
during spring and summer months. Travelers who participate in outdoor
activities in grassy or wooded areas (e.g., trekking, camping,
or going on safari) may be at risk for acquiring tick-borne illnesses,
including those caused by Rickettsia, Anaplasma,
and Neorickettsia species.
Rickettsialpox
Rickettsialpox is an
urban, mite-vectored disease associated with R. akari -infected
house mice. Outbreaks of this illness have occurred shortly after
rodent extermination programs. R. akari-infected rodents
have been found in urban centers in the former Soviet Union, South
Africa, Korea, Croatia, and the United States.
Q Fever
Q fever occurs worldwide,
most often in persons who have frequent contact with goat, sheep,
and cattle carcasses (especially farmers, veterinarians, butchers,
or meat packers). Travelers who visit farms or rural communities
can be exposed to Coxiella burnetii, the agent of Q fever,
through airborne transmission (via contaminated soil and dust),
or possibly through consumption of unpasteurized milk products.
Initially, these infections may result in only mild illnesses,
but if untreated, infections may become chronic, particularly in
persons with preexisting heart valve abnormalities or with prosthetic
valves. Such persons can develop chronic and potentially fatal
endocarditis.
Cat-Scratch Disease and Oroya Fever
Cat-scratch disease is contracted through scratches
and bites from domestic cats infected with Bartonella henselae,
and possibly from their fleas. Exposure can therefore occur wherever
cats are found. Oroya fever can be transmitted by sandflies infected
with B. bacilliformis. The agent of this disease is endemic
in the Andean highlands.
Clinical Presentation
Clinical presentations of rickettsial illnesses
vary (Table 3–17), but early symptoms
are generally nonspecific, involving fever, headache, and malaise.
Rashes are often associated with rickettsioses, and an eschar (thick
blackened scab) is seen in several spotted fever rickettsioses and
in scrub typhus. Illnesses resulting from infection with rickettsial
agents often go unrecognized or are attributed to other causes. Diagnosis
of rickettsial diseases is based on two or more of the following:
1) clinical symptoms and an epidemiologic history compatible with
a rickettsial disease, 2) the development of specific antibodies
reactive with a given pathogen or antigenic group, 3) a positive
polymerase chain reaction test result, or 4) isolation of a rickettsial
agent. Ascertaining the place and the nature of potential exposures
is particularly important for accurate diagnosis, as many rickettsial
diseases have strong geographic links or are associated with exposure
to specific animal reservoir species or arthropod vectors.
Table
3–17. Epidemiologic features, symptoms,
and treatment of rickettsial diseases
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Prevention
With the exception of the louse-borne diseases described
above, for which contact with infectious arthropod feces is the mode
of transmission (via autoinoculation into a wound or inhalation),
travelers and health-care providers are generally not at risk of
becoming infected via exposure to an ill person. Infections result
primarily from exposure to an infected vector or animal reservoir.
Limiting these exposures remains the best means for reducing the
risk for disease. Travelers should be advised that prevention is
based on avoidance of vector-infested habitats, use of repellents
and protective clothing (see Protection
Against Mosquitoes and Other Arthropods), prompt detection and
removal of arthropods on clothing and skin, and attention to hygiene.
Disease management should focus on early detection and proper treatment
to prevent severe complications of these illnesses.
Q fever and Bartonella group
diseases may pose a special risk for persons with abnormal or prosthetic
heart valves and persons who are immunocompromised. Special care
should be taken by these groups of travelers to prevent potential
exposures.
Treatment
Treatments for most rickettsial
illnesses are similar and include administration of appropriate
antibiotics (most often tetracyclines) and supportive care. Treatment
should be initiated on the basis of clinical and epidemiologic
clues, without waiting for laboratory confirmation. No commercially
licensed vaccines are available in the United States, and vaccinations
to prevent rickettsial infections are not required by any country
as a condition for entry.
— Gregory
Dasch, Mary Reynolds
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