RICKETTSIAL DISEASES: INTRODUCTION
The rickettsiae are a heterogeneous group of small, obligately intracellular, gram-negative coccobacilli and short bacilli, most of which are transmitted by a tick, mite, flea, or louse vector. Except for louse-borne typhus, humans are incidental hosts. Among rickettsiae, Coxiella burnetii, Rickettsia prowazekii, and R. typhi have the well-documented ability to survive for an extended period outside the reservoir or vector and to be extremely infectious: inhalation of a single Coxiella microorganism can cause pneumonia. High infectivity and severe illness after inhalation make R. prowazekii, R. rickettsii, R. typhi, R. conorii, and C. burnetii bioterrorism threats.
Clinical infections with rickettsiae can be classified according to (1) the taxonomy and diverse microbial characteristics of the agents, which belong to six genera (Rickettsia, Orientia, Ehrlichia, Anaplasma, Neorickettsia, and Coxiella); (2) epidemiology; or (3) clinical manifestations. The clinical manifestations of all the acute presentations are similar during the first 5 days: fever, headache, and myalgias with or without nausea, vomiting, and cough. As the course progresses, clinical manifestations—including occurrence of a macular, maculopapular, or vesicular rash; eschar; pneumonitis; and meningoencephalitis—vary from one disease to another. Given the 12 etiologic agents with varied mechanisms of transmission, geographic distributions, and associated disease manifestations, the consideration of rickettsial diseases as a single entity poses complex challenges (Table 167-1).
Table 167-1 Features of Selected Rickettsial Infections
| Disease |
Organism |
Transmission |
Geographic Range |
Incubation Period (Days) |
Duration (Days) |
Rash (%) |
Eschar (%) |
Lymphadenopathy
a
|
| Rocky Mountain spotted fever |
Rickettsia rickettsii
|
Tick bite: Dermacentor andersoni
|
United States |
2–14 |
10–20 |
90 |
<1 |
+ |
|
|
D. variabilis
|
United States |
|
|
|
|
|
|
|
Amblyomma cajennense, A. aureolatum
|
Central/South America |
|
|
|
|
|
|
|
Rhipicephalus sanguineus
|
Mexico, United States |
|
|
|
|
|
| Mediterranean spotted fever
b
|
R. conorii
|
Tick bite: R. sanguineus, R. pumilio
|
Southern Europe, Africa, Middle East, Central Asia |
5–7 |
7–14 |
97 |
50 |
+ |
| African tick-bite fever |
R. africae
|
Tick bite: A. hebraeum, A. variegatum
|
Sub-Saharan Africa, West Indies |
4–10 |
? |
50 |
90 |
++++ |
| Rickettsialpox |
R. akari
|
Mite bite: Liponyssoides sanguineus
|
United States, Ukraine, Croatia |
10–17 |
3–11 |
100 |
90 |
+++ |
| Flea-borne spotted fever |
R. felis
|
Flea (mechanism undetermined): Ctenocephalides felis
|
North and South America, Europe |
8–16 |
8–16 |
80 |
15 |
— |
| Epidemic typhus |
R. prowazekii
|
Louse feces: Pediculus humanus corporis, fleas and lice of flying squirrels, or recrudescence |
Worldwide |
7–14 |
10–18 |
80 |
None |
— |
| Tick-borne lymphadenopathy |
R. slovaca
|
Tick bite: Dermacentor marginatus, D. reticularis
|
Europe |
7–9 |
17–180 |
5 |
100 |
++++ |
| Murine typhus |
R. typhi
|
Flea feces: Xenopsylla cheopis, C. felis, others |
Worldwide |
8–16 |
8–16 |
80 |
None |
— |
| Human monocytotropic ehrlichiosis |
Ehrlichia chaffeensis
|
Tick bite: Amblyomma americanum, D. variabilis
|
United States |
1–21 |
3–21 |
36 |
None |
++ |
| Human granulocytotropic anaplasmosis |
Anaplasma phagocytophilum
|
Tick bite: Ixodes scapularis, I. ricinus, I. pacificus
|
United States, Europe, Asia |
1–21 |
3–14 |
Rare |
None |
— |
| Scrub typhus |
Orientia tsutsugamushi
|
Mite bite: Leptotrombidium deliense, others |
Asia, Australia, New Guinea, Pacific Islands |
9–18 |
6–21 |
50 |
35 |
+++ |
| Q fever |
Coxiella burnetii
|
Inhalation of aerosols of infected parturition material (sheep, dogs, others), ingestion of infected milk or milk products |
Worldwide |
3–30 |
5–57 |
<1 |
None |
— |
a
++++, severe; +++, marked; ++, moderate; +, present in a small portion of cases; —, not a noted feature.
b
Eschar is usually present at the bite site.
Establishing the etiologic diagnosis of rickettsioses is very difficult during the acute stage of illness, and definitive diagnosis usually requires the examination of paired serum samples after convalescence. Heightened clinical suspicion is based on epidemiologic data, history of exposure to vectors or reservoir animals, travel to endemic locations, clinical manifestations (sometimes including rash or eschar), and characteristic laboratory findings [including thrombocytopenia, normal or low white blood cell (WBC) counts, elevated hepatic enzyme levels, and hyponatremia]. Such suspicion should prompt empirical treatment. Doxycycline is the drug of choice for most of these infections. Only one agent,
C. burnetii, has been documented to cause chronic illness. One other,
R. prowazekii, causes recrudescent illness (Brill-Zinsser disease) when latent infection is reactivated years after resolution of the acute illness.
Rickettsial infections dominated by fever may resolve without further clinical evolution. However, after nonspecific early manifestations, the illnesses can also evolve along one or more of several principal clinical lines: (1) development of a macular or maculopapular rash; (2) development of an eschar at the site of tick or mite feeding; (3) development of a vesicular rash (often in rickettsialpox and African tick-bite fever); (4) development of pneumonitis with chest radiographic opacities and/or rales [Q fever and severe cases of Rocky Mountain spotted fever (RMSF), Mediterranean spotted fever (MSF), louse-borne typhus, human monocytotropic ehrlichiosis (HME), human granulocytotropic anaplasmosis (HGA), scrub typhus, and murine typhus]; (5) development of meningoencephalitis [louse-borne typhus and severe cases of RMSF, scrub typhus, HME, murine typhus, MSF, and (rarely) Q fever]; and (6) progressive hypotension and multiorgan failure as seen with sepsis or toxic shock syndrome (RMSF, MSF, louse-borne typhus, murine typhus, scrub typhus, HME, and HGA).
Epidemiologic clues to the transmission of a particular pathogen include (1) environmental exposure to ticks, fleas, or mites during the season of activity of the vector species for the disease in the appropriate geographic region (spotted fever and typhus group rickettsioses, scrub typhus, ehrlichioses, anaplasmosis); (2) travel to or residence in an endemic geographic region during the incubation period (
Table 167-1); (3) exposure to parturient ruminants, cats, and dogs (Q fever); (4) exposure to flying squirrels (
R. prowazekii infection); and (5) history of previous louse-borne typhus (recrudescent typhus).
Clinical laboratory findings, such as thrombocytopenia (particularly in spotted fever and typhus rickettsioses, ehrlichioses, anaplasmosis, and scrub typhus), normal or low WBC counts, mild to moderate serum elevations of hepatic aminotransferases, and hyponatremia suggest some common pathophysiologic mechanisms.
Application of these clinical, epidemiologic, and laboratory principles requires a consideration of the possibility of a rickettsial diagnosis and a knowledge of the individual diseases.
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