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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.
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.
[3] The term 'rickettsiae' conventionally embraces the group of microorganisms of the class Proteobacteria, comprising species belonging to the genera Rickettsia, Orientia, Ehrlichia (Anaplasma), Coxiella, and Bartonella. |
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