Rickettsial Infections

Description

Several species of Rickettsia can cause illnesses in humans (Table 4-18). The term "rickettsiae" conventionally embraces a polyphyletic group of microorganisms in the class Proteobacteria, comprising species belonging to the genera Rickettsia, Orientia, Ehrlichia, Anaplasma, Neorickettsia, Coxiella, and Bartonella. These agents are usually not transmissible directly from person to person except by blood transfusion or organ transplantation, although sexual and placental transmission has been hypothesized for Coxiella. Transmission generally occurs via an infected arthropod vector or through exposure to an infected animal reservoir host. 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 usually relatively mild (rickettsialpox, cat scratch disease, and African tick-bite fever) to those that can be life-threatening (epidemic typhus, Rocky Mountain spotted fever, and Oroya fever), and they vary in duration from those that can be self-limiting to chronic (Q fever and bartonelloses) or recrudescent (Brill-Zinsser disease). Most patients with rickettsial infections recover with timely use of appropriate antibiotic therapy.

Travellers 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 Travellers

The geographic distribution and the risks for exposure to rickettsial agents are described below, by disease.

Table 4-18. Epidemiologic features and symptoms of rickettsial diseases

Antigenic group Disease Agent Predominant symptoms1 Vector or Acquisition mechanism Animal reservoir Geographic distribution outside the US
Typhus fevers Epidemic typhus, Sylvatic typhus Rickettsia prowazekii Headache, chills, fever, prostration, confusion, photophobia, vomiting, rash (generally starting on trunk) Human body louse, squirrel flea and louse Humans, flying squirrels (US) Cool mountain- ous regions of Africa, Asia, and Central and South America
Murine typhus R. typhi As above, generally less severe Rat flea Rats, mice Worldwide
Spotted fevers Rocky Mountain spotted fever R. rickettsii Headache, fever, abdominal pain, rash (generally starting on extremities) Tick Rodents Mexico, Central and South America
Mediter- ranean spotted fever2 R. conorii Fever, eschar, regional adenopathy, rash on extremities Tick Rodents Africa, India, Europe, Middle East, Mediter- ranean
African tick-bite fever R. africae Fever, eschar(s), regional adenopathy, rash subtle or absent Tick Rodents Sub- Saharan Africa
North Asian tick typhus R. sibirica As above Tick Rodents Russia, China, Mongolia
Oriental spotted fever R. japonica As above Tick Rodents Japan
Rickett- sialpox R. akari Fever, eschar, adenopathy, disseminated vesicular rash Mite House mice Russia, South Africa, Korea
Tick-borne disease R. slovaca Necrosis erythema, lymphad- enopathy Tick Lago- morphs, rodents Europe
Aneruptive fever R. helvetica Fever, headache, myalgia Tick Rodents Old World
Cat flea rickett- siosis R. felis As murine typhus, generally less severe Cat and dog flea Domestic cats, opossums Europe, South America
Queens- land tick typhus R. australis Fever, eschar, regional adenopathy, rash on extremities Tick Rodents Australia, Tasmania
Flinders Island spotted fever, Thai tick typhus R. honei As above but milder, eschar and adenopathy are rare Tick Not defined Australia, Thailand
Orientia Scrub typhus Orientia tsutsuga- mushi Fever, headache, sweating, conjunctival injection, adenopathy, eschar, rash (starting on trunk), respiratory distress Mite Rodents Indian Sub- continent, Central, Eastern, and Southeast Asia and Australia
Coxiella Q fever Coxiella burnetii Fever, headache, chills, sweating, pneumonia, hepatitis, endocarditis Most human infections are acquired by inhalation of infectious aerosols. Tick3 Goats, sheep, cattle, domestic cats, other Worldwide
Bartonella Cat- scratch disease Bartonella henselae Fever, adenopathy, neuroretinitis, encephalitis Cat flea Domestic cats Worldwide
Trench fever B. quintana Fever, headache, pain in the shins, spleno- megaly, disseminated rash Human body louse Humans Worldwide
Oroya fever B. bacilliformis Fever, headache, anaemia, shifting joint and muscle pain, nodular dermal eruption Sand fly Unknown Peru, Ecuador, Colombia
Ehrlichia Ehrlich- iosis Ehrlichia chaffeensis4 Fever, headache, nausea, occasionally rash Tick Various large and small mammals, including deer and rodents Worldwide
Anaplasma Anaplas- mosis Anaplasma phago- cytophilum4 Fever, headache, nausea, occasionally rash Tick Small mammals, and rodents Europe, Asia, Africa
Neo- rickettsia Sennetsu fever Neorickett- sia sennetsu Fever, chills, headache, sore throat, insomnia Fish, fluke Fish Japan, Malaysia

1This represents only a partial list of symptoms. Patients may have different symptoms or only a few of those listed.
2Includes 4 different subspecies that can be distinguished serologically and by PCR assay, and respectively are the etiologic agents of Boutonneuse fever and Mediterranean tick fever in Southern Europe and Africa (R. conorii subsp. Conorii), Indian tick typhus on Indian subcontinent (R. conorii subsp. Indica), Israeli tick typhus in Southern Europe and Middle East (R. conorii subsp. Israelensis), and Astrakhan spotted fever in the North Caspian region of Russia (R. conorii subsp. Caspiae).
3These arthropods can transmit the pathogen from one animal to another, but are less frequently involved in transmission to humans.
4Organisms antigenically related to these species are associated with ehrlichial diseases outside the continental United States.

Epidemic Typhus and Trench Fever

Epidemic typhus and trench fever, which are caused by 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 recognised recently. Travellers 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. In the eastern United States, campers and wildlife workers can acquire typhus if they come in contact with flying squirrels, their ectoparasites, or their nests, which can be made in houses and tree-holes.

Murine Typhus and Cat-Flea Rickettsiosis

Murine typhus, which is 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, they are most common during the warm summer months. Travellers who visit rat-infested buildings and homes, especially in harbor or riverine environments, can be at risk for exposure to the agent of murine typhus. Similarly, cat-flea rickettsiosis, which is caused by infection with R. felis, occurs worldwide and is responsible for a murine typhus-like febrile disease in humans. The infection may result from exposure to flea-infested domestic cats and dogs or peridomestic animals.

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, Maritime Provinces and Sakhalin Island of Russia, and Tajikistan. Their prevalence, however, fluctuates with temperature and rainfall. Infection may occur on coral atolls in both the Indian and Pacific Oceans, in rice paddies, on oil palm plantations, and in tropical to desert habitats and elevated river valleys. Humans typically encounter the arthropod vector of scrub typhus in recently disturbed terrain (e.g., forest clearings) or other persisting rat-mite foci with rats and other rodents.

Tick-Borne Rickettsioses

Tick-borne rickettsial diseases have a worldwide distribution but are most common in temperate and subtropical regions. These diseases include Rocky Mountain spotted fever (caused by R. rickettsii), Mediterranean spotted fever (caused by R. conorii), African tick-bite fever (caused by R. africae), Queensland tick typhus (caused by R. australis), North Asian tick spotted fever (caused by R. sibirica), Flinders Island spotted fever and Thai tick typhus (caused by R. honei), and ehrlichioses (caused by Ehrlichia spp. and Anaplasma phagocytophilum) (Table 4-18). In general, peak transmission of tick-borne rickettsial pathogens occurs seasonally during spring and summer months. Travellers 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 Ehrlichia species.

Rickettsialpox

Rickettsialpox is generally an urban, mite-vectored disease associated with R. akari-infected house mice, although feral rodent-mite reservoirs also have been described. Outbreaks of this illness have occurred shortly after rodent extermination programs, since the mites seek new hosts. R. akari-infected rodents have been found in urban centers in the former Soviet Union, South Africa, Korea, Croatia, and the United States. Travellers may be at risk for exposure to rodent mites when staying in old urban hostels and cabins.

Q Fever

Q fever occurs worldwide, most often in persons who have frequent contact with goat, sheep, and cattle carcasses and parturient animals (especially farmers, veterinarians, butchers, meat packers, and seasonal workers). Travellers 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 or by exposure to infected ticks. These infections may initially result in only mild and self-limiting influenza-like illnesses, but if untreated, they may become chronic, particularly in persons with preexisting heart valve abnormalities or 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, particularly kittens, infected with Bartonella henselae, and possibly from their fleas. Exposure can therefore occur wherever cats are found. Oroya fever is transmitted by sand flies infected with B. bacilliformis, which is endemic in the Andean highlands.

Clinical Presentation and Diagnosis

Clinical presentations of rickettsial illnesses differ (Table 4-18), but early symptoms, including fever, headache, and malaise, are generally nonspecific. Rashes are often associated with rickettsioses, and an eschar (thick blackened scab) is seen in scrub typhus and several spotted fever rickettsioses. Illnesses resulting from infection with rickettsial agents often go unrecognised or are attributed to other causes. Atypical presentations are common and may be expected with poorly characterised nonindigenous agents, so appropriate samples should be obtained for examination by specialized reference laboratories. A diagnosis of rickettsial disease is based on two or more of the following: 1) compatible clinical symptoms and epidemiologic history, 2) the development of specific convalescent-phase antibodies reactive with a given pathogen or antigenic group, 3) a positive polymerase chain reaction test result, 4) immunohistologic detection of a microorganism, or 5) 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 primary mode of transmission (through autoinoculation into a wound or inhalation), travellers 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. Travellers 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 from clothing and skin, and attention to hygiene. Disease management should focus on early detection and proper treatment to prevent severe complications of these illnesses.

Treatment

Treatments for most rickettsial illnesses are similar and include administration of appropriate antibiotics (most often tetracycline class or chloramphenicol) and supportive care. Treatment should be initiated on the basis of clinical and epidemiologic clues, without waiting for laboratory confirmation. It is advisable to seek specialized infectious disease advice in travel and tropical medicine. 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.

Bibliography
  • Comer JA, Paddock CD, Childs JE. Urban zoonoses caused by Bartonella, Coxiella, Ehrlichia, and Rickettsia species. Vector Borne and Zoonotic Diseases. 2001;1:91-116.
  • Fournier PE, Allombert C, Supputamongkol Y, et al. Aneruptive fever associated with antibodies to Rickettsia helvetica in Europe and Thailand. J Clin Microbiol. 2004;42:816-8.
  • Isaksson HJ, Hrafnkelsson J, Hilmarsdottir I. Acute Q fever: a cause of fatal hepatitis in an Icelandic traveller. Scand J Infect Dis. 2001;33:314-5.
  • Jensenius M, Fournier PE, Raoult D. Tick-borne rickettsioses in international travellers. Int J Infect Dis. 2004;8:139-46.
  • Lewin MR, Bouyer DH, Walker DH, Musher DM. Rickettsia sibirica infection in members of scientific expeditions to northern Asia. Lancet. 2003;362:1201-2.
  • Oteo A, Ibarra V, Blanco JR, et al. Dermacentor-borne necrosis erythema and lymphadenopathy: clinical and epidemiological features of a new tick-borne disease. Clin Microbiol Infect. 2004;10:327-31.
  • Raoult D, Roux V. Rickettsioses as paradigms of new or emerging infectious diseases. Clin Microbiol Rev 1999;10:649-719.
  • Watt G, Parola P. Scrub typhus and tropical rickettsioses. Curr Opin Infect Dis. 2003;16:429-36.
  • Weber DJ, Rutala WA. Risks and prevention of nosocomial transmission of rare zoonotic diseases. Clin Infect Dis. 2001;32:446-56.
  • Zaidi SA, Siger C. Gastrointestinal and hepatic manifestations of tick-borne diseases in the United States. Clin Infect Dis. 2002;34:1397-8.

- Marina Eremeeva and Gregory Dasch

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