RICKETTSIAE

RICKETTSIAE DISEASES

INFECTIOUS AGENTS

Rickettsial infections are caused by a variety of bacteria from the genera Rickettsia, Orientia, Ehrlichia, Neorickettsia, Neoehrlichia, and Anaplasma (Table 3-18). Rickettsia spp. are classically divided into the typhus group and spotted fever group (SFG). Orientia spp. make up the scrub typhus group. The rickettsial pathogens most likely to be encountered during travel outside the United States include R. africae (African tick-bite fever), R. conorii (Mediterranean spotted fever), R. rickettsii (known as both Rocky Mountain spotted fever and Brazilian spotted fever), O. tsutsugamushi (scrub typhus), and R. typhi (murine typhus).

TRANSMISSION

Most rickettsial pathogens are transmitted by ectoparasites such as fleas, lice, mites, and ticks. Organisms can be transmitted by bites from these ectoparasites or by the inoculation of infectious fluids or feces from the ectoparasites into the skin. Inhaling or inoculating conjunctiva with infectious material may also cause infection for some of these organisms. The specific vectors that transmit each form of rickettsiae are listed in Table 3-18. Transmission of some rickettsial diseases after transfusion or organ transplantation is rare but has been reported.

EPIDEMIOLOGY

All age groups are at risk for rickettsial infections during travel to endemic areas. Both short and long-term travelers are at risk for infection. Transmission is increased during outdoor activities in the spring and summer months when ticks and fleas are most active. However, infection can occur throughout the year. Because of the 5- to 14-day incubation period for most rickettsial diseases, tourists may not necessarily experience symptoms during their trip, and onset may coincide with their return home or develop within a week after returning. Although the most commonly diagnosed rickettsial diseases in travelers are usually in the spotted fever or typhus groups, travelers may acquire a wide range of rickettsioses, including emerging and newly recognized species (Table 3-18).

Tickborne spotted fever rickettsioses are the most frequently reported travel-associated rickettsial infections. Game hunting and traveling to southern Africa from November through April are risk factors for African tick-bite fever, and this consistently remains the most commonly reported rickettsial infection acquired during travel. Mediterranean spotted fever infections are less commonly reported but occur over an even larger region, including (but not limited to) much of Europe, Africa, India, and the Middle East. Rocky Mountain spotted fever (also known as Brazilian spotted fever, as well as other local names) is reported throughout much of the Western Hemisphere, including Canada, the United States, Mexico, and various countries in Central and South America. Contact with dogs (in both rural and urban settings) and outdoor activities such as hiking, hunting, fishing, and camping increase the risk of infection.

Scrub typhus, which is transmitted by mites encountered in high grass and brush, is endemic in northern Japan, Southeast Asia, the western Pacific Islands, eastern Australia, China, maritime areas and several parts of south-central Russia, India, and Sri Lanka. More than 1 million cases occur annually. Most travel-acquired cases of scrub typhus occur during visits to rural areas in endemic countries for activities such as camping, hiking, or rafting, but urban cases have also been described.

R. typhi and R. felis, which are transmitted by fleas, are widely distributed, especially throughout the tropics and subtropics and in port cities and coastal regions with rodents. Humans exposed to flea-infested cats, dogs, and peridomestic animals while traveling in endemic regions, or who enter or sleep in areas infested with rodents, are at most risk for fleaborne rickettsioses. Murine typhus has been reported among travelers returning from Asia, Africa, and the Mediterranean Basin and has also been reported from Hawaii, California, and Texas in the United States.

R. akari, the causative agent of rickettsialpox, is transmitted by house-mouse mites, and circulates in mainly urban centers in Ukraine, South Africa, Korea, the Balkan states, and the United States. Outbreaks of rickettsialpox most often occur after contact with infected rodents and their mites, especially during natural die-offs or exterminations of infected rodents that cause the mites to seek out new hosts, including humans. The agent may spill over and occasionally be found in other wild rodent populations.

Epidemic typhus is rarely reported among tourists but can occur in communities and refugee populations where body lice are prevalent. Outbreaks often occur during the colder months when infested clothing is not laundered. Travelers at most risk for epidemic typhus include those who may work with or visit areas with large homeless populations, impoverished areas, refugee camps, and regions that have recently experienced war or natural disasters. Active foci of epidemic typhus are known in the Andes regions of South America and some parts of Africa (including but not limited to Burundi, Ethiopia, and Rwanda). Louseborne epidemic typhus does not regularly occur in the United States, but a zoonotic reservoir occurs in the southern flying squirrel, and sporadic sylvatic epidemic typhus cases are reported. Tick-associated reservoirs of R. prowazekii have been described in Ethiopia, Mexico, and Brazil, but documented human cases are rare.

Ehrlichiosis and anaplasmosis are tickborne infections most commonly reported in the United States. A variety of species are implicated in infection, but E. chaffeensis and A. phagocytophilum are most common. Infections with various Ehrlichia and Anaplasma spp. have also been reported in Europe, Asia, and South America. Neoehrlichia mikurensis is a tickborne pathogen that occurs in Europe and Asia. Sennetsu fever, caused by Neorickettsia sennetsu, occurs in Japan, Malaysia, and possibly other parts of Asia. This disease can be contracted from eating raw infected fish.

CLINICAL PRESENTATION

Rickettsioses are difficult to diagnose, even by health care providers experienced with these diseases. Most symptomatic rickettsial diseases cause moderate illness, but some Rocky Mountain and Brazilian spotted fevers, Mediterranean spotted fever, scrub typhus, and epidemic typhus may be fatal in 20%–60% of untreated cases, so prompt treatment is essential.

Clinical presentations vary with the causative agent and patient; however, common symptoms that typically develop within 1–2 weeks of infection include fever, headache, malaise, rash, nausea, and vomiting. Many rickettsioses are accompanied by a maculopapular, vesicular, or petechial rash or sometimes an eschar at the site of the tick bite. African tick-bite fever is typically milder than some other rickettsioses, but recovery is improved with treatment. It should be suspected in a patient who presents with fever, headache, myalgia, and an eschar (tache noir) after recent travel to southern Africa. Mediterranean spotted fever is a potentially life-threatening rickettsial infection and should be suspected in patients with rash, fever, and eschar after recent travel to northern Africa or the Mediterranean. Rocky Mountain and Brazilian spotted fever are characterized by fever, headache, nausea, abdominal pain, and cough; a rash is commonly reported, but eschars are not. Scrub typhus should be suspected in patients with a fever, headache, and myalgia after recent travel to Asia; eschar, lymphadenopathy, cough, and encephalitis may be present. Patients with murine or epidemic typhus usually present with a severe but nonspecific febrile illness, and approximately half will also present with a rash. Ehrlichiosis and anaplasmosis should be suspected in febrile patients with leukopenia with an exposure history.

DIAGNOSIS

Diagnosis is usually based on clinical recognition and serology; the latter requires comparison of acute- to convalescent-phase serology, so is only helpful in retrospect. Etiologic agents can generally only be identified to the genus level by serologic testing. PCR and immunohistochemical analyses may also be helpful. If ehrlichiosis or anaplasmosis is suspected, a buffy coat may be examined to identify characteristic intraleukocytic morulae. Contact the CDC Rickettsial Zoonoses Branch at 404-639-1075 for further information.

TREATMENT

Treatment of patients with possible rickettsioses should be started early and should never await confirmatory testing, which may take weeks when serology is used. Immediate empiric treatment with a tetracycline is recommended, most commonly doxycycline. Broad-spectrum antibiotics are not usually helpful. Chloramphenicol may be an alternative in some cases, but its use is associated with more deaths, particularly for R. rickettsii. Expert advice should be sought if alternative agents are being considered.

PREVENTION

No vaccine is available for preventing rickettsial infections. Antibiotics are not recommended for prophylaxis of rickettsial diseases.

Travelers should be instructed to minimize exposure to infectious arthropods during travel (including lice, fleas, ticks, mites) and animal reservoirs (particularly dogs) when traveling in endemic areas. The proper use of insect or tick repellents, self-examination after visits to vector-infested areas, and wearing protective clothing are ways to reduce risk. These precautions are especially important for people with underlying conditions that may compromise their immune systems, as these people may be more susceptible to severe disease. For more detailed information, Fallow reference below .......... 

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