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Hello, my name is Rohit Inder Singh. I am a 4th year pathology resident at Rush University Medical Center in Chicago. Welcome to this Pearl of Laboratory Medicine on “Rickettsiae and Rickettsial Diseases.”
Rickettsiae are gram negative coccobacilli that are 0.8 to 2.0 µ long and 0.3 to 0.5 µ in diameter. They are obligate intracellular bacteria belonging to the class of alpha-1 proteobacteria that parasitize eukaryotes, especially arthropods. They can infect vertebrates including humans and can cause diseases. These organisms are named after Howard Taylor Ricketts, who is credited with the discovery of these organisms. Ricketts was devoted to his research and unfortunately died of a rickettsial infection.
Rickettsial infections are caused by bacteria included in the genera Rickettsia, Orientia, Anaplasma, Neorickettsia, and Ehrlichia. Based on robust molecular phylogeny, 4 lineages/clades in genus Rickettsia are now proposed. The first two, i.e. ancestral group and transitional group, which are not indicated in this slide, include organisms such as R. belli and R. felis, respectively. And the other two are the Typhus group and the spotted fever group.
This slide gives some information concerning the mode of transmission. Arthropods such as fleas, lice, mites, and ticks act as vectors for rickettsiae and can transmit them during feeding. Rubbing crushed arthropods or infectious feces while scratching skin can also lead to infection. Dust inhalation and inoculation of conjunctiva with infectious material are other less commonly reported methods of transmission.
Now let’s focus our attention on the epidemiology of these diseases. Rickettsial infections are found to affect all age groups, especially in individuals traveling to endemic areas. Transmission is increased during outdoor activities in the spring and summer months when ticks and fleas are most active. The incubation period is anywhere between 5 days to two weeks and it is not hard to understand that symptoms can occur upon return from an endemic area. Fortunately, immunity develops after recovery from infection and/or receipt of antibiotic treatment.
In the next few slides, I will discuss common and important rickettsial pathogens and the diseases caused by them. The table lists the disease, agent, animal reservoir, symptoms, and geographical distribution. The typhus group includes epidemic typhus and endemic or murine typhus caused by R. prowazekii and R. typhi, respectively. Both these diseases are widely distributed. Epidemic typhus (Primary louse-borne typhus) occurs in communities and refugee populations where body lice are prevalent. Outbreaks often occur during the colder months when infested clothing is not laundered. The clinical presentation includes a non-specific febrile illness with headache, fever, chills, and a rash that begins on the trunk. The symptoms in endemic typhus are similar but milder. Endemic typhus (murine typhus) occurs in the tropical and subtropical areas of the world and in ports and coastal cities with rodents. Sylvatic epidemic typhus cases occur only from direct contact with flying squirrels or their nesting materials and squirrel ectoparasites in the eastern United States.
A few words about the transitional group. It includes R. akari and R. felis (not shown in this slide). R. felis is a flea-borne pathogen and causes cat flea rickettsiosis with symptoms similar to endemic typhus. R. akari causes rickettsialpox. Rickettsialpox, transmitted by house-mouse mites, circulates in 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 symptoms include fever, eschar formation, and rash. An eschar is a piece of dead necrotic tissue that develops at the site of inoculation; it is often black.
The spotted fever group includes a host of tick-borne diseases, a few of which are listed here in this slide. All of these diseases are characterized by a febrile illness with rash and fever and /or eschar formation at the site of inoculation. The animal reservoirs are rodents, dogs, and ruminants. Rocky Mountain spotted fever (RMSF) occurs in North, Central, and South America, and is caused by R. rickettsiosis, and is associated with lack of presence of an eschar. R. rickettsii infects endothelial cells and more rarely infects underlying smooth muscle cells, where rickettsiae multiply freely in the cytoplasm. In RMSF, a rash typically develops in about 90% of patients. It begins as a maculopapular rash (2-5 days after the onset of fever) on the wrists, forearms, ankles, and spreads to the trunk and sometimes, palms and soles. A petechial rash develops in about 40-60% of patients and signifies severe disease. Game hunting and traveling to southern Africa from November through April are risk factors for African tick-bite fever in travelers, which is the most frequently reported travel-associated rickettsiosis.
However, Mediterranean spotted fever infections occur over an even larger region and can be quite severe. This spotted fever group consists of an ever growing list of pathogens and many of them are emerging diseases of public health importance.
This slide shows an adult female lone star deer tick on the left, which is common in Southeastern and South-Central United States, and typical petechial rash that occurs in a patient with RMSF.
Scrub typhus is caused by Orientia tsustsugamushi and is transmitted by trombiculid mites (chiggers). The disease 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.
Anaplasmataceae members, A. phagocytophilum, E. chaffeensis, E. ewingii, Ehrlichia canis, and N. sennetsu, infect humans. Circulating leukocytes are their targets, and the corresponding disease are often named according to the infected leukocyte appended by the bacterial genus; for example, E. chaffeensis infecting monocytes causes human monocytic ehrlichiosis (HME), and A. phagocytophilum infecting granulocytes causes human granulocytic anaplasmosis (HGA). E. ewingii infects granulocytes. Symptoms of ehrlichiosis and anaplasmosis are similar and include fever, leukopenia, and/or thrombocytopenia with raised transaminases. Ehrlichiosis is most commonly reported in the Southeastern and South-Central United States, especially Missouri, Oklahoma, and Arkansas, where the lone star tick, Amblyomma americanum, and white-tailed deer are commonplace. AnapIasmosis is more common in the Northeast and upper Midwest.
Rickettsial diseases are typically diagnosed by a combination of clinical recognition and serology. The historic assay for the diagnosis of rickettsial diseases is the Weil Felix Test, which used circulating antibodies to various proteus species that harbored cross-reacting epitopes to Rickettsial antigens.It is replaced by more accurate serologic methods such as IFA or EIA. Serologic tests can only identify up to genus level and are not very useful in early detection as they require demonstration of a four-fold rise in titer between two acute and convalescent sera. In the case of anaplasmosis, and less often, ehrlichiosis, microscopic examination of peripheral blood smear stained by Wright stain may demonstrate intracytoplasmic inclusions called morulae which are stippled blue inclusions consisting of bacteria.
Recent trends reveal a gradual shift to molecular detection methods such as PCR and sequencing. These aid in early diagnosis and can also aid in epidemiologic studies of vectors.
These pictures illustrate morulae of Ehrlichia chaffeensis and Anaplasma phagocytophilum in Wright- stained leukocytes and cell culture.
Treatment of patients with possible rickettsioses should be started early and should not await confirmatory testing. Treatment usually involves doxycycline. Chloramphenicol, azithromycin, fluoroquinolones, and rifampin may be alternatives, depending on the scenario. Expert advice should be sought if these alternative agents are being considered. Vaccines are not available and antibiotics should not be used for prophylaxis. Travelers are advised to minimize exposure to infecting agents such as arthropods by the use of tick and insect repellants.
Certain rickettsial pathogens such as R. prowazekii have been used as agents of bioterrorism because of certain unique biologic characteristics that are listed here. Fortunately, production of highly purified, virulent, weapon-quality rickettsiae is a daunting task that requires expertise and elaborate, state-of-the art laboratory procedures. Another drawback to developing rickettsial pathogens as biological weapons is their lack of direct transmission from host to host and the availability of very effective therapeutic countermeasures against these obligate intracellular bacteria.
Slide 16: References
These are a list of the references I used to prepare this Pearl and also suggested further reading.
Slide 17: Disclosures\
Thank you for joining me on this Pearl of Laboratory Medicine on “Rickettsiae and Rickettsial Diseases.”