Rickettsia Felis: Facts & Information
Author: Disabled World
Contact : Disabled World
Information regarding rickettsiosis caused by R. felis, also known as flea-borne spotted fever or cat-flea typhus.
Rickettsia felis was first recognized more than twenty years ago and has now been described as, 'endemic,' to all continents with the exception of Antarctica. The rickettsiosis caused by R. felis is known as flea-borne spotted fever or cat-flea typhus. The large number of arthropod species found to carry R. felis, which may act as potential vectors, support the view that it is a pan-global microbe.
Rickettsia felis or cat-flea typhus is a species of Rickettsia that causes flea-borne spotted fever in cats. Fleas are the vector carriers of the bacteria and it has been found in cat flea populations of North and South America, Southern Europe, Thailand and Australia. The bacteria can also infect humans and cause illness. The host is usually infected by flea feces coming into contact with scratched or broken skin.
The main arthropod reservoir and vector is the cat flea, 'Ctenocephalides felis,' although more than twenty other species of ticks, fleas and mite species have been reported to be potential carriers. Few bacterial pathogens of people have been found to be associated with such a diverse range of invertebrates. With the projected increase in global temperatures over the next one-hundred years, there is concern that changes to the ecology, as well as distribution of R. felis vectors might negatively affect public health.
The human pathogenicity of the Spotted Fever group (SFG) rickettsiae differs greatly between species. Even though significant phenotypic variability exists between species, the majority of members of the group are treated as potential human pathogens. Rickettsia felis is a newly described species of the SFG. In the past, R. felis was classified as a member of Typhus group (TG), which continued for around a decade.
The main vectors for R. felis, as well as R. typhi, are fleas. As with all rickettsiae, R. felis is an obligate intracellular Gram-negative alpha-proteobacterium requiring a vertebrate and invertebrate host to survive and reproduce. There are proposals to re-organize the genus, 'Rickettsia,' into four different groups instead of two, adding an Ancestral group (AG) and Transitional Group (TRG) to the accepted TG and SFG. If this new form of genotypic classification becomes accepted in general, R. felis would join the TRG alongside Rickettsia akari, which is related.
The geographic distribution of R. felis in arthropods, particularly the cosmopolitan cat flea, C. felis, reinforces the hypothesis that R. felis may be found in the majority - if not all, human populations where domestic animals are human pets. The world-wide distribution of R. felis is most likely due to the co-migration of people and domestic animals carrying C. felis.
R. felis, first described by Adams in the year 1990 was originally named, 'ELB agent,' after the laboratory from which it was first isolated, El Labs (ELB) in America. Its discovery was by electron microscopy, revealing a rickettsia-like organism similar in morphology to Rickettsia typhi, the only species of Rickettsia known at the time to be carried by fleas. Not long afterwards, Azad described the new variant of rickettsiae, reporting on its 17 kDa and citrate synthase genese and classifying it as a TG rickettsia. The demonstration of the presence of the ampA gene in R. felis by Bouyer resulted in the re-classification of R. felis as a member of the SFG. Re-analysis of the 17 kDa gene revealed greater similarity to the SFG than the TG.
The genome of R. felis has been sequenced - it was the first rickettsia to be described as having, 'plasmids.' Along with the two plasmids, the combined genetic material is the largest among Rickettsia so far. The plasmids of R. felis are greatly variable.
Flea-Borne Cat-Flea Typhus and Spotted Fever
The names used to describe R. felis infection might soon be deemed to be inaccurate with the recognition of other arthropods such as mites and ticks as potential vectors. Until this is confirmed; however, the use of the names, 'flea-borne spotted fever, ' or, 'cat-flea typhus,' are still valid. Clinical manifestations of human R. felis infections include:
- Maculopapular rash
Observation of instances reported in literature reveal a variability of presentation of clinical symptoms that may include a combination of some or all of the signs and symptoms. So far there have been no reports of flea-borne spotted fever causing either serious complications or death. It seems to be milder than other rickettsioses. Due to shared symptoms with other rickettsial and viral infections, it is believed that a number of human instances are currently misdiagnosed.
The highest standard of rickettsial diagnosis is currently serology using and indirect immunoflourescence assay. There is considerable cross-reactivity between TG and SFG antibodies in human sera. Despite this, serology is still the diagnostic tool of choice because it has a rapid turnaround time and is easy to use. R. felis responds serologically as if it were a TG rickettsia, most likely contributing to the earlier misdiagnosis of R. felis infections as R. typhi.
A need exists to incorporate more diagnostic assays such as, 'polymerase chain reaction (PCR),' to supplement serology for diagnosing Rickettsia infections. PCR as a diagnostic tool is not quickly available to clinicians, as few diagnostic laboratories have access to a rickettsial PCR assay. Diagnostic protocols for rickettsiosis continue to depend on serological methods such as, 'immunofluorescence assay (IFA), along with clinical presentation of symptoms and epidemiological knowledge such as a person's travel history.
The self-limiting nature of the majority of rickettsioses may be another reason why these infections, particularly FBSF, are under-reported. The issue is further exacerbated by the overlapping endemic areas of R. typhi and R. felis, along with shared hosts and vectors. The recent appearance in literature and increased reporting of instances and locations supports the designation of FBSF as an emerging disease. Recently, human infections happened in Australia where a cluster of five people between the ages of 4-63 years of age were exposed to fleas originating from the pet house cat. All of the people sero-converted to TG antigens. The detection of R. felis DNA from the cat fleas supported the diagnosis of FBSF.
Infections by R. felis has been attributed to flea saliva instead of feces. In this regard it is not like R. typhi which is commonly transmitted through inhalation of dried flea feces. FBSF was first described in 1990 and its main vector was identified as C. felis. Since its description, more than a dozen flea species have been identified as being hosts. The expansion of R. felis hosts and possible vectors include lice, mites and ticks, which further highlights how young the field of study is. A great amount of work remains to be done to completely understand the bacterium's ecology. The sharing of arthropod hosts between a number of pathogens, particularly bacteria, of different and similar genera is well documented. The diversity of hosts might have contributed to the earlier misdiagnosis of R. felis infections.
The Migration to People
Human migration might have led to the geographical spread of R. felis hosts - particularly C. canis, C. felis, Xenopsylla cheopis and Pulex irritans. Occurrence of flea vectors in people around the world and among animals that are usually associated with human activities such as dogs, cats and rodents support this perception. Increased travel might accompany animals moved between countries.
During the twentieth century and the beginning of the twenty-first century, mass migrations in the form of refugees fleeing prosecution and conflict were common. A new form of transient migration has arisen with the advent of tourism with fast and inexpensive travel. It is not uncommon to find reports of rickettsial infections in travelers returning to their home nation. Returning infected travelers increase the risk of horizontal transmission of rickettsiae to endemic arthropods. The efficiency and rate of horizontal transmission of rickettsial organisms; particularly R. felis, remains less understood.
- All age groups are at risk for rickettsial infections during travel to endemic areas.
- 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.
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