Rickettsia typhi, transmitted by rat fleas, causes most human flea-borne 
rickettsioses worldwide. Another rickettsia, Rickettsia felis, found in cat 
fleas, Ctenocephalides felis, has also been implicated as a potential human 
pathogen. In the continental United States, human cases of flea-borne 
rickettsioses are reported primarily from the southern regions of Texas and 
California where the cat flea is considered the principal vector. In California, 
more than 90% of locally acquired human cases are reported from suburban 
communities within Los Angeles and Orange counties despite the almost ubiquitous 
presence of cat fleas and their hosts throughout the state. The objective of 
this study is to assess the presence and infection rate of Rickettsia species in 
cat fleas from selected endemic and nonendemic regions of California. Cat fleas 
were collected from cats in Los Angeles County (endemic region) and Sacramento 
and Contra Costa counties (nonendemic region). Sequencing of 17 amplicons 
confirmed the presence of R. felis in both the endemic and non-endemic regions 
with a calculated maximum likelihood estimation of 131 and 234 per 1000 fleas, 
respectively. R. typhi was not detected in any flea pools. Two R. felis-like 
genotypes were also detected in fleas from Los Angeles County; Genotype 1 was 
detected in 1 flea pool and Genotype 2 was found in 10 flea pools. Genotype 1 
was also detected in a single flea pool from Sacramento County. Results from 
this study show that R. felis is widespread in cat flea populations in both 
flea-borne rickettsioses endemic and nonendemic regions of California, 
suggesting that a high prevalence of this bacterium in cat fleas does not 
predispose to increased risk of human infection. Further studies are needed to 
elucidate the role of R. felis and the two R. felis-like organisms as etiologic 
agents of human flea-borne rickettsioses in California.