Rickettsia africae in ticks and lice
We demonstrated a high prevalence (78.95%; estimated true prevalence)
of R. africae, the agent of African tick bite fever (ATBF, also
known as African tick typhus) in humans, in Am. variegatum ticks
collected mostly from cattle. Ever since the first description in Kenya
of R. africae in Amblyomma ticks from the Maasai Mara
region (Macaluso et al., 2003), high infection rates in Amblyomma ticks have been reported at SHs in Mombasa and Nairobi (Mutai et al.,
2013), Siaya County, which borders Busia County (Maina et al., 2014),
pastoral communities in North Eastern Kenya (Koka et al., 2017), the
Shimba Hills National Reserve (Mwamuye et al., 2017), Baringo County
(Omondi et al., 2017), and the Maasai Mara National Reserve (Oundo et
al., 2020). Rickettsia africae has similarly been reported in Amblyomma ticks from Cameroon (Ndip et al., 2004), Zimbabwe
(Beati et al., 1995), Senegal (Kelly et al., 2010), and the Central
African Republic (CAR) (Dupont et al., 1995). We also detected R.
africae at much lower prevalence in rhipicephaline ticks and for the
first time we are aware of in H. suis lice. However, this novel
finding is not surprising as lice are known vectors of other SFG
rickettsiae (Hornok et al., 2010), but there is a paucity of studies
that have surveyed rickettsiae in lice in Africa.
Our finding that all 34 livestock blood samples, from which the R.
africae-positive ticks were obtained, were negative for the pathogen
reinforces the notion that Amblyomma ticks are the major
reservoir of the pathogen, but also indicates a low transmissibility to
livestock. Since these ticks mostly parasitise large ruminants,
it is evident that cattle play an important role in the epidemiology of
ATBF by providing an abundant blood-meal source, as described previously
for R. conorii by Kelly et al. (1991).
In travel medicine, ATBF, which is characterised by headaches,
inoculation eschar, rash, and myalgia (Jensenius et al., 2003), is
believed to be only second to malaria as the cause of febrile illness in
travellers to sub-Saharan Africa (SSA). Most acute cases have been
reported in tourists and foreign travellers with some fatal cases
(Rutherford et al., 2004). Its seroprevalence is usually high in native
populations, but few acute cases have been reported (Kelly et al., 1991;
Ndip et al., 2004). This may be due to exposure at an early age leading
to only mild clinical cases that are ignored, poor visibility of
inoculation eschars on pigmented skin, and lack of diagnostic capacity
at most health centres (Jensenius et al., 2003). Alternatively, some R. africae may be more virulent than others. In this study, we
found R. africae variants that have been reported in previous
studies (Kimita et al.,2016; Macaluso et al., 2003; Maina et al., 2014).
The differences found in the nucleotide composition of the omp B
gene, which codes for the most immuno-dominant surface cell antigen of Rickettsia, could possibly affect the virulence of R.
africae variants. Surface cell antigens are involved in cellular
adhesion of Rickettsia and subsequent entry into cells (Blanc et
al., 2003). The hypothesis that variants with an intact omp B
gene are less virulent than those with the deletion (Maina et al., 2014)
may explain the absence of acute ATBF cases in Kenya, despite the high
seroprevalence. This is supported by the evidence that genome reduction
may lead to increased virulence in Rickettsia (Fournier et al.,
2009). However, it remains to be seen if some of these variants can be
detected in febrile patients in our study area. Clearly, there is a need
for studies that focus on the public health aspect of this pathogen in
endemic areas.