Rickettsial diversity and human Rickettsiosis in South Africa

Abstract

Rickettsia are a diverse group of organisms said to fit in somewhere between viruses and bacteria. They are found globally and cause a plethora of diseases in humans, animals and occasionally in arthropod hosts. Rickettsial pathogenicity is variable with a broad spectrum of associated maladies with severity ranging from mild symptoms to a high rate of case fatality. Clinicians are increasingly having to manage patients presenting with diseases caused by one of a wide range of rickettsias incidentally infecting humans bitten by an arthropod vector. Clinicians are encouraged to maintain awareness of the spectrum of diseases caused by Rickettsia species to better recognize and treat afflicted patients. If an arthropod reservoir can opportunistically bite a human, it can transmit its harbored rickettsial species onto the human. In order to better understand the rickettsial diversity and human rickettsiosis in South Africa, a two-part study was conducted: the first part investigated diagnostic aspects including commercial kit comparability and inter-laboratory comparison (ILC); and the second part examined the rickettsial species involved in severe cases of tick bite fever (TBF) in South Africa and the harborage of Rickettsia species by several tick species collected at various locations. The findings are presented herein.

The various serological kits available in South Africa were compared in Chapter 3 to determine whether the various kits would produce consistent results for a panel of specimens. Significant differences between all the kits were observed, thus the null hypothesis was rejected. The deficiencies of commercial diagnostic offerings were highlighted, including the necessity for in-house evaluation, validation and adaptation as required e.g. setting assay cut-off values for an endemic population. In Chapter 4, an inter-laboratory assessment results were reported. The ILC was aimed at gauging performance and agreement of serological TBF results across a standardised clinical specimen panel by several participating laboratories. Although the ILC was a once-off “snap shot” assessment, it was highlighted that South African laboratories would benefit from routine ILC programs, for both serological and molecular-based techniques, to improve proficiency and assess technique reliability. Additionally, passive surveillance programs for known Rickettsia ‘hot spot’ areas would provide data necessary to facilitate test optimization criteria e.g. cutoff values and baseline titers. A case series of patients presenting with suspected Crimean-Congo haemorrhagic fever (CCHF) but diagnosed with TBF in South Africa, between 2011-2018 was described in Chapter 5. Clinical aspects and genetic characterization of the implicated Rickettsia species were presented and it was highlighted that TBF is an important differential diagnosis for CCHF, especially since geographic distribution, primary vectors and symptom presentations overlap for the two diseases. Clinicians are encouraged to submit eschar swabs and/or eschar biopsies, for polymerase chain reaction (PCR) analysis in addition to blood samples submitted, to improve the likelihood of detecting Rickettsia. Finally, in Chapter 6, Rhipicephalus ticks were found to be harboring two recently-identified South African Rickettsia species: ‘Candidatus’ R. muridii and ‘Candidatus’ R. rhabdomydis, and H. rufipes ticks demonstrated a larger Rickettsia species diversity with ticks found to be carrying R. sibirica subsp. mongolitimonae, and possibly two to three currently uncharacterized species.