Epidemiology and ecology of rickettsial infections

Abstract

Rickettsiae are obligatory intracellular bacteria, transmitted to vertebrates by arthropod vectors, primarily by fleas and ticks. A rapid increase in the incidence of four endemic rickettsioses; Rocky Mountain spotted fever, Mediterranean spotted fever, North Asian tick typhus, and Queensland tick typhus was noted since 1970s and for Japanese spotted fever, since its discovery in mid-1980s. As a result, spotted fever group of rickettsiae (SFG) currently include over 25 formally recognized species. Elevated attention to rickettsial diseases, advent and adaptation of new molecular tools used for field and laboratory studies in the 1990s and increase in funding support have lead to this second pronounced increase in the discovery of novel species and the increase in incidence of tick-borne rickettsial diseases in the last 40 years. Change in ecological factors which determine the vector species and their behaviour, particularly those driven by climate change or human activities such as deforestation, human behavioural changes such as recreative activities that involve close association with nature, human population increases and the improvements in surveillance methodologies may contribute to the change in rickettsial disease ecology and epidemiology. Since the use of molecular technologies, numerous rickettsiae from kown to unknown or variable degrees of pathogenicity for humans are being found to co-circulate in overlapping geographic regions and demonstrated in the same tick species. Furthermore, the discovery and description of novel nosological entities caused by previously unknown SFG rickettsiae, ability of most rickettsiae to circulate in diverse sylvatic or peridomestic reservoirs without having obvious impacts on their vertebrate hosts or affecting humans, occurance of rickettsiae in association with a wide range of hard and soft ticks which feed on very different species of large and small animals, their mmaintenance in these vector systems by both vertical and horizontal transmission has lead to a degree where the traditional views of tick-borne rickettsioses as endemic diseases with largely focal distributions, limited host and geographic ranges, predetermined seasonality and defined tick associations to become obsolete or at least very incomplete. Therefore, continuous vigilance, survailance, research and funding are warranted in order to understand the changing ecology and epidemiology of rickettsial diseases