One mode of transmitting infectious diseases is by vectors. By definition, vectors are living organisms that can transmit contagious pathogens between humans or from animals to humans. Accordingly, vector-borne diseases are human diseases caused by parasites, viruses, and bacteria transmitted by vectors .
Infectious diseases have grown dramatically in recent years for various reasons. According to the reports by the US Centers for Disease Control and Prevention (CDC), the number of reports of vector-borne diseases has tripled over 13 years (2004 to 2016). According to the latest figures from the World Health Organization (WHO), these diseases now account for more than 17% of all infectious diseases, killing more than 700,000 people worldwide each year; hence, more than half of the world's population are at risk for these diseases [1, 3]. As one of the most important and known vectors, ticks can transmit different types of bacteria, protozoa, and viruses that cause a wide range of diseases in humans and animals. Rickettsial diseases caused by bacterial agents from the order Rickettsiales are among these diseases .
Ehrlichia spp., Anaplasma spp., and several Rickettsia spp. are tick-borne pathogens in the order Rickettsiales. Rickettsia, the most significant genus, has been classified as four groups based on whole genome analysis data. Among them, the spotted fever group (SFG) and the transitional group (TRG) species are transmitted by ticks of the family Ixodidae and are considered zoonotic . The presence of a high variety of ticks that can transmit Rickettsiae to several countries in the Middle East makes it highly prone to cause diseases. Despite clear evidence, no studies have reported any rickettsial disease in Iran . However, the presence of SFG species in serum samples from humans and animals based on the results of IFA (indirect immunofluorescent antibody) and ELISA (enzyme-linked immunosorbent assay) tests has been confirmed .
Rickettsiae are the most prominent members of the Rickettsiaceae family that belong to the order Rickettsiales. This genus includes Gram-negative, aerobic, and intracellular coccobacilli that are transmitted by arthropods, including ticks, mites, fleas, and lice, and its life cycle involves both vertebrate and invertebrate hosts. Members of the genus Rickettsia can be classified as several groups, including the SFG, the typhus group, R. bellii, and R. Canadensis [8, 9]. The genus Rickettsiae includes 31 different species. Among tick-borne diseases, rickettsiosis is caused by Rickettsiae bacteria belonging to the group of Rickettsiae that cause spotted fever  diseases such as typhoid . Co-morbidity in humans and animals is a significant point concerning this pathogen. Accordingly, the need to study the distribution and diversity of this bacterium species at different geographical levels, and to predict outbreaks of vector-borne diseases can be compelling. This study aimed to identify the genetic species and to investigate the distribution of Rickettsiae in the ticks collected from sheep in Khuzestan province of Iran.