Identification and validation of internal reference genes for real-time quantitative polymerase chain reaction-based studies in Hyalomma anatolicum ticks

Tick-borne pathogens in camels: A systematic review and meta-analysis of the prevalence in dromedaries

Published data on tick-borne pathogens (TBPs) in camels worldwide have been collected to provide an overview of the global prevalence and species diversity of camelid TBPs. Several TBPs have been detected in dromedary camels, raising concerns regarding their role as natural or maintenance hosts for tick-borne pathogens. Insubstantial evidence exists regarding the natural infection of camels with Babesia spp., Theileria spp., Anaplasma spp., and Ehrlichia spp., particularly because most of the camels were considered healthy at the time of sampling. Based on polymerase chain reaction (PCR) testing, a pooled prevalence of 35.3% (95% CI: 22.6-48.1%) was estimated for Anaplasma, which was the most frequently tested TBP in dromedaries, and DNA of Anaplasma marginale, Anaplasma centrale, Anaplasma ovis, Anaplasma platys, and A. platys-like were isolated, of which ruminants and dogs are reservoirs. Similarly, the estimated pooled prevalence for the two piroplasmid genera; Babesia and Theileria was approximately equal (10-12%) regardless of the detection method (microscopy or PCR testing). Nevertheless, Babesia caballi, Theileria equi, and Theileria annulata DNA have frequently been detected in camels but they have not yet been proven to be natural hosts. Scarce data detected Babesia microti, Anaplasma phagocytophilum, and Borrelia burgdorferi sensu lato (s.l.) DNA in blood of dromedaries, although ticks of the genus Ixodes are distributed in limited areas where dromedaries are raised. Interestingly, a pooled seroprevalence of 47.7% (26.3-69.2%) was estimated for Crimean-Congo hemorrhagic fever virus, and viral RNA was detected in dromedary blood; however, their contribution to maintain the viral transmission cycles requires further experimental investigation. The substantially low incidence and scarcity of data on Rickettsia and Ehrlichia species could imply that camels were accidentally infected. In contrast, camels may play a role in the spread of Coxiella burnetii, which is primarily transmitted through the inhalation of aerosols emitted by diseased animals and contaminated environments. Bactrian camels showed no symptoms due to the examined TBPs, meanwhile, clinical disease was seen in alpacas infected with A. phagocytophilum. Similar to dromedaries, accidental tick bites may be the cause of TBP DNA found in the blood of Bactrian camels.

Stable internal reference genes for quantitative RT-PCR analyses in Rhipicephalus microplus during embryogenesis

Studies on the transcriptional control of gene expression are crucial to understand changes in organism's physiological or cellular conditions. To obtain reliable data on mRNA amounts and the estimation of gene expression levels, it is crucial to normalize the target gene with one or more internal reference gene(s). However, the use of constitutive genes as reference genes is controversial, as their expression patterns are sometimes more complex than previously thought. In various arthropod vectors, including ticks, several constitutive genes have been identified by studying gene expression in different tissues and life stages. The cattle tick Rhipicephalus microplus is a major vector for several pathogens and is widely distributed in tropical and subtropical regions globally. Tick developmental physiology is an essential aspect of research, particularly embryogenesis, where many important developmental events occur, thus the identification of stable reference genes is essential for the interpretation of reliable gene expression data. This study aimed to identify and select R. microplus housekeeping genes and evaluate their stability during embryogenesis. Reference genes used as internal control in molecular assays were selected based on previous studies. These genes were screened by quantitative PCR (qPCR) and tested for gene expression stability during embryogenesis. Results demonstrated that the relative stability of reference genes varied at different time points during the embryogenesis. The GeNorm tool showed that elongation factor 1α (Elf1a) and ribosomal protein L4 (Rpl4) were the most stable genes, while H3 histone family 3A (Hist3A) and ribosomal protein S18 (RpS18) were the least stable. The NormFinder tool showed that Rpl4 was the most stable gene, while the ranking of Elf1a was intermediate in all tested conditions. The BestKeeper tool showed that Rpl4 and cyclophilin A (CycA) were the more and less stable genes, respectively. These data collectively demonstrate that Rpl4, Elf1a, and GAPDH are suitable internal controls for normalizing qPCR during R. microplus embryogenesis. These genes were consistently identified as the most stable in various analysis methods employed in this study. Thus, findings presented in this study offer valuable information for the study of gene expression during embryogenesis in R. microplus.

Virus-Derived DNA Forms Mediate the Persistent Infection of Tick Cells by Hazara Virus and Crimean-Congo Hemorrhagic Fever Virus

Crimean-Congo hemorrhagic fever (CCHF) is a severe disease of humans caused by CCHF virus (CCHFV), a biosafety level (BSL)-4 pathogen. Ticks of the genus Hyalomma are the viral reservoir, and they represent the main vector transmitting the virus to its hosts during blood feeding. We have previously shown that CCHFV can persistently infect Hyalomma-derived tick cell lines. However, the mechanism allowing the establishment of persistent viral infections in ticks is still unknown. Hazara virus (HAZV) can be used as a BSL-2 model virus instead of CCHFV to study virus/vector interactions. To investigate the mechanism behind the establishment of a persistent infection, we developed an in vitro model with Hyalomma-derived tick cell lines and HAZV. As expected, HAZV, like CCHFV, persistently infects tick cells without any sign of cytopathic effect, and the infected cells can be cultured for more than 3 years. Most interestingly, we demonstrated the presence of short viral-derived DNA forms (vDNAs) after HAZV infection. Furthermore, we demonstrated that the antiretroviral drug azidothymine triphosphate could inhibit the production of vDNAs, suggesting that vDNAs are produced by an endogenous retrotranscriptase activity in tick cells. Moreover, we collected evidence that vDNAs are continuously synthesized, thereby downregulating viral replication to promote cell survival. Finally, vDNAs were also detected in CCHFV-infected tick cells. In conclusion, vDNA synthesis might represent a strategy to control the replication of RNA viruses in ticks allowing their persistent infection. IMPORTANCE Crimean-Congo hemorrhagic fever (CCHF) is an emerging tick-borne viral disease caused by CCHF virus (CCHFV). Ticks of the genus Hyalomma can be persistently infected with CCHFV representing the viral reservoir, and the main vector for viral transmission. Here we showed that tick cells infected with Hazara virus, a nonpathogenic model virus closely related to CCHFV, contained short viral-derived DNA forms (vDNAs) produced by endogenous retrotranscriptase activity. vDNAs are transitory molecules requiring viral RNA replication for their continuous synthesis. Interestingly, vDNA synthesis seemed to be correlated with downregulation of viral replication and promotion of tick cell viability. We also detected vDNAs in CCHFV-infected tick cells suggesting that they could represent a key element in the cell response to nairovirus infection and might represent a more general mechanism of innate immunity against RNA viral infection.