Dynamics of Theileria equi Infection in Rhipicephalus (Boophilus) microplus during the Parasitic Phase in a Chronically Infected Horse

Abstract

Experimental studies have demonstrated that Rhipicephalus (Boophilus) microplus transmits Theileria equi to horses. However, the degree and dynamics of this protozoan infection in the vector’s organism have not been fully elucidated. Therefore, this study aimed to evaluate the infection rate and parasitic load of T. equi in R. (B.) microplus, the infection dynamics in this arthropod during experimental infestation in a horse chronically infected with T. equi, and to evaluate the trans-stadial and intrastadial transmission competence of T. equi by R. (B.) microplus. The experimental infestation period of R. (B.) microplus on the horse was 33 days, but males were found on the animal up to 60 days post-infestation. After the fifth day post-infestation, ticks and equine blood were collected every two days. Whole ticks from the same developmental stage collected in the same day were pooled. Adult ticks were dissected to extract salivary glands and gut. DNA extraction was performed for all the samples, and they were then submitted to qPCRs for T. equi diagnosis. Freshly molted nymphs collected as larvae in the horse and freshly molted males and females collected as nymphs in the horse showed equal to or greater than 75% positivity for T. equi, indicating a strong possibility of trans-stadial transmission. The longest permanence of the male ticks on the horse associated with the high positivity rate of this type of sample for T. equi indicate that the male may play a role in the intrastadial transmission of T. equi to infection-free horses. The salivary glands displayed 77.78% positivity for T. equi and presented a higher infection rate at the end of the experimental period (100% from 29 to 33 days post-infection). This study shows that R. (B.) microplus has high T. equi infection rates and that the infection rate and parasitic load increased over the experimental period. These findings confirm the importance of chronically infected horses with T. equi as a source of infection for R. (B.) microplus.

Differential Expression of Immune Genes in the Rhipicephalus microplus Gut in Response to Theileria equi Infection

“…This lack of tick immune response may favor T. equi to complete the biological cycle. Other studies have already confirmed that even when ticks feed from a horse presenting low parasitemia, the tick acquires and develops the complete protozoan life cycle [8,40].…”

“…The overall analysis of these two experiments evidenced several genes with significant differences in the expression levels in the groups infected with high loads of T. equi (Figure 3A,C,E), suggesting that the significant differential gene expression of the components of the signaling pathway is dependent on the parasite load. Natural infection in immunocompetent horses presents a chronicle character similar to experiment 2 [8]. When the horse shows low parasitemia, the tick ingests low T. equi load during blood feeding; consequently, there is little stimulus for the immune response in the gut of R. microplus.…”

“…Subsequently, this horse was splenectomized and inoculated intravenously with cryopreserved T. equi strain isolated from a foal of 8 days old at UNESP-FCAV, Jaboticabal, state of São Paulo, Brazil [16]. Since then, it has been stabled in isolated pens and used in experimental studies on interactions between T. equi and R. microplus [8,17].…”

“…The mean values of the quantification cycles (Cq) obtained from the three biological replicates of the infected group were plotted on a standard curve produced by a previous study [8] to attain the gut parasite load of R. microplus obtained from each of the experimental infestations performed.…”

“…Equine piroplasmosis is a vector-borne disease transmitted by several tick species of the Dermacentor, Rhipicephalus, Hyalomma, and Haemaphysalis genera [4,5]. However, in some countries of South America, including Brazil, the only tick species with vector capacity for the transmission of T. equi is the cattle tick R. microplus [5][6][7][8]. The capacity of R. microplus to transmit T. equi has been experimentally confirmed, although the mechanisms that enable this tick to be competent for this pathogen transmission are still unclear [5][6][7][8].…”