Transovarial transmission efficiency of Babesia bovis tick stages acquired by Rhipicephalus (Boophilus) microplus during acute infection

Quantification of Theileria parva in Rhipicephalus appendiculatus (Acari: Ixodidae) confirms differences in infection between selected tick strains

Theileria parva is the etiologic agent of East Coast fever, an economically important disease of cattle in sub-Saharan Africa. This protozoan parasite is biologically transmitted by Rhipicephalus appendiculatus (Neumann) (Acari: Ixodidae). An understanding of the vector-parasite interaction may aid the development of improved methods for controlling transmission. We developed quantitative polymerase chain reaction (qPCR) and nested PCR (nPCR) assays targeting the T. parva-specific p104 gene to study T. parva pathogenesis in two strains of R. appendiculatus that had previously been selected to be relatively more (Kiambu) or less (Muguga) susceptible to infection. Nymphs from both strains were fed simultaneously to repletion on acutely infected calves. Nymphs from the Kiambu strain showed significantly higher engorgement weights compared with Muguga strain nymphs. Immediately after engorgement qPCR confirmed that nymphal Kiambu ticks had significantly higher parasite loads at repletion than Muguga nymphs. By 12 d postengorgement, parasites were below quantifiable levels but could be detected by nPCR in 83-87% (Muguga and Kiambu, respectively) of nymphs. After the molt, adult feeding on naïve cattle stimulated parasite replication in the salivary glands. PCR detected significantly more infected ticks than microscopy, and there was a significant difference between the two tick strains both in the proportion of ticks that develop salivary gland infections, and in the number of parasites within infected salivary glands. These data confirm that although both tick strains were competent vectors, Kiambu is both a significantly more susceptible and a more efficient host for T. parva than Muguga. The mechanisms that contribute to the levels of susceptibility and efficiency are unknown; however, this study lays the groundwork for a comparison of the transcriptome of these tick strains, the next step toward discovering the genes involved in the tick-parasite interaction.

Persistently infected horses are reservoirs for intrastadial tick-borne transmission of the apicomplexan parasite Babesia equi

Tick-borne pathogens may be transmitted intrastadially and transstadially within a single vector generation as well as vertically between generations. Understanding the mode and relative efficiency of this transmission is required for infection control. In this study, we established that adult male Rhipicephalus microplus ticks efficiently acquire the protozoal pathogen Babesia equi during acute and persistent infections and transmit it intrastadially to naïve horses. Although the level of parasitemia during acquisition feeding affected the efficiency of the initial tick infection, infected ticks developed levels of > or =10(4) organisms/pair of salivary glands independent of the level of parasitemia during acquisition feeding and successfully transmitted them, indicating that replication within the tick compensated for any initial differences in infectious dose and exceeded the threshold for transmission. During the development of B. equi parasites in the salivary gland granular acini, the parasites expressed levels of paralogous surface proteins significantly different from those expressed by intraerythrocytic parasites from the mammalian host. In contrast to the successful intrastadial transmission, adult female R. microplus ticks that fed on horses with high parasitemia passed the parasite vertically into the eggs with low efficiency, and the subsequent generation (larvae, nymphs, and adults) failed to transmit B. equi parasites to naïve horses. The data demonstrated that intrastadial but not transovarial transmission is an efficient mode for B. equi transmission and that persistently infected horses are an important reservoir for transmission. Consequently, R. microplus male ticks and persistently infected horses should be targeted for disease control.

Coinfection with antigenically and genetically distinct virulent strains of Babesia bovis is maintained through all phases of the parasite life cycle

Antigenic polymorphism is a defining characteristic of the Babesia bovis variable merozoite surface antigen (VMSA) family. Sequence analysis strongly suggests that recombination between virulent strains contributes to VMSA diversity. While meiosis during the aneuploid stage of the parasite's life cycle in the tick vector Rhipicephalus (Boophilus) microplus is the most probable source of interstrain recombination, there is no definitive evidence that coinfection of the mammalian host or R. microplus ticks with more than one virulent strain occurs. Using allele-specific real-time quantitative PCR, we tested the hypotheses that cattle could support coinfection of two antigenically variant virulent tick-transmissible strains of B. bovis and that R. microplus ticks could acquire and transmit these two divergent strains. The results indicate that both calves and ticks can support virulent B. bovis coinfection through all phases of the hemoparasite's life cycle. Neither strain dominated in either the mammalian or invertebrate host, and larval tick progeny, which could be coinfected individually, were also able to transmit both strains, resulting in virulent babesiosis in recipients. While coinfection of the tick vector provides the context in which allelic antigenic diversity can be generated, recombination of VMSA genes could not be confirmed, suggesting that VMSA allelic changes are slow to accumulate.

Persistently infected calves as reservoirs for acquisition and transovarial transmission of Babesia bovis by Rhipicephalus (Boophilus) microplus

Babesia bovis is a deadly disease of cattle resulting in severe economic losses in the vast regions of the world where it is endemic. If reintroduced into the United States, babesiosis would cause significant mortality in the naïve cattle population. In order to address the risk to U.S. cattle, it is essential to quantify the transovarial transmission efficiency in adult female Boophilus microplus ticks following acquisition feeding on persistently infected cattle. This study tested the hypothesis that infection rates are the same for larval progeny derived from females fed to repletion during persistent or acute infection. Increasing parasite levels during acute infection correlated with an increasing number of females harboring kinetes detectable in hemolymph (r = 0.9). The percent infected larvae ranged from 0 to 20% when derived from females fed to repletion on persistently infected calves and from 4 to 6% when derived from females fed to repletion during acute parasitemia. There was no significant difference in infection rates of larval progeny, implying that the risk associated with the introduction of either persistently infected or acutely infected cattle is equal. Parasite levels ranged from 2.4 x 10(2) to 1.9 x 10(5) in 3-day-fed larvae derived from females fed to repletion on persistently infected cattle. One group of larvae failed to transmit the parasite, suggesting that a threshold level of parasites must be obtained by larval progeny via transovarial transmission in order for larvae to deliver sufficient parasites to infect a naïve host.