Quantification of <I>Theileria parva</I> in <I>Rhipicephalus appendiculatus</I> (Acari: Ixodidae) Confirms Differences in Infection Between Selected Tick Strains

Abstract

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.

Thirty years of tick population genetics: A comprehensive review

“…Although the specific genetic mechanism for this difference remains unknown, an obvious follow-up to this study would be to test this vector and parasite from geographically diverse locations. A genetic factor determining the susceptibility of R. appendiculatus laboratory strains to infection with T. parva is being studied (Buscher and Tangus, 1986;Kubasu, 1992;Odongo et al, 2009), but not yet in a population genetics framework. These studies point out the possibility of biological limitations to disease transmission and provide intriguing targets for future studies.…”

Identification of hard ticks (Acari: Ixodidae) and seroprevalence to Theileria parva in cattle raised in North Kivu Province, Democratic Republic of Congo

“…In the present work, the situation is probably due to the low R. appendiculatus tick burden (4.3±0.17 ticks/ animal) recorded in the enrolled herds. Low-tick abundance and a low rate of infection in the tick population for a given pathogen are related to low seroprevalences to T. parva (Gilioli et al 2009;Odongo et al 2009). However, data about the seasonal abundance and the rate of infected ticks by T. parva are not available in the study area.…”

Environment and farm factors associated with exposure to Theileria parva infection in cattle under traditional mixed farming system in Mbeere District, Kenya

“…Endemic instability implies an ecological imbalance between the host, tick, and environment where only a small proportion (mostly <70%) of cattle in a population become infected and immune by 6 months of age (Norval et al 1992). The low prevalence could have arisen out of low infection challenge, probably due to low levels of vector abundance and distribution and/or low tick infection rates among other reasons (Gilioli et al 2009;Odongo et al 2009). This is supported by the observation that the low seroprevalence existed under open grazing system that normally permits exposure to infected ticks.…”