Comparative vertical transmission of Rickettsia by Dermacentor variabilis and Amblyomma maculatum

Abstract

The geographical overlap of multiple Rickettsia and tick species coincides with the molecular detection of a variety of rickettsial agents in what may be novel tick hosts. However, little is known concerning transmissibility of rickettsial species by various tick hosts. To examine the vertical transmission potential between select tick and rickettsial species, two sympatric species of ticks, Dermacentor variabilis and Amblyomma maculatum, were exposed to five different rickettsial species, including Rickettsia rickettsii, Rickettsia parkeri, Rickettsia montanensis, Rickettsia amblyommatis, or flea-borne Rickettsia felis. Fitness-related metrics including engorgement weight, egg production index, nutrient index, and egg hatch percentage were then assessed. Subsamples of egg clutches and unfed larvae, nymphs, and adults for each cohort were assessed for transovarial and transstadial transmission of rickettsiae by qPCR. Rickettsial exposure had a minimal fitness effect in D. variabilis and transovarial transmission was observed for all groups except R. rickettsii. In contrast, rickettsial exposure negatively influenced A. maculatum fitness and transovarial transmission of rickettsiae was demonstrated only for R. amblyommatis- and R. parkeri-exposed ticks. Sustained maintenance of rickettsiae via transstadial transmission was diminished from F1 larvae to F1 adults in both tick species. The findings of this study suggest transovarial transmission specificity may not be tick species dependent, and sustained vertical transmission is not common.

Role of Sca2 and RickA in the Dissemination of Rickettsia parkeri in Amblyomma maculatum

“…In horizontal acquisition, ticks imbibe an infectious bloodmeal from the vertebrate host, allowing the rickettsiae to enter the gut and then, through undefined mechanisms, disseminate throughout the tick to infect organs central to transmission, including the ovaries (vertical) and salivary glands (horizontal). The ability of individual Rickettsia species to successfully infect and be transmitted by a tick host varies by both Rickettsia and tick species ( 1 ). Transmission of SFG Rickettsia to a vertebrate host during tick feeding can result in disease ranging from a mild, self-limiting infection to death ( 2 , 3 ).…”

Proposing a new hypothesis: Rickettsia spp. as a mechanism maintaining parapatry between two Australian reptile tick species

“…are involved in preventing A. limbatum from moving south. Rickettsia can be transmitted via several pathways (transovarially, transtadially and horizontally), and transmission is dependent on the pathogen and the viability of its vector and host (Harris et al, 2017). Studies on the transmission of R. honei in B. hydrosauri ticks from Flinders Island, Tasmania, showed that Rickettsia is located in the oocytes, immature eggs, Malpighian tubules, midgut epithelium and salivary glands – suggestive of transovarial and horizontal transmission (Stenos et al, 2003; Whitworth et al, 2003).…”

“…Rickettsia spp. have been found to alter host animal behaviour and fitness such as influencing dispersal and thermal tolerance, egg production, moulting success and offspring viability in tick hosts (Harris et al, 2017). Crucially, in one study, R. rickettsii was found to be lethal to its tick vector Dermacentor andersoni following both vertical and horizontal transmission (Niebylski et al, 1999).…”

Neglected aspects of tick-borne rickettsioses

“…However, studies have demonstrated the occurrence of transovarial transmission in only a limited number of species [ 11 ]. Moreover, the efficiency of vertical transmission varies according to the rickettsial and tick species [ 19 , 34 ], and transmission may not occur in female ticks infected by sexual/co-feeding transmission [ 35 ]. Vertebrate hosts may thus be necessary to maintain and perpetuate some rickettsial agents in nature [ 36 ].…”