Effects of Rickettsia amblyommatis Infection on the Vector Competence of Amblyomma americanum Ticks for Rickettsia rickettsii

Abstract

Although Dermacentor spp. ticks are considered the primary vectors of Rickettsia rickettsii in the United States, other North American tick species are also capable of transmitting the agent, including the lone star tick-Amblyomma americanum. The lone star tick is an aggressive human-biting tick abundant in the South, Central, and Mid-Atlantic United States, which has been shown to be a competent vector of R. rickettsii in laboratory studies. However in nature, A. americanum frequently carry Rickettsia amblyommatis-another member of the spotted fever group-with the prevalence of infection reaching 84% in some populations. It has been postulated that the presence of an endosymbiotic Rickettsia in a significant proportion of a vector population would diminish or even block transmission of pathogenic Rickettsia in ticks from generation to generation due to transovarial interference. We measured the ability of R. amblyommatis-infected A. americanum to acquire R. rickettsii from an infected host with a bloodmeal, and transmit it transstadially, horizontally (to a susceptible host), and vertically to the next generation. Larvae from both the R. amblyommatis-infected and R. amblyommatis-free cohorts acquired R. rickettsii from infected guinea pigs, but the presence of the symbiont diminished the ability of coinfected engorged larvae to transmit R. rickettsii transstadially. Conversely, acquisition of R. rickettsii by cofeeding was unaffected in R. amblyommatis-infected nymphs and adults; prevalence of R. rickettsii in engorged adults reached 97% in both R. amblyommatis-infected and R. amblyommatis-free cohorts. In guinea pigs exposed to dually infected nymphs, R. rickettsii infection was milder than in those fed upon nymphs infected with R. rickettsii only. The frequency of transovarial transmission of R. rickettsii in the R. amblyommatis-infected cohort (31%) appeared lower than that in the R. amblyommatis-free cohort (48%), but the difference was not statistically significant. Larval progenies of dually infected A. americanum females transmitted R. rickettsii to naïve guinea pigs confirming viability of the pathogen. Thus, the vector competence of A. americanum for R. rickettsii was not significantly affected by R. amblyommatis.

Current and Future Distribution of the Lone Star Tick, Amblyomma americanum (L.) (Acari: Ixodidae) in North America

“…Lone star ticks are well-known as a vector of human ehrlichiosis [38] and Southern Tick Associated Rash Illness [39], but recent laboratory and field investigations have confirmed that the recently discovered human pathogens Heartland Virus (HRTV) and Bourbon Virus (BRBV) are also transmitted by nymphs of this species [40–42]. Levin et al (2015) [43] recently showed that lone star ticks are competent, at least under laboratory conditions, to maintain and transmit Rickettsia rickettsii , the causative agent for Rocky Mountain spotted fever; and, the increase in incidences of spotted fever group (SFG) rickettsiosis in the US is suspected to be related to the expansion of lone star ticks [44]. Finally, a rapidly emerging and potentially fatal illness, commonly referred as red meat allergy (alpha-gal syndrome), is a condition that affects some individuals bitten by ticks—they develop anaphylaxis following red meat consumption—the tick responsible for this poorly understood malady in North America is most likely the lone star tick [45].…”

Polymicrobial Nature of Tick-Borne Diseases

“…phagocytophilum induces the production of the antifreeze glycoprotein (IAFGP), which in turn makes infection easier (99). Relationships between the symbiont-pathogen and the tick may result in a neutral status quo; for example, the vector competence of A. americanum for R. rickettsii was not significantly affected by R. amblyommatis (100). In other instances, tick responses can be harmful to pathogens.…”

Scoping review of distribution models for selected Amblyomma ticks and rickettsial group pathogens

“…This is particularly evident in the case of rickettsial pathogens, comprising the ehrlichiosis, anaplasmosis, and spotted fever rickettsioses, which compared to Lyme disease remain understudied. Rickettsial pathogens of medical importance may be encountered worldwide, and ticks from the Amblyomma genus are competent vectors for many of these pathogens ( Levin, Schumacher & Snellgrove, 2018 ). Although numbers of documented cases have been increasing in recent years, the true extent of geographic risk for rickettsial pathogens is challenging to delineate due to a lack of consistent, long-term, and widespread surveillance data, and regionally low case detection.…”