Levin Michael L, Schumacher Lauren B M, Snellgrove Alyssa
Rickettsial Zoonoses Branch, Centers for Disease Control and Prevention , Atlanta, Georgia .
Vector Borne Zoonotic Dis. 2018 Nov;18(11):579-587. doi: 10.1089/vbz.2018.2284. Epub 2018 Aug 10.
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.
在美国，尽管革蜱属蜱虫被认为是立氏立克次体的主要传播媒介，但其他北美蜱虫种类也能够传播该病原体，包括孤星蜱（美洲钝眼蜱）。孤星蜱是一种攻击性很强的叮人蜱虫，在美国南部、中部和大西洋中部地区数量众多，在实验室研究中已被证明是立氏立克次体的有效传播媒介。然而在自然环境中，美洲钝眼蜱经常携带斑点热群的另一个成员——美洲钝眼蜱立克次体，在某些种群中感染率高达84%。据推测，由于经卵干扰，在相当比例的媒介种群中存在内共生立克次体，会减少甚至阻止蜱虫体内致病性立克次体在代际间的传播。我们测定了感染美洲钝眼蜱立克次体的美洲钝眼蜱通过吸食感染宿主的血液获取立氏立克次体，并经变态期、水平传播（传给易感宿主）以及垂直传播给下一代的能力。来自感染美洲钝眼蜱立克次体和未感染美洲钝眼蜱立克次体的幼虫群体都从感染的豚鼠身上获取了立氏立克次体，但共生体的存在降低了同时感染的饱血幼虫经变态期传播立氏立克次体的能力。相反，在感染美洲钝眼蜱立克次体的若虫和成虫中，通过共同取食获取立氏立克次体的情况未受影响；在感染美洲钝眼蜱立克次体和未感染美洲钝眼蜱立克次体的群体中，饱血成虫中立氏立克次体的感染率均达到97%。在接触双重感染若虫的豚鼠中，立氏立克次体感染比仅接触感染立氏立克次体若虫的豚鼠要轻。在感染美洲钝眼蜱立克次体的群体中立氏立克次体的经卵传播频率（31%）似乎低于未感染美洲钝眼蜱立克次体的群体（48%），但差异无统计学意义。双重感染的美洲钝眼蜱雌性的幼虫后代将立氏立克次体传播给了未接触过该病原体的豚鼠，证实了该病原体的生存能力。因此，美洲钝眼蜱对立氏立克次体的传播能力并未受到美洲钝眼蜱立克次体的显著影响。
