Levin Michael L, Zemtsova Galina E, Killmaster Lindsay F, Snellgrove Alyssa, Schumacher Lauren B M
Rickettsial Zoonoses Branch, Centers for Disease Control, Prevention, Atlanta, GA, United States.
Rickettsial Zoonoses Branch, Centers for Disease Control, Prevention, Atlanta, GA, United States.
Ticks Tick Borne Dis. 2017 Jun;8(4):615-622. doi: 10.1016/j.ttbdis.2017.04.006. Epub 2017 Apr 12.
Rickettsia rickettsii - the etiologic agent of Rocky Mountain spotted fever (RMSF) - is widely spread across the Americas. In the US, Dermacentor spp. ticks are identified as primary vectors of R. rickettsii and Rhipicephalus sanguineus s.l. has been implicated in transmission of this pathogen in several locations in the Southwest. Conversely, ticks of the genus Amblyomma are recognized vectors of RMSF in Central and South America, but not in the US. A. americanum is one of the most aggressive human-biting ticks in the US, whose geographical range overlaps with that of reported RMSF cases. Despite sporadic findings of R. rickettsii DNA in field-collected A. americanum and circumstantial association of this species with human RMSF cases, its vector competence for R. rickettsii has not been appropriately studied. Therefore, we assessed the ability of A. americanum to acquire and transmit two geographically distant isolates of R. rickettsii. The Di-6 isolate of R. rickettsii used in this study originated in Virginia and the AZ-3 isolate originated in Arizona. Under laboratory conditions, A. americanum demonstrated vector competence for both isolates, although the efficiency of acquisition and transovarial transmission was higher for Di-6 than for AZ-3 isolate. Uninfected larvae acquired the pathogen from systemically infected guinea pigs, as well as while feeding side by side with Rickettsia-infected ticks on non-rickettsiemic hosts. Once acquired, R. rickettsii was successfully maintained through the tick molting process and transmitted to susceptible animals during subsequent feedings. Guinea pigs and dogs infested with infected A. americanum developed fever, scrotal edema and dermatitis or macular rash. R. rickettsii DNA was identified in animal blood, skin, and internal organs. The prevalence of infection within tick cohorts gradually increased due to side-by-side feeding of infected and uninfected individuals from 33 to 49% in freshly molted nymphs to 71-98% in engorged females. Moreover, R. rickettsii was transmitted transovarially by approximately 28% and 14% of females infected with Di-6 and AZ-3 isolates, respectively. Hence, A. americanum is capable of acquiring, maintaining and transmitting R. rickettsii isolates originating from two different geographical regions of the US, at least under laboratory conditions. Its role in ecology and epidemiology of RMSF in the US deserves further investigation.
立氏立克次体——落基山斑疹热（RMSF）的病原体——广泛分布于美洲。在美国，革蜱属蜱虫被确定为立氏立克次体的主要传播媒介，而血红扇头蜱指名亚种在西南部的几个地区被认为参与了这种病原体的传播。相反，美洲钝缘蜱属蜱虫在中美洲和南美洲被认为是RMSF的传播媒介，但在美国并非如此。美洲钝缘蜱是美国最具攻击性的叮咬人类的蜱虫之一，其地理分布范围与报告的RMSF病例的分布范围重叠。尽管在野外采集的美洲钝缘蜱中偶尔发现立氏立克次体DNA，且该物种与人类RMSF病例存在间接关联，但其作为立氏立克次体传播媒介的能力尚未得到充分研究。因此，我们评估了美洲钝缘蜱获取和传播两个地理上相距遥远的立氏立克次体分离株的能力。本研究中使用的立氏立克次体Di - 6分离株起源于弗吉尼亚州，AZ - 3分离株起源于亚利桑那州。在实验室条件下，美洲钝缘蜱对这两个分离株均表现出传播媒介能力，尽管Di - 6分离株的获取和经卵传播效率高于AZ - 3分离株。未感染的幼虫从全身感染的豚鼠身上获取病原体，也在与感染立克次体的蜱虫一起在非立克次体血症宿主上取食时获取病原体。一旦获取病原体，立氏立克次体就能成功地在蜱虫蜕皮过程中存活，并在随后的取食过程中传播给易感动物。感染美洲钝缘蜱的豚鼠和狗出现发热、阴囊水肿、皮炎或斑疹。在动物的血液、皮肤和内脏中鉴定出立氏立克次体DNA。由于感染和未感染个体的并喂，蜱虫群体中的感染率逐渐增加，从新蜕皮若虫中的33%至49%增加到饱血雌蜱中的71%至98%。此外，感染Di - 6和AZ - 3分离株的雌蜱分别约有28%和14%经卵传播立氏立克次体。因此，至少在实验室条件下，美洲钝缘蜱能够获取、维持和传播源自美国两个不同地理区域的立氏立克次体分离株。其在美国RMSF生态学和流行病学中的作用值得进一步研究。
