Unique Strain of Rickettsia parkeri Associated with the Hard Tick Dermacentor parumapertus Neumann in the Western United States

Abstract

In 1953, investigators at the Rocky Mountain Laboratories in Hamilton, MT, described the isolation of a spotted fever group Rickettsia (SFGR) species from Dermacentor parumapertus ticks collected from black-tailed jackrabbits (Lepus californicus) in northern Nevada. Several decades later, investigators characterized this SFGR (designated the parumapertus agent) by using mouse serotyping methods and determined that it represented a distinct rickettsial serotype closely related to Rickettsia parkeri; nonetheless, the parumapertus agent was not further characterized or studied. To our knowledge, no isolates of the parumapertus agent remain in any rickettsial culture collection, which precludes contemporary phylogenetic placement of this enigmatic SFGR. To rediscover the parumapertus agent, adult-stage D. parumapertus ticks were collected from black-tailed jackrabbits shot or encountered as roadkills in Arizona, Utah, or Texas from 2011 to 2016. A total of 339 ticks were collected and evaluated for infection with Rickettsia species. Of 112 D. parumapertus ticks collected in south Texas, 16 (14.3%) contained partial ompA sequences with the closest identity (99.6%) to Rickettsia sp. strain Atlantic rainforest Aa46, an SFGR that is closely related or identical to an SFGR species that causes a mild rickettsiosis in several states of Brazil. A pure isolate, designated strain Black Gap, was cultivated in Vero E6 cells, and sequence analysis of the rrs, gltA, sca0, sca5, and sca4 genes also revealed the closest genetic identity to Rickettsia sp. Atlantic rainforest Aa46. Phylogenetic analysis of the five concatenated rickettsial genes place Rickettsia sp. strain Black Gap and Rickettsia sp. Atlantic rainforest Aa46 with R. parkeri in a distinct and well-supported clade.IMPORTANCE We suggest that Rickettsia sp. Black Gap and Rickettsia sp. Atlantic rainforest Aa46 represent nearly identical strains of R. parkeri and that Rickettsia sp. Black Gap or a very similar strain of R. parkeri represents the parumapertus agent. The close genetic relatedness among these taxa, as well as the response of guinea pigs infected with the Black Gap strain, suggests that R. parkeri Black Gap could cause disease in humans. The identification of this organism could also account, at least in part, for the remarkable differences in severity ascribed to Rocky Mountain spotted fever (RMSF) among various regions of the American West during the early 20th century. We suggest that the wide variation in case fatality rates attributed to RMSF

First molecular detection of Rickettsia sp. strain Atlantic rainforest in Amblyomma ovale ticks from Espírito Santo state, Brazil

“…A recent study provided phylogenetic evidence that the Rickettsia sp. strain Atlantic rainforest represents a genetic variant of the human pathogen Rickettsia parkeri (PADDOCK et al, 2017). In this way, it would be important to conduct an active epidemiological search with the purpose of characterizing new eschar-associated rickettsiosis cases in Brazil.…”

A biosafety level-2 dose-dependent lethal mouse model of spotted fever rickettsiosis: Rickettsia parkeri Atlantic Rainforest-like isolate

“…A study of infection of guinea pigs ( Cavia porcellus ) by intraperitoneal inoculation of 1.9 × 10 8 of R. parkeri Black Gap strain resulted in a nonlethal illness, manifested in most animals by mild fever and mild to moderate swelling and erythema of the scrotum [4]. This strain has a close genetic identity with the Brazilian Atlantic rainforest strain [4] and the Colombian RpARFL isolate [6].…”

“…A study of infection of guinea pigs ( Cavia porcellus ) by intraperitoneal inoculation of 1.9 × 10 8 of R. parkeri Black Gap strain resulted in a nonlethal illness, manifested in most animals by mild fever and mild to moderate swelling and erythema of the scrotum [4]. This strain has a close genetic identity with the Brazilian Atlantic rainforest strain [4] and the Colombian RpARFL isolate [6]. Furthermore, subclinical infection was observed in an additional study that utilized the natural mode of infection of guinea pigs by tick transmission, using A. ovale nymphs infected with the Brazilian Atlantic rainforest strain [17].…”

“…Rickettsia parkeri , a member of the SFG, has been reported across the American continents as infecting several tick species including the Amblyomma maculatum complex ( A. maculatum , A. triste , A. tigrinum ), A. ovale , A. nodosum , A. parvitarsum and Dermacentor parumapertus [3,4]. Distinct R. parkeri strains have been described in the American continents, including the Atlantic Rainforest strain isolated in Brazil [5].…”

“…Distinct R. parkeri strains have been described in the American continents, including the Atlantic Rainforest strain isolated in Brazil [5]. The first isolate of R. parkeri in Colombia is most closely related to the Atlantic Rainforest strain from Brazil [6] and Black Gap strain from Texas [4]. Rickettsia parkeri has been associated with a relatively mild human disease characterized by eschar formation at the tick feeding site, fever, myalgia and rash [7–9].…”