Importation of exotic ticks and tick-borne spotted fever group rickettsiae into the United States by migrating songbirds

Abstract

Birds are capable of carrying ticks and, consequently, tick-transmitted microorganisms over long distances and across geographical barriers such as oceans and deserts. Ticks are hosts for several species of spotted fever group rickettsiae (SFGR), which can be transmitted to vertebrates during blood meals. In this study, the prevalence of this group of rickettsiae was examined in ticks infesting migratory songbirds by using polymerase chain reaction (PCR). During the 2009 and 2010 spring migration season, 2064 northward-migrating passerine songbirds were examined for ticks at Johnson Bayou, Louisiana. A total of 91 ticks was removed from 35 individual songbirds for tick species identification and spotted fever group rickettsia detection. Ticks were identified as Haemaphysalis juxtakochi (n=38, 42%), Amblyomma longirostre (n=22, 24%), Amblyomma nodosum (n=17, 19%), Amblyomma calcaratum (n=11, 12%), Amblyomma maculatum (n=2, 2%), and Haemaphysalis leporispalustris (n=1, 1%) by comparing their 12S rDNA gene sequence to homologous sequences in GenBank. Most of the identified ticks were exotic species originating outside of the United States. The phylogenetic analysis of the 71 ompA gene sequences of the rickettsial strains detected in the ticks revealed the occurrence of 6 distinct rickettsial genotypes. Two genotypes (corresponding to a total of 28 samples) were included in the Candidatus Rickettsia amblyommii clade (less than 1% divergence), 2 of them (corresponding to a total of 14 samples) clustered with Rickettsia sp. "Argentina" with less than 0.2% sequence divergence, and 2 of them (corresponding to a total of 27 samples), although closely related to the R. parkeri-R. africae lineage (2.50-3.41% divergence), exhibited sufficient genetic divergence from its members to possibly constitute a new rickettsial genotype. Overall, there does not seem to be a specific relationship between exotic tick species, the rickettsiae they harbor, or the reservoir competence of the corresponding bird species.

Keywords: Exotic ticks; Migratory songbirds; SFGR; Tick molecular ecology.

Fig 1

Representative captured songbird species infested with immature ticks on various body parts during northward-bound migration at Johnson Bayou, Louisiana (2009–2010). (A) Tick on head of an Acadian flycatcher (Empidonax virescens). (B) Ticks attached to the upper eyelid of a worm-eating warbler (Helmitheros vermivorus). (C) Ticks attached on the left eye of a worm-eating warbler (H. vermivorus). (D) Tick embedded in feathers of an indigo bunting (Passerina cyanea). (E) Cloaca of a wood thrush (Hylocichla mustelina). (F) Ticks attached to the cloaca of a wood thrush (H. mustelina). (G) Partially fed tick attached to the right eye of an indigo bunting (P. cyanea).

Fig 2

Identified tick species collected from migratory songbirds.

Fig 3

(A) PCR amplification of spotted fever group Rickettsia spp. in exotic ticks collected from migratory birds. Lane 1: low DNA mass ladder; lanes 2, 4, 6: empty lanes; lane 3: no-template control; lane 5: Rickettsia ompA-positive control; lanes 7–19: rickettsial ompA gene amplified from extracted tick DNA using gene-specific primers in a nested PCR assay. (B) PCR amplification of spotted fever group Rickettsia spp. in migratory songbird blood samples. Lane 1: low DNA mass ladder; lanes 2, 4: empty wells; lane 3: no-template control; lane 5: Rickettsia ompA-positive control, lanes 6–19: rickettsial ompA gene fragment amplified from extract bird blood DNA using gene-specific primers in a nested PCR assay.

Fig 4

Phylogenetic relationships of the rickettsial strains detected in ticks based on partial ompA gene sequences and inferred by Bayesian analysis. Number near nodes represents posterior probability support.