Bird-tick and human-tick encounters in the Rio Grande Valley (Texas, USA): ecological associations and pathogen detections.

Abstract

Background: The tropical climate and diverse vector community allows the Rio Grande Valley (RGV) of South Texas to support many vector-borne pathogen transmission cycles. It is a key area for monitoring bird ticks, since most of the migratory birds fly through this corridor to move for south tropical latitudes. Some of the tick species that infest birds in Texas can also transmit tick-borne pathogens that concern public health.

Methods: During bird banding activities in 2019-2024, ticks were collected opportunistically from local and migrant birds, as well as from outdoor recreationalists, to explore the presence of tick-borne pathogens. Applying a polymerase chain reaction (PCR)-DNA sequencing approach, ticks were tested for Ehrlichia and Rickettsia species.

Results: Of 375 ticks, eight tick species were identified, including species regarded as locally established (Amblyomma inornatum, Amblyomma maculatum, Amblyomma mixtum, Amblyomma tenellum, and Dermacentor variabilis), neotropical species imported by migratory birds (Amblyomma geayi and Amblyomma longirostre), and for the first time in Texas, Ixodes keiransi, formerly the North American lineage of Ixodes affinis. Amblyomma tenellum was the most abundant tick species (89.3%). All ticks were screened for Ehrlichia, resulting in Ehrlichia chaffeensis detection in three A. tenellum ticks (one nymph and two adults) found on humans, and one positive for Ehrlichia ewingii in an A. inornatum nymph collected from a Clay-colored Thrush (Turdus grayi). Both bacteria can cause human ehrlichiosis, which is infrequently reported in Texas. The Rickettsia screening of ticks resulted in detection of Rickettsia amblyommatis, a potentially pathogenic spotted fever group Rickettsia, in nine ticks: eight A. inornatum ticks (one larva, five nymphs and two adults), seven of which were collected from Long-billed Thrashers (Toxostoma longirostre); and an A. longirostre engorged nymph from an Acadian Flycatcher (Empidonax virescens).

Conclusions: Our results highlight the importance of occupational exposure to ticks and the potential public health impact of the relatively neglected human-biting vector, A. tenellum.. There is also a critical need to investigate the fate of bird-imported A. inornatum and A. longirostre, and the pathogens they carry.