Submitted to: Parasites & Vectors
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/10/2013
Publication Date: 10/23/2013
Citation: Scoles, G.A., Ueti, M.W. 2013. Amblyomma cajennense is an intrastadial biological vector of theileria equi. Parasites & Vectors. 6:306.
Interpretive Summary: Equine piroplasmosis (EP) is a disease of horses and other equines that is endemic throughout most of the world. Prior to the outbreak in Texas in 2009 the United States was considered to be free of this parasite. Occasional cases occurred in the U.S. but these were attributed to false negative serological tests that allowed infected horses to be imported. There was no evidence for endemic vector-borne transmission in the U.S. until the 2009 outbreak in Texas involved more than 270 horses. This outbreak appears to have been the result of endemic tick-borne transmission by the Cayenne tick, Amblyomma cajennense, and the American Dog tick Dermacentor variabilis. Dog ticks are common throughout the U.S., but cayenne ticks are found only in southern Texas and south throughout various parts of Central and South America. Although the cayenne tick is suspected to be a vector throughout South America based on epidemiological evidence, experimental transmission has not been demonstrated. In this study we demonstrate that the cayenne tick is a competent biological vector if T. equi when adult ticks acquisition feed on an infected host and then move to a susceptible host for a subsequent feeding (intrastadial transmission). We were unable to transmit with dog ticks using this route and neither species transmitted when acquisition fed on an infected horse as nymphs and subsequently re-fed on susceptible horses after molting to the adult stage (transstadial or interstadial transmission). Based on these results we propose that the most likely natural mode of transmission for this parasite/vector combination in the Texas outbreak would have been the result of adult cayenne ticks moving between infected and uninfected hosts. It appears that A. cajennense is a natural intrastadial vector of T. equi.
Technical Abstract: Background: The apicomplexan hemoprotozoan parasite Theileria equi is one of the etiologic agents causing equine piroplasmosis (EP), a disease of horses and their relatives that are endemic throughout large parts of the world. Prior to 2009 the United States had been considered to be free of this parasite. Although occasional cases have occurred in the U.S. there was no evidence for endemic vector-borne transmission until a 2009 outbreak in Texas in which Dermacentor variabilis and Amblyomma cajennense were implicated as vectors. Although D. variabilis had previously been shown to be a competent laboratory vector, previous studies suggested A. cajennense was not a competent transstadial vector, even though epidemiologic evidence from South America suggests that presence of this species is correlated with higher prevalence of infection. In this study we test the transstadial and intrastadial vector competence of D. variabilis and A. cajennense for T. equi. Results: Amblyomma cajennense transmitted T. equi intrastadially when adult ticks acquired infection by feeding on an infected horse, and transmitted to a naïve host on subsequent reattachment and feeding, whereas Dermacentor variabilis failed to transmit in the same experiment. Transstadial transmission (nymphal acquisition and adult transmission) was not successful for either tick species. In addition, PCR on DNA isolated from eggs of females that had fed on an infected horse suggests that there is no transovarial passage of this parasite by either of these species. Conclusion: This work confirms that ticks from the Texas population of A. cajennense are competent intrastadial vectors of T. equi. We propose that the most likely natural mode of transmission for this parasite/vector combination in the Texas outbreak would have been biological transmission resulting from adult male ticks moving between infected and uninfected horses. The intrastadial mode of transmission should be considered as one equally possible scenario whenever implicating vectors of T. equi.