Location: Animal Disease Research2012 Annual Report
1a. Objectives (from AD-416):
The primary objective of this proposal is to answer basic questions regarding population structure of cattle fever ticks and prevalence of Babesia sp. in South Texas. The following questions and hypotheses will be addressed: 1) Do cattle fever ticks in the Texas quarantine zone demonstrate population structure, either locally or across the entire landscape? H1: Limited gene flow leads to high levels of population structure and is due to barriers to host dispersal (e.g. fences restrict host movement). H2: Widespread gene flow leads to low levels of population differentiation because of widespread host movement, or other mechanisms of dispersal. 2) Are cattle fever ticks on non-bovine ungulates a source of ticks on cattle? H1: Ticks use hosts indiscriminantly, and thus display no genetic structure across host species. H2: Ticks form separate populations on cattle and other wild ungulates, with little genetic admixture across host types (possibly leading to host-race formation). 3) What geographic sources gave rise to the populations(s) of cattle fever ticks in Texas, and have multiple introductions occurred? H1) Cattle fever ticks in Texas originated from a single introduction event from Mexico, have become permanently established in the quarantine zone, and display a limited subset of the total genetic variation found in Mexico. H2) Texas ticks originate from a wide variety of locations, are continually reintroduced, and retain most of the genetic variation found in Mexico. 4) What is the source of new cattle fever tick populations found outside of the permanent quarantine zone? H1) Recent tick outbreaks originate from nearby areas of the permanent quarantine zone, and therefore geographic proximity can be used to predict genetic similarity. H2) New tick outbreaks originate from random locales within the quarantine zone or from Mexico, and thus geographic proximity does not correlate with genetic similarity. 5) Do cattle fever ticks collected from cattle and non-bovine ungulates within the Texas quarantine zone carry B. bovis and B. bigemina? H1) Babesia has re-emerged in South Texas, and is found in ticks on cattle and/or wild ungulates in this region. H2) Babesia has not re-emerged in South Texas, and is not found in ungulates or cattle in this region. 6) What is the geographic origin of Babesia populations in South Texas? H1) The most likely source of Babesia in the U.S. is Mexico, resulting in high similarity between any US Babesia and pathogen strains from Mexico. H2) Babesia is transported globally, and US Babesia are not similar to strains from Mexico. 7) Do Babesia strains in the permanent quarantine zone demonstrate population structure? H1) B. bovis and B. bigemina are disseminated widely across the permanent quarantine zone, resulting in low levels of population genetic structure. H2) B. bovis and B. bigemina originate from multiple locations but have limited dispersal, resulting in strong population structuring across the quarantine zone.
1b. Approach (from AD-416):
Cattle fever ticks collected from within the permanent and temporary preventive quarantine zone and those provided as part of a collaborative effort with scientists in Mexico will be used as the basis for the proposed tick population genetic studies. Genomic DNA will be isolated from individual tick specimens and used to amplify molecular markers, both published and identified as part of the proposed project, the results of which will be analyzed using standard population biology algorithms. In addition, genomic DNA will be isolated from collected semi-engorged and engorged females for use in amplification of markers specific for Babesia bovis and B. bigemina, to assess the prevalence of the pathogen in South Texas. Existing markers will be evaluated, but the proposed project also aims to identify new species-specific markers. Isolates will be compared between counties of South Texas and also with states within Mexico to determine whether geographic differences can be identified.
3. Progress Report:
This work relates to objective 1 of parent project by provision of data supporting improved control methods for babesia bovis. Our collaborators at Northern Arizona University have begun wrapping up this project and manuscripts are in preparation on population genetic structure of outbreak populations of Rhipicephalus microplus and R. annulatus ticks in Texas. The results reveal the existence of strong genetic structure among tick populations in the Texas quarantine zone. The DNA fingerprinting markers developed in this project will allow us to run population assignment tests to reveal the source of ticks intercepted by USDA-APHIS. Importantly, these methods have allowed us to conclude that ticks collected from cattle and white-tailed deer in the same pasture share a common gene pool, which is consistent with the possibility that deer may be a source of ticks on cattle. Methods to detect and type B. bovis and B. bigemina infection in ticks have also been developed. These genotyping approaches will provide important insights into tick and pathogen dispersal and gene flow in the Texas quarantine zone. The work conducted here at the Animal Disease Research Unit to assess biological differences between populations of R. microplus ticks originating from Deer and from Cattle has also been concluded and a manuscript is also in preparation. Although ticks from either deer or cattle appear to be equally competent vectors of B. bovis, there are small but statistically significant differences between deer and cattle tick populations in feeding and oviposition success, most significantly, the tick population originating from deer had a small fitness advantage over the population originating from cattle. Although these population differences were small, and may have been confounded by isolation by distance and/or founder effects, they demonstrate that populations of R. microplus can differentiate from one another and suggest that small selection pressures could result in adaptation of these isolated outbreak populations.