Movement ecology of exotic nilgai antelope: A threat to the re-emergence of cattle fever ticks in the southern USA

Authors: SLIWA, KATHRYN - Texas A&M University; BAUMGARDT, JEREMY - Texas A&M University; DEYOUNG, RANDY - Texas A&M University; ORTEGA, ALFONSO - Texas A&M University; HEWITT, DAVID - Texas A&M University; Goolsby, John; Lohmeyer, Kimberly - Kim

Submitted to: Ecosphere
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/28/2022
Publication Date: 2/22/2023
Citation: Sliwa, K.M., Baumgardt, J.A., Deyoung, R.W., Ortega, A.J., Hewitt, D.G., Goolsby, J., Lohmeyer, K.H. 2023. Movement ecology of exotic nilgai antelope: A threat to the re-emergence of cattle fever ticks in the southern USA. Ecosphere. https://doi.org/10.1002/ecs2.4401.
DOI: https://doi.org/10.1002/ecs2.4401

Interpretive Summary: Cattle fever ticks (CFT) Rhipicephalus microplus and R. annulatus are invasive livestock pests that are endemic to Mexico and invasive along the Texas – Mexico border. Acaricide resistance, alternate wildlife hosts, and pathogenic landscape forming weeds present challenges for sustainable eradication of this pest in the U.S. CFT are the vector for bovine babesiosis, a lethal disease causing high mortality particularly in cattle. Efforts to eradicate CFT from the United States have been successful; however, in recent years, there has been an increase in CFT infestations outside of the Permanent Quarantine Zone in Texas. One of the alternate wildlife hosts for CFT in South Texas are nilgai (Boselaphus tragocamelus), an exotic Asian bovid. Nilgai are highly mobile with large home ranges and are implicated in the spread of CFT, through the landscape. In this study of nilgai in Cameron County, TX that were fitted with satellite GPS collars we observed varied movement patterns including some that stayed place on the ranches during the entire study (21/40), some were seasonal movers (6/40), and some dispersed long distances from the study ranches (3/40). Two young female nilgai made separate, long-distance movements, where each traveled about 25 miles from their initial capture location. Overall, nilgai in the study had large and highly variable home ranges: annual median home range estimate for females was 1400 acres and for males was 2300 acres. Peak nilgai activity occurred during early morning and evening hours, and median hourly movement for females. This information will help develop effective methods for treatment of nilgai infested with cattle fever ticks.

Technical Abstract: Background: Wildlife play an important role in the emergence of livestock diseases and their movements can complicate disease management efforts. One of the most significant vector-borne diseases of livestock worldwide is bovine babesiosis, spread by the cattle fever ticks (CFT), Rhipicephalus (=Boophilus) microplus (Cannestrini) and R. (B.) annulatus (Say). Although CFT were eradicated from the U.S. by 1943, bovine babesiosis and CFT are prevalent in Mexico. Recently, the management of CFT in the Texas-Mexico region has been complicated by the presence of free-ranging exotic, nilgai antelope (Boselaphus tragocamelus). Nilgai are abundant, a competent host for CFT, and make long-distance movements. The goal of this study was to better understand nilgai movements and space use to inform CFT treatment strategies. Methods: We analyzed hourly locations from 40 GPS-collared nilgai in Cameron County, TX, USA, from April 2019–September 2020. We assigned each nilgai a movement behavior using the net squared displacement metric. We estimated nilgai home range sizes at different temporal scales (monthly, seasonally, and overall) using Brownian bridge movement models. We calculated average movement metrics, activity patterns, and space use of nilgai using the Euclidean distance between consecutive locations. Results: We observed movement patterns consistent with residency (21/40), seasonal movers (6/40), dispersal (3/40), and unclassified (10/40). Two young females made separate, long-distance movements, where each traveled about 40 km from their initial capture location. Overall, nilgai had large and highly variable home ranges: annual median home range estimate for females was 563 ha (range = 105–1,545) and for males was 937 ha (range = 221–1,602). Peak nilgai activity occurred during crepuscular hours, and median hourly movement for females was 57 m/hr and for males was 66 m/hr.