Nilgai antelope display no signs of infection upon experimental challenge with a virulent Babesia bovis strain

Authors: JOHNSON, TAMMI - Texas A&M Agrilife; PERSINGER, KELLY - Texas A&M Agrilife; Taus, Naomi; Davis, Sara; Poh, Karen; Kappmeyer, Lowell; LAUGHERY, JACOB - Washington State University; CAPELLI-PEIXOTO, JANAINA - Washington State University; Lohmeyer, Kimberly - Kim; Ueti, Massaro; Olafson, Pia

Submitted to: Parasites & Vectors
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/3/2024
Publication Date: 6/1/2024
Citation: Johnson, T.L., Persinger, K.A., Taus, N.S., Davis, S.K., Poh, K.C., Kappmeyer, L.S., Laughery, J.M., Capelli-Peixoto, J., Lohmeyer, K.H., Ueti, M.W., Olafson, P.U. 2024. Nilgai antelope display no signs of infection upon experimental challenge with a virulent Babesia bovis strain. Parasites & Vectors. https://doi.org/10.1186/s13071-024-06316-3.
DOI: https://doi.org/10.1186/s13071-024-06316-3

Interpretive Summary: Bovine babesiosis can cause mortality in cattle that are naïve to Babesia bovis, the pathogen that causes the disease. Babesia bovis is transmitted by the southern cattle tick, which primarily feeds on cattle but is known to use nilgai antelope as an alternative host. Since nilgai share the landscape with cattle in south Texas, it is important to understand whether nilgai have a role in the bovine babesiosis disease transmission cycle. In this study, we tested two groups of nilgai antelope for susceptibility to infection by Babesia bovis. One group was inoculated with B. bovis-infected blood and the other group was inoculated with a preparation of B. bovis-infected larval ticks. In each experiment, a beef calf was included as a positive control. Nilgai in both groups did not display clinical signs of infection, and all nilgai were negative for B. bovis using DNA-based and antibody-based diagnostic assays. In contrast, the beef calves had clinical signs of infection (i.e., fever, anemia) and were positive for B. bovis using diagnostic assays. Based on these collective results, nilgai do not appear to be susceptible to infection with a strain of B. bovis that is lethal to cattle. Tick control on these alternative hosts remains a critical priority, especially given their potential to disseminate ticks long distances.

Technical Abstract: Background: Bovine babesiosis is caused by infection with the protozoal parasite Babesia bovis, which is transmitted by Rhipicephalus (Boophilus) spp. It can cause mortality rates up to 90% in immunologically naive Bos taurus cattle. In south Texas, R. (B.) microplus is known to infest nilgai antelope (Boselaphus tragocamelus); however, their susceptibility to infection with B. bovis and their role in the transmission of the parasite remain unknown. In this study, we challenged nilgai antelope with B. bovis and evaluated their susceptibility to infection. Methods: Nilgai were needle inoculated with ˜10^8 B. bovis-parasitized erythrocytes (merozoites) or a homogenate of B. bovis-infected larval ticks (sporozoite) delivered intravenously. Bos taurus beef calves were inoculated in parallel, as this strain of B. bovis is lethal to cattle. Temperature and hematocrit were monitored daily over the course of each study, and whole blood was collected for molecular (PCR) and serological (indirect ELISA) diagnostic evaluation. Histological sections of nilgai cerebral tissue were examined for evidence of infection. Recipient bovine calves were sub-inoculated with blood from nilgai challenged with either stage of the parasite, and they were monitored for clinical signs of infection and evaluated by a PCR diagnostic assay. Red blood cells (RBCs) for pre-challenge nilgai and B. taurus beef cattle were cultured with an in vitro B. bovis merozoite culture to examine colonization of the RBCs by the parasite. Results: Nilgai did not display clinical signs of infection upon inoculation with either the merozoite or sporozoite stage of B. bovis. All nilgai were PCR-negative for the parasite, and they did not develop antibodies to B. bovis. No evidence of infection was detected in histological sections of nilgai tissues, and in vitro culture analysis indicated that the nilgai RBCs were not colonized by B. bovis merozoites. Cattle sub-inoculated with blood from challenged nilgai did not display clinical signs of infection, and they were PCR-negative up to 45 d after transfer. Conclusion: Nilgai do not appear to be susceptible to infection with a strain of B. bovis that is lethal to cattle. Tick control on these alternative hosts remains a critical priority, especially given their potential to disseminate ticks long distances.