Date of Graduation

12-2021

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Poultry Science (PhD)

Degree Level

Graduate

Department

Poultry Science

Advisor/Mentor

Hargis, Billy M.

Committee Member

Tellez, Guillermo

Second Committee Member

Vuong, Christine N.

Third Committee Member

Rochell, Samuel J.

Keywords

blackhead disease; Histomonas meleagridis; histomoniasis; quinine; turkey; vaccine

Abstract

Histomonas meleagridis is the etiological agent of histomoniasis, also commonly known as blackhead disease. This protozoal disease of poultry is detrimental to turkeys with flock mortalities often reaching 80-100%, although other gallinaceous birds are susceptible. Since the voluntary removal of nitarsone in 2015, the poultry industry is suffering with no approved prophylactics, therapeutics, or vaccines for this disease. The objectives of this dissertation were to evaluate multiple methods for prevention or control of histomoniasis, including dietary chemoprophylaxis and vaccination. Specifically, this research evaluated quinine as a chemoprophylactic candidate (Chapter 3) or live-attenuated H. meleagridis as vaccine candidates (Chapter 4) in an experimental challenge model. Quinine is an antiprotozoal phytochemical that has previously shown efficacy against Plasmodium spp., the etiological agent of poultry malaria; therefore, quinine was hypothesized to confer antihistomonal properties. Quinine effectively reduced (P < 0.05) viable histomonads in vitro but did not mitigate histomoniasis when evaluated in vivo, as indicated by similar (P > 0.05) post-challenge body weight gain (BWG), mortalities, and lesion scores (LS) as compared to the positive-challenged control (PC) group. Taken together, these data suggested that phytochemicals should be further evaluated against histomoniasis but both in vitro and in vivo studies should be conducted against multiple isolates. Four experiments were conducted to evaluate age, route, and administration dose of selected live-attenuated H. meleagridis isolates as vaccine candidates (Vacc) against homologous or heterologous wild-type H. meleagridis (WTH) challenge in turkeys. Day-of-hatch administration of Vacc did not confer protection against subsequent WTH-challenge; however, WTH-challenge at day-of-hatch via the oral route induced histomoniasis whereas oral challenge at d21 did not induce disease. When administered intracloacally at d14 at a dose of 2 x 105 cells/turkey, the Vacc isolates generally resulted in reduced (P < 0.05) disease severity during Challenge Phase, as indicated by lower mortalities, lower LS, and improved BWG as compared to the PC group. During Vaccination Phases, the Vacc groups resulted in lower LS and mortalities (P < 0.05) as compared to the PC group and were not different (P > 0.05) as compared to the non-challenged control group, indicating the reduced virulence and apparent safety of Vacc isolates for application to turkeys. Furthermore, d14 intracloacal vaccination offered some protection against homologous and heterologous WTH-challenge. Overall, this research demonstrated vaccination against histomoniasis to be possible although protection was not robust.

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