Long-term protection of turkeys with a live clonal monoxenic <i>Histomonas meleagridis</i> vaccine

Histomonosis, caused by the protozoan parasite <i>Histomonas meleagridis</i>, is a major concern in turkey production due to the lack of licenced drugs and vaccines. Despite various studies on experimental vaccination, the duration of immunity of such a vaccine remains unclear. This study evaluated the long-term efficacy of an attenuated clonal monoxenic <i>H. meleagridis</i> culture in turkeys, focusing on its protective effects. Day-old turkeys were vaccinated orally using a frozen vaccine culture directly, without additional multiplication, and challenged 12 weeks later. The vaccine caused no adverse clinical signs, consistent with prior studies. Instead, vaccinated birds had an improved weight gain and higher body-weight at 42 days. Vaccine uptake was confirmed by the detection of histomonad DNA in faeces starting 14 days post-vaccination, coinciding with the first sampling time point, with 60-70% of birds testing positive by 49 days. Considering all sampling time-points before the challenge, every sampled vaccinated turkey secreted histomonads at least once. Following the challenge, analysed clinical scores showed a more than 20-fold reduction in disease severity in vaccinated birds compared to controls, and survival rates were remarkably higher in the vaccinated group (90%) than in non-vaccinated controls (16%). Overall, this study supports the long-term efficacy of the attenuated <i>H. meleagridis</i> vaccine, providing robust protection against histomonosis, reducing severity of clinical signs and a significant reduction of mortality, organ lesions as well as parasite burden. The vaccine's effectiveness, when administered at day-old, highlights its potential to prevent histomonosis, though challenges remain for widespread use in commercial turkey farming.<b>RESEARCH HIGHLIGHTS</b>Long-term efficacy of attenuated clonal monoxenic <i>H. meleagridis</i> vaccine was evaluated in turkeys.Long-term protection: Vaccine-protected turkeys from histomonosis for up to 84 days.Survival: 90% of vaccinated turkeys survived vs. 16% of non-vaccinated birds.Body-weights: Vaccinated birds weighed more, at 42 and 91-105 days of life.Lesions: Fewer liver/caecal lesions in vaccinated birds.