How can we mitigate blackhead disease in turkeys? New research points to reducing stressors like feed withdrawal and "low-density" or low protein diets. Surprisingly, preventing low-level aflatoxin contamination of feed -- a common stress factor in other poultry species -- does not appear to affect infection or transmission of Histomonas meleagridis. Moreover, stressors associated with bird-to-bird transmission remain a mystery.
Blackhead disease or histomoniasis causes extreme morbidity and mortality in turkeys -- up to 100% flock loss -- thanks to a vector-borne, anaerobic protozoan once treated effectively with nitroimidazoles, roxarsone, and nifursol. However, with the last of these therapeutic drugs outlawed for food animal use in 2016, blackhead disease is on the rise, resulting in more than 700 outbreaks from 2016 to 2022. Histomoniasis also affects chickens, quail, and other gallinaceous birds, but turkeys are most susceptible.
Rates of morbidity and mortality in turkey flocks are unpredictable. Researchers do not yet understand the factors underlying "lateral" transmission of the disease, that is infection resulting from bird-to-bird contact or contact with infected cecal matter in the absence of an obvious vector. So they are particularly interested in stressors that increase disease risk as occurs in other poultry production systems, such as feed withdrawal, microbial infections, transportation, and aflatoxin-contaminated feed.
Previous research pointed to feed restriction and feed composition -- especially protein level and amino acid balance -- acting as stress factors to increase blackhead disease infection and progression. Interestingly, while coccidiosis exacerbated histomoniasis in chickens, an early study suggested it decreased lateral transmission and severity of the disease in turkeys. Thus, researchers have sought to understand in what ways different stressors influence H. meleagridis lateral transmission and infection, which is vital to blackhead disease control and outbreak management in commercial turkey production today.
In this study, a collaborative group of academic and industry researchers conducted three experiments at the Talley Turkey Education Unit research facility at North Carolina State University. Turkeys in Experiment 1 (n = 420) and Experiment 2 (n = 150) were day-of-hatch male Nicholas poults from a commercial Aviagen hatchery in West Virginia. Experiment 3 used five-week-old Nicholas female turkeys (n = 180) donated by a Prestage Farms brooder facility in North Carolina.
Experiments 1 and 3 used floor pens with pine shavings while Experiment 2 used battery cages with brooder paper and pine shavings. Housing temperature and management followed Aviagen guidelines. Transcloacal inoculations of H. meleagridis came from field isolates of blackhead disease outbreaks in Arkansas, Georgia, and Michigan. Experiments 1 and 3 terminated at 10 weeks of age, Experiment 2 at 6 weeks of age. Researchers recorded and necropsied all mortalities to determine cause of death.
In Experiment 1, the 18-hour feed withdrawal (FW) treatment resulted in 72% infection rate while high electrolytes and low-density (reduced protein) diets (HE + LD) had 73% infection rate, both of which were higher than all other stressors (transportation stress, cold stress, delayed placement). The FW treatment resulted in highest mortality (66%) and both FW and HE + LD yielded liver and ceca scores higher than other stressors, "demonstrating an increased histomoniasis severity following these stressors," the researchers noted.
They referenced previous studies showing that feed withdrawal also increased the risk of dysbiosis and overgrowth of Escherichia coli and Salmonella typhimurium, which might facilitate the H. meleagridis growth. In this experiment, the researchers "did not see significant effects on H. meleagridis infections" resulting from transportation stress, cold stress, or delayed placement: "More research is needed to understand the possible impacts of these stressors during a histomoniasis challenge."
Previous research suggested that feed formulation could impact blackhead disease infection rates. Given the impact of the HE + LD diet on histomoniasis in Experiment 1, the researchers used similar high-electrolyte and low-density diets combined with the stress factors in Experiment 2. All low-density diet treatments generated high infection rates (80-100%) along with higher mortality.
"The addition of cocci (Eimeria tenella)," the researchers reported, "led to a significant increase in the liver score (compared to) the birds provided an LD diet alone." However, they also noted that Experiment 2 did not demonstrate lateral transmission of histomoniasis resulting from coccidiosis infection. "The reason is unclear."
Again using the known stressor of low-density diets, in Experiment 3 the researchers tested the effects of naturally occurring aflatoxins (AFLB1 at 0-19.82 ppb). At these relatively low levels, they found no significant differences for histomoniasis infection rate, mortality rate, liver scores, or cecal scores. However, they acknowledged that aflatoxins at higher concentrations could cause liver damage and disrupt immune function in turkeys.
The researchers cautioned that this study looked only at aflatoxins: "Further research should consider the effects of low levels of multiple mycotoxins in feed as a factor affecting histomoniasis."
What does this study mean for producers?
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With blackhead disease -- histomoniasis -- on the rise, focus on stress reduction to support prevention and control.
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Avoid feed restriction or withdrawal and low-density or low-protein diets.
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Don't worry too much about low-level aflatoxin contamination of feed, although total mycotoxin load may cause problems.
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Keep in mind that cocci, E. coli, and salmonella can complicate blackhead disease.
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Look for future research on factors underlying bird-to-bird transmission.
The full paper, titled "Role of stressors in histomoniasis transmission and development in turkeys" can be found in Applied Poultry Research and online here.
DOI: 10.1016/j.japr.2024.100405