The turkey, compared to the chicken, fails to mount an effective early immune response to Histomonas meleagridis in the gut.

Histomonosis is a disease of poultry caused by Histomonas meleagridis. Chickens usually recover while the mortality rate in turkeys is high. The immunological response of both species towards H. meleagridis was investigated. Parasites migrated in greater numbers to the turkey liver compared with that of chicken. Chicken mounted an effective caecal innate response, with increased expression of IL-1beta, CXCLi2 and IL-6 mRNA, resulting in control of parasite numbers. The turkey failed to mount such an effective innate response in the caecal tonsil, allowing greater numbers to migrate to the liver, where a sustained, uncontrolled immune response was mounted, evidenced by the upregulation of mRNA for IL-1beta, CXCLi2, IFN-gamma, IL-13, IL-4 and IL-10. Expression levels of mRNA of the chicken and turkey beta-defensin AvBD2 suggest that this response was not limited to the cytokines. There was an influx of CD4+, CD8alpha+, CD28+ and CD44+ cells into the livers of both species, coinciding with parasite movement. These influxes were more pronounced in the turkey, correlating with a decrease in numbers of the same cells in the spleen, which was not observed in the chicken. Antibody levels in the chicken increased more than those in the turkey, supporting evidence of an adaptive response.