Blackhead Disease (Histomoniasis): Reducing Losses Through Molecular Tracking And Immunization

Molecular techniques were used to track outbreaks of Blackhead Disease, to correlate these outbreaks with specific isolates of Histomonas meleagridis, and to identify genes associated with the virulence of specific isolates. Additionally, the project tested whether a vaccination approach could be used to protect turkeys from this disease. Isolates from outbreaks in turkeys, broiler breeder chickens, layer chickens and bobwhite quail were tested for virulence and categorized by molecular markers.

The objectives of the research were to:

• Survey H. meleagridis field isolates from turkeys, broiler breeder pullets, and other wild birds for variation in virulence. Correlate virulence to molecular markers that classify H. meleagridis stains.
• Investigate the epidemiology of field isolates obtained from H. meleagridis outbreaks using molecular markers. Identify potential disease reservoirs and modes of transport of H. meleagridis between farms.
• Establish and validate a molecular diagnostic test for H. meleagridis.
• Establish a vaccination protocol to stimulate protective immunity in turkeys.

Our data confirm that there are multiple disease reservoirs and that most outbreaks of blackhead arise from distinct isolates of H. meleagridis. Isolates varied considerably in virulence, sensitivity to nitarsone (Histostat-50) and expression of virulence genes.

Variations in mortality and morbidity in outbreaks in the field was a result of the virulence of the associated isolate, particularly in chickens. Based on our data, litter from breeder or layer pullets is likely to contain cecal worm (Heterakis galllinarum) eggs, the known vector for H. meleagridis, is the most likely source of infection in turkeys. Although this confirms earlier work, it emphasizes the increasing infection pressure resulting from overlapping areas of broiler breeder chicken farms and turkey farms. One strain of H. meleagridis isolated from layer pullets resulted in 17 percent mortality in chickens in the lab, emphasizing the potential of this parasite to devastate chicken flocks as well as turkeys.

We have designed a new molecular-based method to diagnosis H. meleagridis in samples obtained from tissue. This PCR-based method is specific to H. meleagridis and allows quick diagnosis without costly DNA sequencing. Vaccination approaches using attenuated live and killed preparations tested by our group failed to produce adequate timely protection in turkeys. Although we saw a delay in the onset of the disease in birds given live attenuated H. meleagridis, complete protection was never attained. While this approach was reported as successful in the literature, our results do not support the use of a vaccination to prevent blackhead.