Reducing Losses from Blackhead Disease23 April 2014
New research from the University of Georgia reveals that chicken litter may pose a risk of transmitting blackhead to turkey flocks, while their work to vaccinate birds against the disease was unsuccessful.
Drs Robert Beckstead and Larry R. McDougald of the University of Georgia report that they used molecular techniques were used to track outbreaks of Blackhead Disease (histomoniasis), 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 categorised by molecular markers.
The objectives of the research, sponsored by the US Poultry & Egg Association, 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 and
- establish a vaccination protocol to stimulate protective immunity in turkeys.
The data from Athens 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 this data, litter from breeder or layer pullets is likely to contain caecal worm (Heterakis galllinarum) eggs, the known vector for H. meleagridis, is the most likely source of infection in turkeys.
Although this confirms earlier work, Beckstead and McDougald stress it emphasises 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 per cent mortality in chickens in the lab, highlighting the potential of this parasite to devastate chicken flocks as well as turkeys.
The researchers report they 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 this group failed to produce adequate timely protection in turkeys. Although the researchers 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, these results do not support the use of a vaccination to prevent blackhead.
This study is the first of its kind funded in the recent years, according to Beckstead and McDougald.
It is clear from the results that a significant part of the biology of H. meleagridis and its interaction with chickens and turkeys is poorly understood. Based on their data, there are many local reservoirs of infection in the environment, increasing the difficulty in preventing blackhead outbreaks.
Careful consideration should be taken when spreading litter from chicken farms near turkey facilities, they continue.
More research is needed to identify insects or other mechanical carriers responsible for survival and spread of the blackhead.
Identification of strains with high virulence in chickens suggests we should use caution in spreading litter from these farms near other poultry operations, Beckstead and McDougald concluded.
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