The_Mountaineer
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Introduction
As many folks are gearing up for hunting spring gobblers, I thought it might be worthwhile to discuss one of most important diseases wild turkeys face. Most game departments and other agencies that deal with wildlife are quick to announce any instances of disease they are aware of prior to the season opener as well as during season. As such, it is worthwhile to check news releases made by such agencies in your hunting area. Generally, however, wild turkeys populations are healthy and don’t suffer from a wide range of diseases. Indeed, the most recent edition of the Field Manual of Wildlife Diseases in the Southeastern United States lists less than a dozen diseases for turkeys (though there are more). Still, hunters may come across some of these maladies in the turkeys they harvest ranging from ticks and lice to pox and lesions. To try and discuss each of these would take some time. Rather than confuse and worry hunters about all the diseases wild turkeys face, I think that a discussion of a disease that is pretty significant to turkey management is the best idea. This disease is known as histomoniasis or blackhead disease.
Importance
The reason I feel that histomoniasis is important for wild turkey management is because of its high rate of mortality. Mortality from blackhead disease runs fairly high compared to other turkey diseases. As much as 75% of wild turkeys suffering from histomoniasis will die. Rates of infection among wild turkey populations remain largely unknown however.
What causes it?
Blackhead disease is ultimately caused by a protozoan parasite known as Histomonas meleagridis - a microscopic single celled organism.
Histomonas meleagridis protozoans shown at the microscopic level
(Photo Courtesy of University of Minnesota – SVM)
The life cycle of this organism is fairly complex and requires an intermediate host. This intermediate host is a parasitic worm known as Heterakis gallinarum.
Heterakis gallinarum – a parasitic worm is an intermediate host in spreading blackhead disease among wild turkeys. (Photo Courtesy of UC Davis Biology of Parasitism and M. Eyre)
Hetakis gallinarum is a nematode that is ingested and often found residing in the turkeys’ caeca (pronounced SEE-kuh); sack-like digestive structures that are attached to the small intestine and thought to aid in water and protein absorption as well as microbial decomposition of fiber. The Histomonas meagridis protozoan often infects the ovaries and hence the fertilized eggs of Hetakis gallinarum. Once laid, the resulting infected worm eggs develop into larvae and release the protozoans. These quickly multiply inside the caeca where they attack the surrounding tissues. They also travel to and attack the liver.
The whole process is fairly complicated as there are intermediate vectors (i.e., hosts) between the original causative agent Histomomas meleagridis and the host turkey. Earthworms, a common and nutritious turkey food, are known hosts to nematodes including Heterakis gallinarum and can thus lead to blackhead disease in turkeys. Droppings from turkeys and especially other birds, both wild and domestic, can contain the full spectrum of hosts – the Histomonas mealgridis protozoan, the cecal worm Hetakis gallinarum, its infected and un-infected eggs as well as larvae.
Life cycle of Heterakis gallinarum nematode (Drawing courtesy of UC-Davis Biology of Parasitism, redrawn from Olsen 1974 pg 429 & 449)
How do I identify Blackhead Disease?
Symptoms for histomoniasis come on quickly, usually within 12 days. Infected turkeys have a general “ragged” look to them. Droopy wings, unkempt feathers, lethargic behavior and movement as well as yellow droppings can signal blackhead disease.
If you have already harvested a turkey, one good identifying sign is a liver with bullseye lesions as shown below, though other diseases can also cause similar symptoms in the liver.
Lesions found on the liver of a turkey with blackhead disease
(Photo courtesy of Michigan Department of Natural Resources)
Another sign are the diseased caeca, which may be difficult to locate for the untrained eye. The diseased caeca grow thick walls, has lesions and erosions on the inside and often contain a yellow-green paste.
Can it hurt people?
Blackhead disease, like many wildlife diseases, is of no known threat to people. Still, precautions should be taken. Using gloves and care, clean your bird quickly and thoroughly so as to prevent spoilage. If you notice anything that might suggest the bird is diseased, promptly report it to your wildlife agency and take the bird with you, entrails and all. Often, if it can be proven that your bird is diseased a wildlife agency will issue you a replacement tag for the bird. Even if they don’t, it’s important that you as an ethical hunter report the incident to wildlife professionals so they may be able to note and monitor the disease. Hunters have a lot more eyes and ears out in the field than wildlife agencies do, at least during season!
Management Implications
Domestic poultry populations in the USA suffering from blackhead disease can be treated with a 0.01875% nitrasone feed up to 5 days before they are sent to market. Essentially, this works as a prophylaxis for the protozoan – stopping the spread but not curing the bird. Conversely, like most wildlife diseases, it is neither feasible nor possible to intervene once blackhead disease has struck a wild turkey population. Rather, the best management plan is prevention.
Baiting is perhaps one of the biggest threats to wild turkeys. It unnaturally concentrates turkeys into small areas and with any concentration of animals, increases the propensity for disease to break out. Baited areas will have higher amounts of droppings and thus increase the risk of ingestion and infection. Thankfully, the practice of baiting is illegal in most areas.
Wild turkeys should not be in close proximity to other birds or their litter. Other birds, wild and domestic, harbor the histomoniasis disease with varying degrees of resistance. Turkeys and grouse seem very susceptible to the disease while quails and partridges don’t suffer as much. The same is true of domestic birds such as chickens which can harbor the disease yet not suffer from it. In the wild, the problem of litter exposure is highly unlikely unless wildlife managers fertilize with domestic poultry litter. While there is no doubt it is a great fertilizer for agricultural purposes, including wild game food plots, there is a risk of turkeys contracting histomoniasis from the litter. The nematode eggs laid in the litter and soil can last for years and present a level of risk. Recent studies show that commercial poultry operations whose litter was used as fertilizer possessed little risk in spreading blackhead disease, while private backyard/barnyard operations posed more significant risk. Regardless, there is some risk that can be avoided altogether by simply using other commercial fertilizers.
Conclusion
Wild turkey populations are generally very healthy throughout their range due to sound management and game laws. However, histomoniasis can be a significant disease to turkeys due to its high mortality rate. Hunters should be watchful of signs of the disease in their harvest and report any findings to their local game department in addition to any conditions, such as baiting, that may be both illegal and threatening to the health of the population.
Sources:
Bates, Kimberly. 1998. Winona State University Department of Biology.
http://bio.winona.edu/bates/Parasitology/histomon.htm
Davidson, William R. (Editor). 2006. Field Manual of Wildlife Diseases in the
Southeastern United States - 3rd Edition by the Southeastern Cooperative Wildlife Disease Study Group. ppg: 264-269.
Jones, Kelly H.. Viagen Technical Update Histomoniasis (Blackhead). June 2005
Volume 2. http://www.aviagen.com/docs/Blackhead_TUVol2.pdf
Liukkonen-Anttila Tuija. 2000. Nutritional and genetic adaptation of galliform birds:
implications for hand-rearing and restocking. University of Oulu, Finland.
Department of Biology. http://herkules.oulu.fi/isbn9514259904/html/index.html
The Merck Veterinarian Manual Online
http://www.merckvetmanual.com/mvm/in.../bc/203000.htm
Michigan Department of Natural Resources Online
http://www.michigan.gov/dnr/0,1607,7...6481--,00.html
University of California – Davis, Biology of Parasitism Online
http://ucdnema.ucdavis.edu/imagemap/...akisgallinarum
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Montani Semper Liber - Mountaineers are always free
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