Progress 08/17/02 to 08/16/09
Outputs
OUTPUTS: The extensive findings in this project on Blackhead Disease were disseminated to other professionals in the field, diagnosticians, and others working with the poultry industries in several ways. Seminars were given in the department of poultry science, as guest lecturer at Texas A&M University, and at industry conferences held at Pennsylvania State University, and North Carolina State University; also at the Free University of Berlin in Germany, and as invited presentations at the University of Vienna College of Veterinary Medicine. We mentored visiting scientists from several european university and government organizations on laboratory techniques for working with the causative agent, Histomonas meleagridis. Frequent questions via telephone and electronic communications were fielded from veterinarians and poultry producers experiencing field outbreaks of this disease. Articles were written for the popular press for publication in poultry-industry focused magazines and newsletters. PARTICIPANTS: Nothing significant to report during this reporting period. TARGET AUDIENCES: This project, aimed at producers of turkeys and broiler breeder chickens in the USA, developed significant new understanding of blackhead disease, caused by the protozoan parasite Histomonas meleagridis. In particular, new knowledge of the biology and molecular biology, as well as the dynamics of transmission from bird to bird in turkeys and chickens, has aided in the control of outbreaks without intervention with drugs. As there is no vaccine or other means for treatment of the disease, management issues are more important. Analysis of strains of parasites isolated from field outbreaks showed that a variety of genotypes is involved, and this helps to explain the differences in virulence in different outbreaks. Molecular analysis targeting genes involved in virulence and variations in virulence help to understand how the parasite adapts to growth under different conditions (cecum of the turkey vs. liver of turkey). PROJECT MODIFICATIONS: Not relevant to this project.
Impacts
Through the broad range of activities under this project, we made considerable progress in the following ways: 1. An understanding of the epidemiology of blackhead disease, particularly the role of broiler breeder chickens as reservoirs of infection for other birds, particularly turkeys. 2. The manner in which disease is introduced into turkey growing farms from outside, and the necessity of direct contact between birds for transmission within flocks. As a result, many have been able to limit the scope of outbreaks by restricting the migration of turkey poults within flocks. 3. The diverse molecular genetics of strains of Histomonas meleagridis involved in outbreaks in turkeys, chickens and other birds. Tracking of molecular types is possible, resulting in the identification of strains from one outbreak involved in other later outbreaks. 4. The culture conditions important for the maintenance of the organism in vitro, with its dependence on bacteria for continued virulence and its need for a starch source, and the impact of oxygen and pH on growth. 5. A genomic project has resulted in identification of approximately 90% of the genes in Histomonas meleagridis, by comparison of genomic sequences with those known from other protozoa. The target of this work was identification of genes responsible for virulence and survival in tissues or in the intestinal tract, and for evoking an immune response.
Publications
- Lollis LA, Gerhold RW, McDougald LR, and Beckstead RB. 2011 Molecular characterization of Histomonas meleagridis in the United States using the 5.8S, ITS-1, and ITS-2 rRNA regions to identify genetic variation. International Poultry Scientific Forum, Atlanta, GA.
- McDougald, LR. 2003. Research on the epidemiology and pathogenicity of Histomoniasis in chickens and turkeys. In H. M. Hafez, ed. Turkey Production: Balance act between consumer protection, animal welfare and economic aspects. Proc. International Meeting of the Working Group 10 (Turkey), World Poultry Science Association, Federation of European Branches, Berlin Germany. Pp. 42-47.
- Lynn B, Gerhold RW, McDougald LR, and Beckstead RB. Determining virulence factors in Histomonas meleagridis. Southeastern Society of Parasitologists. Asheville, NC, April 7-8, 2010.
- Lollis LA, Gerhold RW, McDougald LR, and Beckstead RB. 2010. Molecular characterization of the ITS-1, 5.8S, and ITS-2 rRNA regions of Histomonas meleagridis. Southeastern Society of Parasitologists, Asheville, NC.
- Gerhold RW, Lollis LA, McDougald LR, and Beckstead RB. 2010. Molecular characterization of Histomonas meleagridis. American Association of Veterinary Parasitologists.
- Gerhold RW, Beckstead RB, Lollis LA, and McDougald LR. Establishment of culture conditions for the survival of Histomonas meleagridis in transit. American Association of Veterinary Laboratory Diagnosticians. Minneapolis, MN, November 16, 2010.
- Lynn E, Gerhold RW, McDougald LR, and Beckstead RB. 2011. Determining virulencefactors in Histomonas meleagridis, International Poultry Scientific Forum, Atlanta, GA.
- McDougald, L. R. 2004. The importance of parasitic diseases and interactions. In H. M. Hafez, ed., Proc. 5th International Symposium on Turkey Diseases, Berlin Germany. Pp 245-249.
- McDougald, L. R. 2007. Control of blackhead disease (Histomoniasis) for today and in the future. In H. M. Hafez, ed. Turkey Production: Current Challenges. Proc. 4th international symposium on turkey production, Working Group 10 (Turkey), World Poultry Science Association, Federation of European Branches. Berlin, Germany. pp. 240-246.
- Hauck, R. and L. R. McDougald. 2007. Interaction between Histomonas meleagridis and bacteria. In H. M. Hafez, ed. Turkey Production: Current Challenges. Proc. 4th international symposium on turkey production, Working Group 10 (Turkey), World Poultry Science Association, Federation of European Branches. Berlin, Germany. pp. 247-254.
- McDougald, L. R. 2007. Recent advances in research on Histomonas meleagridis. Symposium on Histomoniasis and Coccidiosis in Poultry. University of Vienna Vet. Med. School, Vienna, Austria.
- Hauck, R., A. L. Fuller, and L. R. McDougald. 2010. Evaluation of nifurtimox for potential use in control of histomoniasis in turkeys. Avian Diseases, 54: 28-32.
- Hauck, R., P. A. Armstrong and L. R. McDougald. 2010. Histomonas meleagridis (Protozoa, Trichomonadidae): Analysis of growth requirements in vitro. J. Parasitology 96:1-7.
- Gerhold, R. W., A. L. Fuller and L. R. McDougald. 2010. Establishment of culture conditions for survival of Histomonas meleagridis in transit. Avian Dis.54:948-950.
- Lollis, L., R. W. Gerhold, L. R. McDougald and R. Beckstead. 2011. Molecular characterization of Histomonas meleagridis in the United States using the ITS-1, 5.8s, and ITS-2 rRNA regions to identify genetic variation. J. Parasitol. 97:610-615.
- Gerhold, R. W., Lori A. Lollis, Larry R. McDougald, and Robert B. Beckstead. 2011. Partial sequences of the alpha-tubulin gene from Histomonas meleagridis isolates from the United States. J. Parasitology 97:354-356.
- Armstrong, P. L., and L. R. McDougald. 2011. The infection of turkey poults with Histomonas meleagridis by contact with infected birds or contaminated cages. Avian Dis. 55: 48-50.
- Hu, J., M. Brooks, , A. Fuller, P. Armstrong, and L. R. McDougald. 2008. Histomonas meleagridis (Parabasala, Trichomonadea, Monocercomonadidae): Presence of natural agglutinins in horse serum. Parasitology Research, 102: 365-369.
- Gerhold RW, Lollis LA, Fuller F, Beckstead RB, and McDougald LR. 2009. Development of molecular diagnostic techniques to detect and distinguish various Eimeria species in North American game birds. Wildlife Disease Association Meeting.
- Lollis LA, Beckstead RB, Gerhold RW, and McDougald LR. Blackhead disease(Histomoniasis): strain variation as a factor in virulence. American Society of Parasitologists, 84th Annual Meeting, August 14-17, 2009.
- Gerhold RW, Lollis LA, Fuller L, Beckstead RB, and McDougald LR. Development of molecular diagnostic techniques to detect and distinguish various Eimeria species in North American game birds. American Association of Veterinary Laboratory Diagnosticians Annual Meeting, October 11, 2009.
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Progress 01/01/08 to 12/31/08
Outputs
OUTPUTS: During this project I was able to make significant contributions to understanding of the biology, pathology, etiology and dissemination of Blackhead Disease of poultry. Owing to the lack of interest by researchers over the past 35 years, important questions on the way the disease is spread between flocks and within flocks were answered. These findings have been reported to the scientific community and to the poultry industry community by means of a series of scientific papers published in refereed journals (Avian Diseases), papers given at professional society meetings (AAAP, Poultry Science, Am. Soc. Parasitologists, Soc. Protozoologists), invited seminars within the University of Georgia and at other universities (Texas A&M, Penn State U., Berlin Free University, U. of Vienna Veterinary School), and at industry meetings (Pennsylvania, Virginia, North Carolina, Georgia). Invited presentations made at national meetings and in other countries (Germany and Austria) were used as venues to reach the national and international scientific communities. Papers were written for the popular press in the USA and Europe. Two Ph.D. students, one master's degree candidate, and several undergraduate and graduate research participants received training through this program. Additional research papers and seminars are planned or submitted, from work that is in late stages of completion. PARTICIPANTS: The individuals who contributed to this project, other than the principal investigator, included Jinghua Hu, A. L. Fuller, M. Brooks, Rudi Hauck, and R. Gerhold. Jinghua Hu, a recent immigrant from China, received his Ph.D. for work done on the project, after which he accepted a position in research at Eli Lilly and Co., Indianapolis IN. Dr. A. L. Fuller, who has worked in the Poultry Science Department at the University of Georgia for more than 30 years has made significant contributions to all aspects of this work. Michelle Brooks, an undergraduate student at UGA at the time, contributed significantly to the laboratory work with Histomonas, and after graduation accepted an assistantship for graduate work in microbiology at Ohio State University. Rick Gerhold, DVM, is presently a Ph.D. student in the Poultry Science department at UGA, focusing on PCR tests for detection of Histomonas. Dr. Rudi Hauck, of Berlin Free University, Institute for Poultry Diseases, Berlin Germany, contributed significantly to the studies on chemotherapy and laboratory studies on culture of Histomonas in vitro. Ms. Lori Lollis is working on the classification of strains of Histomonas using PCR and gene sequencing studies for her Master's degree. TARGET AUDIENCES: This project is of interest to veterinarians and scientists conducting research on poultry diseases, particularly those caused by the enteric protozoa. Diagnosticians and personnel working with poultry companies where there is a risk of exposure of turkeys or chickens to blackhead disease are particularly interested in this work. Blackhead disease has affected production of turkeys and broiler breeder layers, not only in North America, but in most European and Eastern European countries. As a result, personnel working with poultry production in these localities are eager for new information. Scientists in firms hoping to manufacture and sell products for treatment of blackhead disease are interested in . Firms manufacturing vaccines are interested in potential protection of turkeys against blackhead disease using a vaccine. PROJECT MODIFICATIONS: Not relevant to this project.
Impacts
This project has impacted the way poultry and disease are managed in the USA and internationally, particularly in Europe. The discovery that severe outbreaks of blackhead disease affecting large flocks of turkeys were a result of direct contagion between birds rather than requiring the use of an invertebrate vector led to the recommendation that outbreaks be contained by erection of fences within rearing facilities to prevent commingling of infected and uninfected birds. This was successful in field experience, where outbreaks were contained by this simple technique. This direct bird-bird infection technique was developed as a laboratory model for study of the epidemiology. Studies in chickens showed that an entirely different system was responsible, and that the reservoir for infection of chickens was almost entirely in the contaminated soil of the facility. Laboratory studies demonstrated the presence of hydrogenosomes, organelles responsible for anaerobic energy metabolism in the causative protozoan Histomonas meleagridis. Testing of antibiotics and other drugs available for use in animals showed that there was no effective substitute for the highly effective antihistomonals which had been de-registered by FDA. A continuing problem is the use of The requirements for growth in vitro were studied extensively. All previously employed means for culture in vitro were evaluated, showing that the common elements to all were a starch source (rice or other grain flour), serum from any animal, and a suitable physiological saline. As the parasites are anaerobic, it was important to use closed tubes. Growth could be stimulated by addition of nutritient-rich cell culture medium. Histomonas would not grow in media containing glucose at 5 mg/L, because of the low pH produced in supplemented cultures. The growth requirements to enable Histomonas to be grown without the presence of bacteria are still elusive. However, it was shown that the tissue invasive form was significantly different from the intestinal lumen form in the requirement for bacteria. Studies on the immunity resulting from infection with H. meleagridis showed that the turkey was slow to respond to vaccination with killed organisms, but that protective immunity generally resulted from live infections. This work confirmed the difficulty in vaccination of poultry against histomoniasis, but also showed that under some conditions the spread of infection from bird to bird was curtailed. Future work with development of antigens will be directed toward this method of protection.
Publications
- Hu, J., Brooks, M., Fuller, A. L., Armstrong, P., and McDougald, L. R. 2008. Histomonas meleagridis (Parabasala, Trichomonadea, Monocercomonadidae): presence of natural agglutinins in horse serum. Parasitology Research 102, 365-369.
- Hauck, R., Armstrong, P. L., and McDougald, L. R. 2009. Histomonas meleagridis (Protozoa), Trichomonadidae): Analysis of growth requirements in vitro. Journal of Parasitology, 95: (In Press).
- Hauck, R., Fuller, A. L., Greif, G., and McDougald, L. R. 2009. Evaluation of nifurtimox for potential use in control of histomoniasis in turkeys. Avian Diseases, 54: (in press).
- Brooks, M., Fuller, A. L., and McDougald, L. R. 2009. Hydrogenosomes, ornithine and arginine decarboxylases, and polyamine transport in Histomonas meleagridis (Parabasala; Trichomonadea). Journal of Parasitology, 95: (in press).
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Progress 01/01/07 to 12/31/07
Outputs
OUTPUTS: The results of our studies on blackhead disease were communicated to the scientific and lay communities of animal health and production by participating in several scientific and poultry industry meetings. In particular, presentations were made at the World Poultry Science turkey working group in Berlin, Germany in June of 2007, at the international meeting on coccidiosis and blackhead disease at the University of Vienna, Vienna Austria in July of 2007, and at the Northeast Conference on Avian Diseases in Lancaster, PA, in September of 2007. Other informal presentations were given to industry groups, or to departmental seminars in the Poultry Science department, and in the Avian Medicine section of the Department of Population Health, University of Georgia. PARTICIPANTS: Not relevant to this project. TARGET AUDIENCES: The target audience for information on this project would include poultry producers (broiler breeder and turkey), as well as game bird producers. Professionals who work with these industries would be interested, including veterinary diagnosticians, research workers, parasitologists, and other academics. PROJECT MODIFICATIONS: Not relevant to this project.
Impacts
Recommendations have been made to the poultry industry based on the results of our studies and on general observations on the course of this disease in poultry. Increasingly, the turkey and chicken industries of European countries and large producers in the USA have adopted these recommendations, with the result that the impact of outbreaks has been lessened. Specifically, I recommended that the contagiousness of blackhead disease in turkeys could be curtailed by limiting the migration of turkeys through the house by means of barriers erected periodically through the growing barns. Producers have reported that this does indeed help to contain the outbreak. Further recommendations were made on the use of facilities for turkey growing that were previously used for chicken production, which helped growers to understand the risks of this type of husbandry. The control of blackhead in broiler breeder chickens was aided by recommendations for more frequent and earlier application of anthelminthics. My discovery that blackhead disease was not contagious in chickens in the way it is in turkeys is supported by this recommendation.
Publications
- No publications reported this period
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Progress 01/01/06 to 12/31/06
Outputs
The progress of blackhead disease in chickens was studied with several breeds of chickens including broilers, leghorns, Rhode Island reds, and Polish. Polish chickens were markedly more susceptible to the development of liver lesions than the other breeds. Immunization for protection against blackhead disease was studied in turkeys. Vaccines were prepared from cultured histomonads and injected by subcutaneous or intramuscular routes. Initial tests suggested that turkeys could become immune to infection with blackhead after 2-3 injections spaced 2-3 weeks apart. Further studies showed that a single inoculation was not successful in stimulating protective immunity. Turkeys developed circulating antibodies which lysed histomonads in vitro. An agglutination factor in serum of horses was also found in other sera but in much lower activity. Since the antibodies developing from infection of turkeys was of the lysin type, it was considered that agglutinins in horse sera were
not antibodies but acted through some other mechanism. A study of histomonads in vitro and by electron microscopy showed the dependence of these parasites on anaerobic metabolism in specialized organelles called hydrogenosomes for the production of energy. A series of tests in vitro with known enzyme inhibitors demonstrated the presence and importance of the ornithine decarboxylase pathway and the dependence on polyamines such as putrescine. Several tests were conducted with natural products thought to have some value in control of blackhead disease, but with marginal results. An experimental chemotherapeutic from Bayer AG of Germany showed excellent activity at 400 ppm in feed. Further tests are scheduled to follow up on this activity.
Impacts
These results showed for the first time that it was possible to immunize birds against blackhead disease using a vaccine prepared from killed cells cultured in vitro. Knowledge of the key metabolic pathways in these parasites are vital to pursuing new leads in the search for medicines to replace those which are no longer available. The good success with one candidate drug gives us hope that a new product may be found which is capable of stopping losses in affected flocks.
Publications
- Hu, J., L. Fuller, P.L. Armstrong and L. R. McDougald. 2006. Histomonas meleagridis in chickens: Attempted transmission in the absence of vectors. Avian Dis. 50:277-279.
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Progress 01/01/05 to 12/31/05
Outputs
Further studies on the epidemiology of histomoniasis in chickens revealed significant differences from that previously reported for the disease in turkeys. In the absence of any evidence that blackhead disease spread from bird to bird within a flock without the aid of vectors, it was concluded that most if not all infections in commercial chicken flocks result directly from the intake of worm eggs (Heterakis gallinarum) carrying the protozoan parasites. This places significant weight on programs for intensive and frequent worming of breeder pullets or cockerels during the periods of exposure (1-8 wks of age) and on disinfection or removal of contaminated litter materials. Experiments on immunization of turkeys against histomoniasis also gave unexpected results: Inoculation of young turkeys with live or killed cells of H. meleagridis conferred some measure of protection as measured by challenge by the cloacal route. Immunizing inoculations could be given by
subcutaneous or intramuscular routes. Experiments are under way to further characterize the response of turkeys to immunizing inoculations. Tests of immune serum showed the presence of factors which would lyse cultured cells in vitro. Lysis activity was most pronounced in serum which had not been heat-treated to inactivate complement. Further work on the ultrastructure of H. meleagridis demonstrated the lack of mitochondria, which had been expected because of its relationship to certain other protozoa, and the presence of hydrogenosomes, which are specialized organelles responsible for anaerobic energy conversion.
Impacts
The results of these studies will enable poultry and turkey producers to better understand histomoniasis and to institute preventive maintenance programs to minimize losses. Broiler breeder producers can focus on the control of intermediate hosts for the parasites (by chemotherapy and management)as the primary means of prevention/control of this disease. Researchers can use discoveries on cellular ultrastructure as targets for potential new agents of chemotherapy. Further work can be planned on unexplored avenues of immunization for preventive control.
Publications
- Hu, J., Fuller, L. and McDougald, L. R. 2005. Blackhead disease in turkeys: Direct transmission of Histomonas meleagridis from bird to bird in a laboratory model. Avian Dis. 49:328-331.
- McDougald, L. R. 2005. Blackhead disease (Histomoniasis) in poultry: A critical review. Avian Dis. 49:462-476.
- McDougald, L. R. 2005. Blackhead Disease: The Continuing Saga. In Turkey Production: Prospects on future developments. Ed by H. M. Hafez. Proc. 3rd International Symposium on Turkey Production, Berlin Germany. Pp. 244-249.
- Hafez, H. M., Hauck, R., Lueschow, D. and McDougald, L. 2005. Comparison of the specificity and sensitivity of PCR, nested PCR, and real-time PCR for the diagnosis of histomoniasis. Avian Dis. 49:366-370.
- Heijden, H.M.J.F. van der, McDougald, L. R. and Landman, W. J. M. 2005. High yield of parasites and prolonged in vitro culture of Histomonas meleagridis. Avian Pathol. 34: 505-508.
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Progress 01/01/04 to 12/31/04
Outputs
The epidemiology of histomoniasis in chickens and turkeys was studied in laboratory and floor pen experiments. A laboratory model was devised to infect turkeys by the cloacal drop method, wherein cultured histomonads were applied to the cloacal lips with a pipette. Natural muscular movement of the vent sucks the liquid inside, where it is taken directly to the bursa of Fabricius and to the ceca. Infections proceed from this exposure. In further experiments, it was shown that histomoniasis spreads from infected to uninfected birds in the same cage, if the cage floors are covered with paper to cause accumulation of feces during the patent period. Exposure for only 2 days was adequate for the infection to be passed to uninfected birds. These results were observed when 25% or more of the birds in a cage were infected. Infection did not spread as readily within groups of chickens on litter-covered floor pens. There was no evidence of increased contagiousness in pens of
chickens when feed was restricted on a skip-a-day basis to mimic broiler breeder pullet rearing conditions. The lack of spread of disease in these pens suggests that chickens differ markedly from turkeys in the importance of bird-to-bird contact in transmission of infections. Oral infections with cultured histomonads were totally unsuccessful in chickens, as in turkeys. Attempts to culture histomonads in the absence or reduced contamination by bacteria were successful: Cultures were treated with gentamycin, which essentially eliminated visible bacteria. Histomonads grew to large numbers, 1-2 million/ml in the treated cultures. Treatment of cultures with oxerase, a product designed to produce anaerobic conditions, had no significant effect on growth.
Impacts
The demonstration of differences in the epidemiology of blackhead disease in chickens and turkeys will help to guide poultry producers in control programs. In turkeys, control may be achieved by institution of migration control in poultry houses to limit bird-to-bird spread, while in chickens, more emphasis will have to be placed on cecal worm control. The new biological characters of Histomonas meleagridis in vitro will aid an on-going program for development of immunization techniques for blackhead in chickens and turkeys, and in producing pure protozoal material for biochemical and ultrastructural studies.
Publications
- Hu, J., Fuller, L. and L. R. McDougald. 2004. Infection of turkeys with Histomonas meleagridis by the cloacal drop method.
- Hu, J., and L. R. McDougald. 2004. The efficacy of some drugs with known antiprotozoal activity against Histomonas meleagridis in chickens. Vet. Parasitol. 121:233-238.
- McDougald, L. 2004. Enteric disorders in turkeys: The importance of parasitic diseases and interactions. In H. M. Hafez, ed. Proc. 5th International Symposium on Turkey Diseases, Berlin DR. Pp. 245-249.
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Progress 01/01/03 to 12/31/03
Outputs
Several drugs with known antiprotozoal activity were tested against Histomonas meleagridis in chickens. Those drugs in the class nitro-imidazole were active, while those in other classes were not. The method of infection under practical conditions for histomoniasis in chickens and turkeys was investigated. Turkeys inoculated from cultured H. meleagridis became ill as expected. Other birds in the pens also became ill, a result of exposure to the infected birds. This occurred in the absence of vectors. Investigating the means of spread, we tested oral inoculation (with and without alteration of crop pH by withholding feed) without success. Further studies were done with the "cloacal drop method," to test the effects of cloacal drinking and infection by retrograde peristalsis. Birds readily became infected by this route. These results suggest that the normal route of spread of blackhead in a flock is by cloacal exposure to liquid feces from other infected birds, rather
than by oral ingestion of voided material.
Impacts
The results of these studies will guide future work on chemotherapy away from ineffective classes of compounds, toward the more effective types of nitroimidazoles. Further, the studies clarify the epidemiology of histomoniasis within a flock, demonstrating that the disease can spread rapidly through a flock without the presence of vectors (cecal worm eggs) and provide poultry diagnosticians and veterinarians with a better understanding of the importance of gut physiology in transmission.
Publications
- Hu, J. and McDougald, L. R. 2003. Direct lateral transmission of Histomonas meleagridis in turkeys. Avian Dis. 47:489-492.
- Cai, X. Fuller, A. L., McDougald, L. R. and Zhu, G. 2003. Apicoplast genome of the coccidian Eimeria tenella. Gene: 39-46.
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Progress 01/01/02 to 12/31/02
Outputs
The epidemiology of histomoniasis (blackhead disease) in turkeys was studied in floor pens in the absence of demonstrable vectors such as cecal worms, coccidia, earthworms, and insects. An infection introduced into the pens by inoculating seeder birds with cultured Histomonas meleagridis was able to spread through a pen of 40 birds within 30 days of exposure. With high exposure (25% of birds infected from cultures) all of the exposed birds died within 23 days. Lower exposure (10% of birds infected) produced similiar results by 30 days. Further studies on the means of acquisition of infections included direct oral inoculation of cultured Histomonas. This route of infection was not successful. Several antiprotozoal compounds selected from the literature were tested for efficacy against blackhead disease in chickens. Of the several types of compounds tested, only the nitroimidazoles were highly effective, similiar to known antihistomonal products. None of the feed-grade
anticoccidials or antibiotics tested was effective.
Impacts
Veterinarians will have a better understanding of the mechanism of lateral transmission of histomoniasis in turkey and chicken flocks, as it was previously thought that infections were acquired only from ingesting eggs of the vector Heterakis gallinarum (cecal worm).
Publications
- Hu, J. and L. R. McDougald. 2002. Effect of anticoccidials and antibiotics on the control of blackhead disease in broiler breeder pullets. J. Appl. Poult. Res. 11:351-357.
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