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Symposium "Histomonas meleagridis: Update on Life Cycle, Control and Diagnostics"

The diagnostics of Histomonas meleagridis


Wil J.M. Landman, DVM, PhD
Animal Health Service Ltd., P.O. Box 9, 7400 AA Deventer, the Netherlands
E-mail: w.landman@gdvdieren.nl

Histomoniasis is a parasitic disease mainly affecting Galliformes, most prominently turkeys (Meleagris gallopavo). Its causative agent is the protozoan Histomonas meleagridis, which may induce necrotic foci in liver and necrotic typhlitis. In severe cases other organs may also be affected. A presumptive diagnosis can be made relatively easy based on macroscopic lesions, especially when subacute to chronic liver lesions characterized by a circular or oval depressed area of necrosis of about one centimeter in diameter occur in combination with inflammation, ulceration and cecal necrosis. Because of the variable aspect of lesions, especially in early stages and/or severe infestations, confusion with other diseases is possible. Therefore, diagnosis should be confirmed by histopathology. Routine staining techniques such as H&E easily lead to identification of the tissue form of H. meleagridis. Individual or clustered histomonads may appear as round or ovoid eosinophilic bodies laying in lacunae at the periphery of lesions. Other stains like periodic acid-Shiff’s and Grocott’s stain may be useful to differentiate histomonads from fungi. Fluorescent labeled antibody techniques can be used to demonstrate H. meleagridis in tissue samples and culture. As a confirmation test histomonads may also be cultured in special media such as Dwyer’s medium and modifications. A requirement for the culture is that samples of cecal contents are gathered from readily sacrificed birds before cooling. The medium, which consists of 85% medium 199 with Hank’s salts, 10% heat-inactivated horse serum, 5% chicken embryo extract and 10 to 12 mg of rice powder per 12.5 ml medium, is inoculated and incubated at 40°C. Histomonads may also be demonstrated in cecal fluid and scrapings using a phase contrast microscope. Applying a warm stage will enable observation of typical ‘rocking’ movement of histomonads. More recently, a number of molecular techniques have further expanded above mentioned list of diagnostic possibilities. Several PCR techniques have been mentioned including nested and quantitative PCR. In all cases, primers were designed based on the nucleotide sequence of small subunit ribosomal RNAs. PCR techniques have been successful at detecting H. meleagridis in organs as well as faeces samples. Finally, in situ hybridization for H. meleagridis has been described. This technique, regarded as complementary to the histological analysis, enables detection of specific RNA sequences and at the same time histomorphological analysis of tissue samples.



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