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.