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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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