Symposium
"Histomonas meleagridis: Update on Life Cycle,
Control and Diagnostics"
Histomonas
meleagridis: life cycle and epidemiology
Maarten De Gussem
Janssen Animal Health
mdgussem@janbe.jnj.com
H. meleagridis is a member of
the class Sarcomastigophora. This protozoal parasite
of poultry bears characteristics both of both amoeba
(Sarcodina) and flagellates (Mastigophora). H. meleagridis
is a pleomorph organism, as morphology is adapted
to the location in the final or intermediate host.
When present in caecal cavities
or in cultures, one or two flagellae can be found.
This form is also able to form pseudopodes, which
are used for feeding with bacteria, cell debris and
other small particles. Their size is more or less
10 to 20 µm. Reproduction is through simple
binary division.
When present in tissues such
as the caecal wall or the liver, only the intercellular
amoebal form will be found. In the intermediate host
Heterakis gallinarum another, smaller form is found.
Fresh Histomonas is inactivated within hours when
outside its final or intermediate hosts. Unlike Eimeria
species for example, no cyst, that allows survival
longer than approximately one day, is formed. The
inability of Histomonas, whenever not protected by
the eggs of the intermediate host Heterakis gallinarum,
to pass the pH barrier of the stomach emphasizes the
importance of Heterakis in the life-cycle of Histomonas.
Only when a turkey sits on the
caecal dropping within hours after discharge, and
cloacal drinking allows the re-introduction into a
new bird’s caeca, direct transmission can be
successful.
Histomonas meleagridis is an
obligatory parasite. When introduced in the caeca
of its host, either by transmission through the intermediate
host or directly through cloacal drinking, they start
dividing in the lumen.
When Heterakis is present in
the caecum, the adult worms can again ingest the histomonads.
An active invasion of the ovum causes the inclusion
in the intermediate host..
After loosing its flagellum
the histomonads invade the caecal wall, using pseudopodia.
In this phase, the invasive forms release proteolytic
enzymes, which aid in phagocytosis of partly digested
host tissue. The histomonads are transported towards
the liver and other organs, most probably via the
portal vein system.
Pathogenicity largely
depends on whether the histomonads reach the liver
or not. This depends more on host factors than on
strain factors, although pathogenicity differences
between strains are evident. The host factors include
species, breed, strain, age, intestinal flora and
concurrent infections such as coccidiosis.