Histomonosis (blackhead) Clinical signs Control Veterinary Information Note

Histomonosis (blackhead)
Veterinary Information Note
Histomonosis (blackhead) is caused by the protozoan parasite Histomonas meleagridis.
Clinical signs
Control
Infection results in lesions in the caecum followed shortly
afterwards by the development of lesions of variable
appearance in the liver. Lesions are occasionally seen in
other organs such as the spleen and bursa.
No medications are
currently licensed in
Great Britain for the
control or treatment
of
histomonosis.
The risk of disease
can be reduced with
anthelmintic treatment
to control Heterakis
worms. The effects of
histomonosis are known
to be exacerbated in
chickens by caecal
coccidiosis.
The clinical signs are non-specific and may include
drowsiness, a stilted gait when walking, bright yellow
droppings (particularly in turkeys), anorexia and death.
Cyanosis of the skin and wattles of the head may
occasionally be seen and was the origin of the name
‘Blackhead’.
Diagnosis of the disease is usually on the basis of gross
pathology, supported if appropriate by histopathology.
The use of PCR testing has been reported for diagnosing
disease in the early stages, but no PCR test is currently
available in this country.
Pathogenicity
Turkeys are particularly susceptible to the disease, but
blackhead is also encountered in other gallinaceous
birds including chickens (particularly free-range layers),
pheasants and partridges. Peafowl also appear to be
particularly susceptible to the disease.
Histomonas meleagridis is mainly transmitted by the
caecal worm Heterakis gallinarum, which protects
the organism within its eggs during transmission from
bird to bird. The organism can also be harboured and
transmitted by earthworms. Infection can also be spread
by mechanical vectors, including people particularly as a
result of contamination of footwear with Heterakis eggs.
Infection can be transmitted directly between turkeys
within an infected flock via the cloaca, in the absence
of Heterakis worms, but this has not been demonstrated
in chickens. There is no evidence that humans are
susceptible to the disease.
Bacteria
such
as
Clostridium perfringens
also play a role in the
development of disease, which may explain an apparent
response to antibiotic treatment in some outbreaks.
Tiamulin has been reported to reduce mortality in
affected turkey flocks and activity of tiamulin against
Histomonas has been demonstrated in vitro.
Histomonosis is endemic in the UK although relatively
small numbers of incidents are diagnosed each year,
mainly in small turkey flocks. The risk of disease in
turkeys is increased if they are reared on the same site as
chickens (or other gallinaceous birds) as these birds may
harbour Heterakis worms, which can act as a reservoir
for the organism.
Infection in chickens may be asymptomatic or may
be severe and cause mortality, and the outcome may
be affected by the presence of concurrent and/or
immunosuppressive diseases. Turkeys should not be
reared on ground that has held chickens in previous years
because Histomonas remains infective in its nematode
and earthworm hosts for long periods of time, although it
is short lived outside these hosts.
It is also preferable not to rear turkeys on earth floors.
Good biosecurity is important in excluding infection from
turkey flocks.
For further advice on general biosecurity measures in
poultry, see the leaflet ‘Biosecurity and preventing
disease (PB 11380) available on the Defra website.
Revised April 2011
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AHVLA Avian Expert Group
www.defra.gov.uk/ahvla/