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Protozoa are single celled parasites that inhabit principally the guts of game birds. They are capable of causing disease and this is particularly likely when birds are intensively reared. The protozoa that affect game birds include:Coccidia
HexamitaHistomonasTrichomonasBlastocystisProtozoal
parasites of the Eimeria genus are responsible for the disease known as coccidiosis. The
differing Eimeria are host specific with those responsible for disease in pheasants not
responsible for disease in partridges and vice-versa. Birds become immune through exposure
to small numbers of these parasites and the presence of a few coccidial oocysts (the form
of the parasites that passes in the faeces of the bird) is not indicative of clinical
disease.
Large
numbers of these parasites will result in disease and these large numbers are most
commonly found where birds are reared intensively. Affected birds generally become
lethargic and may scour prior to death but in some cases sudden death can occur in birds
that previously showed no clinical signs of disease. Sudden deaths are more common in
partridges than in pheasants.
Standard
game bird feeds should contain drugs to control coccidiosis. These drugs called
coccidiostats, obtained on veterinary prescription, are given at levels that prevent
disease but allow sufficient parasites to survive to allow the birds' natural immunity to
develop. If birds meet with an overwhelming challenge of infection from infected ground
(this may occur if birds are overstocked) or if food intake decreases so that the intake
of coccidiostat is insufficient then clinical disease may still occur.
The
oocysts passed in birds’ faeces form spores that can remain many months or possibly
years on the ground to infected other birds. They are resistant to many disinfectants but
are killed by freezing. Infection therefore remains on land after birds have been removed
and the sporulated oocysts are easily spread on footwear between pens.
These
parasites are motile when seen under the microscope and are seen in both pheasants and
partridges. They do not appear to be host specific so may be spread from one species to
another. Hexamita organisms can be seen in large numbers in healthy birds so their
presence may not be an indication that the birds are diseased.
Previously
the presence of Hexamita organisms in sick birds was taken as an indication that they were
the cause of the sickness. This is now not thought to be the case although it is still
possible that they may contribute to the disease process.
Histomoniasis
or Blackhead disease can occur in all species of game birds and lesions are seen in both
the gut and the liver of affected birds. All ages of birds may be affected, with disease
being more chronic in the older birds. Affected birds are lethargic and rapidly loose
condition. Some birds may become scoured and in young birds the sudden death of birds in
good condition may occur.
Histomonas
parasites do not form cysts but their lifecycle involves the parasites being protected by
being ingested by the Heterakis roundworm. The parasites enter the egg of the roundworm
where they can survive for several years. Ingesting infected roundworm eggs infects
further birds. Earthworms can also be implicated in the spread of this parasite as they
may ingest Heterakis larvae that are themselves infected with Histomonas and the earthworm
then becomes a source of the parasites if it is eaten.
The routine
worming of intensively reared birds may help control this parasite.
These
parasites are motile when seen under the microscope and are believed to cause disease in
both pheasants and partridges. Like Hexamita these parasites may be 'normal' gut
inhabitants and the presence of a few parasites is not indicative of clinical disease.
Large
numbers of parasites can result in clinical disease and affected birds become lethargic,
rapidly loose condition, go off their food and often pass yellow frothy droppings.
Clinical disease is most commonly seen as birds enter the release pens but disease can
also be seen in younger birds.
The
Trichomonas parasite does not produce cysts and it survives in small numbers in clinically
normal birds. These are the source of infection for young birds in subsequent years.
Good
management techniques can help reduce the incidence of disease. When infection is present
treatment of the birds to prevent dehydration, in addition to possible antibiotic
medication to prevent secondary bacterial problems, is important if the birds are to
recover.
This
non-motile parasite is found in the caecae of game birds. It is frequently seen when
microscopic examination of gut contents is undertaken to determine the presence of other
protozoal parasites. As yet it has not been shown to cause clinical disease itself and it
may simply be a normal inhabitant of the gut of game birds.
See also: Causes of Scour in Game Birds
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