Syngamus trachea
This is the gapeworm of poultry, found in the trachea of chickens, turkeys, guinea fowl
and many species of wild birds. It is of particular importance in farm-raised pheasants.
Although Syngamus is found throughout the world, conversion of domestic poultry
raising from outdoors to indoors has significantly reduced its prevalence in the
United States.
Morphology
The most distinctive feature of this nematode is that males and females are joined
together in a state of permanent copulation forming a Y shape as seen in the image on the
right. Females are much larger (up to 20mm long) than males (up to 6mm long).
Life cycle
The life cycle is complicated in both its preparasitic and
parasitic phases. In the preparasitic phase L3s develop inside the eggs at which time they
may hatch. Earthworms play an important role in the life cycle, serving as transport
(paratenic) hosts. Larvae have been shown to remain viable for more than three years
encapsulated in earthworm muscles. Other invertebrates may also serve as paratenic
hosts and these include terrestrial snails and slugs as well as the larvae of Musca
domestica (the common house fly) and Lucilia sericata (the greenbottle fly
responsible for cutaneous myiasis). The parasitic phase involves substantial
migration in the definitive host to reach the predilection site.
Pathogenesis
Young birds are most severely affected with migration of larvae and adults through the
lungs causing a severe pneumonia. Lymphoid nodules form at the point of attachment of the
worms in the bronchi and trachea. Adult worms appear also to be blood suckers. Worms in
the bronchi and trachea provoke a hemorrhagic tracheitis and bronchitis with formation of
large quantities of mucus, plugging the air passages and, in severe cases, causing
asphyxiation.
Pheasants appear to be particularly susceptible to Syngamus infections resulting in
mortality rates as high as 25% during outbreaks.
Epidemiology
Earthworm transport hosts are important factors in the transmission of Syngamus
trachea where poultry and game birds are reared on soil. The longevity of L3s in
earthworms (up to 3 years) is particularly important in perpetuating the infection from
year to year.
Wild birds may serve as reservoirs of infection and have been implicated as the sources
of infections in outbreaks on game-bird farms as well as poultry farms. Wild reservoir
hosts may include pheasants, ruffed grouse, partridges, turkeys, magpies, meadowlarks,
robins, grackles, jays, jackdaws, rooks, starlings and crows.
There is also evidence to suggest that strains of Syngamus trachea from wild
bird reservoir hosts may be more infective for domestic birds if they first pass through
an earthworm transport host rather than by direct infections via ingestion of L3s or eggs
containing L3s.
Clinical signs
Blockage of the bronchi and trachea with worms and mucus will cause infected birds to
gasp for air. They stretch out their necks, open their mouths and gasp for air producing a
hissing noise as they do so. This "gaping" posture has given rise to the common
term "gapeworm" to describe Syngamus trachea. These clinical signs
first appear approximately 1-2 weeks after infection. Severely affected birds,
particularly young ones, will deteriorate rapidly; they stop drinking and become
anorexic. At this stage, death is the usual outcome. Adult birds are usually less severely
affected and may only show an occasional cough or even no obvious clinical signs.
Diagnosis
A diagnosis is usually made on the basis of the classical clinical signs of
"gaping". Subclinical infections with few worms may be confirmed at
necropsy by finding copulating worms in the trachea and also by finding the
characteristic eggs in the feces of infected birds.
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Syngamus trachea - bioperculate egg
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Syngamus trachea - adult worms in the trachea of an
infected turkey
Image courtesy of Professor Max Murray |
