320
Important Helminth Parasites of Horses
THE LARGE STOMACH WORMS
The life-cycles of the round-worms so far considered are direct, the host
acquiring infection through contaminated feed or water. There are, however,
several round-worms, in the life-cycle of which an intermediate host is inter¬
colated. The most important of such worms are the ' large stomach worms
of the horse ' belonging to the genus Habronema. There are three species
of these : H. muscae (Carter's stomach worm) occurring free in the stomach
or attached to its wall; H. microstoma occurs free in the stomach but is capable
of penetrating the wall of the stomach and producing small ulcers ; and H.
megastoma which is the smallest of the three and occurs in tumours in the
stomach.
The eggs of these worms have a very flexible shell. The eggs and larvae
are passed out in the faeces and if ingested by maggots of suitable flies, which
ordinarily develop in the manure, they undergo further development which
keeps pace with the development of the maggots. The infective stage is
reached in the maggot at about the time when the latter pupates. H. muscae
and H. megastoma develop in the house-fly while H. microstoma develops in
the stable-fly. In the adult fly the larvae occur free in the abdominal cavity
and may pass forward into the proboscis. The horse acquires infection by
swallowing live infected flies or infected flies which have been dead only a
short time and still contain live larvae. Another and possibly more common
mode of infection is : As the flies suck the moisture of the lips and nose, the
larvae present in the mouth parts of the fly wriggle out, being stimulated
by the warmth and moisture of the horse's body. The larvae thus deposited
on the lips are readily swallowed while those which get into the nasal cavity
wriggle into the pharynx and are swallowed. On reaching the stomach of
the final host they develop to maturity (Plate XXV, fig. 3).
The free living forms are injurious on account of their tendency to attach
themselves to and penetrate into the stomach wall causing irritation which
results in chronic catarrhal gastritis with the formation of much mucus.
The large mouthed worm—H. megastoma—produces tumours of varying
sizes which seriously interfere with the function of the stomach. If present
at the junction of the stomach and intestine, the tumours interfere mechanically
with the passage of food. The larvae are responsible for a skin disease known
as "Summer sores", " Granular dermititis ", or ''Bursati". A nodular
conjunctivitis of similar origin is also known and has been produced
experimentally with the larvae of H. muscae.
Gastric infestation is difficult to diagnose as the larvae are not readily
found in the faeces, though gastric lavage through a stomach tube may help.
The worms in tumours are inaccessible to any method of treatment known at
present, but the other two species can be successfully treated. The animal
should be starved for 18 to 24 hours and the stomach washed out by
injecting through a stomach tube 8 to 10 litres of 2 per cent solution
of sodium bicarbonate to loosen and remove the thick layer of tenacious
mucus under which the worms lie. This alkaline solution may be siphoned off,