316
Important Helminth Parasites of Horses
to heat also and migration is more active in warm than in cold weather.
Certain amount of moisture is necessary to enable the larva to crawl up blades
of grass. Under unsuitable conditions, the larvae penetrate into the upper
layer of the soil. They can sink to the bottom of water and may remain alive
for a month or so. Under favourable conditions of shade, moisture and
temperature they can remain alive for a couple of years. The larvae ordi-
narily remain in or near the manure where they hatch but they may be scat-
tered by wind and rain over large areas.
On being ingested by a suitable host the infective larvae cast off their
sheath and, though the course taken by the larvae of the different species
has not been definitely ascertained, it is known that practically none of them
directly settle down in the caecum and colon to attain maturity. They
usually penetrate into the mucosa of the intestine, are taken up by lymph
and blood and undergo extensive migrations through the various organs or
tissues, such as liver, pancreas, spleen, lungs, kidneys and other organs of
the host before they return to the large gut. During this course of migration
some of the larvae get caught in the various organs as erratic forms, specially
in the interstitial connective tissues of the pancreas.
The larvae of S. edentatus burrow through the wall of the intestine and
reach the connective tissue underlying the peritoneal lining of the abdeninal
cavity where they remain for about three months and increase much in size.
They then migrate towards the root of the mesentery and finally reach the
wall of the intestine, into which they penetrate. In the wall of the intestine
they become lodged in large haemorrhagic nodules which finally break open
into the lumen and release the worms.
Opinion is divided with regard to the normal route of migration of the
larvae of S. vulgaris. After reaching the intestine some reach the liver and
lungs, via the blood stream, while others are found in the wall of the anterior
mesenteric artery, where they give rise to the formation of aneurysm and the
deposition of a thrombus on the altered intima of the vessel. Some bel eve
that the latter is the normal course and the larvae are transported back to
the intestinal wall by the blood, after they have escaped from the aneurysm
into the lumen of the artery. They finally penetrate into the lumen of the
gut. Others maintain that the latter course is taken by those larvae which
have gone astray, the normal route of migration being via the blood through
the lungs, like ascarid larvae.
The worms live attached to the wall of the gut which they injure. They
draw into the buccal capsule the intestinal mucosa in the form of a plug (Plate
XXV, fig 2) and rupture the blood vessels and suck blood, after digesting
the superficial layers, or cutting into the mucosa by means of their teeth. The
secretions of the oesophageal glands of the worms act on the mucosa, digesting
and liquefying the cells. As the worms suck blood, the loss of blood may be
considerable in cases of heavy infestations and may lead to anaemia, weak-
ness and oedema of the various parts. Digestive disturbances of various
sorts, such as diarrhoea and loss of appetite, et cetera develop and the animal