Numerous organisms derived from the animal and also from the plant world inhabit the digestive system of the horse. The majority of them may be passed over with very slight notice, as it has not yet been proved that they are responsible for any morbid conditions, although it is extremely probable that some of them do produce various forms of derangement which are referred to other causes. Among the vegetable parasites are numerous fungi, such as the common mould, and others which belong to the same family. These fungi are found in the mouth, and thence quite through the digestive track.

The parasites which are derived from the animal world are extremely numerous throughout the digestive system.

Beginning with the lowest forms of life, there are found many of the sporozoa and infusoria, some families of which, the Coccidia, are met with in the liver and the epithelial cells of the mucous membrane. But in the horse it has not yet been demonstrated that any special disease attends their presence.

Coming to the more important parasites, there are first to be considered the worms which infest the stomach of the horse.

In this country the minute nematode discovered by Professor J. Wortley Axe in the stomach of the ass, and two varieties described by Professor Peuberthy in the horse, and the larvae of the AEstrus equi, or stomach bot (fig. 273), are the only parasitic worms of the stomach of the equidae; but two varieties of spiroptera, the megastoma and microstoma, arc described by Continental helminthologists. The spiroptera, like the Strongylus axei (Cobbold), form small round tumours in the mucous membrane. No special signs of illness appear to attend the presence of these worms in the stomach. With regard to the larvae of the bot-fly, which are found in clusters attached to the cuticular membrane of the stomach, opinions are very much divided, some authorities contending that they produce irritation in the stomach, and sometimes even bore their way completely through the coats of that organ. The rule, however, is that they simply penetrate the mucous membrane sufficiently to enable them to retain their hold until the time comes for them to quit their temporary habitation and assume the pupa, or chrysalis stage, in which the perfect fly is developed.

The next illustration was taken from a portion of the stomach, showing the small tumours of the spiroptera and a number of bots attached to the membrane.

A smaller variety of the bot, of a reddish colour, is sometimes seen clinging to the anus; it is known as the Œstrus haemorrhoidalis.

The eggs of the bot-fly are deposited on the hairs of the horse's legs in the early summer, and they are soon hatched by the sun. The newly-hatched larvae by their movements cause a certain amount of irritation and are licked off by the horse, and in this way are quickly conveyed to the stomach, to the mucous membrane of which they at once attach themselves by means of the hooked mandibles with which they are provided. The larvae remain attached to the cuticular membrane of the stomach for some months, probably the greater part of a year. When sufficiently developed they relax their hold and are expelled. It may be noted here, however, that treatment is of no avail; there are no safe remedies which will cause the bots to abandon their position until the proper time. But the fact of quitting their hold naturally calls attention to their existence, and the use of any remedy at this period is sure to be attended with an apparent success.

Fig. 273. - Metamorphoses of the Bot-Fly.

A, Egg of Bot-Fly as it appears stuck on a hair. B, Egg of Bot-Fly, showing the Operculum or lid. C, Lid pushed aside and the larva escaping. D, Larva fully developed after having been in the stomach of the horse. E, Head of Larva, showing Hooklets by which it anchors on to the mucous membrane of the stomach. F, Bot-Fly developed from D. G, Larvae of Bot-Fly anchored on to mucous membrane of the stomach of the horse.

The smaller red variety, the Œstrus hoemorrhoidcdis, deposits its eggs on the lips of the horse, whence the larvae are transferred to the stomach. They are much less common than the larvae of the Œstrus equi.

Of the nematodes which infest the intestines of the horse a large white or pale-yellow worm, the Ascaris megalocephala, is the most common.

The worm varies in length from 3 or 4 inches up to a foot or more, and it is occasionally present in such enormous numbers in the small intestines as to fill them completely, necessarily causing a good deal of disturbance. Occasionally a worm passes up the pancreatic duct, and now and then one is found in the duct leading to the liver. When in sufficient numbers to cause obstruction in the intestinal canal they are said to cause symptoms of colic, giddiness, epilepsy, and tetanus; but when only a few are present they do not seem to cause any inconvenience. The illustration represents the general form of these worms of small dimensions (fig. 274).

Various remedies are used for the expulsion of these worms, among them arsenic, calomel, tartar emetic, carbolic acid, turpentine; and recently san-tonine has been employed, and has proved to be more reliable than any of the other agents. The dose for a horse is 15 grains in a ball, with 3 or 4 drachms of aloes. The medicine should be repeated in a week.

Another of the nematode worms is the whipworm (Oxyurus curvula), which infests the large intestines and occasionally excites irritation of the anus, which is indicated by the horse constantly rubbing the tail. A marked symptom of the presence of oxyurides is the accumulation of yellow dust-like matter around the anus. This is made up of the eggs of the parasite.

The expulsion of these worms is much assisted by occasional enemas of salt and water. An illustration of this worm is given in fig. 275.