A form of ophthalmia which is known as periodic, or sometimes as "moon-blindness", is peculiar to the horse, and in all probability is chiefly due to heredity. The cause to which it was formerly attributed, i.e. irritating gases and badly ventilated stables, is evidently inadequate to explain its presence. The fact that other animals are constantly exposed to the influence of the same conditions without suffering is in itself sufficient to throw considerable doubt on the once-accepted explanation.
It is undoubtedly true that the disease is much less frequent than it formerly was, and it is also true that for many years past great care has been taken at horse shows to reject all animals which gave evidence of having suffered from the malady. In this way breeding animals affected with the disease have been marked out, and their use largely curtailed in consequence.
Symptoms of periodic ophthalmia are in the first instance closely allied to those of simple ophthalmia, but a great contrast is observed in the more advanced stages. Swelling of the eyelids, redness of the conjunctiva, and discharge of tears are the chief indications of the early stage of the disease. To the experienced examiner the diseased eye presents a characteristic appearance even at the outset of the malady, which becomes more and more pronounced with each succeeding attack. A slight contraction in the centre of the upper lid, the edge of which is drawn upward (fig. 254), gives a peculiar triangular appearance to the opening between the two lids. This is an appearance which is not met with in simple ophthalmia; the swelling is less marked also, and when there is a sufficient space between the partially closed lids to allow the surface of the cornea to be seen, a peculiar amber tinge of the part is observed. This is caused by a yellow exudation into the anterior chamber, which at once distinguishes the disease from simple ophthalmia.
Fig. 254. - Recurrent Ophthalmia.
Eye showing an angular condition of the upper lid the result of a succession of attacks of Specific Ophthalmia.
During the progress of periodic ophthalmia various changes take place. The swelling of the eyelids and the congestion of the conjunctiva gradually decrease. The amber tint, which at first extended over the whole of the cornea, becomes concentrated at the lower part, owing to the deposit of lymph gradually falling to the bottom of the chamber. In a week or two the whole of the eye begins to become clear, and in from three to six weeks there may be no trace of the disease left, and the organ is to all appearance sound. This condition may possibly continue for the space of a month, or sometimes much longer, but a recurrence of the attack is a matter of certainty. Generally the disease reappears in the same eye, but occasionally the eye originally attacked remains apparently healthy and the opposite eye becomes affected, the course of the disease in it being as nearly as possible identical with that which has been described.
Probably owing to peculiarities in the system of the horse, the disease at different times assumes an extremely acute, or a sub-acute, or chronic form. In some instances it appears to be concentrated in the eye first attacked, which suffers from repeated reappearances of the disease at short intervals, each attack leaving some morbid changes behind it affecting the internal structures, and ending in total blindness of the affected eye, either from the opacity of the crystalline lens or from the deposit of a large quantity of inflammatory material in the anterior chamber, and ultimately the formation of a false membrane lining the inside of the cornea causing a permanent yellow opacity. In this case it is impossible to ascertain what may be the condition of the structures behind the iris, but post-mortem examinations in a few cases have shown that the whole of the internal structures are implicated in the changes which are apparent in the front of the eyes, the vitreous body and the crystalline lens being the seat of the same kind of yellow deposit which occurred in the anterior chamber.
In some instances black pigment spots are noticed in front and towards the outer margin of the lens. These indicate that the iris has been adherent to the lens as a result of the inflammatory attack, and the pigment behind the former has been left on the latter. Sometimes the two remain permanently united.
When periodic ophthalmia was much more common than it is in the present day, it was a matter of observation that if the disease assumed the acute form, and repeated attacks occurred in one eye, ending in blindness, the other eye remained unaffected; but when the affection appeared first in one eye and then in the other, alternately, the result was the gradual impairment of the vision until the sight was entirely lost in both.
Treatment of the recurrent form of ophthalmia will be the same as that which has been recommended for the simple form of the disease, including a frequent application of warm water while the inflammation exists, and the use of a mild astringent lotion afterwards. It will, however, be understood from what has been said in reference to the course of the disease, that all treatment must be looked upon as merely palliative. There is no remedy known which has any influence in preventing the recurrence of the affection or in checking those morbid changes which sooner or later terminate in total loss of vision.