Reynal records the appearance of the disease in alternate generations, the stallion offspring of blind parents remaining sound himself but producing foals which became victims of the disease in large numbers. A partial explanation may be found in the better conditions under which the stud horse was kept, while under less favorable surroundings his offspring developed the disease.
It must be added that every condition which induces debility or ill health is favorable to the development of the malady. The presence of worms in the bowels is a familiar example. Any debilitating disease like strangles, influenza, contagious pneumonia, indigestion, etc., and overwork or insufficient or indigestible food act in the same manner.
Again, local irritants may rouse the latent tendency. Street or stable dust, sand, hay seeds, chaff, blows on the eye with whip or other object, wounds, irritant gases, smoke, fierce light, cold draughts, storms, and whatever determines inflammation of the eye may be the occasion of an outbreak of recurrent ophthalmia.
Microbiology. It will be recognized that none of these causes fully account for the specific and recurrent nature of this affection, and it is felt that something more is wanted to furnish a full and satisfactory explanation of the malady. This explanation is sought in a direct infection, but in spite of extended investigations by many observers no specific microbe has been demonstrated as uniformly present in all cases.
Potapenke found in the blood of the affected horses a plasmodium like that of ague. This agrees with the damp regions in which the malady prevails and no less with its intermittent or periodic character.
Vigezzi found in the aqueous and tissues a micrococcus (ophthalmo-coccus) which, cultivated on gelatin agar and inoculated in the anterior chamber or under the conjunctiva, produced an affection which he recognized as recurrent ophthalmia.
Trinchera found in the aqueous of the affected animals a bacillus and cocci. Drawing this aqueous into a sterilized syringe and injecting it into the anterior chamber of sound horses produced in 12 to 48 hours characteristic periodic ophthalmia. This was repeated by Schütz and Schwartznecker.
Robert Koch found in the affected aqueous, cocci, singly or in chains and bacilli with rounded ends. Injection of the latter into a sound horse’s eye led to characteristic inflammation and loss of vision. In the cornea of the rabbit it had no effect.
Richter found in the eyes of a foal born with recurrent ophthalmia, of sound parents, diplococci and triplococci.
These observations do not demonstrate the constant presence of one definite microbe, nor that the disease is invariably due to any one particular organism, yet they may be held as strongly suggestive that any one of a variety of microörganisms may prove an exciting etiological factor in a susceptible system, or that, along with the organisms heretofore demonstrated, there exists an essential microbian cause which has up to the present eluded detection.