But as yet no specific pathogenic microbe has been demonstrated so that this doctrine must still be held as a mere plausible hypothesis.
Many veterinarians with long experience in such cases absolutely deny contagion. The hypothetical contagion undoubtedly extends slowly, and uncertainly from animal to animal, probably, like actinomycosis, taking place mainly through the soil, or some outside medium, rather than by direct contact; or a special susceptibility on the part of the individual animal may be necessary to render it effectual.
Accessory Causes can be spoken of more confidently but even of these no one, nor small group, can be advanced as essential. The process of bone nutrition is readily disturbed by a variety of conditions, and such disturbances may easily become the occasion of weakening the resisting power and mayhap of admitting the hypothetical microbe to get in its pathogenic work.
Faulty food has been a favorite explanation. A lack of lime in the soil and fodder seems, at times, to have had a baneful effect, if only, in lowering the general tone and impairing the nutrition. Yet we see osteoporosis on limestone soils (New York, etc.), and in animals generously fed on grain. The same remarks apply to phosphorus and phosphates. Their deficiency apparently contributes to the production of the disease, and yet under other conditions, their abundance is no barrier to its development. The excess of free phosphorus produces osteitis and it is held by some that an over-abundance of phosphates acts in the same way. It has been sought to incriminate a too nitrogenous diet in some cases, and in others one too rich in fat or carbohydrates. The many cases in Philadelphia and Pennsylvania were mostly in animals that had been well fed and were in good condition when attacked (Marshall).
Special food may be the direct cause, bran diet has been already noted. Hinebauch found an acute osteitis with bone softening and arthritis in horses fed on millet, green, partially matured and ripe. Horses elsewhere have fed on millet, without such results, but not perhaps, in the same environment, nor in presence of the hypothetical microbe. Millet is not the sole nor common cause of osteoporosis, but there is reason to suspect that it is at times an important accessory cause.
Of all prejudicial conditions none is to be so dreaded as unwholesome stables. Of 200 cases reported by Berus, in Brooklyn, almost all were in cellar stables or those with floors laid on the soil. Meyer finds that “most all cases can be traced to an unwholesome atmosphere, or gases arising from vaults, sewers, cellars, filthy streams, or from a hollow space under the floor.” Harbaugh says every case was stabled in damp, ill-drained, unventilated and badly lighted buildings. The worst outbreak was in a basement with a damp wall, on one side, and none suffered except those that stood next to this wall. The horses standing on the opposite side, which was on a level with the ground outside, escaped. Removal from a cellar stable to the floor above, put a sudden stop to the appearance of new cases. James, of St. Louis, found 20 successive cases in a stable on a dirt floor, and Jasme, of Charlestown, finds nearly all his many cases on earth floors in malarial regions.
Malaria has been blamed, especially by southern observers, accustomed to see the disease on the warm alluvial seaboard and river bottoms. That this environment predisposes to the disease, by undermining the health, is doubtless the case, but in spite of occasional remissions in the symptoms, malarial germs cannot be set down as the constant cause. One of the worst cases I ever saw, with every bone in the body soft, spongy and light, developed at Inglis Green laundry, Edinburgh, where malaria is absolutely unknown, but where the brook received large quantities of chlorine.
Cold is an undoubted factor, though the disease is most prevalent in our warm southern states. Many veterinarians have noticed its coincidence with rheumatism, in which cold is so often the dominating accessory cause. Some have even suspected that it is only a modified type of the rheumatic condition. Hinebauch found his cases of millet disease in cold basement barns, or with leaking roofs, so that the floor and bedding were constantly wet. He found that cold always aggravated the disease, and bad air even more so, while salicylates seemed to have a marked curative influence.
Damp soils should be named in this connection. These not only chill the air by evaporation, and condense the cold dews at night, contributing to produce the extremes of hot noon day and cold night temperature, with corresponding disturbances of the bodily health, but they favor the preservation of the infinitesimal forms of life (bacteria, protozoa) and therefore of the hypothetical microbe of the disease.
City life is a most potent cause. Berus tells us that hundreds of horses die yearly in Brooklyn of osteoporosis, and that if sent early to the country a large proportion recover. The same is true of New York City and Philadelphia. Of Cincinnati, Meyer says that he has failed to find a case more than five miles outside the city limits, and that cases sent to the country make a partial or complete recovery. If returned to their former city stables, nearly all contract the disease anew within a year.