Many cases, however, are found in the country and often within a circumscribed area. These indicate, as in the city cases, a localized cause, bacteridian or otherwise.
Early life predisposes, the majority of cases taking place before the sixth or seventh year, yet the disease occurs at all ages up to twenty years and over.
Breed does not seem to make a material difference, and though Shetland ponies have acquired a bad reputation, their propensity to become fat and soft, their too often idle, pampered life, and the confined quarters in which they are frequently kept, account for much of the mortality.
Asses and mules habitually escape, even in the South, where the latter animal is so numerous and often so poorly kept. In Hinbauch’s millet disease, the mules suffered more than the horses.
Breeding horses, male and female, have often acute attacks and die early. The drain on the system and confinement seem to act injuriously.
Nature. Until we know the essential cause or causes of osteoporosis, we must be in doubt as to its pathology. We are even debarred from pronouncing authoritatively upon the essential identity or difference of the various forms of softening, or rarefaction of bone. In obedience to the clinical manifestations and structural changes, rachitis, fragilitas ossium and osteoporosis have been separately described, but we cannot positively say that they are not all due to one essential cause, manifesting itself differently according to the activity of trophic processes in the bones of the victim. In the growing foal the active developmental processes in the epiphyseal ligament and periosteum may determine that the symptoms shall be pre-eminently those of rickets, yet we often see these complicated by the facial and other lesions of osteoporosis. Both may be the result of one etiological factor, or there may be a complex disease resulting from the presence of two. Again in the pregnant cow in which the relaxation of the ischio-pubic symphysis and pelvic ligaments means a profound change in the bone nutrition at this point, the presence of the hypothetic microbe, or other essential factor, may determine a decalcifying and fragility of the pelvic bones generally. Again in the mature male and non-breeding female, in the absence of the disturbing conditions of nutrition just named, the vascular elements in the Haversian canals and cancelli may determine the simple rarefaction and expansion of the bone which characterizes osteoporosis. When present in the bone in any of these conditions, acids doubtless fulfill an important rôle in the decalcifying and softening process, but behind these it may be surmised that there is an unknown cause or causes, which it is for the bacteriologist, chemist or botanist to discover.
Symptoms. These are largely the same as in brain disease. There may be first a period of illness, with poor appetite, lack of spirit and energy, early perspiration and fatigue, or if at large, leaving the herd, soon followed by some stiffness of gait and lameness, which may be intermittent, disappearing under exertion, or shifting from one joint or limb to another after the manner of rheumatism. Sometimes it shows in stiffness of the neck, so that the patient finds difficulty in lowering the head to graze; in others the back and loins are stiff and arched so that the animal has difficulty in rising and turns slowly and painfully; in still other cases the dorsal and lumbar vertebræ are depressed so that the back is hollow. Even before the manifestation of lameness, the affected limb may stand forward at the fetlock or knee, the gait is clumsy and awkward, and the patient may suddenly stumble and fall, showing little or no power of prompt recovery of balance. A horse, and especially a young horse, with this habit of stumbling is always to be suspected. The long bones of the limbs tend to enlarge or thicken, and this is likely to be more uniform than in rachitis, and not to be confined so much to the epiphysis. The implication of the stifle, hock or other joint, with marked synovial distention, and mobility or dislocation of the patella, is common and may be the earliest manifestation of illness. The bones of the face usually show early changes. The superior maxillary and nasal bones, beneath the zygomatic spine and infra-orbital foramen and along the line of the molar alveoli become especially bulging and rounded, the other facial and cranial bones suffering to a lesser degree. In the lower jaw, also, the disease predominates along the region of the molar alveoli, and the loosening of the molars permits them to deviate inward so that the grinding surfaces come perceptibly nearer to the median line, and the outer half of the tooth is rapidly worn while the inner edge projects as a sharp cutting ridge (chisel teeth).
For the same reason, the softening branches of the lower jaw deviate inward, tending to still further destroy the due approximation of the upper with the lower molars, and to diminish the breadth of the intermaxillary space. The great thickening of the rami of the maxilla tends still further to reduce the intermaxillary furrow.
Fractures and detachment of tendons and ligaments are common results of the rarefaction, a portion of the bone often remaining adherent to the tendon.
Cary gives the following statistics of fifteen cases: lame in the fore limbs 11 (mostly shoulder), in hind limbs 11 (mostly hip and stifle), stiff in loins 8, unable to rise without aid 3, had indented ribs 4, had shifting lameness 8, had chronic indigestion 6; mares 5; geldings 8; mules 2; ages were—one 3 years, five 6 years, three 7 years, one 8 years, two 9 years and three 10 years.